El Niño/Southern Oscillation - October 2008
Neutral ENSO conditions still prevail in the Pacific Basin
Sea surface temperatures remained near normal across most of the equatorial Pacific. Since February 2008, negative sea surface temperature anomalies have weakened over the equatorial Pacific Ocean, and positive anomalies have expanded westward into the east-central equatorial Pacific Ocean. During October, SSTs continued to be slightly above average over a large portion of the equatorial Pacific Ocean. The ENSO-neutral conditions indicated by the near normal SSTs are also due to the above average upper-ocean heat anomalies and normal thermocline slope index. Monthly ENSO indices are all near normal.
The Niño 3.4 Index region (map of Niño regions) SST anomaly was -0.26°C (-0.47°F), which was a decrease of -0.06°C (-0.13°F) compared with the September index value of -0.20°C (-0.36°F). The Niño 4 Index region (western Pacific) SST anomaly warmed 0.24°C (0.43°F) relative to September to -0.11°C (-0.20°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 3-month (August-October) SST anomaly running mean in the Niño 3.4 region was 0.0°C (0.0°F), indicating neutral conditions (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition).
While not ruling out a small chance of El Niño development by late this year, the CPC's most recent ENSO Diagnostic Discussion indicated that ENSO-neutral conditions are likely to remain in place through the boreal spring (austral fall) 2009. The ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) also reflected the presence of ENSO-neutral conditions and expected continuation of these conditions across the tropical Pacific basin through the end of 2008 (see the Australian BoM ENSO Wrap-Up).
Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The atmospheric winds and convection patterns exhibited a high degree of week-to-week variability across the tropical Pacific during October in response to the Madden-Julian Oscillation (MJO). The cumulative effects of the MJO were above-average convection over Indonesia, and enhanced low-level easterly winds, enhanced upper-level westerly winds, and suppressed convection over the western equatorial Pacific.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite also reflected ENSO-neutral conditions across the tropical Pacific Ocean in October. Much of the central equatorial Pacific was near normal in October. The tropics west of the Date Line also had near average sea level heights for the month. An area of negative sea level anomalies persisted into October north and south of the Equator in the central Pacific, and substantial positive anomalies were present in the far western Pacific region.
In October, OLR values jumped to +1.1 Wm-2 from +0.3 Wm-2 in September. The map below on the left shows the spatial pattern of global OLR (in Wm-2) measured by satellite during October. 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 October marked the 21st 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.
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 October. The standardized monthly averaged value was +1.3, compared to +1.5 in September. These SOI values generally correspond with ENSO-neutral conditions over the equatorial Pacific. [Consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions.]