El Niño/Southern Oscillation - August 2005
ENSO CONDITIONS REMAIN NEUTRAL ACROSS THE EQUATORIAL PACIFIC
Temperatures (SSTs) and
Over the past month, SSTs decreased across the western and central equatorial Pacific Ocean. Below normal SSTs have persisted along the South American coast, as reflected in the observations from the Niño 1+2 region during August. Mixed-layer temperatures also decreased, as a layer of anomalously cold water (1-2 °C below normal) developed at 50-200 m depth along the equatorial cold-tongue in the eastern Pacific region between 160°W and 110°W .
For the month, the SST anomalies remained near average in the Niño index regions, with an August value of +0.05°C (+0.09°F) above average in the Niño 3.4 region in the central equatorial Pacific Ocean, and -0.03°C (-0.05°F) below the mean in the Niño 4 Index region in the western equatorial Pacific Ocean (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for August 2005.
Over the past year, above average SST anomalies had persisted in the western and central equatorial Pacific basin, with the largest SST anomalies of the 2004-2005 El Niño event observed during November 2004. However, SSTs began to cool in early 2005 and the monthly anomalies in the Niño 3.4 region exceeded +0.5°C only during April of this year, after which the warm event dissipated. At the end of August 2005, the 3-month running mean of the Niño 3.4 Index continued to decline and remained below +0.5°C. (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 easterly trade winds were stronger-than-normal across the eastern and central equatorial Pacific basin during August, enhancing equatorial upwelling in the mixed-layer. In the far western equatorial Pacific basin west of the dateline, westerly wind anomalies developed during the latter half of the month (see the August zonal wind loop). At monthly and seasonal time-scales, this pattern of stronger than normal trade winds in the eastern Pacific and weaker than normal trade winds in the western Pacific has persisted for several months.
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 overpass of the Jason-1 satellite on August 21st showed that no large-scale fluctuations in sea-level were present across the equatorial Pacific basin near the end of this past month. Since late April, there have been no significant sea-level anomalies associated with oceanic Kelvin waves in the equatorial Pacific basin.
Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during August. No large-scale OLR anomalies were observed along the equatorial region in the tropical Pacific. The 3-month averaged OLR anomalies during the June-August period show that positive OLR anomalies were present in the central and eastern Pacific basin, and therefore tropical convection was suppressed along the Inter-Tropical Convergence Zone (ITCZ).
The monthly averaged OLR Index shifted signs in August, with a mean value of +0.6 averaged across an area centered over the dateline in the western Pacific (between 160° E and 160° W). The OLR Index has shifted sign several times over the past year, with no persistent trend in the index observed so far in 2005. 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 remained near-neutral in August, despite a slight decrease in the monthly averaged index to a value of -0.8. The SOI has been negative during 5 months so far in 2005, with an extraordinarily low value of -4.1 occurring back in February 2005 (which was the lowest SOI value since the peak of the 1982-1983 El Niño event). However, since March the SOI has been either neutral or near-neutral, which reflects the lack of any large-scale anomalies in mean sea-level pressure (MSLP) across the tropical Pacific basin.