The El Niño/Southern Oscillation (ENSO) began 2008 in a cold phase (i.e., La Niña) which developed during September of 2007, peaked in February 2008, began to dissipate in March, and completely disolved by June. During the first half of 2008, below average sea-surface temperature (SST) anomalies were observed in the central and eastern equatorial Pacific Ocean, with the coldest SST anomalies measured in February in the Niño 3.4 region. By June 2008, the near-equatorial SST anomalies had warmed to near-normal in the central Pacific region as the ENSO transitioned to a neutral phase.
In the western equatorial Pacific, SSTs returned to near-average temperatures by July, remaining near average through November. This can be seen in the monthly averaged SST anomalies in the Niño 4 region. Cooling in the upper-ocean along the equatorial Pacific in the latter part of 2008 did give some indication of the possible late onset of an ENSO cold event. However, neutral ENSO conditions remained at the end of the year, with below-average SSTs.
A slight cooling began in late October when colder SST anomalies developed along the equatorial zone due to stronger than average trade-wind flow, which enhanced upwelling and decreased SST anomalies in both the eastern and central tropical Pacific.
The Southern Oscillation Index (SOI) was positive for the entirety of 2008, save the month of May when it was only slightly negative. The stronger positive numbers in the first part of the year reflect the conditions during the La Niña. Prolonged periods of positive SOI values coincide with abnormally cold ocean waters across the eastern tropical Pacific typical of La Niña episodes.
At the beginning of 2008, the Outgoing Longwave Radiation (OLR) Index was strongly positive as tropical convection was suppressed across the equatorial Pacific in response to the cold SSTs that had developed in association with the La Niña present at that time. Outgoing Longwave Radiation values have been positive since January 2007 when there was a warm phase (i.e., El Niño) present.