December 25, 2012
JAMSTEC
A new type of interannual variability in the equatorial Atlantic
1. Summary
Dr. Ingo Richter, a member of the Climate Variability Predictability and Applica-bility Research Program (CVPARP) of the Research Institute for Global Change (RIGC), JAMSTEC, has re-investigated the causes for interannual warm events in the equatorial Atlantic, also known as Atlantic Niņos. While Atlantic Niņos have long been considered to follow the same equatorial dynamics that govern El Niņo events in the tropical Pacific, Dr. Richter and his colleagues at JAMSTEC have found that some warm events are caused by a different mechanism. This mechanism relies on warm ocean temperature anomalies just north of the equator, which are transported toward the equator by the mean ocean circulation. The initial north-equatorial ocean warming is ultimately linked to sea-surface temperature (SST) anomalies in the sub-tropics, more than 1000 km away from the equator. The results thus suggest a link between the subtropical and equatorial regions. It remains to be seen whether similar processes are active in the tropical Pacific and Indian Oceans, but if so, the results of this study might have broader implications for our understanding of tropical air-sea interaction. The results are to appear on-line on December 16 in the British journal Nature Geoscience under the title "Multiple causes of interannual sea surface temperature variability in the equatorial Atlantic Ocean". The co-authors are Dr. Swadhin Behera, Prof. Yukio Masumoto, Dr. Bunmei Taguchi, Dr. Hideharu Sasaki, and Prof. Toshio Yamagata.
2. Background
The mean conditions in the equatorial Atlantic and Pacific are fairly similar. In both basins the surface winds blow from east to west year round and SST are high in the west (28-30ēC) and cool in the east (23-24ēC). In some years the surface winds weaken, which allows the warm waters in the west to move toward the center of the basin. In the Pacific basin, these events are called El Niņo and are associated with worldwide climate impacts. Warm events in the Atlantic have been considered to rely on the same mechanism and therefore are called Atlantic Niņos. The temperature anomalies occurring during Atlantic Niņos are weaker than those during El Niņo, but nevertheless have significant climatic impacts on the surrounding continents including floods and droughts in tropical South American and Africa. Moreover, recent research suggests that the equatorial Atlantic can contribute to the development of Pacific El Niņos. For the last few decades the scientific consensus has been that Atlantic Niņos are controlled by the same mechanism as El Niņo in the Pacific. Our analysis, how-ever, shows that not all warm events in the tropical Atlantic follow this mechanism.
3. Summary of scientific methods
Of particular interest to our study is the relation between surface winds and SSTs in the equatorial Atlantic. El Niņo events in the Pacific are characterized by a weaken-ing of the equatorial trades (i.e. westerly surface wind anomalies) and warming of SSTs. To re-examine this relation for equatorial Atlantic warm events our study uses ship observations and reanalysis products. For a more detailed understanding of the oceanic mechanisms involved we rely on numerical ocean simulations performed on JAMSTEC's high performance computer, the Earth Simulator.
4. Results and discussion
Our analysis of equatorial surface winds and SST reveals that the relation between the two is much weaker in the Atlantic than in the Pacific. A detailed inspection shows that in the Atlantic some warm events occur even though surface winds in the preceding months are stronger than normal. This is inconsistent with ENSO dynamics, which would predict cooling under such conditions.
In order to examine the oceanic processes responsible for the unexpected warming we use a numerical ocean model forced with observed wind patterns. Our analysis suggests that warm ocean waters north of the equator are a crucial component in such events. Subsurface currents transport these warm water masses toward the equator where they trigger an equatorial warm event (Fig. 1). The warm waters north of the equator are the direct consequence of the overlying wind stress patterns, which essen-tially pump the warm surface waters downward into the ocean interior. The wind stress patterns, in turn, are related to warm SST anomalies in the subtropics, more than a thousand kilometers away from the equator (Fig. 2). The subtropical SST anomalies are long-lived and, to some extent, independent of the equatorial dynamics. Thus our results suggest that the subtropics can influence Atlantic Niņos through a combination of atmospheric and oceanic influences.
Most seasonal prediction models have little skill in predicting Atlantic Niņos, which is in stark contrast to the success these models have in the Pacific. We hope that our results can contribute to a better understanding of the dynamics involved in Atlantic Niņos and that this will ultimately result in better predictions. It has long been known that El Niņo has a global influence but recent studies suggest that the equatorial Atlantic can also influence El Niņo and other remote regions, such as east-ern Asia.
5. Outlook
In future research Dr. Richter would like to examine whether a similar link be-tween the subtropical and equatorial regions exists in the Pacific and Indian Oceans. If so, this might add an important component to the current understanding of tropical air-sea interaction.
Figure 1. Figure 1. Anomalous ocean temperatures (ºC) during northern hemisphere spring, which is the time when Atlantic Niņos start developing. The fields are from an ocean simulation per-formed at the Earth Simulator Center and show the situation for new-type Atlantic Niņos (left panel) and "normal" Atlantic Niņos (right panel). During new-type Atlantic Niņos, ocean temperatures warm up just north of the equator, with the maximum anomalies occurring be-tween 20-40m depth. The mean ocean currents at this depth are directed southward (indicated by black arrows) and therefore transport the warm waters to the equator. During "normal" Atlantic Niņos, on the other hand, ocean temperatures warm up right on the equator due to the weakening of the surface winds. The mean ocean circulation (indicated by the upward point-ing arrow) then brings these warm waters to the surface.
Figure 2. Schematic of the mechanism behind the new-type Atlantic Niņos: warm SST anomalies centered at 15ēN weaken the northern trade winds, which usually blow southwest-ward toward the equator. At the same time the westward directed winds on the equator inten-sify. These surface wind anomalies (indicated by the black arrows) result in strong wind shear just north of the equator, which drives convergent currents (blue arrows) at the ocean surface. Due to the convergence, warm waters from the ocean surface are pumped downward into the interior of the ocean, where they can be transported to toward the equator (see Fig. 1).
