National Climatic Data Center
15 May 2008
La Niña conditions weakened considerably in April, as sea surface temperature (SST) anomalies warmed and the ocean heat content increased in the eastern and central equatorial Pacific.
Atmospheric conditions in April were also indicative of a weakening of the La Niña during April: the Outgoing Longwave Radiation (OLR) index had a monthly value of +1.5, while the Southern Oscillation Index (SOI) was +0.6 for the month — both of which were a signficant decline from the observed indices in March.
Therefore, both the oceanic and atmospheric indicators reflected a considerable weakening of La Niña conditions across the Pacific basin in April.
KEYWORDS: ENSO; Niño Regions; Southern Oscillation Index (SOI); Kelvin Waves; Thermocline; Sea Surface Temperatures (SSTs); Madden-Julian Oscillation (MJO); Walker Circulation
Sea surface temperature (SST) anomalies colder than -1.0°C (-1.8°F) remained across the central and western equatorial Pacific this month, but the monthly averaged anomalies decreased in April as the oceanic heat content increased significantly compared with March. Water temperatures in the mixed layer also warmed in April, with an area of positive temperature anomalies in the top 50 meters of the mixed layer in the eastern equatorial Pacific. Warmer-than-average sub-surface temperatures remained in the western equatorial Pacific.
For the month, the SST anomaly in the Niño 3.4 Index region was -0.75°C (-1.35°F) in April, which was a warming of +0.28°C (+0.50°F) compared to the March anomaly. The SSTs in the Niño 4 Index region of the western equatorial Pacific also warmed during April to a monthly anomaly of 0.87°C (1.57°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 April 2008.
Despite the warming of the SSTs in the Niño 3.4 index region over the past month, the 3-month running mean remained below -0.5°C (-0.9°F) in April, 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 Climate Prediction Center's most recent ENSO Diagnostic Discussion indicated that the cold event (La Niña) is weakening, but will continue to persist over the next several months. The ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) also reflected the significant weakening of cold event conditions across the tropical Pacific basin (see the Australian BoM ENSO Wrap-Up).
Anomalous easterlies (i.e., Trade winds) were observed across the western equatorial Pacific during April, while in the central equatorial Pacific the trade winds were near-normal, and in the eastern equatorial Pacific they were weaker-than-normal for the month. Significant week-to-week variability in the near-surface winds has been observed along the equatorial region of the Pacific in April, which can be seen in the loop of April zonal winds.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite also reflected a weakening of La Niña conditions, as the sea level anomalies continued to transition to near-normal in the central equatorial Pacific in mid-April. Below-normal sea level anomalies remained north and south of the equatorial zone, reflecting the cooler-than-average ocean temperatures associated with the current La Niña event (see the most recent loop of Pacific basin sea level anomalies).
The map on the left below shows the spatial pattern of global OLR (in W m-2) measured by satellite during April. An area of positive OLR anomalies was observed in the western equatorial Pacific primarily west of the Dateline, illustrating the suppressed tropical convection in this region. The lack of convection along the equator near the Dateline has persisted since the development of the cold event. The suppression of convection in the western Pacific is more clearly seen in the 3-month average OLR anomalies (below right).
The monthly OLR index for April was +1.5 W m-2 averaged across an area in the western Pacific between 160° E and 160° W. This was the fifteenth consecutive month that the OLR index had a positive monthly value. Persistently high positive OLR indices are typical of the mature phase of a La Niña episode, although the decline of the OLR index in April was indicative of the weakening cold phase conditions.
As of early April, NOAA's Climate Prediction Center (CPC) has forecasted the current La Niña episode to transition to neutral during June-July 2008 (click here for CPC's ENSO Diagnostic Advisory Archive). It is expected that the monthly OLR indices will eventually decrease toward neutral (i.e., near-zero) as the cold event in the equatorial Pacific continues to weaken.
Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO, which is 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.
The standardized SOI had a monthly averaged value of +0.6 in April. This was a significant decline compared with the March value of +1.1, and also from the February index value of +2.7, which was the highest SOI of the 2007/2008 La Niña. Overall, April was the ninth consecutive month with a positive SOI value [consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions].
Therefore, the decline of the SOI during April indicated the continued weakening of the La Niña across the equatorial Pacific basin.
Sea surface temperature (SST) anomalies colder than -1.5°C (-2.7°F) shifted further to the west during the month, with the coldest anomalies from approximately 160°E to 140°W in March. Water temperatures in the mixed layer warmed in March, with an area of -2.0°C (-5.4°F) and slightly colder temperature anomalies between 50 and 150 meters depth of the mixed layer in the eastern equatorial Pacific. Warmer-than-average temperatures remained in the mixed layer in the western equatorial Pacific, primarily west of the Dateline.
For the month, the SST anomaly in the Niño 3.4 Index region was -1.03°C (-1.85°F) in March, which was an increase of +0.71°C (+1.28°F) compared to the February anomaly. The SSTs in the Niño 4 Index region of the western equatorial Pacific also warmed during March to a monthly anomaly of 0.98°C (1.76°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 March 2008.
