Global and European climate impacts of a slowdown of the AMOC in a high resolution GCM

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Global and European climate impacts of a slowdown of the AMOC in a high resolution GCM Article in Climate Dynamics · March 2015 DOI: 10.1007/s00382-015-2540-2

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Abstract The impacts of a hypothetical slowdown in the Atlantic Meridional

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Overturning Circulation (AMOC) are assessed in a state-of-the-art global climate

3

model (HadGEM3), with particular emphasis on Europe. This is the highest reso-

4

lution coupled global climate model to be used to study the impacts of an AMOC

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slowdown so far. Many results found are consistent with previous studies and can

6

be considered robust impacts from a large reduction or collapse of the AMOC.

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These include: widespread cooling throughout the North Atlantic and northern

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hemisphere in general; less precipitation in the northern hemisphere midlatitudes;

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large changes in precipitation in the tropics and a strengthening of the North

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Atlantic storm track.

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The focus on Europe, aided by the increase in resolution, has revealed pre-

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viously undiscussed impacts, particularly those associated with changing atmo-

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spheric circulation patterns. Summer precipitation decreases (increases) in north-

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ern (southern) Europe and is associated with a negative summer North Atlantic

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Oscillation (NAO) signal. Winter precipitation is also affected by the changing

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atmospheric circulation, with localised increases in precipitation associated with

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more winter storms and a strengthened winter storm track. Stronger westerly

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winds in winter increase the warming maritime effect while weaker westerlies in

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summer decrease the cooling maritime effect. In the absence of these circulation

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changes the cooling over Europe’s landmass would be even larger in both seasons.

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The general cooling and atmospheric circulation changes result in weaker peak

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river flows and vegetation productivity, which may raise issues of water availabil-

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ity and crop production.

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Keywords Climate · impacts · AMOC

Climate Dynamics manuscript No. 2(will be inserted by the editor)

L.C. Jackson et al.

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Global and European climate impacts of a slowdown of

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the AMOC in a high resolution GCM

27

L.C. Jackson · R. Kahana · T. Graham ·

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M.A. Ringer · T. Woollings · J.V. Mecking ·

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R.A. Wood

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Received: date / Accepted: date

L.C. Jackson Met Office Hadley Centre, Exeter, UK. Tel.: +44 1392 885680 Fax: +44 1392 885681 E-mail: laura.jackson@metoffice.gov.uk

R. Kahana Met Office Hadley Centre

T. Graham Met Office Hadley Centre

M.A. Ringer Met Office Hadley Centre

T. Woollings University of Reading

Climate impacts over Europe of a slowdown of the AMOC

J.V. Mecking University of Southampton

R.A. Wood Met Office Hadley Centre

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L.C. Jackson et al.

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1 Introduction

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The Atlantic Meridional Overturning Circulation (AMOC) plays a very impor-

33

tant role in the climate system by transporting heat northwards in the Atlantic

34

(∼ 1 PW at 30o N, Bryden and Imawaki (2001)). This circulation is predicted to

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weaken as the climate warms and the surface waters of the North Atlantic become

36

less dense, inhibiting wintertime convection. There is uncertainty as to the extent

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of this weakening since the predictions from global climate models (GCMs) vary

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substantially (Collins et al, 2013) and the AMOC is known to be sensitive to ad-

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ditional fresh water input not fully represented in these GCMs (such as melting

40

from the Greenland ice sheet). Although experiments with estimates of likely addi-

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tional fresh water input in GCMs have shown only moderate additional weakening

42

(Swingedouw et al, 2013), and none of these have shown a rapid shutdown of the

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AMOC, studies of paleoclimate data proxies have suggested that large and rapid

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reorganisations of the ocean currents may have occured in the past (Rahmstorf,

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2002; Clement and Peterson, 2008). There have been suggestions that many GCMs

46

have a bias which might make the AMOC more stable and less sensitive to fresh

47

water inputs (de Vries and Weber, 2005; Jackson, 2013), although this bias has

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been removed in some of the current GCMs (Weaver et al, 2012).

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Although a rapid collapse of the AMOC has been assessed to be very unlikely

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within the 21st century (Collins et al, 2013), such a collapse would cause large

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and rapid changes in the climate. Hence it is regarded as a ’low probability-high

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impact’ event, making it important to assess the risk of those impacts. In addition,

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the magnitude of AMOC decrease in projections of 21st century greenhouse gas-

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induced climate change are very varied, with GCMs in the most extreme climate

Climate impacts over Europe of a slowdown of the AMOC

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change scenario of the fifth IPCC report suggesting a weakening of 12-54% by

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2100 (Collins et al, 2013). Both Good et al (2014) and Drijfhout et al (2012) have

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shown that temperature changes over Europe when greenhouse gases are increased

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are dependent on the magnitude of AMOC reduction, and Woollings et al (2012a)

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and Woollings et al (2012b) have shown that the same is true for changes in the

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Atlantic storm track and reduction in polar lows. Hence a greater understanding of

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impacts associated with a decrease in the AMOC will improve our understanding

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of uncertainties around impacts of anthropogenically forced climate change.

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Previous studies have examined the impacts of a decrease in the AMOC through

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the addition of fresh water into the North Atlantic. These studies have included

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realistic values or more idealised fresh water forcing set-ups, and have been applied

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in preindustrial (Vellinga and Wood, 2002; Stouffer et al, 2006), future (including

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increasing greenhouse gases, Vellinga and Wood (2008); Kuhlbrodt et al (2009))

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and paleoclimate conditions (Manabe and Stouffer, 1997; Kageyama et al, 2012).

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Although most studies have concentrated on large scale temperature and precipita-

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tion changes, some have investigated the influence on sea level (Levermann et al,

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2005), ecosystems and agriculture (Kuhlbrodt et al, 2009) and regional climate

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changes (Jacob et al, 2005; Chang et al, 2008; Parsons et al, 2014). This study is

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unique in that it uses the highest resolution coupled global model for such a study.

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This increased resolution is important for a number of reasons. Firstly, the response

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of the ocean circulation may be affected by increases in the ocean resolution which

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permit eddies. Secondly, there are suggestions that increased resolution can im-

77

prove the atmospheric transport of moisture (Demory et al, 2013) and synoptic

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variability over Europe (Scaife et al, 2011, 2014). Thirdly, the increased resolution

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L.C. Jackson et al.

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allows a much better representation of orography and local weather patterns. In

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particular this can improve our assessment of localised impacts.

