El Niño/Southern Oscillation - October 2005
ENSO CONDITIONS REMAIN NEUTRAL IN THE EQUATORIAL PACIFIC
Temperatures (SSTs) and
Since April 2005, the SSTs across the equatorial Pacific Ocean have cooled to near-normal in the different Niño regions. The coldest anomalies have developed along the South American coast, as reflected in the SST observations from the Niño 1+2 region. Temperatures in the upper ocean and mixed-layer have also decreased, as a layer of anomalously cold water (~1°C or ~2°F below normal) has persisted at 50-200 m depth in the equatorial Pacific.
For the monthly means, the October SST anomaly was -0.06°C (-0.108°F) below average in the Niño 3.4 Index region in the central equatorial Pacific Ocean, and +0.24°C (+0.43°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 October 2005.
The monthly SST anomalies in the Niño 3.4 region have exceeded +0.5°C only during April of this year, after which the 2004-2005 ENSO warm event dissipated. At the end of October 2005, the 3-month running mean of the Niño 3.4 Index continued to decrease below the long term mean. (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.)
Zonal Winds (U-Component Winds) and Sea-Level
The easterly trade winds were above-normal during October across the tropical Pacific basin, which enhanced equatorial upwelling in the mixed-layer. Monthly zonal wind anomalies exceeding 1 m/s were observed along the equator near the South American coast and between 170° E and 150° W. In the western equatorial Pacific basin, anomalous westerly winds that were present at the beginning of the month relaxed and easterly trade winds developed west of the dateline by mid-October (see the October zonal wind loop).
Satellite altimetry of ocean surface topography from the NASA/JPL Jason-1 satellite over the Pacific basin and global oceans are shown to the left. The overpass of the Jason-1 satellite on October 2nd showed that increased sea level associated with tropical convection developed near the equator in the western Pacific at the beginning of the month.
Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during October. No large-scale OLR anomalies (positive or negative) were observed along the equator or within the equatorial region in the tropical Pacific. The 3-month averaged OLR anomalies during the August-October period show that positive OLR anomalies were observed immediately north of the equator along the Inter-Tropical Convergence Zone (ITCZ) and therefore tropical convection was below average in the central and eastern equatorial Pacific.
The monthly averaged OLR Index remained postive in October, with a 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.
Oscillation Index (SOI):
The standardized SOI increased in October, to a monthly averaged index value of +1.1. This was the largest positive SOI value since May 2004. So far in 2005, the SOI has been negative during 5 months. However, since March of this year the SOI has been either neutral or near-neutral, which reflects the lack of any large-scale fluctuations in mean sea-level pressure (MSLP) or the Walker Circulation across the tropical Pacific basin. Therefore, atmospheric indicators during the past few months have continued to reflect the near-neutral status of ENSO in the equatorial Pacific.