El Niño/Southern Oscillation - November 2007
LA NIÑA CONDITIONS PERSIST:
SSTs CONTINUE TO COOL ACROSS THE EQUATORIAL PACIFIC
SSTs CONTINUE TO COOL ACROSS THE EQUATORIAL PACIFIC
Equatorial Pacific Ocean surface and subsurface temperatures have been cooler-than-average over the past several months, due to the development of a La Niña event. The observed cooling trend continued in November as the SST anomalies continued to decrease across the entire equatorial Pacific region. Water temperatures in the upper ocean also remained below normal, with an area of -2.0°C (-4.4°F) and cooler temperature anomalies in the mixed-layer 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 November.
For the month, the SST anomaly in the Niño 3.4 Index region was -1.27°C (-2.29°F), which was a decrease of -0.20°C (-0.36°F) compared to the October value. The SSTs in the Niño 4 Index region of the western equatorial Pacific were also below normal in November, with an anomaly of -0.81°C (-1.46°F) for the month (map of Niño regions). For the most recent global ocean surface temperatures, please see the loop of satellite-derived weekly SST anomalies for November 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 November, 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 continued to be 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 stronger-than-normal across most of the tropical Pacific during November. The above normal easterly winds along the equatorial zone increased upwelling of colder water in the mixed-layer, which maintained the below normal SST anomalies.
The above normal easterly winds along the equatorial zone were especially evident in the western Pacific region, while in the eastern equatorial Pacific the Trade winds were slightly above normal.
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 November zonal winds. Stronger-than-normal easterly winds occurred across the far western Pacific region in early-November, but these weakened significantly later in the month.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite remained significantly below average across the equatorial Pacific in November. The mid-November satellite overpass measured a broad area of negative sea level anomalies, which reflect the cooler-than-normal ocean surface and mixed-layer temperatures along the equator (see the most recent image of 12 November 2007 Pacific sea level anomalies).
Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) measured by satellite during November. An area of positive OLR anomalies was observed in the western Pacific just north of the equator along the Inter-Tropical Convergence Zone (ITCZ).
Positive OLR anomalies were also observed over the past 3 months (September-November), 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 equatorial Pacific region is a typical atmospheric signal associated with La Niña conditions, and is associated with a reduction of deep tropical convection due to cooler SSTs in this region.
The monthly OLR index for November 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 decrease of -0.6 from the October value. Despite the slight decrease in the OLR index, the November value was the tenth consecutive month with a positive OLR index value, which 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 +0.9 in November. Despite a brief excursion to a negative SOI in July (-0.5), five of the past six 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, as well as the increase in the SOI in November, indicates the development of a La Niña sea level pressure pattern across the equatorial Pacific basin.