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The model and methods used in this study are presented in Section 2. Global

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impacts are discussed in Section 3 and those over Europe in Section 4. Section 5

83

presents the conclusions.

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2 Model and Methods

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2.1 HadGEM3 GC2

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The GC2 (Williams et al, 2015) configuration of the HadGEM3 model (Hewitt

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et al, 2011) is used, consisting of atmosphere, ocean, sea-ice and land-surface mod-

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els. Fluxes of heat, freshwater and momentum are passed between the atmosphere

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and ocean-ice components every three hours while fluxes are passed between the

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ocean and ice models every ocean model time step (22.5 minutes).

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The atmosphere model is version Global Atmosphere vn6.0 (GA6.0; Walters

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and et al (2015)) of the Met Office unified model with a horizontal resolution of

93

N216 (approximately 60 km in mid-latitudes). Demory et al (2013) showed that

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increased horizontal resolution up to approximately 60 km resulted in an increase

95

in the large-scale water transport from the ocean to the land and that models with

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resolution of 60 km and finer agreed well with reanalyses. This was interpreted as

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showing that large-scale storms penetrate further over land in higher-resolution

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models while at lower resolution there is much more local recycling of water (i.e.

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it tends to be evaporated and precipitated locally rather than being advected

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horizontally). Compared to previous generations of climate models, GC2 is notable

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for a very realistic representation of jet stream variability and does not suffer from

Climate impacts over Europe of a slowdown of the AMOC

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102

an underestimation of midlatitude cyclone intensity ((Williams et al, in prep).

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Scaife et al (2014) have shown that the seasonal forecast system GloSea5, which is

104

based on HadGEM3 at N216 resolution, can successfully predict the winter North

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Atlantic Oscillation (NAO) and aspects of European winter variability, although

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the magnitude of the variability is underestimated. The skill of GloSea5 may be

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due to the increased resolution since the previous version of the forecast system (at

108

lower resolution) showed low skill outside the tropics (Arribas et al, 2010). There

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are 85 levels in the vertical meaning that compared to earlier Met Office climate

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models, GC2 has improved resolution of the stratosphere which is thought to

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be important for resolving teleconnections including to the Eurasian atmospheric

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circulation (Ineson and Scaife, 2009).

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The ocean model is the Global Ocean 5 (GO5; Megann et al, 2013) version of

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the ORCA025 configuration of the NEMO model (Madec, 2008) with a nominal

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horizontal resolution of 0.25â—Ś . In the southern hemisphere the grid is an isotropic

116

mercator grid: the meridional grid spacing decreases at high latitudes such that the

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grid cells remain approximately square at high latitude. In the northern hemisphere

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the grid is a quasi-isotropic bipolar grid with poles over land. The eddy permitting

119

horizontal resolution has been shown to reduce a cold bias off the coast of Grand

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Banks as a result of an improved Gulf Stream and North Atlantic Drift path in

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comparison to lower resolution models. The reduction of the cold bias has led to

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an improved simulation of Atlantic and European winter blocking (Scaife et al,

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2011). Because of the increased horizontal resolution the model no longer uses the

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GM (Gent and Mcwilliams, 1990) parameterisation for horizontal eddy mixing of

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tracers.

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L.C. Jackson et al.

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The ocean model has 75 model levels with a thickness of 1m at the surface

127

increasing to approximately 200m at a depth of 6000m. The increased vertical

128

resolution in the upper ocean compared to previous Hadley Centre climate models,

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along with the increased coupling frequency, leads to improved simulation of the

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diurnal cycle (Bernie et al, 2008). Near surface vertical mixing is carried out using

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a modified version of the turbulent kinetic energy scheme (TKE; Gaspar et al,

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1990; Madec, 2008). Unresolved vertical mixing processes are represented by a

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constant background vertical diffusivity of 1.2 × 10−5 m2 s−1 .

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The sea ice model is version 4.1 of the Los Alamos National Laboratory sea ice

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model (CICE Hunke and Lipscomb, 2010) and has the same resolution as the ocean

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model. The model uses Elastic-Viscous-Plastic ice dynamics (Bitz and Lipscomb,

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1999), energy conserving thermodynamics and five sea ice thickness categories.

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The land model used in GC2 is the GL6 configuration of the Joint UK Land

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Environment Simulator (JULES). The model simulates the state of the soil and

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5 vegetation types (broadleaf trees, needleleaf trees, C3 (temperate) grass, C4

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(tropical) grass, and shrubs) in each land grid box. The distribution of these plant

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types is fixed throughout the simulation and represent the world vegetation circa

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1980. For this reason the changes from one type of plant to another, that would

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naturally occur under changing climate are not presented here. We will analyse

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the vegetation Net Primary Productivity (NPP) as a measure of plant productiv-

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ity. NPP is calculated in the model as the difference between the gross primary

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productivity and respiration rate and its changes do not affect the atmospheric

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CO2 or other carbon stores since the model does not include a carbon cycle.

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The model provides estimates of runoff and river flows for each grid cell, which

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are then routed across the domain to the river outflow points. The river’s network

Climate impacts over Europe of a slowdown of the AMOC

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output is at 1 degree resolution which is lower than the atmospheric resolution

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and therefore only enables us to analyse Europe’s biggest rivers. The simulations

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provide an estimation of the natural flow and do not include the effects of dams,

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irrigation or other extractions. This should be taken into consideration when com-

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paring with observational data.

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2.2 Experimental Design

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To examine the impact of a shutdown or large reduction of the AMOC we com-

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pare a simulation where the salinity is perturbed to cause a reduction of AMOC

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strength with a long control run. The model was initialised from a development

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version of GC2 (with only minor model differences) that had been initialised from

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climatology and spun up for over 36 years. The control run of GC2 has been run

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for a further 150 years from this time. Since the spin up time was short there is

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a drift in temperatures and salinities in the control experiment. Drifts in upper

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ocean temperatures and salinities (calculated as area means in separate basins)

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range up 0.8o C/century and 0.1 PSU/century which are smaller than the changes

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experienced in the experiment where the salinity is perturbed (and much smaller

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in the case of salinity). To take into account this drift, anomalies are calculated

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with reference to the same period in the control experiment.

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To reduce the length of simulation required we adopt the method of Vellinga

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and Wood (2002) to induce a collapse of the AMOC. This methodology involves

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perturbing the salinity in the upper layers of the North Atlantic to inhibit deep

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convection and hence quickly shut down the AMOC. Vellinga and Wood (2002)

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found that this method had a similar impact to the more traditional ’hosing’ exper-

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iments where additional freshwater fluxes are applied to reduce salinity. Although

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this method of collapsing the AMOC is unrealistic, it is adequate for the purposes

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of investigating the impacts of a shutdown, but not for giving a credible assessment

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of the rate of change.

