NOAA:Ā February Update
By:Ā Emily Becker
Many thanks to Tom for covering my post last month, allowing me to focus on the American Meteorological Societyās annual meeting (where I talked about the blog, and enjoyed my company on the plane ride home). Itās good to be back, and Iām glad to see our La NiƱa is still hanging aroundā¦ although probably not for much longer. In fact, the current ENSO forecast from the Climate Prediction Center and IRI predicts a 55% chance that neutral conditions will be in place by spring 2018. Letās get to it!
Sea shanties
During January, the sea surface temperature in the NiƱo3.4 region of the tropical Pacificāour primary measurement for ENSOās ocean componentāwas close to 1.0Ā°C cooler than the long-term average. This places it comfortably in La NiƱa territory.
The NovemberāJanuary temperature in the NiƱo3.4 region was also 1.0Ā°C cooler than the long-term average. Our double-dip La NiƱaās second year has been stronger than the first, as the greatest departure from average during 2016ā17 was -0.7Ā°C.Ā As Nat showed in his excellent post last week, this is somewhat unusual for a double-dip La NiƱa, as the second year tends to be a bit weaker than the first. However, as he discussed, the impacts on North American temperature and precipitation tend to be stronger in La NiƱaās second yearā¦ if you havenāt read Natās post, go do it now! Iāll wait.
Carrying the tune
The atmosphere continues to respond to the cooler-than-average surface waters in the tropical Pacific, showing all the signs of a strengthened Walker Circulation. Those cooler-than-average waters lead to less rising air and cloud formation in the central Pacific than average, with more rising air and storms forming over the far western Pacific and Indonesia.
More evidence of the strengthened Walker Circulation during January was provided by stronger-than-average near-surface winds (the trade winds), as well as stronger west-to-east winds in the upper atmosphere. The stronger trade winds help to keep the surface cooler, and to keep warmer water piled up in the far western Pacificāpart of the critical feedback processes that make up ENSO. However, this feedback doesnāt go on forever, and weāre seeing signs that La NiƱaās swan song is imminent.
Bass notes
We keep track of whatās going on under the surface of the tropical Pacific, too, as significant temperature changes in the subsurface waters can provide a valuable clue about the evolution of ENSO events. From early December to late January, an area of warmer-than-average water centered between about 50 and 200 meters (~160ā650 feet) under the surface traveled from the western Pacific to the east-central Pacific. This downwelling Kelvin wave chased away most of the cooler-than-average subsurface waters, leaving La NiƱa without itās steady supply of cooler waters.
This movement toward more neutral subsurface temperatures is one of the factors forecasters are looking at as we anticipate the decline of La NiƱa. Most of the computer models also foresee this transition, and overall forecasters have come to the consensus of a 55% chance that La NiƱa conditions will dissipate by MarchāMay, as the tropical Pacific transitions to neutral conditions.
World musicā¦ or, a little off-key?
While this La NiƱaās effect on precipitation and temperature has been generally in-line with expectations over North America, especially regarding the dry conditions across the southern half of the US, global impacts havenāt been entirely as expected during the past few months.
La NiƱa during NovemberāJanuary is associated with more rain than average in eastern Australia, northern South America, and southern Africa. On the other hand, La NiƱa ordinarily tends toward drier conditions in southern Brazil/northern Argentina/Uruguay, as well as southeastern China. (These maps show the expected patterns, but the colors are reversed, since theyāre showing El NiƱoā¦ sorry!) November 2017āJanuary 2018, however, did not conform well to the expected patterns.
La NiƱaās global effects on temperature have historically been associated with cooler temperatures in Australia, southern Africa, and much of South America, but, due to human-caused global warming, itās increasingly rare to see a lot of cooler-than-average areas over the globe.
This doesnāt mean ENSO is useless as a forecasting tool, but it does further illustrate the complexity of the global climate system, and why forecasts are probabilistic. Over the past few months, La NiƱa just wasnāt the loudest voice in much of the worldās climate acapella group. (Check out Angel Munozās post about La NiƱaās effects in South America for a much more effective musical analogy!)
Regarding other voicesāstay tuned for Michelleās later February post on the Madden-Julian Oscillation, whoās been outperforming himself over the past few weeks, likely affecting global weather and climate. Also, the MJO is currently substantially weakening the trade winds, potentially hastening La NiƱaās curtain call. Donāt touch that dial! Thereās more to come on Radio ENSO, after this message from our sponsors.