El Niño/Southern Oscillation - June 2004
Temperatures (SSTs) and
In the central equatorial Pacific, SST anomalies increased slightly in June, which led to an increase in the monthly averaged Niño 3.4 index. Above average ocean temperatures were also observed in the mixed-layer, with the largest anomalies east of 140°W. In the western equatorial Pacific, above normal SST anomalies decreased slightly during June, and this was reflected in the monthly averaged SST anomaly in the Niño 4 Index (map of Niño regions). For the most recent ocean surface temperature conditions, please see the loop of satellite-derived weekly SST anomalies for June.
The observed gradient in SSTs across the equatorial Pacific has increased over the past several months, and this can be seen in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST version 2). For June, the Niño 3.4 index increased to +0.32°C (+0.58°F) above normal, and the 3-month running mean increased as well. (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 their latest official assessment of ENSO conditions.)
Winds (U-Component Winds) and Sea-Level Topography:
Above average trade wind flow was observed during June in the equatorial Pacific, resulting in negative zonal wind anomalies for the month. The increase in the trade winds enhanced equatorial and coastal upwelling, which increased the colder than average SST anomalies in the eastern equatorial Pacific and along the South American coast. The June average zonal wind field clearly illustrates that stronger than average easterly anomalies were present in the equatorial Pacific from South America to the dateline (180°longitude), with the strongest anomalies observed south of the equator.
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. Decreases in sea-level and negative sea-level anomalies developed in the central equatorial Pacific in late June (see the 27 June 2004 overpass).
Outgoing Longwave Radiation (OLR):
The monthly averaged OLR index value for June was negative across the region centered over the dateline in the western Pacific between 160°E and 160°W. Since the beginning of 2004 there has been no consistent trend in the OLR index, which has shifted sign several times after a nine-month period of positive values. The map to the left shows the spatial pattern of global OLR anomalies in June. The negative OLR anomalies in the western Pacific reflected areas of enhanced convection north of Indonesia in the northwestern tropical Pacific, as well as an area of enhanced convection associated with the South Pacific Convergence Zone (SPCZ), while positive OLR anomalies and supressed tropical convection was observed in June over the central and eastern Pacific just north of the equator.
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). During June, there was a lack of MJO related activity across the Pacific basin, which was reflected in the positive OLR anomalies observed across the central and eastern Pacific. However, a moderate MJO event in the Indian Ocean moved into the far western Pacific at the end of June, likely enhancing convection in the western Pacific region. The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices.
Southern Oscillation Index (SOI):
Similar to the OLR index, there has been no consistent trend in the SOI in 2004. The standardized SOI shifted from a positive value in May to a negative one in June, with an averaged index value of -1.3 for the month. Since November 2003, the SOI has switched signs numerous times, reflecting the lack of any consistent mean-sea-level pressure pattern across the tropical Pacific associated with ENSO.