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The perturbed experiment is initialised after 42 years of the control run. In-

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stantaneous salinity perturbations are applied to the upper 536 m of the Atlantic

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and Arctic Ocean north of 20â—Ś N each December for the first 10 years of the per-

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turbation run (see Fig. 1). Each salinity perturbation is equivalent to continuously

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adding freshwater at a rate of 10 Sv (1 Sv=106 m3 s−1 ) for 10 years (total of 100

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SvYr). Tapering is applied to the perturbation over 350-536 m to reduce numerical

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problems that could be caused by unrealistically large density gradients. Similarly,

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the perturbation was spread over ten years to reduce the model shock associated

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with the perturbation. As is common practice in hosing experiments the salinity

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in the rest of the ocean is also perturbed such that the total freshwater content of

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the global ocean remains constant.

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In all of the runs the atmospheric CO2 concentration is kept constant (at 1980’s

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levels). This means that changes to the vegetation productivity are caused only by

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the climatic effects of the AMOC decrease (i.e. changes in temperature, and the

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hydrological cycle) and not by the more direct effect that changes in CO2 levels

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have on plant photosynthesis and transpiration (Wiltshire et al, 2013; Hemming

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et al, 2013). This is an important difference between this experiment and future

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projections of plant productivity from increasing greenhouse gases, in which plants

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are affected by both the changing climate and by the physiological CO2 effects.

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All averages presented (unless otherwise stated) are for years 60-90 after the

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start of the freshwater perturbations (50-80 years after the perturbations have

Climate impacts over Europe of a slowdown of the AMOC

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199

ended). Anomalies are presented relative to a 30 year average of the control at

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the same period. Significance is tested by taking 30 year means of the control

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simulation with start dates every 10 years and calculating the standard deviation

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of these mean values. Anomalies are considered significant if they are greater than

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2 standard deviations.

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3 Global response to an AMOC weakening

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The large freshening of the subpolar North Atlantic causes a cessation of convection

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over the deep water formation regions and a rapid decrease in the strength of the

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overturning circulation (see Fig. 2). At the end of the freshwater perturbations

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(year 10), the AMOC strength at 26o N (blue line) partly recovers from a complete

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shutdown to a level of âˆź5 Sv, about a third of its original value. The AMOC

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decrease and recovery is more gradual in the southern hemisphere (30o S, red line).

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The AMOC recovery is different from that in the similar experiment of Vellinga

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et al (2002) and Vellinga and Wood (2002) where the AMOC strength recovered

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steadily over âˆź120 years back to its control strength. In this study the AMOC

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shows no signs of recovery over the first 100 years of the experiment. This difference

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may be caused by the greater perturbation of the salinity field (equivalent to 100

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SvYr rather than 16 SvYr), or may be a result of different dynamical behaviour in

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this model, and will be investigated in a future study.

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Although there is still a weak overturning cell in the Atlantic, there has been a

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dramatic decrease in winter mixed later depth in the subpolar gyre. The maximum

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winter mixed layer depth after the Atlantic freshening is less than 250m (not

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shown) suggesting that there is no longer any deep convection. The northwards

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L.C. Jackson et al.

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transport of heat in the Atlantic ocean (measured at 30o N) is more than halved

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from 1.0 to 0.4 PW (Fig. 2 bottom middle). There is also a reversal in the south

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Atlantic, with the ocean transporting heat southwards rather than northwards

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when the AMOC is reduced. As a result, there is widespread surface cooling of the

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ocean throughout the North Atlantic, cooling of the atmosphere over the North

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Atlantic and throughout the northern hemisphere (Fig. 2 bottom right and Fig.

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3 top) and the northern hemisphere sea ice extends further south in all seasons,

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even reaching as far south as the northern tip of the UK and the Grand Banks at

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its greatest extent in March. The cooling is stronger in winter, particularly in the

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Arctic where the increased ice insulates the atmosphere from the warmer ocean.

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Cooling is particularly intense in the Barents, Greenland-Iceland-Norway (GIN)

233

and Labrador seas which have experienced the greatest increase in sea ice, with

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widespread coverage even in summer when the AMOC is reduced. Temperatures

235

in those regions are reduced by up to 15o C, although the temperature decreases

236

over the Atlantic are more typically 2-5o C in the subtropical gyre and 5-10o C

237

in the subpolar gyre. Here we find that the southwards shift of the ITCZ in the

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Atlantic extends to the other ocean basins causing a global shift to the south in

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the ITCZ.

240

Previous studies have reported similar decreases in surface air temperature

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caused by a reduction of the AMOC, though the amount and extent of cooling, and

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the reduction of the AMOC vary with model and forcing scenario (Stouffer et al,

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2006; Kageyama et al, 2012; Vellinga and Wood, 2002). All show the largest cooling

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over the North Atlantic subpolar gyre, Labrador, GIN and Barents sea regions with

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some experiments in Stouffer et al (2006) showing temperature increases north of

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the hosing regions as a result of northwards shifts in convection. There is a wide

Climate impacts over Europe of a slowdown of the AMOC

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247

range in the zonal extent of the cooling in the northern hemisphere in these studies,

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with most finding cooling over Europe, some with cooling over parts of Asia and

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very few with cooling over North America. The results in this study show a greater

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zonal extent than most, if not all, previous experiments. In common with other

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studies there is a slight warming across the southern hemisphere, particularly in

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the south east Atlantic.

253

Precipitation patterns indicate a southwards shift of the Intertropical Conver-

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gence Zone (ITCZ) caused by changes in the Atlantic sea surface temperature

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gradients. This results in large equatorial precipitation anomalies in the Atlantic,

256

extending into the Pacific and Indian Oceans and affecting precipitation over Cen-

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tral and South America, Africa and south east Asia (Fig. 3 middle). Over much

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of the northern hemisphere precipitation is decreased, consistent with a cooling-

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induced decrease in atmospheric moisture content. Other studies have also found

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a drying over the North Atlantic and a southwards shift of the Atlantic ITCZ (Vel-

261

linga and Wood, 2002; Stouffer et al, 2006; Kageyama et al, 2012), however the

262

precipitation anomalies over the tropical Pacific and Indian oceans vary substan-

263

tially between studies. One striking difference in this study is the large drying seen

264

over the Amazon. The large reduction of precipitation over the Amazon occurs in

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its wet season (DJF) although this is partly compensated by an increase in pre-

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cipitation in the dry season (JJA). This reduction in seasonality over the Amazon

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was also seen in Parsons et al (2014). They found that although the annual mean

268

precipitation decreased over the Amazon (as did the soil moisture and terrestrial

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water storage), there was actually an increase in primary productivity since the

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water stress in the dry season was reduced. In this study we find a decrease, rather

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L.C. Jackson et al.

