4)
Oscillations & Circulation : ENSO, PDO, NAO, AMO, A(A)O, QBO, AMOC
The climate is not driven by the short-term SW or LW absorption within the atmosphere’s relatively trivial mass, as its heat capacity is only the equivalent of a 2.5 m layer of seawater, but rather by the long-term ‘storage function’ of the memory of the accumulated radiative equilibrium that resides in the ocean (Ellsaesser, 1984). In the intermediate term, the atmosphere is driven by variations in ocean dynamics in accordance with the various oscillations that we are dealing with in this section, the El Niño phenomenon being probably one of the most conspicuous. In the longer term, it is driven by variations in solar irradiance associated with variations in the Earth’s orbital motion about the Sun and the variations in the solar activity (and cycles) and how they might influence cloudiness and thus the albedo. Most of the natural oscillations discussed in this section, if limited to their atmospheric component, could be considered as such short-term changes that would have a more direct relationship to meteorology than to climatechange strictly speaking, though climate as already said is just the sum over time (the integral) of the weather, thus their indissociable connection. Most of these oscillations concern the oceans to the notable exception of the mainly atmospheric QBO (Quasi-Biennial Oscillation) and some are mixed (e.g. ENSO), thus showing the complex intricacies happening in this highly coupled-system ocean-troposphere-stratosphere not to mention the role played by the topography (and of course the major mountain-belt systems) which should also be integrated as they deviate or orient in some way the atmospheric circulation. ENSO (El Niño Southern Oscillation, alternatively El Niño - La Niña), AMO (Atlantic Multidecadal Oscillation), NAO (North Atlantic Oscillation), PDO (Pacific Decadal Oscillation), AO (Arctic Oscillation), AAO (AntArctic Oscillation), and QBO will be briefly reviewed. Even though these oscillations take place on short-term scales (as far as the climate is concerned) they can be traced back for the entire Holocene and so belong to the climate and furthermore contribute to physical phenomenons having an impact on longer term mechanisms, e.g. the importance at all time-scales of the role of the oceans in regulating the CO 2 atmospheric content as per Henry's law “We used space-based CO2 observations to confirm that the tropical Pacific Ocean does play an early and important role in modulating the changes in atmospheric CO 2 concentrations during El Niño events” (Chatterjee et al., 2017). AMOC will be considered last and AMO in more details in another section with the Arctic. ENSO is the first global system of climate variability, in fact the largest perturbation to the climate system on an interannual time scale with a period of 2 to 7 years. The Southern Oscillation is an associated (atmospheric) pressure oscillation between northern Australia and the central Pacific. The warm phase is designated El Ninõ (El Niño de Navidad), it is the term used by Peruvian fishermen who named the weather phenomenon after the newborn Christ, as they mainly noticed it after Christmas. It is an intensive warming of the ocean in the Eastern Pacific at the level of the tropics for about 5 months. The opposite cold phase is called La Ninã and the system oscillates between warm and cold conditions over a return period of about 4 years, on average with large deviations (e.g. no ENSO occurred in between 1927 to 1940). The climatic impact of ENSO is spatially and temporally complex and involves time delays and each El Ninõ event is distinct from another one in terms of precipitation, temperature, etc. (Jacobs et al., 1994). There exists a negative correlation between the indexes of these oscillations, i.e. the sea surface temperatures (SST) averaged over the tropical east-central Pacific on the one hand and the Southern Oscillation Index (SOI), i.e. the normalized pressure difference between Tahiti, in the mid-Pacific, and Darwin, Australia on the other hand. The SOI measures the pressure gradient across the tropical Pacific, an indicator of equatorial wind variations. When the SOI index reaches low negative values, a strong El Niño is in progress with air pressure low in the eastern Pacific and high in the western Pacific. Reversely when the SOI index goes highly positive, this indicates a La Niña episode, with air pressure high in the eastern Pacific and low in the western Pacific, corresponding to a strengthening of the Walker circulation and to the upwelling of cold deep sea water which cools the sea surface to below average temperatures. Initially, it was thought that the ENSO variability affected only the Pacific ocean, but the severe ENSO event of 1982/1983, when the sea surface off Peru warmed by more than 7° C, demonstrated that there are strong links to weather in other regions , e.g. floods in California, intensified drought in Africa, etc. but it was further discovered that the effects were even much broader and that planetary-scale oceanic waves crossed the Pacific and that effects of El Nino events can be extremely long-lived (Jacobs et al., 1994). The observation of this global connection implied that the oceanic and atmospheric anomalies of the equatorial Pacific might be the key to accurate seasonal weather forecasts in other regions. ENSO has major regional impacts, but the most obvious is that the displacement of warm water from the west Pacific and the Indian Ocean to the east Pacific takes the rain with it, causing extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific and for example, Singapore experienced the driest February in 2014 since records began in 1869, with only 6.3 mm of rain falling in the month and temperatures hitting as high as 35 °C on 26 February. There are
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