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El Niño/Southern Oscillation - July 2005


SST ANOMALIES COOL ACROSS THE EQUATORIAL PACIFIC







Sea-Surface Temperatures (SSTs) and Mixed-Layer Conditions:
SSTs decreased across the western and central equatorial Pacific Ocean this past month. Anomalously cold ocean surface temperatures persisted along the South American coast in July, as reflected in the observations from the Niño 1+2 region during this past month. Surface and subsurface temperatures in the mixed-layer decreased during July, as a layer of anomalously cold water developed near 140°W and along the equatorial cold-tongue in the eastern Pacific region.

For the month, the SST anomalies decreased in the Niño index regions: the July value was -0.1°C (-0.18°F) below average in the Niño 3.4 region in the central equatorial Pacific Ocean, and +0.1°C (+0.18°F) above the mean in the Niño 4 Index region in the western equatorial Pacific Ocean (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for July 2005.

Over the past year, above average SST anomalies had persisted in the western and central equatorial Pacific basin, with the largest SST anomalies observed during November 2004. However, SSTs cooled at the beginning of 2005 with monthly anomalies exceeding +0.5°C only during April of this year. At the end of July, the 3-month running mean of the Niño 3.4 Index continued to decrease and remained below +0.5°C. (NOTE: A running 3-month mean SST anomaly above +0.5°C in the Niño 3.4 region is one indicator that an El Niño is occurring. For the official NOAA 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 stronger-than-normal across the eastern and central equatorial Pacific basin during July, which helped enhance equatorial upwelling in the mixed-layer. In the far western equatorial Pacific basin west of the dateline, westerly wind anomalies developed during the latter half of the month (see the July zonal wind loop). Several high frequency fluctuations in the trade winds also occurred during July, as the strength of the easterly flow varied considerably along the equatorial zone.

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 overpass of the Jason-1 satellite on July 25th showed that no large-scale fluctuations in sea-level were present across the equatorial Pacific basin near the end of this past month.



Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during July. Negative OLR anomalies, which are associated with enhanced tropical convection, were observed along the equator west of the dateline in the western Pacific. The 3-month averaged OLR anomalies during the May-July period show that tropical convection was suppressed in the central and eastern Pacific basin along the Inter-Tropical Convergence Zone (ITCZ).

The July OLR Index was near-neutral, with a monthly mean value of -0.3 averaged across an area centered over the dateline in the western Pacific (between 160° E and 160° W). The OLR Index has shifted sign several times over the past year, with no persistent trend in the index observed so far in 2005. 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 was neutral in July, with a monthly averaged index value of 0.0. Similar to the OLR Index, the SOI displayed unusual behavior during the relatively weak 2004-2005 El Niño event. For example, during the latter-half of 2004 the monthly SOI fluctuated several times between a near-zero value (indicating near-neutral conditions) and a more negative index (indicating El Niño conditions), although 2004 ended with seven consecutive negative monthly values. During the past few months the SOI has been near-neutral, which reflects the neutral state of the atmosphere across the tropical Pacific basin.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: El Niño/Southern Oscillation for July 2005, published online August 2005, retrieved on April 19, 2014 from http://www.ncdc.noaa.gov/sotc/enso/2005/7.