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than increase, of primary productivity (not shown) which might be explained by

272

the greater decrease in precipitation found here.

273

4 Impacts on Europe

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The climate of Europe, particularly western Europe, is significantly influenced by

275

Atlantic sea temperature due to the prevailing westerly weather systems. Hence,

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large temperature decreases over the Atlantic ocean have widespread impacts

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throughout Europe.

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4.1 Cooling

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Europe experiences widespread cooling when the AMOC is reduced (Fig. 4) with

280

the greatest cooling in the far north of Europe in winter which appears to be

281

greatly affected by the amplified winter cooling in the Arctic, with temperature

282

decreases in excess of 10o C. Western Europe shows a more uniform cooling year

283

round. These results are consistent with previous studies that have mainly shown

284

cooling over Europe of a few degrees, with the greatest annual mean cooling over

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western and northern Europe (Vellinga and Wood, 2002, 2008; Jacob et al, 2005;

286

Laurian et al, 2010). One exception is Kuhlbrodt et al (2009) who found little

287

cooling over northern Europe when using a downscaling technique to give regional

288

projections on a finer resolution (0.5o x0.5o ) from a coarse resolution global climate

289

model. This result may be a consequence of their downscaling technique since their

290

global model shows wide scale cooling over the whole region. Generally cooling over

291

land is less than at similar latitudes over the ocean, with the exception of eastern

292

Europe in winter. The greater cooling of surface air temperature over the ocean

Climate impacts over Europe of a slowdown of the AMOC

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293

is to be expected because of the large reduction in surface heat flux out of the

294

ocean, consistent with a large reduction of meridional ocean heat transport (Fig.

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2). However the European land mass is strongly affected by the marine climate

296

and several factors that might affect the land-sea contrast in the cooling pattern

297

are discussed below.

298

Laurian et al (2010) propose that the land-sea contrast in the cooling pattern

299

is maintained by changes in cloud cover. They find that the oceanic cooling causes

300

a more stable marine boundary layer and more low clouds over the ocean (see also

301

Klein and Hartmann (1993)). They also show that there is reduced transport of

302

moisture from the ocean to the land resulting in a decrease in clouds over land.

303

Consistent with Laurian et al (2010), in the annual mean we see an increase in

304

cloud over the ocean and a reduction over land (Fig. 5f). This results in radiative

305

changes at the top-of-atmosphere (TOA), with the spatial pattern of the cloud

306

radiative changes being dominated by the shortwave (rather than longwave) com-

307

ponent (Fig. 5b,d), suggesting that it is primarily driven by changes in low-level

308

cloud. These changes in the cloud radiative components of the TOA flux generally

309

warm the land and cool the ocean. The land-sea contrast in the cloud forcing is

310

opposed by that in the clear sky (no cloud) radiative components. Over land there

311

is an increase in surface albedo due to increased snow cover (Fig. 10) which was

312

also seen by Jacob et al (2005), resulting in more reflected shortwave radiation,

313

and a cooling. This mitigates the shortwave warming over land resulting from the

314

reduction in cloud and reduces the land-sea contrast in the total shortwave radia-

315

tive component (Fig. 5c). There is also less outgoing longwave clear sky radiation,

316

particularly over the ocean, since the ocean has cooled more (Fig. 5a). Hence the

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L.C. Jackson et al.

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net (shortwave+longwave) TOA radiation balance shows no consistent contrast

318

between the changing fluxes over land and ocean (Fig. 5e).

319

There are also important seasonal variations in these changes. Cloud and

320

albedo changes in winter (Fig. 5h) are similar to those in the annual mean, al-

321

though the albedo changes dominate the net TOA flux leading to a net cooling

322

over land (not shown). In summer the cloud changes are different (Fig. 5g), with

323

increased cloud over southern Europe and reduced cloud over northern Europe.

324

There is still a relative increase in cloud over ocean compared to that over the

325

land, however. Greater solar insolation leads to an enhancement of the shortwave

326

cloud radiative effect and hence the cloud changes dominate the radiative impact

327

in summer (not shown).

328

Another process that can affect the constrast in land-ocean temperature changes

329

is the change in thermal advection to continental Europe. Colder ocean tempera-

330

tures would suggest that this advection should have a net cooling effect, however

331

the change in advection will also be affected by changing wind patterns. In winter

332

there is a strengthening of the southwesterly winds and in summer a weakening.

333

This is consistent with changes in sea level pressure (SLP) shown in Fig. 8 and

334

discussed further in Section 4.2.

335

The change in thermal advection between the perturbed and control experi-

336

ment shows anomalous warming over land in both summer and winter (Fig. 6; see

337

Appendix for definition). In winter the predominantly southwesterly winds nor-

338

mally have a warming effect on Europe since the ocean is warmer than the land.

339

When the AMOC is weak, although the ocean cools more than the land, the ocean

340

is still warmer than the land in absolute terms, so the increase in southwesterly

341

winds acts to warm Europe. In summer the southwesterly winds normally have a

Climate impacts over Europe of a slowdown of the AMOC

17

342

cooling effect on Europe since the land is warmer than the sea. When the AMOC

343

is weak this temperature gradient is stronger, but there is also a weakening of the

344

westerly winds. The net effect is a relative warming.

345

Although the changes in seasonal mean thermal advection suggest a warm-

346

ing in both seasons, the total thermal advection is complex and also depends on

347

high frequency wind fluctations. Periods of time where the wind is not from the

348

prevailing southwesterly direction will also have an impact on temperatures: par-

349

ticularly when the wind is from the much colder north. The importance of changes

350

in thermal advection to the actual surface temperature change also needs to be

351

demonstrated. To show how advection from the prevailing winds affects European

352

surface temperature (TS ), a regression model was built between area averaged TS

353

and seasonal mean advection at 850 hPa (see Appendix). This gives significant

354

correlations of 0.5-0.7 for central European regions giving us confidence in the im-

355

portance of the seasonal mean advection. This regression model can also be used

356

to separate the changes in thermal advection from temperature changes and wind

357

changes. Applying the regression model developed between thermal advection and

358

TS to the advection calculated with no wind changes (using winds from the con-

359

trol experiment) suggests that the cooling over Europe in the absence of the wind

360

response would be of order 10-30% stronger. Hence the atmospheric circulation

361

changes result in a reduction in the cooling over land in Europe.

