What everyone would like to know, of course, is what will happen ENSO-wise later in the year, following the neutral conditions that are likely to remain through the summer. This means that there are limited sources of cooler-than-average water to replenish the surface over the next few months, adding some confidence that La Niña is done… for now. As an atmospheric scientist, I’d actually be very surprised if the atmosphere coupling occurred immediately upon crossing the ENSO threshold!įinally, the water below the surface of the tropical Pacific is warmer than average and a downwelling Kelvin wave is moving eastward under the surface. The atmosphere-ocean system is vastly complicated, of course, and there are always many different things going on at once, so a delayed response isn’t surprising. For example, a couple of months preceding the last El Niño featured sea surface temperature anomalies near or slightly exceeding the thresholds, but a coherent atmospheric response didn’t kick in right away. Hence, the ENSO forecast team has concluded that La Niña is over, despite the sea surface temperature anomaly.Ī sea surface temperature anomaly of -0.5☌ (or, in the case of El Niño, +0.5☌) isn’t a magic switch that causes an instant atmospheric reaction, as we’ve seen many times before. The Southern Oscillation Index and Equatorial Southern Oscillation Index, both of which measure the atmospheric component of ENSO (more details here), are also indicating neutral conditions. Other signifiers, such as stronger-than-average trade winds, have also diminished across most of the Pacific Ocean in the past month. The drier-than-average signature of La Niña in the central tropical Pacific is visible as the large orange area that ends in April 2021. NOAA image, based on data provided by the Climate Prediction Center. Each row in this type of image is the departure from average (1974-2020) at that time. Regions with more clouds and rain than average are shown in purple areas with fewer clouds and less rain are shown in orange. Outgoing long-wave radiation anomalies for the past year. This pattern was evident for the past several months, especially the drier-than-average region over the central tropical Pacific, but has dissipated as of early April. That strengthened Walker circulation is characterized by reduced rain and clouds over the central Pacific and more over the far western Pacific and Indonesia. Over the past few weeks, atmospheric conditions over the tropical Pacific no longer resemble the strengthened Walker circulation pattern that we expect during La Niña. The atmospheric component is just as important, as it serves to reinforce the surface temperature anomaly and transmit ENSO’s impacts across the globe. However, as we know from long experience, ENSO (El Niño/Southern Oscillation) is more than just the ocean surface temperature anomaly (the difference from the long-term mean). April 2021 sea surface temperature departure from the 1981-2010 average (note that this image still uses the previous averaging period).
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