El Niño/Southern Oscillation - April 2007


NEUTRAL ENSO PHASE PERSISTS:
SSTs REMAIN COOL ACROSS THE EASTERN EQUATORIAL PACIFIC






Sea-Surface Temperatures (SSTs) and Mixed-Layer Conditions:
Ocean surface and subsurface temperatures have been near- or cooler-than-average over the past several months, as the 2006/2007 El Niño warm event dissipated. The cooling has been most pronounced in the mixed-layer across the eastern equatorial Pacific. During April, water temperatures in the mixed-layer remained below normal, with an area of -3.0°C (-5.4°F) and cooler temperature anomalies between 50-150 meters depth in the eastern equatorial Pacific. However, warmer than average upper ocean temperatures remained in the far western equatorial Pacific west of the Date Line in April, as well as in the central tropical Pacific where the monthly SSTs were near-normal.

For the month, the SST anomaly in the Niño 3.4 Index region was +0.09°C (+0.16°F), which was a change of +0.20°C (+0.36°F) compared to the monthly anomaly for March. The SSTs in the Niño 4 Index region of the western equatorial Pacific cooled slightly during April, which had a monthly anomaly of +0.34°C (+1.21°F) above 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 2007.

The recent cooling of SST anomalies in the Niño 3.4 index region brought the 3-month running mean below +0.5°C (+0.9°F) in April. (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 phase of ENSO was neutral. The latest ENSO update from the Australian Bureau of Meteorology (BoM) also reflected the transition to neutral ENSO conditions in the equatorial Pacific basin. Both CPC and the BoM have indicated an elevated probability of a La Niña cold event developing over the next several months (see the Australian BoM ENSO Wrap-Up).





Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly Trade winds were above normal across most of the central and western equatorial Pacific during April. In contrast, weaker than normal Trade winds were observed this past month in the eastern tropical Pacific region.

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 April zonal winds. Periods of anomalous westerly flow occurred in the eastern equatorial Pacific region during April, as easterly Trade winds were below average across the eastern equatorial Pacific.

Pacific sea levels measured by the NASA/JPL Jason-1 satellite remained below average across the central equatorial Pacific, reflecting the cooling ocean surface temperatures in this region (see the most recent image of 22 April 2007 sea level anomalies).



Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) measured by satellite during April. A region of positive OLR anomalies was measured in the eastern Pacific along the equatorial zone, illustrating the supression of tropical convection due to the cold SSTs observed in this region over the past month. This pattern of positive OLR anomalies along the equator in the eastern Pacific was observed over the February-April period as well.

The monthly OLR index for April was +0.1 W m-2 averaged across an area in the western Pacific near the Date Line between 160° E and 160° W. Therefore, the April value was very close to zero, reflecting the neutral ENSO 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.


Southern Oscillation Index (SOI):
The standardized SOI was -0.4 in April. The SOI remains negative, a pattern that has persisted over the past 5 months [consistently negative (positive) values of the SOI are typical of El Niño (La Niña) conditions]. However, the monthly indices have been slowly evolving toward neutral since October when it fell to -1.7 (the lowest index value of the 2006/2007 El Niño event).

Note, the SOI may transition to positive values over the next several months as NOAA's Climate Prediction Center (CPC) has forecasted a possible development of La Niña (cold event) conditions during May-July 2007.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: El Niño/Southern Oscillation for April 2007, published online May 2007, retrieved on October 20, 2014 from http://www.ncdc.noaa.gov/sotc/enso/2007/4.