Hurricane Season 2006: an unusual year?

Hurricane Season 2006: an unusual year?

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Copyright 2006 Partner Re 90 Pitts Bay Road Pembroke HM 08, Bermuda Author Dr. Jan Kleinn, Atmospheric Scientist Catastrophe Research, PartnerRe, Zurich jan.kleinn@partnerre.com Quinquennalis fiducias aegre Editor verecunde amputat Geologist pretosius Dr. Sara Thomas, concubine, quamquam Corporate Communications, PartnerRe Zurich optimus perspicax catelli aegre sara.thomas@partnerre.com celeriter imputat Pompeii.

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For moreoratori copies of this publication or Pretosius senesceret for permission to reprint, Caesar, etiam optimus fragilisplease contact: Celia Powell rures comiter suffragarit Coporate Communications apparatus bellis, quamquam 90 Pitts Bay Road parsimonia zothecas praemuniet Pembroke HM 08, Bermuda saetosus ossifragi. celia.powel@partnerre.com Telephone (1 441) 292 0888 Telefax (1 441) 292 7010 This publication is also available for download under www.partnerre.com

Editorial

Dear reader, this is the second issue of our Research bulletin and it already has a new layout, as we were asked to comply with a corporate communi足 cations template. Expect some further changes in layout in the next issues as corporate communications is currently developing a new corporate identity (CI) and this includes new layouts for internal newsletters. The name Partner足 ReCycle was kept but will most probably also be changed as soon as the new CI strategy is defined. The whole bulletin is work in progress and there足 fore there might be further changes in the format of this newsletter in the future.

Please accept my apologies that this second issue of the bulletin has taken more than six months to appear. I nevertheless hope you enjoy the two 足articles about the 2006 Atlantic hurricane season and the Java Earthquake of May 2006.

PartnerRe 2006 Hurricane Season 2007: an unusual yaer?

Hurricane Season 2006: an unusual year?

After the two intense hurricane seasons of 2004 and 2005, the June forecasts in 2006 predicted an above-average number of tropical storms for the season. The hurricane season (which is officially defined to last from June until November) had an average start in terms of number of tropical storms, but the first hurricane occurred only mid-September, about one month later than usual (see Figure 1). In the end, there were ten tropical storms, five hurricanes, and two intense hurricanes (category 3 and above). All these numbers are below the long term average of eleven, six, and three, respectively, but also far below any of the June forecasts.

In contrast, the June 2005 forecasts pointed towards a season with slightly above-average activity and the June 2004 forecasts towards a season with

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cumulative number of storms cumulative number of storms cumulative number of storms

Figure 1: Seasonal evolution of the hurricane seasons 2004, 2005, and 2006. The solid lines show the long term average evolution of a hurricane season (average taken from 1950–2005) with regard to the number of tropical storms, hurricanes, and intense hurricanes. The season starts of with no storms and ends with an average of about 11 tropical storms, 6 hurricanes, and 3 intense hurricanes. The dashed lines show the actual seasons and the vertical bars to the right show the range of seasonal­ forecasts published before the start of the hurricane season, i.e. end of May.

about average activity. The 2005 hurricane season not only started unusually early, but it also lasted longer than normal with the last tropical storm forming on December 30th. The 2005 season was an exceptionally active season with a total of 28 tropical storms, 15 hurricanes, and seven intense hurricanes. That year, the hurricane season activity was much higher than the June forecasts. In 2004, the season started late with the first tropical storm forming end of July but it quickly reached above average activity. With 15 tropical storms, 9 hurricanes and 6 intense hurricanes, the season 2004 not only ended above average, but also above any of the June forecasts.

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Tropical Storms Hurricanes Intense Hurricanes

PartnerRe 2006 Hurricane Season 2007: an unusual yaer?

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Looking at the storm tracks, the last three seasons showed very different behaviour (see Figure 2).

In 2004, one part of the hurricanes headed towards the US coast while another part stayed out in the open Atlantic. There was no distinct pattern

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in the hurricane tracks in 2005, almost all regions of the Atlantic were covered. Finally, in 2006, only a few tropical storms came close to the US coast, all the hurricanes and the other tropical storms turned north-east and headed to the open Atlantic.

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Figure 2: Storm tracks of the seasons 2004, 2005, and 2006. The storm tracks are colour-coded according to their intensity. Note that in 2004, storms either went towards the Caribbean and the US or they went straight north. In 2006, most strong storms turned north before coming close to the US coast.

