Late last week the Climate Prediction Center issued an El Niño Advisory, meaning that El Niño conditions have been observed or are expected to continue. There is an 80% chance that a weak El Niño will continue through the spring, 60% chance into the summer.
Back in February, the atmosphere finally began to show signs of responding to the warmer-than-average sea surface temperatures in the central equatorial Pacific. (El Niño is defined by the warming of the tropical Pacific waters. An atmospheric response then follows.) It is believed that an atypical sea surface temperature gradient delayed the atmospheric response. In the fall warmer water temperatures were observed in the western Pacific, while it was cooler in the east. This is not the typical set-up for an El Niño event. During the late fall and winter, we failed to see the eastward movement of the warmer surface waters. Things are on the move now.
The amount of warmer-than-average water above and below the surface of the tropical Pacific increased substantially February. This occurred after dropping over the previous few months. ENSO (El Niño-Southern Oscillation) isn’t just an oceanic reaction, the atmosphere also responds to the warming water temperatures. The Walker Circulation is beginning to weaken, and the trade winds near the Equator as starting to slow. Weaker trade winds allow the surface waters to warmer further. This would explain the considerable warming lately.
What’s next? Climate scientists expect the positive trend in sea surface temperatures to persist for the next several months. Weak El Niño conditions could linger through early fall, however forecasts made in the spring are less accurate. Why? What’s happening across the equatorial Pacific in March is just a quarter of what will occur in the fall and winter when the influence of El Niño/La Niña is stronger. The influence of ENSO is generally weaker in the spring and summer. However El Niño or La Niña can influence the upcoming hurricane season. If El Niño persists into the summer it could mean a quieter Atlantic hurricane season. During a typical “El Niño hurricane season” there are fewer hurricanes in the Atlantic basin due to stronger wind shear and trade winds along of more atmospheric stability, all three act to limit tropical development. Meanwhile, it could be a more active season for the eastern Pacific.