El Niño/Southern Oscillation - March 2008
LA NIÑA CONDITIONS WEAKEN TO MODERATE:
SSTs WARM ACROSS THE EASTERN EQUATORIAL PACIFIC
Sea surface temperature (SST) anomalies colder than -1.5°C (-2.7°F) shifted further to the west during the month, with the coldest anomalies from approximately 160°E to 140°W in March. Water temperatures in the mixed layer warmed in March, with an area of -2.0°C (-5.4°F) and slightly colder temperature anomalies between 50 and 150 meters depth of the mixed layer in the eastern equatorial Pacific. Warmer-than-average temperatures remained in the mixed layer in the western equatorial Pacific, primarily west of the Dateline.
For the month, the SST anomaly in the Niño 3.4 Index region was -1.03°C (-1.85°F) in March, which was an increase of +0.71°C (+1.28°F) compared to the February anomaly. The SSTs in the Niño 4 Index region of the western equatorial Pacific also warmed during March to a monthly anomaly of 0.98°C (1.76°F) below the mean (map of Niño regions). For the most recent global ocean surface temperatures, please see the loop of satellite-derived weekly SST anomalies for March 2008.
Despite the significant warming of the SST anomalies in the Niño 3.4 index region over the past month, the 3-month running mean remained below -0.5°C (-0.9°F) in March, which is the Oceanic Niño Index (ONI) threshold for a cold event (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition).
The Climate Prediction Center's most recent ENSO Diagnostic Discussion indicated that the cold event (La Niña) is weakening, but they forecast it to persist over the next several months. The ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) also reflected the weakening of cold event conditions in the eastern tropical Pacific basin (see the Australian BoM ENSO Wrap-Up).
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
Anomalous easterlies (i.e., trade winds) were observed across the western equatorial Pacific during March, while in the central equatorial Pacific the trade winds were near-normal, and in the eastern equatorial Pacific they were below-normal for the month. Significant week-to-week variability in the near-surface winds has been observed along the equatorial region of the Pacific over the past month, which can be seen in the loop of March zonal winds.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite also reflected a weakening of La Niña conditions as sea level anomalies had transitioned to close to normal along the equator in the central equatorial Pacific in March. Below-normal sea level anomalies remained north and south of the equatorial zone, reflecting the cooler-than-average ocean temperatures associated with the La Niña event (see the most recent loop of Pacific basin sea level anomalies).
The map on the left below shows the spatial pattern of global OLR (in W m-2) measured by satellite during March. A large region of positive OLR anomalies was observed in the western equatorial Pacific mainly west of the Dateline, illustrating the suppressed tropical convection in this region. The lack of convection along the equator and near the Dateline has persisted since the development of the cold event. The suppression of convection in the western Pacific is also clearly seen in the 3-month average OLR anomalies (below right).
The monthly OLR index for March was +2.4 W m-2 averaged across an area in the western Pacific between 160° E and 160° W. This was the fourteenth 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.
As of early March, NOAA's Climate Prediction Center (CPC) has forecasted the current La Niña episode to continue through the Boreal spring (March-May) 2008 (click here for CPC's ENSO Diagnostic Advisory Archive). It is expected that the monthly OLR indices will eventually decrease toward neutral (i.e., zero) as the cold event in the equatorial Pacific begins to weaken over the next several months.
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 standardized SOI had an observed value of +1.1 in March. This was a significant decline compared with the February index value of +2.7, which was the highest monthly SOI of the 2007/2008 La Niña. Overall, March was the eighth consecutive month with a positive index value [consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions].
Therefore, the recent decline of the SOI during March indicates the weakening of the La Niña to moderate intensity across the equatorial Pacific basin.