Wednesday, November 18, 2015

El Niño of 2015 Hits All-Time Record Strength for a 1-Week Period

Dr. Jeff Masters writes:

Incredibly warm waters continue to build across the equatorial Pacific, and the El Niño event of 2015 has just set a record for the warmest waters ever observed in the equatorial Pacific over a 1-week period. Sea-surface temperatures (SSTs) in the Pacific’s Niño3.4 region, between 90°W and 160°E longitude and 5° north/south latitude, are considered the benchmark for rating the strength of an El Niño event. The weekly departure of SST from average in this region hit +3.0°C (5.4°F) over the past week, NOAA announced in their November 16 El Niño update. This exceeds the previous 1-week record warmth in the equatorial Pacific of 2.8°C above average set during the week of November 26, 1997; accurate El Niño records extend back to 1950. However, the standard measure for the strength of an El Niño event is the three-month average Niño 3.4 SSTs, and the El Niño of 2015 is not yet officially considered the strongest on record. The August-September-October 2015 three-month average Niño 3.4 SSTs were 1.7°C above average, good for only the 2nd warmest on record, behind 1997. Judging from the trajectory of SST anomalies in Figure 1, though, it is likely that one of the late-year three-month average Niño 3.4 SST values in 2015 will end up upending 1997's record warmth and claim for the 2015 the title as strongest El Niño event on record. El Niño can't get much stronger than it is now, though, since there simply isn't enough warm water available in the Western Pacific to transport to the Eastern Pacific; wunderblogger Steve Gregory speculated in his Friday post that El Niño may now be peaking, and will begin a slow decline over the the next three months. Even so, El Niño will decline only gradually, and we can expect significant global impacts on weather during the coming winter. 
And Bob Henson writes:
If you’re looking for an old-fashioned holiday, you may be out of luck across large parts of the U.S. and Canada, at least when it comes to December cold. El Niño climatology and seasonal forecast models are pointing toward high odds of a very mild December across most of the continent east of the Rockies and north of the Deep South. We wouldn’t expect every day to be unusually balmy--and in December, “warmer than average” can still be quite chilly--but the analogue years and the model forecasts do raise the possibility of at least a few days of record-melting weather across a vast area... 
The dramatic month-by-month evolution seen in Figure 1 for autumn settles down by January. At that point, the Pacific jet stream is much more likely to fall into the midwinter pattern typical of strong El Niños, with storm systems barreling into California and across the South while mild air spreads across much of the northern U.S. and eastern Canada. These atmospheric effects can persist through much of winter even if the warm sea-surface temperatures associated with El Niño begin to cool. One potential fly in the ointment is the possibility of a negative North Atlantic Oscillation, which is highlighted in the latest Arctic Oscillation outlook produced by Judah Cohen and colleagues at Atmospheric and Environmental Research. Snow cover advanced more quickly than usual over Siberia during October, and Cohen’s work has related this variable to an enhanced risk of negative NAO conditions by early winter, especially in January and February. Running counter to the warming effect of a strong El Niño, a period of negative NAO would favor intrusions of cold air over the northeast U.S. In the past, the combination of a negative NAO and a strong El Niño has led to some noteworthy winter storms along and near the East Coast, so it will be interesting to see if this pairing actually develops.
Month-to-month variations in average temperature during strong and “super” El Niño events between 1895 and 2014. Temperature departures are shown in blue/green colors (cooler than average) and red/orange colors (warmer than average), as calculated against the long-term average for the period 1895-2000. The El Niño events in these composites (peak Niño3.4 indices of at least 1.5°C above average for at least three overlapping three-month periods) include 1896-97, 1902-03, 1930-31, 1940-41, 1957-58, 1965-66, 1972-73, 1982-83, 1987-88, 1991-92, and 1997-98. Image credit: Eric Webb, @webberweather, using a mapping/analysis tool from NOAA’s Earth System Research Laboratory.


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