![]() ![]() These stronger-than-normal winds, in turn ( here’s the feedback!), then help to reinforce the ocean temperature anomalies by increasingly pushing water away from the eastern Pacific to the western Pacific Ocean. schematic by Emily Eng and inspired by NOAA PMEL. These feedbacks between ocean and atmosphere strengthen the La Niña event. Over the cool waters, it is less buoyant, favoring subsidence (downward gray arrow) and less rainfall. Over the warm waters, the air becomes more buoyant (upward thick arrow), and rainfall increases. With the increases in sinking motion over the eastern Pacific and rising motion over the western Pacific, the trade winds that typically blow from east-to-west along the equator become even stronger.ĭuring La Niña, the surface trade winds (small gray arrows) blow more strongly than normal, piling up warm surface water in the western Pacific and drawing up deep, cool water in the eastern Pacific. Over the cooler eastern Pacific, air begins to sink more and dry out (also why basements are colder). Over the western Pacific, warmer temperatures cause air near the surface to rise, moisten, and become convective, leading to thunderstorms and more rain (just like how warm air rises into an attic, though thankfully, thunderstorms won’t form in a house). This means the difference (or gradient) in temperature between the relatively warmer western Pacific and the cooler eastern Pacific becomes even larger than normal. We often don’t know exactly what initiates the feedback loop-sometimes sea surface temperatures start to cool and then the trade winds begin to strengthen, or vice versa (3). But, we have to start somewhere, so I’ll start with the ocean cooling (4). As I write, we’re in a La Niña, so I’ll outline the feedback starting there. The Bjerknes feedback is a positive feedback between tropical Pacific surface wind and ocean temperature, which essentially means processes in the ocean and atmosphere goad each other to bring on an ENSO event. In climate, feedbacks do not refer to comments on performance reviews or the squeak of a microphone, but rather physical processes that reinforce each other (a positive feedback) or destroy each other (a negative feedback). As the Dough Risesīut how does this coupling allow ENSO to emerge and grow? The magic is in feedbacks and, in particular, what we now call the Bjerknes feedback. He brilliantly realized it was all one big system and that the ocean and atmosphere were “coupled,” meaning that the pressure and temperature patterns rely on interactions between the ocean and atmosphere.ĮNSO-Neutral or average conditions across the tropical Pacific Ocean. By using new satellites and observations from Canton Island in the South Pacific Ocean, Bjerknes was able to link changes in the pressure pattern that Walker discovered with fluctuations in temperature in the tropical Pacific Ocean (2). No one really knew why the Southern Oscillation existed, and they certainly didn’t grasp the larger implications of his discovery until a guy by the name of Jacob Bjerknes had his “ah ha!” moment in the 1960s (1). Unfortunately for him, at the time, Walker’s discoveries were treated more as scientific curiosities than anything else. He called this large-scale seesaw in pressure the Southern Oscillation to draw contrast with the more northern locales of other climate phenomena he also identified and named, like the North Atlantic Oscillation (yes, same guy!). While working in India from 1903-23, he observed “the swaying of pressure on a big scale backwards and forwards between the Pacific Ocean and Indian Ocean” (Adamson, 2020). We have described the Walker Circulation many times before, which is named after Gilbert Walker because he is very important to the field of meteorology and climate. Assemble the Ingredientsīut how do the ocean and atmosphere come together to make the bread ENSO? Good question and, incredibly, this problem took nearly fifty years to solve, even after all of the basic ingredients were uncovered. That’s right, I’m basically saying ENSO is fresh baked bread (Mmm, ENSO, you’re so tasty). Flour and yeast also would be pretty boring and inert on their own, but when put together, they reinforce each other, the combined product increases in size, and eventually it releases the heavenly scents of fresh baked bread. Various ingredients of the ocean have to be combined with the atmosphere in order for ENSO to blossom and grow. Without it, ENSO would not exist, and it would be considerably more difficult to predict climate impacts seasons in advance. The key is in the ocean-atmosphere coupling across the tropical Pacific Ocean. ![]() We often get asked how El Niño or La Niña events form and increase in strength. ![]()
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