El Niño/Southern Oscillation - April 2008
LA NIÑA CONDITIONS WEAKEN CONSIDERABLY:
SSTs CONTINUE TO WARM ACROSS THE EQUATORIAL PACIFIC
Sea surface temperature (SST) anomalies colder than -1.0°C (-1.8°F) remained across the central and western equatorial Pacific this month, but the monthly averaged anomalies decreased in April as the oceanic heat content increased significantly compared with March. Water temperatures in the mixed layer also warmed in April, with an area of positive temperature anomalies in the top 50 meters of the mixed layer in the eastern equatorial Pacific. Warmer-than-average sub-surface temperatures remained in the western equatorial Pacific.
For the month, the SST anomaly in the Niño 3.4 Index region was -0.75°C (-1.35°F) in April, which was a warming of +0.28°C (+0.50°F) compared to the March anomaly. The SSTs in the Niño 4 Index region of the western equatorial Pacific also warmed during April to a monthly anomaly of 0.87°C (1.57°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 April 2008.
Despite the warming of the SSTs 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 April, 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 will continue to persist over the next several months. The ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) also reflected the significant weakening of cold event conditions across the 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 April, while in the central equatorial Pacific the trade winds were near-normal, and in the eastern equatorial Pacific they were weaker-than-normal for the month. Significant week-to-week variability in the near-surface winds has been observed along the equatorial region of the Pacific in April, which can be seen in the loop of April zonal winds.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite also reflected a weakening of La Niña conditions, as the sea level anomalies continued to transition to near-normal in the central equatorial Pacific in mid-April. Below-normal sea level anomalies remained north and south of the equatorial zone, reflecting the cooler-than-average ocean temperatures associated with the current 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 April. An area of positive OLR anomalies was observed in the western equatorial Pacific primarily west of the Dateline, illustrating the suppressed tropical convection in this region. The lack of convection along the equator near the Dateline has persisted since the development of the cold event. The suppression of convection in the western Pacific is more clearly seen in the 3-month average OLR anomalies (below right).
The monthly OLR index for April was +1.5 W m-2 averaged across an area in the western Pacific between 160° E and 160° W. This was the fifteenth 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, titlehough the decline of the OLR index in April was indicative of the weakening cold phase conditions.
As of early April, NOAA's Climate Prediction Center (CPC) has forecasted the current La Niña episode to transition to neutral during June-July 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., near-zero) as the cold event in the equatorial Pacific continues to weaken.
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 a monthly averaged value of +0.6 in April. This was a significant decline compared with the March value of +1.1, and also from the February index value of +2.7, which was the highest SOI of the 2007/2008 La Niña. Overall, April was the ninth consecutive month with a positive SOI value [consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions].
Therefore, the decline of the SOI during April indicated the continued weakening of the La Niña across the equatorial Pacific basin.