El Niño/Southern Oscillation - December 2004
WEAK WARM EVENT CONDITIONS PERSIST IN CENTRAL PACIFIC
Temperatures (SSTs) and
SST anomalies remained above normal in December across the central equatorial Pacific, although there was a slight increase in the monthly averaged Niño 3.4 index. Above average ocean temperatures were also observed in the mixed-layer, with the largest temperature anomalies reaching 3°C at approximately 100 m depth near 120°W. In addition, the depth of the 20°C isotherm increased in December. In the central equatorial Pacific, above normal SST anomalies remained during December, which were reflected in the monthly averaged SST anomalies in the Niño 3.4 Index. The western Pacific SST anomaly warmed slightly in December, as observed 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 December 2004.
Above average SST anomalies across the equatorial Pacific have persisted 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 December, the Niño 3.4 Index decreased slightly to +0.81°C (+1.46°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 December, with westerly zonal wind anomalies developing the western Pacific during the month. The anomalous westerly flow during the month weakened equatorial upwelling, which led to a slight increase in the monthly averaged SST anomaly across the western equatorial Pacific region. Despite the below average zonal wind field from Indonesia eastward past the dateline to 150°W, 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. Anomalous westerly wind activity in December appears to have generated another oceanic Kelvin wave in the mixed-layer that propogated eastward during the month. The most recent Pacific overpass of the Jason-1 satellite shows the sea level rise associated with this Kelvin wave in the central equatorial Pacific at the end of December (see both the 12 December 2004 and 27 December 2004 overpasses).
Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite in December. In general, the majority of the equatorial Pacific region had near-normal OLR and only small departures from the 1979-1995 base period during December. The 3-month averaged OLR anomalies were also near-normal or slightly positive along the equator from the dateline to the South American coast.
The monthly averaged OLR Index value switched signs again in December, resulting in a monthly index value of +0.5 averaged across the region centered over the dateline in the western Pacific between 160°E and 160°W. The shift in sign of the OLR Index from a negative value in November to a positive one in December reflected the lack of persistence in the OLR Index this past year, during which the index shifted several times between positive and negative values. Therefore, the convective response of the atmosphere to the above average SST anomalies in the western and central equatorial Pacific has been limited, since the OLR Index has shown no consistent trend throughout all of 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.
Southern Oscillation Index (SOI):
The standardized SOI remained negative for the seventh consecutive month in December, with an average index value of -0.9 for the month. Negative SOI values are consistent with ENSO warm event (i.e. El Niño) conditions. The trend in SOI during the past six months has fluctuated several times between a near-neutral value and a more negative index associated with warm event conditions, which suggests that the atmosphere has lagged behind the ocean in response to the above average SSTs during the past few months. Therefore, El Niño conditions in the equatorial Pacific remained weak at the end of the year.
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