El Niño/Southern Oscillation - September 2004
Weak El Niño Conditions Present
Temperatures (SSTs) and
SST anomalies increased significantly in September 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 in September. In the western equatorial Pacific, above normal SST anomalies also increased during September, 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 September.
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 September, the Niño 3.4 index increased to +0.84°C (+1.51°F) above normal, and the 3-month running mean of the Niño 3.4 index was warmer than +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 their latest official assessment of ENSO conditions.)
Winds (U-Component Winds) and Sea-Level Topography:
Easterly trade wind flow relaxed across much of the central and eastern equatorial Pacific basin during September as compared to conditions in August. The decrease in the trade winds resulted in weaker equatorial upwelling, which led to an increase in the average SSTs in the central and eastern equatorial Pacific, though SSTs were still cooler than average immediately adjacent to the South American coast. The September average zonal wind field clearly illustrates that westerly zonal wind anomalies were present in the equatorial Pacific from Indonesia eastward to 170°W, with moderate trade wind flow and negative or near average zonal wind anomalies from approximately 170°W to the South American coast, though these were weaker than the August monthly averaged zonal wind anomalies in the central and eastern Equatorial Pacific.
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 in the central equatorial Pacific in September (see the 19 September 2004 overpass).
Outgoing Longwave Radiation (OLR):
The monthly averaged OLR index value for September was negative across the region centered over the dateline in the western Pacific between 160° E and 160°W for the second consecutive month. 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 in September. The lower OLR anomalies in the western Pacific east of Japan 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 September.
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 fourth consecutive month in September, with an averaged index value of -0.6 for the month. The negative SOI the past four 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 September.