El Niño/Southern Oscillation - October 2007
LA NIÑA CONDITIONS INTENSIFY:
SSTs CONTINUE TO COOL ACROSS EQUATORIAL PACIFIC
SSTs CONTINUE TO COOL ACROSS EQUATORIAL PACIFIC
Equatorial Pacific Ocean surface and subsurface temperatures have been cooler-than-average over the past several months, as ENSO was in transition from a neutral to a cold phase. The observed cooling trend continued in October, as the SST anomalies decreased across the entire equatorial Pacific basin. Water temperatures in the mixed-layer also remained below normal, with an area of -3.0°C (-4.4°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 October.
For the month, the SST anomaly in the Niño 3.4 Index region was -1.07°C (-1.93°F), which was a decrease of -0.20°C (-0.36°F) compared to the September value. The SSTs in the Niño 4 Index region of the western equatorial Pacific were also below normal in October, with 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 October 2007.
The cooling of the SST anomalies in the Niño 3.4 index region over the past several months kept the 3-month running mean below -0.5°C (-0.9°F) in October, which is the Oceanic Niño Index (ONI) threshold for a cold event (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 officially in a cold phase (La Niña), while the latest ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) concurred that La Niña conditions were present 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 October. 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 eastern 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 October zonal winds. Periods of anomalous westerly winds occurred across parts of the western Pacific region in early October, and in the eastern Pacific region later in the month.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite remained significantly below average across the equatorial Pacific in October. The mid-October overpass measured a broad area of negative sea level anomalies, which reflected the cooler-than-normal ocean surface and mixed-layer temperatures along the equator (see the most recent image of 16 October 2007 Pacific sea level anomalies). The negative sea level anomalies expanded significantly along the equatorial zone, as the cold SST anomalies intensified in October.
Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) measured by satellite during October. An area of positive OLR anomalies was observed across the entire Pacific basin just north of the equator along the Inter-Tropical Convergence Zone (ITCZ). Positive OLR anomalies were also observed over the past 3 months (August-October), 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 October was +1.4 W m-2, averaged across an area in the western Pacific near the Date Line between 160° E and 160° W. This was an increase of +0.8 from the September value. The persistence of positive OLR index values is also 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 +0.6 in October. Despite a brief excursion to a negative SOI in July (-0.5), four of the past five months have had positive index values [consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions]. Therefore, the persistence of positive monthly SOI values over the past several months indicates the development of a La Niña sea level pressure pattern across the equatorial Pacific basin.