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El Niño/Southern Oscillation - May 2007


ENSO REMAINS IN NEUTRAL PHASE:
COOLER SSTs PERSIST IN THE EASTERN EQUATORIAL PACIFIC






Sea-Surface Temperatures (SSTs) and Mixed-Layer Conditions:
Equatorial Pacific 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 and the phase of ENSO transitioned to neutral. The cooling has been most pronounced at the surface and in the mixed-layer across the eastern equatorial Pacific. During May, 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 May, while in the central tropical Pacific the monthly SSTs were near-normal.

For the month, the SST anomaly in the Niño 3.4 Index region was -0.37°C (-0.66°F), which was a change of -0.46°C (-0.83°F) compared to the April value. The SSTs in the Niño 4 Index region of the western equatorial Pacific also cooled slightly during May, which had a monthly anomaly of 0.17°C (0.36°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 May 2007.

The cooling of the SST anomalies in the Niño 3.4 index region over the past several months decreased the 3-month running mean below the long-term average for May. (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 current phase of ENSO was neutral, while the latest ENSO update from the Australian Bureau of Meteorology (BoM) also reflected the ENSO-neutral 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 1-3 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 the eastern equatorial Pacific during May. In contrast, weaker than normal Trade winds were observed this past month in the western tropical Pacific region.

Over the past month, 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 May zonal winds. Periods of anomalous westerly flow occurred across parts of the equatorial Pacific region during May, as the easterly Trade winds were weaker-than-average across the western 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 May 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 May. 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. This pattern of positive OLR anomalies along the equator in the eastern Pacific was observed during the March-May period as well.

The monthly OLR index for May was +0.2 W m-2 averaged across an area in the western Pacific near the Date Line between 160° E and 160° W. Therefore, the May value of the OLR Index 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 May. This was the 3rd consecutive month with an SOI value of -0.4, and the 6th straight month with a negative index value [consistently negative (positive) values of the SOI are typical of El Niño (La Niña) conditions]. However, the overall trend in the monthly SOI values has slowly evolved toward neutral since October, when the SOI fell to -1.7 (the lowest index value of the 2006/2007 El Niño event).

Note, the SOI will potentially transition to positive values over the next several months, as NOAA's Climate Prediction Center (CPC) has forecasted the likely development of La Niña (cold event) conditions during June-August 2007.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: El Niño/Southern Oscillation for May 2007, published online June 2007, retrieved on April 20, 2014 from http://www.ncdc.noaa.gov/sotc/enso/2007/5.