The equatorial Pacific Ocean remains in an El Niño state, and during October (especially in the last two weeks of the month) the atmosphere appeared to have started responding to the ocean. Equatorial sea surface temperature anomalies are positive across much of the Pacific, with the largest anomalies now in the central Pacific, in and near the NINO4 region. NINO 3 & 4 anomalies were +0.9 °C and +1.2 °C in October (3–month ASO means +1.1 °C and +1.0 °C). Subsurface oceanic heat content remains above average, with the largest anomalies in the central Pacific, where a significant positive subsurface temperature anomaly has developed during October (+3 °C for the month, +5 °C in the latest 5-day mean).
Overall, the regional diagnostics suggest the current situation is a Central-Pacific or Modoki El Niño. In the atmosphere, the SOI has been dropping rapidly since mid-October, with the October average around –1.4 (ASO mean –0.6). Westerly wind anomalies began to penetrate east of the Dateline during October, reaching 120°E by late in the month, while at the same time intensifying west of the Date Line. Convection over the Tropical Pacific was enhanced west of the Date Line in October, and suppressed over the Indian ocean and Indonesia.
Equatorial OLR anomalies remain weak east of the Date Line. The TRMM ENSO precipitation index is near-zero at +0.04. The MJO has been muted in October, but a convective pulse is predicted to develop and propagate over the Indian Ocean over the next two weeks, associated with suppressed convection over Indonesia and the western Pacific.
All dynamical models (and all but one statistical model) show warm conditions through to the end of January 2010, with a general easing to neutral conditions by mid-autumn. The NCEP ENSO discussion from 8 October suggests the El Niño peaking at moderate strength and persisting through southern summer. The IRI summary indicates a 90% probability for El Niño persisting through to the end of January, and at least an 80% probability of El Niño through March.
(Mean sea surface temperatures (ºC) for October 2009)
(Sea surface temperature anomalies (ºC) for October 2009)