The progression of oceanic conditions in the Equatorial Eastern Pacific towards El Ni�o has been interrupted by a marked cooling over the past two weeks, and the onset of a full-fledged El Ni�o event this fall and winter is now in considerable doubt. Sea surface temperatures in the Equatorial Eastern Pacific were approximately 0.5°C above average--the threshold for a weak El Ni�o event--from the beginning of July through mid-September. However, for the past two weeks, these temperatures have fallen to just 0.2°C above average--solidly in the neutral category. In addition, over the past three months, winds, pressures, and cloud cover over the Pacific have not responded in the fashion typically associated with an El Ni�o (one exception: some stronger westerly surface winds than usual have developed near New Guinea and Indonesia, which could act to push warm water eastwards towards South America in coming months and tip the ocean more towards El Ni�o.) NOAA's Climate Prediction Center (CPC) classified conditions as being neutral in their monthly El Ni�o discussion, issued October 4, but continued their El Ni�o watch, giving a 55% chance that an El Ni�o event will be in place for the October-November-December period. This is a big reduction from 69% odds given in their September forecast. NOAA's final take on the matter:

Due to the recent slowdown in the development of El Ni�o, it is not clear whether a fully coupled El Ni�o will emerge. The majority of models indicate that borderline ENSO-neutral/ weak El Ni�o conditions will continue, and about half suggest that El Ni�o could develop, but remain weak. The official forecast therefore favors the continuation of borderline ENSO-neutral/ weak El Ni�o conditions into Northern Hemisphere winter 2012-13, with the possibility of strengthening during the next few months.

The lack of a progression towards El Ni�o so far this October means that the Atlantic hurricane season is likely to extend into November, as has been the norm over the past decade. El Ni�o events tend to increase wind shear over the tropical Atlantic, suppressing hurricane activity. However, the latest 2-week wind shear forecast from the GFS model shows continued near-average wind shear levels over the Caribbean and tropical Atlantic through mid-October. Given the recent faltering of El Ni�o, I expect that near-average wind shear levels will continue over the tropical Atlantic into November, and that we will see one or two more tropical storms in the Atlantic this hurricane season.

Figure 1. Departure of sea surface temperature (SST) from average between October 2011 and October 2012 in the Equatorial Eastern Pacific between 5°S - 5°N, 170°W - 120°W (the Ni�o 3.4 region.) A La Ni�a episode occurs when SSTs in the Ni�o 3.4 region are 0.5°C cooler than average for an extended period (below the thick blue line.) La Ni�a conditions were in place between October 2011 - March 2012. El Ni�o conditions occur when SSTs in the El Ni�o 3.4 region are more than 0.5°C warmer than average (above the thick red line.) El Ni�o conditions developed in early July, but have fallen below the threshold for a weak El Ni�o event over the past two weeks. Image credit: NOAA's Climate Prediction Center (CPC).

Jeff Masters