El Niño/Southern Oscillation - May 2005
NEUTRAL ENSO CONDITIONS DEVELOP - SSTs COOL IN THE EASTERN PACIFIC
Temperatures (SSTs) and
SSTs decreased across the eastern equatorial Pacific Ocean and along the South American coast in May, as reflected in the Niño 1+2 region, although the monthly averaged SST anomalies increased slightly in the central and western parts of the Pacific basin. Above average temperature anomalies that have persisted over the past six months in the mixed-layer decreased during May, as below average water temperatures expanded along the equatorial cold-tongue in the eastern Pacific region.
For the month, the SST anomaly in the Niño 3.4 Index region in the central equatorial Pacific Ocean increased to +0.53°C (+0.94°F) above the mean, while in the western Pacific region the SST anomaly also increased in May, with a monthly averaged Niño 4 Index value of +0.55°C (+0.99°F) (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for May 2005.
Above average SST anomalies persisted throughout the latter-half of 2004 and into early 2005 in the western and central equatorial Pacific basin, which can be seen in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST version 2). Despite the slight increase in the monthly averaged SSTs in the central equatorial Pacific this past month, the 3-month running mean of the Niño 3.4 Index remained below +0.5°C in May. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.)
Winds (U-Component Winds) and Sea-Level
The easterly trade winds were stronger-than-normal across the eastern equatorial Pacific basin during May. In the western parts of the basin near the date line, westerly wind anomalies developed during the latter half of the month (see the May zonal wind loop). Several high frequency fluctuations in the trade winds also occurred in the central tropical Pacific during May, as the strength of the easterly flow varied considerably along the equatorial zone.
Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. The May 29th overpass of the Jason-1 satellite shows that there were no large-scale sea-level anomalies present across the equatorial Pacific basin at the end of the month.
Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during May. Negative OLR anomalies were measured west of the date line in the western tropical Pacific and along the South Pacific Convergence Zone (SPCZ) during the past month. In addition, negative OLR anomalies were observed just north of the equator in the central Pacific, while in contrast positive OLR anomalies occurred along the equator in the central and eastern portions of the Pacific basin. A similar pattern is evident in the most recent 3-month averaged OLR anomalies, with large areas of negative OLR anomalies west of the dateline and along the SPCZ. Enhanced tropical convection also occurred in the eastern Pacific immediately north of the equator along the Inter-Tropical Convergence Zone (ITCZ).
The monthly-averaged OLR Index was near-neutral in May, with a value of -0.1 averaged across an area centered over the date line in the western Pacific (between 160° E and 160° W). The recent shifts in sign of the OLR Index reflected the lack of persistence of El Niño conditions in the atmosphere since it developed in mid-2004. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity 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.
Southern Oscillation Index (SOI):
The standardized SOI decreased in May to a monthly averaged value of -1.2 (note that negative SOI values are consistent with El Niño conditions). Despite the negative SOI value the past three months, none of these monthly values approached the unusually low index in February 2005, which was the lowest monthly value since the peak of the 1982-1983 El Niño event.
The SOI has shown some unusual behavior during the relatively weak 2004-2005 El Niño event. For example, during the latter-half of 2004 the monthly SOI fluctuated several times between a near-zero value (indicating near-neutral conditions) and a more negative index (indicating warm event conditions), although 2004 ended with seven consecutive negative monthly values.
Therefore, despite the recently observed negative SOI values and the persistence of anomalously warm SST anomalies in the Niño 3.4 and 4 regions during May, the majority of oceanic and atmospheric indices suggest that the current ENSO warm event had transitioned from weak El Niño conditions to neutral conditions during the past month.