Tropical Storms Hurricanes Intense Hurricanes

What is driving hurricane activity and tracks? In the long term, the heat content of the ocean plays a vital role in hurricane activity. For a tropical storm to form, the sea surface temperature (SST) must have a minimum temperature, i.e. a minimum energy content. The SST in the mid-Atlantic was above average in June and July of 2005, whereas in 2004 and 2006 the June-July SST was about normal in the mid-Atlantic (see Figure 3). This explains the late start of the 2004 and 2006 seasons compared to

June–July 2004

the early start of the season 2005. The mean temperature over the whole North Atlantic is a proxy for the ocean heat content in the North Atlantic and turns out to be a good proxy for hurricane activity in the North Atlantic. Since about 1995, the ocean heat content in the North Atlantic is higher than it was in the period 1965-1995 (see Figure 4).

There were also more hurricanes in the Atlantic basin since 1995 compared to the 1965–1995 period.

June–July 2005

June–July 2006

Figure 3: Maps with June and July sea surface temperature anomalies for 2004, 2005, and 2006 showing areas with unusually warm and unusually cold sea surface temperatures.

The box denotes the main development region (MDR) of hurricanes, i.e. the region where most hurricanes form. −2

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PartnerRe 2006 Hurricane Season 2007: an unusual yaer?

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Hurricane activity is also influenced by vertical shear in the atmosphere, i.e. the difference in wind speed at the sea surface and at the top of the atmosphere.

High shear inhibits the proper development of a hurricane and can also reduce the intensity of an existing storm, as the tower of the storm is tipped over. Shear over the Atlantic usually has a very regular oscillation with high shear every other season (the climate signal is therefore called Quasibiannual

Figure 5: Yearly counts of tropical storms, hurricanes, and intense hurricanes since 1950 in the entire Atlantic basin, based on data from the Hurricane Re-Analysis Project of the Hurricane Research Division by NOAA/AOML and best track data from the National Hurricane Center by NOAA. Years with El Niño events are marked to show the influence of El Niño on the hurricane activity.

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Figure 4: Smoothed Atlantic Multidecadal Oscillation (AMO) since 1950. The AMO is a measure of the mean North Atlantic sea surface temperature and hence a measure of the ocean heat content. The more heat is available, the more hurricanes can form and the more intense hurricanes can get. It is still a heated debate in the scientific community, if we only see a natural cycle of warm and cold phases or whether a trend is at least partially responsible for the increase of the AMO.

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Oscillation, QBO). Finally, the irregularly appearing El Niño brings dry air and further shear to the Caribbean, which further inhibits hurricane formation. The low activity in the 2006 hurricane season was mostly attributed to the appearance of El Niño this winter. El Niño has already shown to reduce hurricane activity over the Atlantic in the past (See Figure 5).

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Hurricane Season 2006: an unusual year?

PartnerRe 2006 Hurricane Season 2007: an unusual yaer?

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In addition to these factors, the number of storms that hit the U.S. is also governed by large scale weather patterns. In 2004, there was one group of storms that was geared towards the U.S. coast and one group of storms that stayed way out in the

August–September 2004 Figure 6: Maps with the hurricane tracks of August and September of 2004, 2005, and 2006. The grey contours depict the pressure anomalies, i.e. dashed contours represent lower than average pressure and solid contours represent higher than average pressure.

Regions with lower than average pressure act like low pressure systems on the mean flow, i.e. the flow is counterclockwise. Regions with higher than average pressure feature a clockwise flow. In 2004, the Bermuda high and its counteracting lowpressure system to the East steered hurricane tracks either to the US coast or directly North. In 2006, the strong low pressure over the US coast steered the hurricane tracks to the North and hence away from the coast.

August–September 2005

In 2005 there was no distinct and persistent large scale weather pattern, which would have directed the storm tracks in one direction or the other. In 2006, there was a persistent low pressure system over the Eastern U.S., which pushed the storm tracks far away from the U.S. coast. Only three tropical storms came close to the U.S. coast or made landfall.

PartnerRe 2006 Hurricane Season 2007: an unusual yaer?

Atlantic. The reason for the split in tracks was the ‘Bermuda high’, which inhibited any storm from passing through a region just east of Bermuda (see Figure 6).

August–September 2006

The first forecasts for the hurricane season 2007, published in December, all point towards an aboveaverage hurricane season. Given the lack of skill of the June forecasts in 2004, 2005, and 2006, these December forecasts have to be taken with a pinch of salt.

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