El Niño/Southern Oscillation - September 2007
LA NIÑA CONDITIONS DEVELOP:
SSTs CONTINUE TO COOL ACROSS EQUATORIAL PACIFIC
SSTs CONTINUE TO COOL ACROSS EQUATORIAL PACIFIC
Equatorial Pacific Ocean surface and subsurface temperatures have been near or slightly cooler-than-average over the past six months, as ENSO was in transition from a neutral to a cold phase. However, during the past several months the SSTs decreased across the entire equatorial Pacific region, and this trend continued in September. Water temperatures in the mixed-layer remained below normal, with an area of -2.0°C (-3.6°F) and cooler temperature anomalies between 50-200 meters depth in the central and eastern equatorial Pacific. Warmer-than-average upper ocean temperatures remained in the far western equatorial Pacific west of the Date Line in September.
For the month, the SST anomaly in the Niño 3.4 Index region was -0.87°C (-1.57°F), which was a decrease of -0.14°C (-0.25°F) compared to the August value. The SSTs in the Niño 4 Index region of the western equatorial Pacific also decreased in September, which resulted in a monthly anomaly of 0.47°C (0.85°F) below 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 September 2007.
The cooling of the SST anomalies in the Niño 3.4 index region over the past several months decreased the 3-month running mean below the long-term average during September. (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition).
The NOAA Climate Prediction Center's most recent ENSO Diagnostic Discussion indicated that ENSO was in a cold phase (La Niña), while the latest ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) concurred that La Niña conditions had developed in the equatorial Pacific basin. Both CPC and the BoM have indicated that the La Niña will persist over the next several months, at least into early 2008.
Winds (U-Component Winds) and Sea-Level Topography:
The easterly Trade winds were above normal across most of the tropical Pacific during September. The above normal easterly winds along the equatorial zone increased upwelling in the mixed-layer, which was especially evident in the central Pacific region. Slightly weaker than normal Trade winds were observed in the far western equatorial Pacific.
Significant week-to-week variability in the near-surface winds has been observed along the equatorial region of the Pacific, as shown in the animation of September zonal winds. Periods of anomalous westerly winds occurred across parts of the far western Pacific region during late September, as the Trade winds weakened over Indonesia and New Guinea.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite remained significantly below average across the equatorial Pacific in September. The negative sea level anomalies reflected the below normal ocean surface and mixed-layer temperatures along the equator (see the most recent image of 10 September 2007 Pacific sea level anomalies). The negative sea level anomalies expanded significantly along the equatorial zone, as the cold SST anomalies intensified in September.
Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) measured by satellite during September. An area of positive OLR anomalies was observed across the eastern and central Pacific just north of the equator along the Inter-Tropical Convergence Zone (ITCZ). Positive OLR anomalies were also observed over the past 3 months (July-September), with a broad area of suppressed tropical convection north of the equator from the western Pacific to the South American coast. The persistence of positive OLR anomalies in the Pacific equatorial region is a common atmospheric signal associated with La Niña conditions.
The monthly OLR index for September was +0.8 W m-2, averaged across an area in the western Pacific near the Date Line between 160° E and 160° W. This was a slight increase from the August value of +0.6. The persistence of positive OLR index values is consistent with La Niña conditions.
Note: high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO), which is an intra-seasonal oscillation in 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 value of the SOI was +0.2 in September. Despite a brief excursion to a negative SOI in July (-0.5), three of the past four months have had positive index values [consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions]. Therefore, the recent shift to a positive SOI was indicative of a La Niña sea level pressure pattern across the equatorial Pacific basin.