The end result is relatively warmer surface water in the west and cooler surface waters in the east. Thanks to Earth’s rotation on its axis, trade winds at the equator blow relentlessly from east to west, pushing sea water towards the west side of most ocean basins. What creates such a broad, large-scale circulation along the equator? Well, we already sorta went over that here, when talking about the importance of a sea surface temperature gradient. Convection associated with rising branches of the Walker Circulation is found over the Maritime continent, northern South America, and eastern Africa. Generalized Walker Circulation (December-February) during ENSO-neutral conditions. ![]() In contrast, downward branches of dry, stable air sink strongly over the eastern Pacific Ocean and more weakly over the Arabian Sea. These rising branches coincide with bull’s-eyes of precipitation during December through February. During neutral ENSO conditions, upward branches of the Walker Circulation rise strongly across the Maritime Continent and more weakly over eastern Africa and northern South America. SO (pun intended) what does the entire Walker Circulation look like normally and how is it maintained? Figure 1 shows a simplified version of what the airflow over the equator looks like during the northern hemisphere winter, when ENSO events-El Niño and La Niña-are strongest. ENSO can affect this atmospheric circulation, the Walker Circulation, over the entire tropics, impacting rainfall near the equator across multiple continents from Africa to Asia to South America! The post EN…SO? describes how El Niño can affect this Pacific Ocean “overturning cell,” but unlike the popular saying in that desert southwest tourist town, what happens in the Pacific, does not just stay in the Pacific. Rising air in the west and sinking air in the east connect the flow in one big, continuous loop. The lower part of the loop flows east to west across much of the tropics near the surface the upper part flows west to east at higher altitudes. The focus of Emily’s post was on the Pacific Ocean atmospheric circulation, which we also call the Pacific “overturning cell” of an atmospheric “loop” called the Walker Circulation. ![]() We have previously touched on both aspects of this with a post on the importance of a sea surface temperature (SST) gradient in the equatorial tropical Pacific Ocean (and the lack of gradient may be currently hindering the development of El Niño) as well as a post on the atmospheric response, the SO part of ENSO, where Emily Becker described the Southern Oscillation, especially the lack of typical response in that Southern Oscillation during June 2014. ![]() And just like there would be no “Kimye” without Kanye West and Kim Kardashian, there is no ENSO event without both an atmospheric and oceanic response. If the ocean is Abbott, then the atmosphere is Costello the ocean…Laverne, the atmosphere…Shirley the ocean…Kanye, the atmosphere…Kim. Reading back over the many excellent (if I do say so myself) posts here at the ENSO blog, there have been several re-occurring themes-the biggest of which is that ENSO is not just an ocean phenomena but an ocean-atmospheric interaction.
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