El Niño/Southern Oscillation - Annual 2005
The El Niño/Southern Oscillation (ENSO) began 2005 in a weak warm phase (i.e., a weak El Niño), which had originally developed during July and August 2004. During the first half of 2005, above average sea-surface temperature (SST) anomalies were observed in the central equatorial Pacific Ocean, with a peak in the SST anomalies in May in the Niño 3.4 region. Over the past six months these have cooled significantly, especially in the eastern Pacific region. In the western equatorial Pacific, SST anomalies have also cooled throughout most of 2005, decreasing to near-average by August. During the last few months of 2005, the monthly mean SSTs in the Niño 4 region remained slightly above or near-normal, with the most recent monthly mean anomaly in December 2005 decreasing below the long-term average.
The observed cooling of the upper ocean conditions during latter half of 2005 was reflected in the classical El Niño region of the eastern Pacific and along the South American coast (i.e. the Niño 1+2 region). The cooling trend of the eastern equatorial Pacific SSTs began as early as January, although there was a temporary warming in late April and early May. Since the end of May, colder waters have developed at the surface and in the mixed-layer along the South American coast and spread slowly westward into the central Pacific.
More pronounced cooling began in late August, 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. By the end of December, these cold anomalies had spread further west, as part of the overall cooling of the western Pacific SSTs to near-normal in 2005.
The Southern Oscillation Index (SOI) has shown considerable variability during the latter half of 2005. During the first half of the year, the SOI was primarily negative, reflecting the weak El Niño conditions in the Pacific basin. A pronounced negative anomaly was observed during February, when the SOI dropped dramatically to -4.1, which was the lowest monthly value since the peak of the strong 1982-1983 El Niño event. However, this was short-lived as the monthly SOI values remained negative, but near-neutral, over the next few months. Since June, no consistent trend in sea-level pressure has developed in the equatorial Pacific, which has led to several changes in sign in the index, although it has remained primarily near-neutral. The most recent value of the SOI was -0.2 for the December monthly average.
At the beginning of 2005, the Outgoing Longwave Radiation (OLR) Index showed only limited signs of a convective response to the warm SSTs in the equatorial Pacific associated with the weak warm event. However, this changed in February when the OLR Index had an exceptionally low value for the month, reaching a low index for the year of -2.3 (which is indicative of enhanced convection associated with El Niño conditions). Afterwards the index switched signs several times, although it remained slightly positive or near-zero (i.e. near-neutral) from March through October. In December, the OLR Index reached its largest positive value for the year of +1.5. Therefore, at the end of 2005 the OLR index appears to have finally responded to the development of cooler-than-normal SSTs in the eastern and central equatorial Pacific, with the index in December 2005 reflecting the suppression of atmospheric convection along the near-equatorial zone associated with the development of colder-than-average SSTs.