El Niño/Southern Oscillation - August 2004
Temperatures (SSTs) and
SST anomalies increased significantly in August across the central equatorial Pacific, which led to an increase in the monthly averaged Niño 3.4 index. Above average ocean temperatures were also observed in the mixed-layer, with the largest anomalies east of 140°W. In addition, the depth of the 20°C isotherm increased as well in August, with the anomalies reaching 15-20 m in the eastern Pacific. In the western equatorial Pacific, above normal SST anomalies also increased during August, which was reflected in the monthly averaged SST anomaly in the Niño 4 Index (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for August.
The SSTs across the equatorial Pacific have increased over the past several months, and this can be seen in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST version 2). For August, the Niño 3.4 index increased to +0.80°C (+1.44°F) above normal, and the 3-month running mean of the Niño 3.4 index was warmer than +0.5°C for the first time since December 2003. (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 their latest official assessment of ENSO conditions.)
Winds (U-Component Winds) and Sea-Level Topography:
Easterly trade wind flow weakened across the western equatorial Pacific basin during August, resulting in positive zonal wind anomalies for the month. The decrease in the trade winds resulted in weaker equatorial upwelling, which led to an increase in the average SST anomalies in the western and central equatorial Pacific. Trade winds were near or above normal in the eastern Pacific basin, where the monthly averaged SSTs remained below normal. The August average zonal wind field clearly illustrates that westerly zonal wind anomalies were present in the equatorial Pacific from Indonesia eastward to 170°W, with trade wind flow and negative zonal wind anomalies from approimately 170°W to the South American coast.
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. Increases in sea-level, shown on these images as positive sea-level anomalies, developed again in the western equatorial Pacific in late August (see the 23 August 2004 overpass).
Outgoing Longwave Radiation (OLR):
The monthly averaged OLR index value for August was negative across the region centered over the dateline in the western Pacific between 160°E and 160°W. Since the beginning of 2004 there has been no consistent trend in the OLR index, which has shifted sign several times. The map to the left shows the spatial pattern of global OLR anomalies in August. The negative OLR anomalies in the western Pacific north of New Guinea reflected areas of enhanced tropical convection associated with several Northwest Pacific typhoons. However, most of the equatorial Pacific had near-normal OLR for the month of August.
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):
Since November 2003, the SOI has switched signs numerous times, reflecting the lack of any persistent mean-sea-level pressure pattern across the tropical Pacific associated with ENSO. This trend appears to have changed over the Northern Hemisphere summer, since the standardized SOI remained negative for the third straight month in August, with an averaged index value of -0.2 for the month. The negative SOI the past three months is consistent with ENSO warm event (i.e. El Niño) conditions and the above normal SSTs that were observed in the western and central equatorial Pacific during August.