ENSO & SST

ENSO and Sea Surface Temperatures

The tropical Pacific Ocean remains in a neutral but warm state (which may also be described as a weak El Niño state), much as it was in April. The rapid changes occurring in late April/early May, towards a possible El Niño state, appear to have been a short-lived perturbation. The Southern Oscillation Index (SOI) dropped slightly last month to -1.5, making the March-May mean -1.0. The NINO3 sea surface temperature (SST) anomaly rose rapidly in early May, with the May monthly average about +1.0 °C, while NINO4 was steady near +0.6 °C, and NINO3.4 was +0.7 °C. The March-May mean for NINO3.4 was about +0.6 °C. Subsurface temperature anomalies have eased in the equatorial Pacific, after a large positive anomaly surfaced in the east from late April. A significant Madden-Julian Oscillation (MJO) event came through the western Pacific in the first half of May, associated with positive surface zonal wind anomalies and a small rise in SST anomalies.

Activity weakened again towards the end of the month. Outgoing longwave radiation (OLR) anomalies for May show suppressed convection over Indonesia and northern Australia, but weaker anomalies further east.

Most available models indicate neutral conditions (mostly with positive NINO3.4 anomalies) through winter and spring. The Australian POAMA model and the UK Meteorological Office model have both eased towards neutral forecasts (from El Niño and La Niña forecasts respectively last month). The latest US National Center for Environmental Prediction (NCEP/CPC) statement calls for neutral conditions through June-August, with an uncertain outlook for spring. The International Research Institute for Climate Prediction (IRICP) summary gives a 70% chance of neutral conditions persisting through July 2005, with a 30% chance of an El Niño developing.

Sea surface temperature anomalies (°C) for May 2005

Mean sea surface temperatures (°C) for May 2005