El Niño/Southern Oscillation - June 2008
BORDERLINE ENSO NEUTRAL CONDITIONS PRESENT ACROSS PACIFIC BASIN
Sea surface temperatures (SST) continued to warm across the entire equatorial Pacific Ocean in June, with a remaining pocket of cool La Niña anomalies weakening in the western Pacific. Above normal SSTs persisted along the South American coast, although those anomalies have decreased slightly to values below +1.0°C (+1.8°F). Monthly ENSO indices are all now near normal.
The Niño 3.4 Index region (map of Niño regions) SST anomaly was -0.31°C (-0.56°F), which was a warming of +0.27°C (+0.49°F) compared with the May index value. Likewise, the Niño 4 Index region (western Pacific) SST anomaly warmed +0.19°C (+0.34°F) relative to May to -0.57°C (-1.03°F). For the most recent equatorial Pacific Ocean surface temperatures, please visit NOAA's Tropical Atmosphere Ocean (TAO) project.
With the continued warming of SSTs in the Niño 3.4 index region over the past month, the 3-month (April-June) running mean was -0.50°C (-0.9°F), which is the Oceanic Niño Index (ONI) threshold between a cold event and neutral conditions (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 ENSO-neutral conditions should persist over the next several months. The ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) also reflected the presence and expected continuation of ENSO-Neutral conditions across the tropical Pacific basin into Austral spring (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 further in June, with speeds varying about the long-term mean throughout the month. Decreased trades continued their presence in the eastern equatorial Pacific as indicated by the loop of weekly-averaged zonal winds.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite also reflected the continued development of ENSO-neutral conditions. A pool of slightly negative sea level anomalies developed in the western equatorial Pacific basin around 165°E by early June. Meanwhile, the region of positive anomalies that developed along the west coast of South America during May persisted into June.
Cloudiness over the central equatorial Pacific was near normal in June, contributing to a continued decrease of OLR values toward normal conditions 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 June. An area of positive OLR anomalies observed in the western equatorial Pacific near the Dateline in May had nearly disappeared in June. The lack of convection along the Equator near the Dateline had 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 June was +0.4 W m-2 averaged across an area in the western Pacific between 160° E and 160° W. This was the seventeenth consecutive month that the OLR index had a positive monthly value. However, this is the lowest standardized OLR value since May 2007, signifying a return to neutral conditions. 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.
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 slightly positive in June; however, conditions were near the long-term mean. The standardized monthly averaged value was +0.3, as compared to -0.3 in May. SOI values have generally been declining since December, 2007. These near-normal SOI values support the development of ENSO-neutral conditions over the equatorial Pacific. [Consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions.]