362

4.2 Precipitation and weather patterns

363

The high horizontal resolution in this study reveals specific features such as en-

364

hanced precipitation over topographical features and western coasts (Fig. 7), sug-

18

L.C. Jackson et al.

365

gesting that we can have greater confidence in regional and local projections. When

366

the AMOC is reduced there is a general reduction in precipitation over Europe as

367

colder temperatures reduce the amount of water that is held in the atmosphere,

368

however there are also local changes in precipitation that are likely caused by

369

changes in atmospheric circulation.

370

Summer precipitation changes show a clear distinction between the Mediter-

371

ranean, which becomes wetter, and northern Europe, which becomes drier. Vel-

372

linga and Wood (2002) and Jacob et al (2005) report drying in both seasons across

373

Europe, although in Vellinga and Wood (2008) there is some signal of increased

374

annual mean precipitation over the Mediterranean sea, but not land. It should

375

be noted that the increase in precipitation in the Mediterranean in both this

376

study and that of Vellinga and Wood (2008) is small in absolute terms (about 0.5

377

mm/day) although this can mean an increase of about 35% in summer rainfall.

378

These regional changes can be contrasted with projected changes in future sce-

379

narios with increased CO2 where summer rainfall is predicted to decrease across

380

Europe (Collins et al, 2013). Hence precipitation changes associated with a large

381

AMOC reduction could be expected to reinforce the drying over northern Europe

382

and oppose the drying over southern Europe.

383

The different responses for northern Europe and the Mediterranean resem-

384

ble the observational findings of Sutton and Dong (2012). They found increased

385

(decreased) summer precipitation over northern Europe (the Mediterranean) as-

386

sociated with warm surface Atlantic temperatures during years with a positive

387

Atlantic Multidecadal Oscillation (AMO) index. They attribute their patterns to

388

a positive summer North Atlantic Oscillation (NAO) pattern (Folland et al, 2009)

389

with anomalously low sea level pressure over the UK. Since our experiment has

Climate impacts over Europe of a slowdown of the AMOC

19

390

anomalously cold, rather than warm, surface temperatures over the Atlantic we

391

find the signal with the signs reversed: decreased (increased) precipitation over

392

northern Europe (the Mediterranean). Also consistent with the Sutton and Dong

393

(2012) findings is an anomalous high pressure signal over the UK and Scandi-

394

navia in summer resulting in a negative summer NAO (Fig. 8 left). Magnitudes

395

of both the precipitation and pressure anomalies are of similar size to those found

396

in observations by Sutton and Dong (2012), although the sea surface temperature

397

(SST) changes in the Atlantic are 5 to 10 times larger in this study. Other studies

398

have also suggested that the atmosphere responds too weakly to North Atlantic

399

sea surface temperatures in models (Gastineau et al, 2013; Scaife et al, 2014),

400

although a stronger response has been found in models with higher resolution

401

(Minobe et al, 2008; Kirtman et al, 2012). This suggests that the magnitude of

402

the summer precipitation response to Atlantic cooling could be much larger.

403

In winter there is a general drying over Europe apart from over few localised

404

regions, in particular western coasts of the UK and Norway. These regions are

405

affected by a strengthening and eastwards extension of the winter storm track

406

(measured by the variance of filtered 6 hourly winter SLP, as in Brayshaw et al

407

(2009)) shown in Fig. 9. Although there is less water content in the atmosphere, the

408

intensification of the storm track implies more and/or more intense winter storms

409

resulting in localised increases in precipitation where the storms make landfall.

410

This strengthened storm track also results in an increase in mean winter wind

411

speed along its path (Fig. 9, bottom right) with increases of 10-20% over land.

412

Brayshaw et al (2009) similarly found an increase in strength and eastwards

413

penetration of the storm track in an experiment with a weakened AMOC. They

414

find that these changes in the storm track are caused by a strengthening of the

20

L.C. Jackson et al.

415

SST gradients over the northern Atlantic (as the subpolar gyre cools more than

416

the subtropics) which increases the baroclinicity of the near surface atmosphere.

417

Associated with this strengthening of the storm track we see a strengthening of the

418

Azores high which projects onto the winter NAO pattern resulting in a shift to a

419

more positive winter NAO (Fig. 8 bottom right). There is also evidence from both

420

observations and models of a consistent relationship between multidecadal Atlantic

421

cooling variations and the positive NAO, especially reflecting a strengthening of

422

the Atlantic jet (Peings and Magnusdottir, 2014; Woollings et al, 2014).

423

The wintertime upper tropospheric circulation response is almost identical to

424

that in Figure 5 of Brayshaw et al (2009), so is not shown here. This response

425

comprises a strengthening of both the eddy-driven jet at about 55o N and the sub-

426

tropical jet at about 25o N, enhancing the split jet structure over the eastern North

427

Atlantic. As in Brayshaw et al the meridional wind shows that the strengthened

428

subtropical jet is driven by a local enhancement of the Hadley Cell over the trop-

429

ical SST anomalies. The strengthening of the eddy-driven jet is consistent with

430

the increase in baroclinicity, and hence storm activity over the mid-latitude SST

431

anomalies. There is potential for the tropical SST anomalies to have an additional

432

influence on the eddy-driven jet, however further experiments would be needed

433

to investigate this. In summer (JJA) the subtropical response is similar to win-

434

ter but in the extratropics the eddy-driven jet shifts to the north rather than

435

strengthening.

436

Projections of winter precipitation change under increasing CO2 show increased

437

precipitation over northern Europe and reduced precipitation over southern Eu-

438

rope (Collins et al, 2013). In a scenario with an additional strong decrease of

439

the AMOC we might then expect the precipitation patterns associated with the

Climate impacts over Europe of a slowdown of the AMOC

21

440

AMOC decrease to oppose the increased precipitation in northern Europe, apart

441

from on western coasts, and reinforce the drying across southern Europe.

442

The rate of snowfall increases in most regions and seasons, despite the decrease

443

in total precipitation, and the proportion of precipitation falling as snow increases

444

everywhere. SON (Fig. 10 top left) and JJA (not shown) show increased rates of

445

snowfall everywhere, though in the latter these are still very small. In DJF (Fig. 10

446

top right) and MAM (not shown) there is an increase in snowfall rates over western

447

Europe and particularly along the path of the strengthened storm track where

448

rates nearly double over Western Scotland and Norway. Parts of Scandinavia and

449

Eastern Europe show decreased snowfall. The increased snowfall, particularly in

450

SON and MAM indicates a prolonged snowy season that starts earlier and finishes

451

later in the year. This might be expected for a colder climate and has been noted

452

in previous studies (Vellinga and Wood, 2008; Jacob et al, 2005). There is also an

453

increase in snow depth everywhere and an increase in the number of months with

454

significant snow cover (depth > 5cm). In particular central Europe and Scotland

455

experience a large increase in the length of the season with snow cover (Fig. 10

456

bottom).

457

4.3 Rivers and surface runoff

458

The cooling and drying over Europe and the change into a more snowy regime

459

lead to a clear and consistent reduction in surface runoff and river flow across

460

Europe (Fig. 11 and 12). The runoff anomaly maps reflect the spatial pattern of

461

changes to the hydrological cycle, while the river flows enable us to examine the

462

changes over an entire river basin. Changes to river flows in southern European

22

L.C. Jackson et al.

463

rivers mainly reflect the direct rainfall, while in northern and Eastern Europe

464

they are also driven by changes to snowmelt amount and timing. The large-scale

465

decrease in runoff is most pronounced in winter (Fig. 11a). In summer, however,

466

runoff increases in southern Europe and the Mediterranean (consistent with the

467

seasonal precipitation changes shown in Fig. 7) and in northern Europe, where it

468

might indicate a snowmelt peak later in the season.

469

The long-term average monthly flow for the Garonne and the Danube rivers

470

is shown in Fig. 12a,b. We have extracted the simulated river flows for the con-

471

trol and the perturbed experiments from grid cells adjacent to river gauges that

472

represent, as much as possible, the entire basin, and present it along with the

473

observational data from GRDC river gauges (GRDC). The model captures the

474

flow reasonably well but does not always reproduce the seasonality or the variabil-

475

ity of the observations, possibly due to the relatively coarse model resolution as

476

well as human interventions to the natural flow by means of dams and irrigation.

477

Changes to the flow in the Garonne in Southern France are typical for rivers in

478

western Europe and show a decrease of the winter high flow of about 30%. The

479

maximum flow in early spring is shifted from March to April and the differences

480

in low flow during late-summer are smaller, suggesting a less pronounced seasonal

481

cycle of the flow. The considerable decrease in discharge is typical for large rivers

482

throughout Europe, and the stronger decrease in winter is more pronounced in

483

rivers in southwest Europe. For example, rivers In the Iberian peninsula show

484

very strong decreases (up to 80%) of predicted winter high flow while late summer

485

low flow values decreased only by 0-30%. This is consistent with the pattern of

486

precipitation change in southern Europe (Fig. 7).

Climate impacts over Europe of a slowdown of the AMOC

23

487

Rivers in central and eastern Europe show a more uniform reduction of river

488

flow under a weaker AMOC climate (Fig. 12b) and in some, both the high and weak

489

flows are shifted forward by a month. The magnitude of flow reduction (30-80%) is

490

comparable, if not larger than changes predicted under the most severe projections

491

of future climate change (Milly et al, 2005; Falloon and Betts, 2006; Hurkmans

492

et al, 2010). Future climate change simulations predict an annual increase in runoff

493

in northern Europe, mostly in winter and earlier spring due to earlier snowmelt

494

(Falloon and Betts, 2010). A weakened AMOC, however, results in a decreased

495

flow throughout Europe, which can have implications for natural systems and a

496

range of human activities.

497

4.4 Vegetation

498

The AMOC weakening causes a strong reduction in plant productivity across the

499

northern hemisphere. The decrease is most pronounced in high latitudes (> 50o

500

N) and in the growing season (spring and summer). The decrease of Net Primary

501

Productivity (NPP) due to the colder and drier conditions over Europe is shown

502

in Fig. 13 and generally follows regional patterns of change to the net precipitation

503

(precipitation - evapotranspiration, Fig. 3).

504

The decline in plant productivity is marked over western Europe temperate

505

forest regions in spring and over higher latitude boreal forest regions in summer

506

(Fig. 13a,b). The model is capable of separating NPP of different types of vege-

507

tation (not shown) and shows that under the ’weakened AMOC’ climate regime

508

there are substantial (30-50%) reductions in productivity of broadleaf forests in

509

the Baltic and Mediterranean, and needleleaf forests in northern Europe. There

24

L.C. Jackson et al.

510

is an increase in summer productivity, of about 10-40% along parts of the north

511

Mediterranean coast in Turkey and Greece, (associated mainly with shrubs), and

512

a small (<30%) increase in summer productivity of grasses and shrubs in France

513

(Fig. 13a,b). Plants in these regions might have benefitted from the marked de-

514

creases in evapotranspiration and increases in precipitation.

515

Lower productivity of land vegetation at the time of reduced AMOC was found

516

in Zickfeld et al (2008) and Vellinga and Wood (2002) and for some regions in

517

Kuhlbrodt et al (2009) (see their Fig. 10). Plant productivity reductions over

518

Europe in this experiment are much more uniform and an order of magnitude

519

larger then those in Kuhlbrodt et al (2009) which is consistent with the smaller

520

cooling and spatially variable precipitation changes they present. The scenario

521

they used included much higher levels of CO2 which enhanced plant productivity

522

and dominated any effect from the AMOC weakening. The direct CO2 fertilisation

523

effect was shown by Falloon and Betts (2010) and Wiltshire et al (2013) to increase

524

plant productivity under CO2 -induced future climate change scenarios and is not

525

modelled here.

526

Our estimated annual vegetation productivity decrease over Europe is some-

527

what larger than the 0.9 Gt Carbon per year (about 16% decrease) found by

528

Vellinga and Wood (2002). We estimate an annual decrease of 3.7 Gt C or a re-

529

duction of 26% in primary productivity over Europe. These larger reductions are

530

consistent with a stronger magnitude of change in most other parameters exam-

531

ined here and are likely to be a response to the greater cooling found in this study

532

(see Section 4.1)

Climate impacts over Europe of a slowdown of the AMOC

25

533

4.4.1 Possible effects on food production

534

To identify possible impacts on agricultural productivity and crop yield, we exam-

535

ine the changes in productivity of the modelâ&#x20AC;&#x2122;s temperate and tropical (C3 and C4)

536

grasses in main agricultural areas (defined by Ramankutty et al (2008)) as a crude

537

measure of crop yield. This approach enables us to estimate only the large-scale

538

response in European major cropland areas and does not include important agri-

539

cultural aspects such as irrigation, pests control or diseases (see a fuller discussion

540

in Wiltshire et al (2013)). Furthermore, the ecosystem changes shown here are

541

only related to changes in the hydrological cycle and temperature changes, and do

542

not include direct CO2 effects.

543

Crop productivity in the main agricultural regions in western Europe (Spain,

544

France and Germany), the UK and northern Europe (most notably Denmark) and

545

in central and eastern Europe (Poland and Ukraine) decreases dramatically. This

546

decline is probably due to the combined effect of colder temperatures and water

547

stress caused by changes in the water cycle. The lower productivity is marked

548

in spring, when we find an overall drying and very small or no reduction to the

549

evapotranspiration rates (Fig. 13c). However, there is also a strong reduction in

550

summer crop productivity in eastern Europe and mainly around the Black sea

551

(Bulgaria, Romania, Ukraine), where agriculture is currently an important part of

552

the economy (Fig. 13d).

553

Grass productivity in main grazing regions is also reduced in spring and sum-

554

mer by up to 50% in Ireland and western UK and to a lower degree (10-20%) in

555

mountain ranges from the Carpathians and Balkans in the east through to the

556

Alps, the Pyrenees and the Iberian mountains in the west.

26

L.C. Jackson et al.

557

The strong reduction of crop yield and pasture over Europe in a ’weakened

558

AMOC’ climate regime is consistent with the strong cooling and drying. The

559

decrease is stronger in the north but all regions will become less attractive for

560

cropland than today. Globally, however, we found enhanced grass productivity

561

in the southern hemisphere and areas in Australia, southern Africa and eastern

562

Brazil that might become potentially suitable for crops.

563

Simulations of crop productivity under future (CO2 -induced) climate change

564

scenarios show a similar decrease for the Mediterranean, due to the warming and

565

drying of this region. In northern Europe, however, crop productivity is shown to

566

increase, with cropland extending further northwards, both because of the direct

567

fertilisation effect of the higher atmospheric CO2 concentration and because of the

568

annual temperature increase that might lead to a longer growing season (Falloon

569

and Betts, 2010).

570

5 Conclusions

571

We have assessed the impacts to climate from a collapse of the Atlantic Meridional

572

Overturning Circulation (AMOC) using the Met Office Hadley Centre’s most re-

573

cent global climate model (GCM) HadGEM3, with particular focus on the impacts

574

in Europe. This is a state-of-the-art climate model and is the highest resolution

575

model used for such a study.

576

577

578

579

Many results found are consistent with previous studies and can be considered robust impacts from a large reduction or collapse of the AMOC. These include: • Widespread cooling throughout the North Atlantic and northern hemisphere

in general, with cooling in Europe of several degrees.

Climate impacts over Europe of a slowdown of the AMOC

27

580

• Much greater sea ice coverage in the North Atlantic.

581

• Less precipitation and evaporation in the northern hemisphere mid-latitudes.

582

• Large changes in precipitation in the tropics with a southwards shift of the

583

584

585

586

Atlantic Intertropical Convergence Zone.

• A strengthening of the North Atlantic storm tracks.

The focus on Europe and increased resolution has revealed new impacts that have previously not been discussed. The most significant are:

587

• Summer precipitation decreases across much of Europe, however there is an

588

increase in precipitation around the Mediterranean. This pattern is associated

589

with a negative summer NAO.

590

• In winter, the storm tracks across the North Atlantic and into Europe are

591

strengthened and penetrate further over land (as seen before). The higher

592

resolution of this model enables us to see very localised regions of increased

593

precipitation likely caused by the increase in winter storms, despite the general

594

signal of decreased winter precipitation across Europe.

595

596

• Greater proportion of precipitation falling as snow over all Europe. There is

also an increase in the number of months with significant snow cover.

597

• In both winter and summer the atmospheric circulation response moderates the

598

cooling over central Europe: in winter the westerly winds strengthen, enhanc-

599

ing the winter maritime warming effect, while in summer the westerly winds

600

weaken, which weakens the summer maritime cooling effect. The cooling over

601

Europe is also moderated by reduced cloud cover over land that allows more

602

shortwave radiation to reach the surface, but this is offset by increased albedo

603

from greater coverage of snow and ice.

28

L.C. Jackson et al.

604

â&#x20AC;˘ River flow and surface water runoff in Europe are significantly reduced be-

605

cause of the reduction in precipitation, however they are also affected by the

606

distribution of precipitation changes and changes to the snowmelt timing.

607

â&#x20AC;˘ Vegetation and crop productivity show strong decreases over Europe in re-

608

sponse to the cooling and decrease in available water.

609

Many of these new results are associated with changes in the atmospheric cir-

610

culation and might be model dependent, however the general patterns of change

611

found here are supported by previous studies, including evidence from observa-

612

tional records. A summer NAO pattern in the sea level pressure has previously

613

been associated with changes in sea surface temperature of the North Atlantic,

614

and has been linked to a similar pattern of precipitation change as found in this

615

study (Sutton and Dong, 2012; Folland et al, 2009). A negative summer NAO sig-

616

nal would also be expected to weaken the prevailing westerly winds over Europe.

617

Likewise, changes in winter sea level pressure have also previously been associated

618

with changes in North Atlantic sea temperatures, with colder temperatures and

619

stronger temperature gradients producing a positive NAO pattern (Peings and

620

Magnusdottir, 2014) (resulting in stronger westerly winds) and a strengthening

621

of the North Atlantic storm track (Brayshaw et al, 2009). Hence the patterns of

622

response found here to a weakened AMOC are plausible, although there are sug-

623

gestions that the magnitudes of response might be stronger in a reality (Kirtman

624

et al, 2012; Scaife et al, 2014).

625

The consequences of an AMOC collapse and the potential impacts on human

626

and natural systems are very large and would affect not only Europe but large

627

regions across the globe. The 5th Intergovernmental Panel on Climate Change has

Climate impacts over Europe of a slowdown of the AMOC

29

628

assessed a complete collapse of the AMOC to be very unlikely within the 21st

629

century, however a weakening of the AMOC has been assessed to be very likely

630

(Collins et al, 2013). Even though the likelihood of a collapse is low, the severity

631

of the consequences makes a careful assessment of the impacts expedient.

