El Niño/Southern Oscillation - March 2009
La Niña Conditions Weaken Further in March
Below-average sea surface temperatures (SST) across the equatorial Pacific Ocean continued during March, but La Niña conditions weakened. All four Niño regions had negative anomalies for the sixth consecutive month. However, the SST anomalies in the Niño 3.4 and Niño 4 regions (central and western equatorial Pacific) experienced warming. The Niño 4 region negative SST anomalies weakened in March with a value of -0.37°C (-0.67°F)—an increase of 0.27°C (0.49°F) as compared to February. The SST anomalies in the Niño 3.4 region warmed by 0.10°C (0.18°F).
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 weakened further across the eastern half of the equatorial Pacific Ocean. Positive temperature anomalies in the western and central Pacific expanded eastward. Negative temperature anomalies persisted in the far eastern Pacific.
Even though the Niño 3.4 region SST anomalies experienced warming in March, the latest three-month (January-February-March) SST anomaly running mean (-0.7°C [-1.3°F]) was still below the Oceanic Niño Index (ONI) La Niña conditions threshold of -0.5°C (-0.9°F) for the third consecutive month (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition). Five consecutive values at or below this threshold constitutes a La Niña episode.
The majority of dynamic computer models forecasts for the Niño-3.4 region show that once ENSO-neutral conditions are reached, they will continue through the remainder of 2009. Based on current observations, recent trends, and model forecasts, a transition to ENSO-neutral conditions is expected during April 2009.
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
Enhanced low-level easterly winds and upper-level westerly winds decreased across almost the entire breadth of the tropical Pacific during March. Collectively, these oceanic and atmospheric anomalies are consistent with a weakening La Niña.
Pacific sea levels measured by the NASA/JPL Jason satellite reflected ENSO-neutral conditions across the tropical Pacific Ocean in March. Much of the central equatorial Pacific was near normal during the month. An area of negative sea level anomalies persisted into March north of the Equator in the central Pacific, and positive anomalies were present in the far western Pacific region again this month.
Outgoing Longwave Radiation (OLR) values decreased for the third consecutive month from +1.7 Wm-2 in February to +1.4 Wm-2 in March. The map below on the left shows the spatial pattern of global OLR (in Wm-2) measured by satellite during March. 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 March marked the 26th 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.
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.
In March, the Southern Oscillation Index (SOI) was negative for the first time since May of 2008. The standardized monthly averaged value fell from +1.8 in February to -0.1 in March.