El Niño/Southern Oscillation - October 2004
ENSO WARM EVENT CONTINUES TO DEVELOP IN THE PACIFIC
Temperatures (SSTs) and
SST anomalies continued to increase in October across the equatorial Pacific, which led to an increase in the monthly averaged Niño indices. Above average ocean temperatures were also observed in the mixed-layer, with the largest temperature anomalies east of 140°W. In addition, the depth of the 20°C isotherm increased in October. In the central and western equatorial Pacific, above normal SST anomalies also increased during October, which were reflected in the monthly averaged SST anomalies in both the Niño 3.4 and Niño 4 indices (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for October.
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 October, the Niño 3.4 index increased to +0.87°C (+1.57°F) above normal, and the 3-month running mean of the Niño 3.4 index was well above +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:
The easterly trade winds were below normal across much of the western equatorial Pacific basin during October. The below normal easterly flow during the month weakened equatorial upwelling, which led to an increase in the average SSTs across the entire equatorial Pacific basin. Despite the below average zonal wind field from Indonesia eastward past the dateline, moderate trade wind flow and above average easterly zonal wind anomalies were present from the central Pacific 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 in the western equatorial Pacific in early October, and subsequently propagated eastward. This suggests the development of a Kelvin wave in the mixed layer (see the most recent altimetry measurements over the Pacific Ocean from the 31 October 2004 overpass).
Outgoing Longwave Radiation (OLR):
The monthly averaged OLR Index value for October was zero (i.e. neutral) 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 persistent trend in the OLR Index, which has shifted signs several times.
The map to the left shows the spatial pattern of global OLR observed by satellite in October. The above average OLR anomalies in the western Pacific over Indonesia reflected areas of suppressed tropical convection, while the below average OLR anomalies northwest of Australia were associated with enhanced convection along the South Pacific Convergence Zone (SPCZ). In general, most of the equatorial Pacific region had near-normal OLR and small departures from normal during October.
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 remained negative for the fifth consecutive month in October, with an average index value of -0.3 for the month. Negative SOI values are consistent with ENSO warm event (i.e. El Niño) conditions. However, the trend in SOI during the past five months has been toward a neutral value, as the index increased from -1.3 in June to -0.3 in October, suggesting that the atmosphere has lagged significantly behind the ocean during the past few months. Therefore, El Niño conditions in the equatorial Pacific remain relatively weak at the end of October, and any intensification of the current ENSO warm event will most likely depend on the impacts of the oceanic Kelvin wave in the central Pacific as it propagates eastward toward the South American coast during November.