Despite the significant warming of the SST anomalies in the Niño 3.4 index region over the past month, the 3-month running mean remained below -0.5°C (-0.9°F) in March, 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 Climate Prediction Center's most recent ENSO Diagnostic Discussion indicated that the cold event (La Niña) is weakening, but they forecast it to persist over the next several months. The ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) also reflected the weakening of cold event conditions in the eastern tropical Pacific basin (see the Australian BoM ENSO Wrap-Up).
Anomalous easterlies (i.e., trade winds) were observed across the western equatorial Pacific during March, while in the central equatorial Pacific the trade winds were near-normal, and in the eastern equatorial Pacific they were below-normal for the month. Significant week-to-week variability in the near-surface winds has been observed along the equatorial region of the Pacific over the past month, which can be seen in the loop of March zonal winds.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite also reflected a weakening of La Niña conditions as sea level anomalies had transitioned to close to normal along the equator in the central equatorial Pacific in March. Below-normal sea level anomalies remained north and south of the equatorial zone, reflecting the cooler-than-average ocean temperatures associated with the La Niña event (see the most recent loop of Pacific basin sea level anomalies).
The map on the left below shows the spatial pattern of global OLR (in W m-2) measured by satellite during March. A large region of positive OLR anomalies was observed in the western equatorial Pacific mainly west of the Dateline, illustrating the suppressed tropical convection in this region. The lack of convection along the equator and near the Dateline has persisted since the development of the cold event. The suppression of convection in the western Pacific is also clearly seen in the 3-month average OLR anomalies (below right).
The monthly OLR index for March was +2.4 W m-2 averaged across an area in the western Pacific between 160° E and 160° W. This was the fourteenth consecutive month that the OLR index had a positive monthly value. Persistently high positive OLR indices are typical of the mature phase of a La Niña episode.
As of early March, NOAA's Climate Prediction Center (CPC) has forecasted the current La Niña episode to continue through the Boreal spring (March-May) 2008 (click here for CPC's ENSO Diagnostic Advisory Archive). It is expected that the monthly OLR indices will eventually decrease toward neutral (i.e., zero) as the cold event in the equatorial Pacific begins to weaken over the next several months.
Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO, which is 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.
The standardized SOI had an observed value of +1.1 in March. This was a significant decline compared with the February index value of +2.7, which was the highest monthly SOI of the 2007/2008 La Niña. Overall, March was the eighth consecutive month with a positive index value [consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions].
Therefore, the recent decline of the SOI during March indicates the weakening of the La Niña to moderate intensity across the equatorial Pacific basin.
The area of sea surface temperature (SST) anomalies colder than -1.5°C (-2.7°F) continued to shift westward and intensify during the month, with the coldest anomalies from approximately 165°E to 120°W in February. Water temperatures in the mixed layer also decreased in February, with an area of -3.0°C (-5.4°F) and colder temperature anomalies between 50 and 150 meters depth of the mixed layer in the eastern equatorial Pacific. Warmer-than-average temperatures remained in the mixed layer in the western equatorial Pacific this month, primarily west of the Dateline.
For the month, the SST anomaly in the Niño 3.4 Index region was -1.74°C (-3.13°F) in February, which was a slight increase of 0.03°C (0.05°F) compared to the January anomaly. The SSTs in the Niño 4 Index region of the western equatorial Pacific also cooled during February to a monthly anomaly of 1.46°C (2.29°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 February 2008.
Significant cooling of the SST anomalies in the Niño 3.4 index region since the beginning of 2008 has kept the 3-month running mean well below -0.5°C (-0.9°F) in February, 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 Climate Prediction Center's most recent ENSO Diagnostic Discussion indicated that the cold event (La Niña) had reached maturity by the end of 2007, and the SST anomalies over the past two months have cooled further in the western and central equatorial Pacific. The ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) also reflects mature cold event conditions in the tropical Pacific basin, which they forecast to persist for several months (see the Australian BoM ENSO Wrap-Up).
Anomalous easterlies (i.e., trade winds) were observed across the western equatorial Pacific during February, while in the central equatorial Pacific the trade winds were near-normal, and in the eastern equatorial Pacific they were below normal for the month. Significant week-to-week variability in the near-surface winds has been observed along the equatorial region of the Pacific over the past month, which can be seen in the loop of February zonal winds.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite were below average across the central equatorial Pacific in February, reflecting the cooler-than-average ocean temperatures and the mature phase of the La Niña event. The positive sea level anomalies have moved slowly westward over the past couple of months (see the most recent loop of Pacific basin sea level anomalies).
The map on the left below shows the spatial pattern of global OLR (in W m-2) measured by satellite during February. A large region of positive OLR anomalies was observed in the western equatorial Pacific centered on the Dateline, illustrating the suppressed tropical convection in this region. The lack of convection along the equator and near the Dateline has persisted for several months, which is clearly seen in the 3-month average OLR anomalies (below right).