632

These results are illustrations of impacts rather than predictions or projections

633

since they are based on an idealised AMOC collapse in a present day climate. If

634

an AMOC reduction occurs in the future, the climatic impacts would be com-

635

bined with those of increasing greenhouse gases, and would depend on the relative

636

magnitudes and timing of an AMOC reduction and global warming. Many im-

637

pacts from an AMOC collapse could be of comparable size to, or larger than those

638

from global warming, and although some impacts from global warming would be

639

lessened, others would be reinforced by an AMOC collapse.

640

6 Appendix

641

The thermal advection is calculated as −u850 · ∇T850 using the wind vectors (u)

642

and atmospheric temperatures (T ) at 850 hPa. The gradients were calculated as

643

centred differences over 20◦ in both latitude and longitude to smooth out small

644

scale noise.

To show how advection from the prevailing winds affects European surface temperature (TS ), a regression model was built between seasonal mean TS and advection at 850hPa : p TSp = −Aup850 · ∇T850 + residual. p where TSp and up850 · ∇T850 were area averaged over central European regions show-

ing strong thermal advection changes in Figure 6 (0-30◦ E, 50-65◦ N in DJF and

30

L.C. Jackson et al.

0-30◦ E, 45-60◦ N in JJA). The superscript p indicates that the data were taken from seasonal means from the 30 year period of the perturbation run where the AMOC was reduced in strength, though similar relationships are found in the equivalent 30 year period in the control run. This gives correlations between TSp p and −up850 · ∇T850 of 0.5 in DJF and 0.7 in JJA, and values of A of 2.7x105 s and

1.8x105 s respectively. The significant correlations support the importance of the seasonal mean advection in affecting surface temperature.

The cooling associated with the temperature changes alone can be assessed using the regression model above with the wind vector replaced by that from the control experiment: p .

TSp∗ = −Auc850 · ∇T850

645

Hence the thermal advection is calculated using wind velocities from the control

646

experiment and temperature from the perturbed experiment, effectively assuming

647

the wind field does not change in response to the forcing. Applying this regression

648

model to estimate TSp∗ suggests that the cooling over Europe in the absence of

649

the wind response would be 12% stronger in winter and 34% stronger in summer.

650

Hence the atmospheric circulation changes result in a reduction in the cooling over

651

land in Europe.

652

Acknowledgements This work was supported by the Joint DECC/Defra Met Office Hadley

653

Centre Climate Programme (GA01101). We would like to thank M. Mizielinski for technical

654

assistance in setting up and running HadGEM3 and C. Mathison and K. Williams for assistance

655

with the river flow analysis. The authors would also like to thank Pete Falloon and Richard

656

Betts for useful discussions during the preparation of this paper. Finally we wish to thank two

657

anonymous reviewers for their comments which improved this manuscript.

Climate impacts over Europe of a slowdown of the AMOC

31

658

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Freshwater added to North Atlantic Perturbed Control

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Fig. 1 Cumulative volume of freshwater added to the North Atlantic Ocean (Sv Yr). 10 Sv Yr are added each year for the first 10 years. No further freshwater is added after this time.

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Fig. 2 The Atlantic Meridional Overturning Circulation (AMOC). Top left: AMOC streamfunctions (Sv) for the control (average of years 60-90). Top left: AMOC streamfunction (Sv) in the perturbed experiment (average of years 60-90). Note the different scales. Bottom left: timeseries of AMOC strength at 26o N, 1000m depth (blue) and at 30o S, 1000m depth (red) for the control (dashed) and perturbed experiment (solid). Bottom middle: Meridional ocean heat transport at 30o N (blue) and 30o S (red) in the Atlantic for the control (dashed) and perturbed experiment (solid). Bottom right: Anomalies of area averaged SAT for the whole of the northern hemisphere (blue) and over the Atlantic (60-0o W, 20-60o N, red).

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Climate impacts over Europe of a slowdown of the AMOC

Latitude

Fig. 3 Anomalies of surface air temperature (top left, o C), sea surface temperature (top right, o C),

precipitation (bottom left, mm/day) and net precipitation (precipitation - evapotranspi-

ration) (bottom right, mm/day). Anomalies that are not significant compared to the control variability (see text) are white. The top right panel also shows the extent of the March sea ice (concentrations > 15 %) for the control (white lines) and perturbed experiment (grey lines).

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Anomalies at all grid points shown are significant compared to the control variability (see

text)

Climate impacts over Europe of a slowdown of the AMOC

a) Ann: LW TOA flux

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Fig. 5 Anomalous TOA radiative fluxes and cloud fractions. Top left: annual mean longwave flux. Positive values indicate net energy flux downwards (W/m2 ). Top right: cloud component of annual mean longwave flux (W/m2 ). Second row left: annual mean shortwave flux. (W/m2 ).

Second row right: cloud component of annual mean shortwave flux (W/m2 ). Third row left: annual mean net flux (W/m2 ). Third row right: Anomaly in annual mean cloud fraction.

Bottom row: anomaly in JJA (left) and DJF (right) cloud fraction. Anomalies that are not significant compared to the control variability (see text) are white.

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Latitude Latitude Fig. 6 Anomalies in thermal advection (−u.∇T ) at 850hPa (10−5

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JJA (left) and DJF (right). Positive values indicate a warming by the advection. Overlain are the anomalous wind vectors.

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Fig. 7 Precipitation in the control (top) and anomalies in the perturbation experiment (bottom) for JJA (left) and DJF (right) over Europe. Precipitation is measured in mm/day in the top panels and shown as a relative or fractional change in the bottom panels. Anomalies that are not significant compared to the control variability (see text) are white.

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Fig. 8 Sea Level Pressure (SLP) in the control (top) and anomalies in the perturbed experiment (bottom) for seasons DJF (left) and JJA(right). Units are hPa. Anomalies that are not significant compared to the control variability (see text) are white.

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control (blue) and perturbed (orange) experiments. Simulated river flow is from the grid cell closest to available historical river gauges data from GRDC (black lines). The modelled river flow annual cycle is calculated over 30 years and the shaded areas and dashed black lines reflect one standard deviation.

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Fig. 13 Relative fractional changes to net primary productivity (NPP) between the perturbed run and the control. Spring (MAM, panel a) and Summer (JJA, panel b) NPP anomaly for total vegatation. c) and d) Same as a and b but for grasses only. Dotted areas in c and d are regions where present day cropland is large that 40 % Ramankutty et al (2008). Anomalies that are not significant compared to the control variability (see text) are white.

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C) GRASS NPP anomaly: MAM

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