El Niño/Southern Oscillation - January 2009
Cooler than normal equatorial Pacific sea surface temperatures persist, but with some warming in the east
Below-average sea surface temperatures (SST) across the equatorial Pacific Ocean continued during January, but with the central and east-central equatorial SSTs showing slight warming. All four Niño regions had negative anomalies for the fourth consecutive month. The Niño 3.4 region negative SST anomalies continued to strengthen in January with a value of -0.99°C (-1.78°F)—a decrease of -0.26°C (-0.47°F) as compared to December. However, the Niño 1+2 region (eastern equatorial Pacific) experienced warming with an increase in SST anomalies from -0.41°C (-0.74°F) in December to -0.18°C (-0.32°F) in January.
For the most recent equatorial Pacific Ocean surface temperatures, please visit NOAA's Tropical Atmosphere Ocean (TAO) project and for weekly or monthly Niño region average SST and anomaly values, visit CPC's Atmospheric and SST Index values page.
Negative subsurface oceanic heat content anomalies persisted in the eastern equatorial Pacific, but weakened as positive subsurface temperature anomalies from the western Pacific expanded eastward into the central Pacific between late December and January.
The three-month (November-December-January) SST anomaly running mean in the Niño 3.4 region was -0.6°C (-1.1°F), which is below the Oceanic Niño Index (ONI) threshold of -0.5°C (-0.9°F), indicating La Niña conditions (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition).
A majority of the model forecasts for the Niño 3.4 region, and the eastward propagation of warmer sub-surface water from the western equatorial Pacific, indicate the cooler surface conditions in the Pacific may not persist. Therefore, based on current observations, recent trends, and model forecasts, anomalously cooler conditions are expected to continue into the boreal spring, but the most likely scenario is for the central and eastern Pacific to warm further over the coming months.
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
Trade winds in the central and eastern equatorial Pacific returned to near-normal in January. Easterlies west of the Date Line diminished somewhat as compared to December, but still remain anomalously strong. Generally, strong low-level easterlies over the equatorial Pacific suggest the possibility of anomalous cooling of the SSTs.
Outgoing Longwave Radiation (OLR) values decreased from +2.3 Wm-2 in December to +1.8 Wm-2 in January. The map below on the left shows the spatial pattern of global OLR (in Wm-2) measured by satellite during January. The lack of convection along the Equator near the Date Line has persisted since the development of the cold event in late May 2007.
The monthly OLR anomaly for January marked the 24th consecutive month that the OLR index had a positive monthly value. Persistently high positive OLR indices are typical of the mature phase of a La Niña episode, while negative values indicate the presence of increased convection and warm phase conditions.
Convection remained enhanced over the western equatorial Pacific and suppressed near the International Date Line. However, in recent months intraseasonal variability has contributed to episodic strengthening and weakening of convection over the western equatorial Pacific.
Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO), which is convective activity that propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days. The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices.
The Southern Oscillation Index (SOI) was once again positive in January for the 8th consecutive month. The standardized monthly averaged value, though slightly weaker this month (+1.2) as compared to December (+1.5), remains firmly positive. Consistently positive values of the SOI are typical of La Niña conditions.