El Niño/Southern Oscillation - February 2006


WEAK LA NIÑA CONDITONS PERSIST:
BELOW AVERAGE SST ANOMALIES REMAIN ACROSS THE CENTRAL EQUATORIAL PACIFIC





Sea-Surface Temperatures (SSTs) and Mixed-Layer Conditions:
SSTs remained below average across most of the equatorial Pacific Ocean in February. The trends in the observed SST anomalies during February were below the long-term means in the central and western parts of the tropical Pacific basin. In contrast, across the eastern Pacific the observed SST anomalies increased to above-normal by the end of the month in the Niño 1+2 region. In the mixed-layer, below average water temperatures persisted in the eastern and central Pacific, with the largest monthly anomalies between -2 to -3°C centered near 120°W and at ~100 m depth.

In the central equatorial Pacific, the SST anomaly in the Niño 3.4 Index region was -0.64°C (-1.15°F) below the mean. The western Pacific SST anomaly decreased during February, with a monthly averaged Niño 4 Index of -0.65°C (-1.17°F) (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for February 2006.

Below average SST anomalies have persisted throughout the latter half of 2005 and into the first two months of 2006 in the central equatorial Pacific basin, and this can be seen in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST version 2). Due to the persistence of the below average SSTs, the 3-month running mean of the Niño 3.4 Index remained below -0.5°C in February. (NOTE: A running 3-month mean SST anomaly below -0.5°C in the Niño 3.4 region is a significant indicator that a La Niña is occurring. For the NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition and see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.)





Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly trade winds were above normal across parts of the equatorial Pacific basin during February, with the largest easterly zonal wind anomalies exceeding ?? m/s in the central Pacific during the month. The stronger-than-average trade winds increased equatorial upwelling in the mixed-layer, which maintained the observed colder-than-average SSTs in both the Niño 3.4 and 4 regions.

Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans is shown to the left. The most recent Pacific overpass of the Jason-1 satellite showed a number of large positive sea level anomalies in the southwest and northwest Pacific, most likely associated with enhanced tropical convection, while the negative anomalies that were measured in the central equatorial Pacific during February are directly related to the cold SSTs associated with La Niña conditions (see the 17 February 2006 overpass).





Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during February. Positive OLR anomalies (typically associated with La Niña) have persisted over the past several months in the equatorial western Pacific region, centered near the dateline, as tropical convection was suppressed along the equator during February. The 3-month averaged OLR anomalies were also positive in the same region of the western Pacific basin near the dateline. In contrast, enhanced convection was observed further west in Indonesia and the maritime continent region, where large-scale tropical convection and several tropical cyclones developed and impacted Western Australia during the past month.

The monthly-averaged OLR Index value remained positive in February, with a value of +0.6 W m-2 averaged across an area centered over the dateline in the western Pacific between 160° E and 160° W (positive OLR Index values are typically associated with La Niña conditions). February was the seventh consecutive month with a positive OLR index value, and during this time large-scale tropical convection has been suppressed in the western and central equatorial Pacific associated with the development of La Niña conditions and the persistence of below average SSTs in the ocean.

Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO) (MJO related convective activity 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.


Southern Oscillation Index (SOI):
The standardized SOI switched signs from a positive to a negative index in February, with an average index value of -0.2 for the month. The SOI has shown no persistent signal during the current weak La Niña event, and it has fluctuated several times between a near-neutral value and both positive and negative index values. Therefore, the SOI value in February suggests that the atmosphere has yet to reflect the below average SSTs over the past several months.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: El Niño/Southern Oscillation for February 2006, published online March 2006, retrieved on October 2, 2014 from http://www.ncdc.noaa.gov/sotc/enso/2006/2.