El Niño/Southern Oscillation - May 2004
Temperatures (SSTs) and
Mixed-Layer Conditions: Above normal SST anomalies
increased across the western equatorial Pacific in
May, and this was reflected in the monthly averaged SST anomaly
in the Niño
4 Index region. In the central equatorial Pacific, the SST
anomalies decreased slightly in May, which led to a decrease in the
monthly averaged Niño
3.4 index (map of
Niño regions). For the most recent ocean surface
temperature conditions, please see the loop of satellite-derived
weekly SST anomalies for May.
The observed changes in the SST anomalies in these two Niño regions were evident in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST v. 2). In May, the Niño 3.4 anomaly decreased to +0.14°C (+0.252°F) above normal, and the 3-month running mean decreased as well. Despite this slight cooling, the May value for the Niño 3.4 index continued the trend of above average equatorial SST conditions that began in June 2003. 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).
Sub-surface temperatures in the mixed-layer warmed across the equatorial Pacific region in May due to the passage of an oceanic Kelvin wave. Warmer water from the western Pacific propagated eastward during the month, and this eastward movement was evident in the loop of May sub-surface ocean temperatures and anomalies. The warmer surface and sub-surface conditions in the eastern Pacific during May were reflected in an increase of the monthly average depth of the 20°C isotherm, which is an approximate depth of the center of the oceanic thermocline (the oceanic thermocline in the eastern Pacific is anomalously deeper during El Niño episodes, and shallower during La Niña episodes).
Winds (U-Component Winds) and Sea-Level Topography: Zonal
wind anomalies were negative across the eastern and central
equatorial Pacific Ocean, and were the result of slightly above
average easterly trade wind flow in May. The observed increase in
the trade winds, which is clearly shown in the loop of
May 5-day averaged zonal winds, helped to enhance equatorial
and coastal upwelling and the development of the cold anomaly in
the eastern equatorial Pacific and along the South American coast
during the first half of the month. The
May average zonal wind field clearly illustrates that stronger
than average easterly anomalies were present in the equatorial
Pacific from South America to 160°W.
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. In the eastern equatorial Pacific, the observed decrease in sea-level and the negative sea-level anomalies began in March and continued into early May 2004, but the sea-level anomalies since that time have returned to near-normal along the equatorial zone in the eastern Pacific.
Outgoing Longwave Radiation (OLR):
The average OLR index value for May was positive across the region centered over the western Pacific between 160°E and 160°W. The OLR Index has gone through a series of abrupt shifts that began in January 2004 when the index switched from a nine-month period of positive values. The map to the left shows the spatial pattern of global OLR anomalies in May. These negative OLR anomalies 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 (PCZ), while positive OLR anomalies and associated tropical convection was suppressed over the eastern Pacific along 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). Enhanced convection related to MJO activity propagated eastward across the equatorial Pacific in April and early May, and a weaker MJO event moved into the western Pacific basin in mid-May. It is likely that the MJO event in April generated the oceanic Kelvin wave that slowly propagated eastward across the Pacific basin in May. The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices.
Southern Oscillation Index (SOI):
The standardized SOI shifted from a negative value in April to a positive one in May, with an averaged index value of 1.1 for the month. Since November 2003, the SOI has switched signs several times, reflecting the lack of any consistent mean-sea-level pressure pattern across the tropical Pacific associated with ENSO.
In summary, the observed oceanic and atmospheric indices in May suggest that near-neutral ENSO conditions have continued to dominate the observed climatic conditions in the equatorial Pacific basin. However, significant changes in the oceanic mixed-layer did occur in May due to the passage of the Kelvin wave, which suggests that above normal SSTs will likely continue along the equator in the central Pacific and in the Niño 3.4 region.