Synopsis: “La Niña conditions have returned and are expected to gradually strengthen and continue into the Northern Hemisphere winter 2011-12.” (El Niño/Southern Oscillation (ENSO) Diagnostic Discussion issued by the Climate Prediction Center/NCEP/NWS September 8, 2011.)
La Niña conditions are characterized by a decrease in average equatorial sea surface temperatures (SST) by half a degree Celsius or more. Looking at the image above you can see the blue strip with the three colder patches indicating SST anomalies in the Eastern Pacific of -0.5 ° C or colder than the average SST.
The SST anomalies observed in late August were occurring in the Eastern El Niño regions 1-3 but not so much in the Western most region 4.
A reduction in the subsurface oceanic heat content also supports the return of La Niña conditions. The reduction in subsurface temperatures is attributed to “increased upwelling and further shoaling of the thermocline across the Eastern Pacific Ocean.” The image below is representative of average temperature anomalies in the upper 300 meters of the ocean. You can see the colder trends in the Eastern Pacific.
Most models for SSTs in the El Niño 3.4 region are still predicting ENSO neutral conditions (El Niño Southern Oscillation neutral) heading into Fall 2011 (see below image), i.e. no SST variations greater than 0.5° C from the norm. However, the NCEP (National Center for Environmental Prediction) Climate Forecast System (CFS) has performed well in recent months predicting the recent decrease in SST anomalies.
According to the NCEP report:
The better model performance, combined with the historical tendency for significant La Nina episodes (as in 2010-11) to be followed by relatively weaker La Niña episodes, leads to increased confidence that La Niña will persist into the winter. While it is not yet clear what the ultimate strength of this La Niña will be, La Niña conditions have returned and are expected to gradually strengthen and continue into the Northern Hemisphere winter 2011-12.
Across the contiguous United States, temperature and precipitation impacts associated with La Niña are expected to remain weak during the remainder of the Northern Hemisphere summer and early fall, and to generally strengthen during the late fall and winter. During September-November 2011, there is evidence that La Niña favors an increased chance of above-average temperatures across the mid-section of the country, [below average temperatures throughout much of Alaska and the Pacific Northwest] and an increased chance of above-average precipitation across the Pacific Northwest.
Temperature Probability 3 Month Outlook (Climate Prediction Center)
Probability of Precipitation 3 month outlook (Climate Prediction Center)
See the 3-month seasonal outlook released on 18 August 2011. Or check out the full September 8th update here: (El Niño/Southern Oscillation (ENSO) Diagnostic Discussion issued by Climate Prediction Center/NCEP/NWS September 8, 2011.)
More Snow for the Pacific North West!? More Snow Stoke and the ever present chance of a big winter for Tahoe? Can’t lose! Bring it La Niña!
This just makes me want to lawn dart into winter so I’m throwing it in here again. Attack of La Niña coming to Squaw October 7th and Whistler on Sept 30th. Enjoy!