ENSO & SST

ENSO and Sea Surface Temperatures

The tropical Pacific Ocean is in a borderline El Niño state, with the monthly Southern Oscillation Index (SOI) back to near zero after its strong negative excursion in February. Surface zonal winds have returned to near normal across the Equatorial Pacific, and outgoing longwave radiation (OLR) anomalies now show suppressed convection across most of the tropical Pacific south of the Equator. Equatorial sea surface temperature (SST) anomalies have not changed much since February: NINO3 is near average (zero anomaly), NINO4 is about +0.8°C anomaly, and the NINO3.4 anomaly is about +0.3°C. However, a pulse of strongly positive subsurface temperature anomalies still lies across the central Equatorial Pacific, and was nearing the South American coast towards the end of March. As this feature rises in the eastern Equatorial Pacific it may have a significant effect on SSTs, at least in the short term.

Most available models indicate neutral conditions (with positive NINO3.4 anomalies) through June 2005, and over the winter. As usual at this time of year, there is a large spread in model forecasts, and considerable uncertainty in the El Niño/Southern Oscillation (ENSO) outlook for the second half of the year. Three or four models predict a warm event redeveloping in the next few months, while one predicts significant cooling over the winter. The latest NCEP/CPC statement calls for weak El Niño conditions easing to a neutral state, and neutral conditions persisting over the southern hemisphere winter. The IRI summary describes the present situation as neutral and gives a 65% chance of neutral conditions persisting through May 2005, with a 35% chance of an El Niño re-developing. The chance of a La Niña developing is close to zero at present, and La Niña conditions are expected to remain unlikely through the rest of 2005.

Therefore, it is important to monitor the ENSO indicators in the tropical Pacific Ocean over the coming months, especially the Kelvin Wave and the Madden-Julian Oscillation.

Sea surface temperature anomalies (°C) for March 2005

Mean sea surface temperatures (°C) for March 2005