El Niño/Southern Oscillation - August 2007
ENSO IN TRANSITION TO LA NIÑA:
SSTs COOL ACROSS EQUATORIAL PACIFIC
SSTs COOL ACROSS EQUATORIAL PACIFIC
Equatorial Pacific Ocean surface and subsurface temperatures have been near or slightly cooler-than-average over the past several months, as the phase of ENSO was neutral. However, during August the SSTs decreased across the entire equatorial Pacific region. In addition, 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 August.
For the month, the SST anomaly in the Niño 3.4 Index region was -0.73°C (-1.31°F), which was a decrease of -0.44°C (-0.79°F) compared to the July value. The SSTs in the Niño 4 Index region of the western equatorial Pacific also decreased in August, which resulted in a monthly anomaly of 0.11°C (0.19°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 August 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 August. (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 the current phase of ENSO was in a transition from neutral conditions to a cold phase (La Niña), while the latest ENSO Update from the Australian Bureau of Meteorology (BoM) also reflected the transition to La Niña conditions in the equatorial Pacific basin. Both CPC and the BoM have indicated that this transition to La Niña will continue over the next 1-3 months (see the Australian BoM ENSO Wrap-Up).
Winds (U-Component Winds) and Sea-Level Topography:
The easterly Trade winds were above normal across most of the tropical Pacific during August. 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 along the South American coast and in the far western equatorial Pacific this past month.
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 August zonal winds. Periods of anomalous westerly winds occurred across parts of the far eastern Pacific region during mid-August, as the Trade winds weakened off the South American coast.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite remained significantly below average across the equatorial Pacific in August. The negative sea level anomalies reflected the below normal ocean surface and mixed-layer temperatures along the equator (see the most recent image of 26 August 2007 Pacific sea level anomalies). The negative sea level anomalies expanded significantly along the equatorial zone, as the cold SST anomalies intensified in August.
Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) measured by satellite during August. An area of positive OLR anomalies was observed in the eastern Pacific just north of the equator along the Inter-Tropical Convergence Zone (ITCZ). Positive OLR anomalies were also observed over the past 3 months (June-August) as well, with a broad area of suppressed tropical convection from the western Pacific to the South American coast. The persistence of positive OLR anomalies along the equatorial zone in the Pacific is a common signal associated with La Niña conditions.
The monthly OLR index for August was +0.6 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 decrease from the July value of +1.1, although it remained positive and consistent with a transition from a neutral phase to cold event (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 +0.1 in August. Despite a brief excursion to a negative SOI in July (-0.5), two of the past three months have had positive index values [consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions].
Over the past six months, the SOI was inconsistent with the other atmospheric and oceanic indicators, as it remained mostly negative. However, the shift to a positive index in August was indicative of a transition in the sea level pressure pattern across the equatorial Pacific basin.