El Niño/Southern Oscillation - March 2005
ANOMALOUSLY WARM SSTs HANGING ON IN THE CENTRAL PACIFIC
Temperatures (SSTs) and
SSTs decreased along the South American coast and across the eastern equatorial Pacific Ocean in March, although the monthly anomalies increased slightly in the central parts of the Pacific basin. In the mixed-layer, above average temperatures persisted in the central equatorial Pacific, with the largest anomalies reaching over 4°C at 100-200 m depth between 170°W and 130°W. The depth of the 20°C isotherm increased in March, with the largest depth anomalies in the central equatorial Pacific.
For the month, the SST anomaly in the Niño 3.4 Index region in the central equatorial Pacific Ocean increased slightly to +0.32°C (+0.58°F) above the mean. In the western Pacific region, the SST anomaly decreased slightly in March, with a monthly averaged Niño 4 Index value of +0.67°C (+1.21°F) (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for March 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). Due to the recent decreases in the monthly averaged SSTs, the 3-month running mean of the Niño 3.4 Index declined below +0.5°C in March. (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.)
Zonal Winds (U-Component Winds) and Sea-Level
The trade winds were above normal across much of the eastern equatorial Pacific basin. A significant transition occurred in the western half of the tropical Pacific during during March (see the March zonal wind loop), as westerly wind anomalies during the first half of the month gave way to stronger than normal easterly trade winds. This led to the observed decreases in the monthly averaged SST anomalies in the Niño 1+2 region.
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 March 21st overpass of the Jason-1 satellite shows that the Kelvin wave that propagated across the equatorial Pacific in late February and early March dissipated as it moved into the eastern parts of the equatorial Pacific near 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 March. In contrast to the anomalous convection and tropical cyclone activity observed in February, there were no large regions of enhanced convection and associated negative OLR anomalies present in the equatorial Pacific in March. Positive OLR anomalies were observed in the central and eastern portions of the Pacific basin and along the South American coast during the month. However, the convection that occurred in February is reflected in the most recent 3-month averaged OLR anomalies, with large areas of negative OLR anomalies near the dateline and westward along the Equator, and also along the South Pacific Convergence Zone (SPCZ).
The monthly-averaged OLR Index shifted sign to a positive value in March, with an index of +0.4 averaged across an area centered over the Dateline 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. Therefore, the change in sign in March has been part of a longer period of inconsistent OLR indices since early 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.
Oscillation Index (SOI):
The standardized SOI was near-neutral in March, with a monthly averaged value of -0.2 (note that negative SOI values are consistent with El Niño conditions). This transition to near-neutral conditions in the atmosphere followed a strongly negative SOI in February.
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-neutral value and a more negative index associated with warm event conditions (although 2004 ended with seven consecutive negative monthly values).
In summary, the SST anomalies and other indices suggest that the current ENSO warm event remained weak and confined to the western and central parts of the equatorial Pacific at the end of March.
Additional El Niño/Southern Oscillation Links
- ENSO Monitoring
- NOAA El Niño Observations Page
- NOAA El Niño / La Niña Index Definition
- NOAA's Pacific Marine Environmental Laboratory (PMEL):
- NOAA's Climate Prediction Center (CPC):
- NOAA's Climate Diagnostics Center (CDC)
- NASA/JPL Ocean Surface Topography from Space
- Australian Bureau of Meteorology (BoM) ENSO Wrap-Up
- IRI - International Research Institute