El Niño/Southern Oscillation - Annual 2007
The El Niño/Southern Oscillation (ENSO) began 2007 in a weak warm phase (i.e., weak El Niño), which developed during September and October of 2006. The El Niño warm event peaked in December of 2006 and began to dissipate during January of 2007. As a result, the equatorial Pacific sea-surface temperature (SST) anomalies decreased during the first two months of 2007, eventually declining to near average by the end of March.
Neutral ENSO conditions persisted in the equatorial Pacific Ocean through the Boreal summer, and eventually the SST anomalies declined to signficantly below average in July. The cooling trend continued in August, as below normal SST anomalies spread westward from the eastern tropical Pacific and the South American coast. The cooling ocean temperatures eventually reached the western equatorial Pacific, as the SST anomalies decreased to below-average in August, breaking 15 consecutive months with above normal SST anomalies in the Niño 4 region.
Ocean surface and subsurface temperatures have continued to cool throughout the remainder of 2007. This can be seen in the monthly averaged SST anomalies in both the Niño 3.4 and Niño 4 regions, which show that the cooling trend in the equatorial zone in the Pacific basin has continued to develop over the last few months of 2007.
The observed cooling of the equatorial Pacific was most pronounced in the classical El Niño region of the eastern Pacific and along the South American coast (i.e. the Niño 1+2 region) during May and September of 2007. The cooling of the eastern equatorial Pacific SSTs began as early as March.
By the end of December 2007, ocean surface temperatures in the equatorial Pacific had decreased to the lowest monthly anomalies of the La Niña event so far. SST anomalies from the TAO buoy array were below -2.0°C (-3.6°F) across the equatorial Pacific from the South American coast to 150°W, with monthly averaged anomalies declining below -2.5°C (-4.5°F) in the eastern Pacific. Sub-surface temperature anomalies were also significantly below normal in December.
The Southern Oscillation Index (SOI) reflected the transition from a weak El Niño to a moderate La Niña during 2007. During the first half of the year, the SOI was primarily negative, reflecting the relatively weak El Niño conditions in the Pacific basin. In fact, the SOI was negative for the first 5 months of 2007. Beginning in June, the SOI switched to a positive monthly value. Since August, the SOI has had positive monthly values, and these have continued through the end of 2007. In fact, the SOI in December reached it's highest value of the 2007/2008 cold event, with an index of +1.8 for the month.
At the beginning of 2007, the Outgoing Longwave Radiation (OLR) Index was strongly negative, which reflected the enhanced tropical convection and cloudiness across the equatorial Pacific. However, this changed significantly in June when the OLR Index increased to a monthly value of +0.6. Subsequently, the OLR Index has been positive, and the index increased to +1.4 in October, which was the highest value of the year. The most recent monthly averaged OLR Index was again positive, but with a slightly lower value of +0.8 in November. The OLR Index increased significantly in December to a positive monthly value of +2.2, and similar to the SOI this was the highest value of the 2007/2008 La Niña.