El Niño/Southern Oscillation - May 2008
LA NIÑA CONDITIONS CONTINUE TO WEAKEN:
SSTs CONTINUE TO WARM ACROSS THE EASTERN PACIFIC
Sea surface temperatures (SST) continued to warm across the equatorial Pacific Ocean in May, while some areas of the western Pacific still showed some weak cool (negative) SST anomalies left from the La Niña that began the year. Along the South American coast, larger warm (positive) SST anomalies, some exceeding +1.0°C (+1.8°F) were evident. These conditions support a return to ENSO-neutral conditions across the basin over the next few months.
The Niño 3.4 Index region (map of Niño regions) SST anomaly was -0.59°C (-1.06°F), which was a warming of +0.26°C (+0.47°F) compared with the April index value. Likewise, the Niño 4 Index region (western Pacific) SST anomaly warmed +0.20°C (+0.36°F) to -0.77°C (-1.39°F) relative to April. For the most recent global ocean surface temperatures, please see the loop of satellite-derived weekly SST anomalies for May 2008.
Despite the warming of the SSTs in the Niño 3.4 index region over the past month, the 3-month running mean (-0.84°C [-1.5°F]) remained below -0.5°C (-0.9°F) in May, 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 and will likely become ENSO-neutral in June and July. 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:
Trade winds in the central and western Pacific weakened during May, with speeds varying about the long-term mean near the end of the month. Decreased trades continued their presence in the eastern equatorial Pacific, and showed some westward expansion throughout the month, as shown in the loop of May zonal winds.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite also reflected a weakening of La Niña conditions. The pool of negative sea level anomalies present across the central and eastern equatorial Pacific at the end of April shrunk appreciably through May. Additionally, a sizable region of positive sea level anomalies developed along the South American coast in May.
Cloudiness remained below average over the central equatorial Pacific in May, contributing to above average OLR values for portions of the equatorial Pacific near the Dateline. The map on the left below shows the spatial pattern of global OLR (in W m-2) measured by satellite during May. The area of positive OLR anomalies observed in the western equatorial Pacific primarily west of the Dateline in April continued through May, 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 in late May, 2007. 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 May was +1.2 W m-2 averaged across an area in the western Pacific between 160° E and 160° W. This was the sixteenth 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 continued decline of the OLR index in May is indicative of the weakening cold phase conditions.
As of early May, NOAA's Climate Prediction Center (CPC) continued to forecast a transition to ENSO-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 Southern Oscilation Index (SOI) was negative for the first time since July, 2007. The standarized monthly averaged value was -0.3, as compared to +0.6 in April. SOI values have been declining since December, 2007. This continued decrease and negative SOI value is further indication of the breakdown of La Niña conditions over the equatorial Pacific. [consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions].