Easterlies/Westerlies convergence in the tropical Pacific triggering El Niño initiation?

Climate change and climate variability play a relevant role on the occurrence of conflicts in several parts of the world, including the tropics, where events of flooding and droughts are dictated by El Niño Southern Oscillation (ENSO). In order to better analyze possible relations between conflicts hot-spots and ENSO impacts on society and security, a better understanding of the dynamics of the latter is needed. ENSO is the result of an ocean-atmosphere feedback, which produces an irregular oscillation between a warm (El Niño) and a cold (La Niña) phase, peaking in boreal winter and recurring every 2-5 years. During La Niña phase, intensified trade winds accumulate warm water in to the west of the Pacific, leading to droughts in the northern US and catastrophic floods in regions such as northern Australia. During El Niño warm phase, westerly winds advect warm waters eastward towards the coasts of America, generating dry conditions in northern US and Canada, and wetter periods in the US Gulf Coast areas. Most of the studies on ENSO focused on the coupling between changes in the depth of the main thermocline, heat content in the surface layer of the water column and oceanic feedback on the zonal wind pattern. According to these works, the subsurface memory of the ocean (i.e. the heat stored in the surface layer), depends on the depth of the thermocline and the zonal shape of the isothermal surfaces is sustained by the dynamical balance between the zonal pressure gradient and the trade winds. This process systematically transfers heat westward and “charges” the western Pacific, which is then “discharged” through the action of eastward propagating internal Kelvin Waves (KW). While westerly wind events are known to play an important role in the generation of KW associated with El Niño, much less is known on the role of easterly winds. Here we show that the encountering between Westerlies and Easterlies determines the convergence, providing the initial forcing exciting internal, downwelling Rossby and Kelvin waves. Only KW formed east of 175oE reach the eastern Pacific boundary and determine an El Niño events, that become the more intense the more the waves are formed eastward, indicating a “zonal position” triggering of El Niño. It is shown here that the zonal shifts of the Easterlies/Westerlies convergence region displaces zonally in phase with region of the deep atmospheric convection and with the Southern Oscillation Index, indicating that changes in the large scale pressure system, the zonal position of westerly wind events, the easterly wind variability, the position of the deep atmospheric convection and El Niño are all intimately related features of the whole tropical Pacific climate system.Funding from the STO Office of Chief Scientist 907EUR30 is gratefully acknowledged.

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