El Niño/Southern Oscillation - December 2008
Negative sea surface temperature anomalies persist in the central to eastern equatorial Pacific
The central and eastern equatorial Pacific Ocean cooled further during December as negative equatorial sea surface temperature (SST) anomalies strengthened across the central and east-central Pacific Ocean.
All four Niño regions had negative anomalies for the third consecutive month. The Niño 3.4 region SST anomaly fell over half a degree Celsius this month with a value of -0.73°C (-1.31°F); a decrease of -0.51°C (-0.92°F) compared with the November value of -0.22°C (-0.40°F). For the most recent equatorial Pacific Ocean surface temperatures, please visit NOAA's Tropical Atmosphere Ocean (TAO) project and for weekly or monthly Niño region average SST and anomaly values, visit CPC's Atmospheric and SST Index values page.
The subsurface oceanic heat content anomalies became increasingly negative as below-average temperatures at thermocline depth strengthened in the central and eastern Pacific. However, there has been a build-up of warmer sub-surface water in the western equatorial Pacific.
The 3-month (October-November-December) SST anomaly running mean in the Niño 3.4 region was -0.3°C (-0.5°F), which is within the Oceanic Niño Index (ONI) threshold of -0.5°C (-0.9°F) for neutral conditions (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition).
While a number of models suggest the development of La Niña during the next few months, the majority of the SST forecasts indicate ENSO-neutral conditions will continue into the first half of 2009. The ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) also indicates the potential for a return to La Niña conditions.
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
During December, low-level easterly winds and upper-level westerly winds strengthened across the equatorial Pacific Ocean. The recent strengthening of the low-level easterlies over the equatorial Pacific suggests the possibility of additional anomalous cooling of the SSTs. However, the magnitude of cooling remains uncertain and it is possible the La Niña threshold will not be met. Overall, the ocean-atmosphere system during December remained consistent with ENSO-neutral conditions, but exhibited several atmospheric characteristics typical of weak La Niña conditions.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite reflected ENSO-neutral conditions across the tropical Pacific Ocean in December. Much of the central equatorial Pacific was near normal during the month, however, cooler anomalies were evident across the equitorial Pacific. An area of negative sea level anomalies persisted into December north and south of the Equator in the central Pacific, and substantial positive anomalies were present in the far western Pacific region again this month.
OLR values increased to +2.3 Wm-2 in December from +1.2 Wm-2 in November. The map below on the left shows the spatial pattern of global OLR (in Wm-2) measured by satellite during December. The lack of convection along the Equator near the Date Line has persisted since the development of the cold event in late May 2007.
The monthly OLR anomaly for December marked the 23rd consecutive month that the OLR index had a positive monthly value. Persistently high positive OLR indices are typical of the mature phase of a La Niña episode, while negative values indicate the presence of increased convection and warm phase conditions.
Convection remained enhanced over the western equatorial Pacific and suppressed near the International Date Line. However, in recent months intraseasonal variability has contributed to episodic strengthening and weakening of convection over the western equatorial Pacific.
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.
The Southern Oscillation Index (SOI) was once again positive in December. The standardized monthly averaged value remained the same this month at +1.5. Consistently positive values of the SOI are typical of La Niña conditions.