El Niño/Southern Oscillation - July 2008


ENSO NEUTRAL CONDITIONS CONTINUE ACROSS EQUATORIAL PACIFIC

Sea Surface Temperatures (SSTs) and Mixed Layer Conditions:

Since February 2008, negative sea surface temperature anomalies have weakened over the equatorial Pacific Ocean, and positive anomalies have expanded westward into the east-central equatorial Pacific Ocean. Over the past month, SST have continued to be slightly above average over a large portion of the equatorial Pacific Ocean. The ENSO-neutral conditions indicated by the near normal SSTs are also borne out by the above average upper-ocean heat anomalies and normal thermocline slope index. Monthly ENSO indices are all now near normal.

The Niño 3.4 Index region (map of Niño regions) SST anomaly was 0.12°C (0.22°F), which was a warming of +0.44°C (+0.79°F) compared with the negative June index value of -0.32°C (-0.58°F). Likewise, the Niño 4 Index region (western Pacific) SST anomaly warmed +0.23°C (+0.41°F) relative to June to -0.32°C (-0.58°F). For the most recent equatorial Pacific Ocean surface temperatures, please visit NOAA's Tropical Atmosphere Ocean (TAO) project and for weekly or monthly Niño region average SST and anomaly values, visit CPC's Atmospheric and SST Index values page.

With the continued warming of SSTs in the Niño 3.4 index region over the past month, the 3-month (May-July) running mean was -0.26°C (-0.47°F), which is within the Oceanic Niño Index (ONI) threshold of -0.5°C (-0.9°F) for neutral conditions (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition).

The CPC's most recent ENSO Diagnostic Discussion indicated that ENSO-neutral conditions should persist over the next several months, and while not ruling out a small chance of El Niño development by late this year, the CPC feels that neutral conditions are likely to remain in place until Boreal Spring (Austral Fall) 2009. The ENSO Wrap-Up from the Australian Bureau of Meteorology (BoM) also reflected the presence and expected continuation of ENSO-Neutral conditions across the tropical Pacific basin through the end of 2008 (see the Australian BoM ENSO Wrap-Up).

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Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:

July trade winds in the central and western Pacific fluctuated around the long-term mean, although low-level flow in the far western Pacific was stronger than average. Decreased trades continued their presence in the central and eastern equatorial Pacific as indicated by the loop of weekly-averaged zonal winds.

Pacific sea levels measured by the NASA/JPL Jason-1 satellite also reflected the ENSO-neutral conditions present across the tropical Pacific Ocean in July. The region of above average sea levels in the eastern Pacific decreased somewhat between late June and early July, while much of the central equatorial Pacific was near normal in July. The tropics west of the Date Line also had near average sea level heights for the month. An area of negative sea level anomalies persisted into July north and south of the equator in the central Pacific, and substantial positive anomalies were present in the far western Pacific region.

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Outgoing Longwave Radiation (OLR):

Cloudiness further decreased over the central equatorial Pacific in July, contributing to an increase of OLR values and decreased convection for portions of the equatorial Pacific around and east of the Dateline. This pattern has contributed to the return to normal SST for much of the central Pacific. The map on the left below shows the spatial pattern of global OLR (in W m-2) measured by satellite during July. The lack of convection along the Equator near the Dateline had persisted since the development of the cold event in late May 2007. The suppression of convection in the western Pacific is more clearly seen in the 3-month average OLR anomalies map (below right).

The monthly OLR index for July was +0.9 W m-2 averaged across an area in the western Pacific between 160° E and 160° W. This was the 18th consecutive month that the OLR index had a positive monthly value, and was an increase of 0.5 W m-2 from June. However, July's value remains less than those experienced during the most recent La Niña event. Persistently high positive OLR indices are typical of the mature phase of a La Niña episode, while negative values indicate the presence of increased convection and warm phase conditions.

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.

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Southern Oscillation Index (SOI):

The Southern Oscillation Index (SOI) was once again slightly positive in July and decreased slightly from June; however, conditions were near the long-term mean. The standardized monthly averaged value was +0.2, as compared to +0.3 in June. These near-normal SOI values generally correspond with ENSO-neutral conditions over the equatorial Pacific. [Consistently positive (negative) values of the SOI are typical of La Niña (El Niño) conditions.]


Citing This Report

NOAA National Climatic Data Center, State of the Climate: El Niño/Southern Oscillation for July 2008, published online August 2008, retrieved on October 31, 2014 from http://www.ncdc.noaa.gov/sotc/enso/2008/7.