El Niño/Southern Oscillation - December 2006
MODERATE EL NIÑO EVENT MATURES:
SSTs COOL SLIGHTLY IN THE EASTERN EQUATORIAL PACIFIC
SSTs COOL SLIGHTLY IN THE EASTERN EQUATORIAL PACIFIC
A large area of Sea-Surface Temperature (SST) anomalies greater than +1.0°C (+1.8°F) stretched from the Date Line in the west-central Pacific to the South American coast in December, with anomalies greater than +1.5°C (+2.7°F) near the Dateline between 170°E and 150°W. Water temperatures in the mixed-layer also remained warm over the past month, with a large area of +3.0°C (+5.4°F) and greater temperature anomalies as deep as 150 meters in the eastern equatorial Pacific. In addition, a layer of cooler water below 100 m depth developed in mid-November in the western Pacific and moved eastward in December.
For the month of December, the SST anomaly in the Niño 3.4 Index region was +1.37°C (+2.47°F), which was an increase of +0.29°C (+0.52°F) compared to the November anomaly. The SSTs in the Niño 4 Index region of the western equatorial Pacific also remained warm during December, although they decreased slightly from the previous month to an anomaly of +1.06°C (+1.91°F) above the mean (map of Niño regions). For the most recent global ocean surface temperatures, please see the loop of satellite-derived weekly SST anomalies for December 2006.
With warmer SSTs in the Niño 3.4 index region in December, the 3-month running mean remained above the +0.5°C (+0.9°F) threshold that indicates the presence of an El Niño episode (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition). The Climate Prediction Center's most recent ENSO Diagnostic Discussion indicated that the warm event (El Niño) had reached maturity and has now shown signs of weakening slightly over the past month. The ENSO forecast from the Australian Bureau of Meteorology (BoM) also continues to reflect the persistence of warm event conditions in the tropical Pacific basin into the first part of 2007 (see the Australian BoM ENSO Wrap-Up).
Winds (U-Component Winds) and Sea-Level Topography:
Anomalous westerly winds were observed across a portion of the far western equatorial Pacific during December, while the easterly Trade winds were slightly above normal across the central and eastern equatorial Pacific.
Significant week-to-week variability in the near-surface winds has been observed along the equatorial region of the Pacific over the past month, as shown in the loop of December zonal winds. A period of anomalous westerly flow occurred in the equatorial Pacific region during early December, followed by a return to easterly Trade winds across the Pacific basin later in the month.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite were above average across the eastern equatorial Pacific in early and mid-December, reflecting the warmer-than-average ocean temperatures and the maturing El Niño event (see the most recent image of 17 December 2006 sea level anomalies).
Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) measured by satellite during December. A region of negative OLR anomalies was observed in the western equatorial Pacific near the Date Line, which suggests that enhanced tropical convection has developed in this region.
The monthly OLR index for December was -0.3 W m-2 averaged across an area in the western Pacific between 160° E and 160° W. This was the fifth consecutive month that the OLR index was below the long-term mean, although an OLR Index value of -0.3 is considered near-neutral. Persistently negative OLR indices are typical of the mature phase of a warm event.
At present, NOAA's Climate Prediction Center (CPC) has forecasted the current El Niño episode to persist into 2007. Therefore, it is expected that the OLR Index will decrease further as the Walker Circulation shifts to the east and tropical convection in the central equatorial Pacific intensifies.
Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO, which is convective activity that 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 was -0.5 in December, switching signs from a positive value in November. The near-neutral SOI in November followed six consecutive months with negative index values. Note that consistently negative (positive) values of the SOI are typical of El Niño (La Niña) conditions. However, negative SOI values are expected to redevelop by the end of the year, as NOAA's Climate Prediction Center (CPC) continues to forecast the persistence of the current El Niño episode into the first few months of 2007.
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