The monthly OLR index for February was +2.5 W m-2 averaged across an area in the western Pacific between 160° E and 160° W. This was the thirteenth consecutive month that the OLR index had a positive monthly value. Persistently high positive OLR indices are typical of the mature phase of a La Niña episode.
As of early February, NOAA's Climate Prediction Center (CPC) has forecasted the current La Niña episode to continue through the Boreal spring (March-May) 2008 (click here for CPC's ENSO Diagnostic Advisory Archive). It is expected that the monthly OLR indices will eventually decrease toward neutral (i.e., zero) as the cold event in the equatorial Pacific begins to weaken over the next several months.
Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO, which is 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.
The standardized SOI was +2.7 in February, which was the highest monthly SOI of the 2007/2008 La Niña so far. In fact, the February SOI was the highest index value since January 1974 when it last reached 2.7. Overall, February was the seventh consecutive month with a positive index value [consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions].
Therefore, the anomalously high monthly SOI values during the December to February period indicates the persistence of the mature phase of La Niña across the equatorial Pacific basin.
The area of sea surface temperature (SST) anomalies colder than -1.5°C (-2.7°F) moved westward and intensified during the month, with the coldest anomalies from approximately 170°E to 130°W in January. Water temperatures in the mixed layer also decreased in January, with an area of -2.0°C (-3.6°F) and colder temperature anomalies between 50 and 150 meters depth of the mixed layer in the eastern equatorial Pacific. Warmer-than-average temperatures remained in the mixed layer in the western equatorial Pacific this month, primarily west of the Dateline.
For the month, the SST anomaly in the Niño 3.4 Index region was -1.77°C (-3.19°F) in January, which was a decrease of 0.45°C (0.81°F) compared to the December anomaly. The SSTs in the Niño 4 Index region of the western equatorial Pacific also cooled during January to a monthly anomaly of 1.27°C (2.29°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 January 2008.
Significant cooling of the SST anomalies in the Niño 3.4 index region over the past two months kept the 3-month running mean well below -0.5°C (-0.9°F) in January, 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 Climate Prediction Center's most recent ENSO Diagnostic Discussion indicated that the cold event (La Niña) had reached maturity at the end of 2007, and the SST anomalies have cooled during the month. The ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) also reflects the maturing of cold event conditions in the tropical Pacific basin, which they forecast to persist for the first few months of 2008 (see the Australian BoM ENSO Wrap-Up).
Anomalous easterlies (i.e., trade winds) were observed across the western equatorial Pacific during January, while in the central and eastern equatorial Pacific the trade winds were near-normal for the month.
Significant week-to-week variability in the near-surface winds has been observed along the equatorial region of the Pacific over the past month, as shown in the loop of January zonal winds. A period of anomalous westerly flow occurred in the eastern Pacific during mid-January, as the easterly trade winds weakened in the equatorial zone.
Pacific sea levels measured by the NASA/JPL Jason-1 satellite were below average across the central equatorial Pacific in January, reflecting the cooler-than-average ocean temperatures and the mature phase of the La Niña event. The positive sea level anomalies have moved slowly westward over the past couple of months (see the most recent loop of Pacific basin sea level anomalies).
The map on the left below shows the spatial pattern of global OLR (in W m-2) measured by satellite during January. A large region of positive OLR anomalies was observed in the western equatorial Pacific near the Dateline, illustrating the suppressed tropical convection in this region. The lack of convection along the equator and near the Dateline has persisted for several months, which is clearly seen in the 3-month average OLR anomalies (below right).
The monthly OLR index for January was +1.0 W m-2 averaged across an area in the western Pacific between 160° E and 160° W. This was the twelfth consecutive month that the OLR index had a positive monthly value. Persistently high positive OLR indices are typical of the mature phase of a La Niña episode.
As of early February, NOAA's Climate Prediction Center (CPC) has forecasted the current La Niña episode to continue through the Boreal spring (March-May) 2008 (click here for CPC's ENSO Diagnostic Advisory Archive). Therefore, it is expected that the monthly OLR indices will eventually decrease toward neutral (i.e., zero), as the cold event in the equatorial Pacific begins to weaken over the next several months.
Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO, which is 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.
The standardized SOI was +1.9 in January, which was the highest monthly SOI of the 2007/2008 La Niña so far. Overall, January was the sixth consecutive month with a positive index value [consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions].
Therefore, the persistence of positive monthly SOI values, along with the high monthly SOI values in December and January, indicate that the La Niña has entered its mature phase across the equatorial Pacific basin.
For all climate questions other than questions concerning this report, please contact the National Climatic Data center's Climate Services Division:
Climate Services DivisionFor further information on the historical climate perspective presented in this report, contact:
Karsten Shein-or-
David Levinson
http://lwf.ncdc.noaa.gov/oa/climate/research/2008/enso-monitoring.html 
