El Niño/Southern Oscillation - September 2009
Weak El Niño Conditions Persist
This report was issued on 15 October 2009
Above average sea surface temperatures (SST) across the equatorial Pacific Ocean in September continued to support the presence and persistence of the El Niño conditions that developed earlier this past summer. The most recent three–month (July – September) SST anomaly for the Niño 3.4 region remained above the El Niño threshold of 0.5°C (0.9°F) for the third consecutive month. However, some corresponding indicators of El Niño, such as the Southern Oscillation Index (SOI), have been relatively ambiguous compared to what might be expected as part of an established El Niño event. Overall, the timing and progression of primary El Niño conditions in the equatorial Pacific Ocean have been consistent with the development of a weak to moderate El Niño event by the end of 2009.
Historically, impacts over the United States from El Niño tend to be weak during the summer and strengthen toward the winter months. Fall Atlantic hurricane activity may be suppressed during an El Niño due to increased vertical wind shear above the tropical Atlantic. For a comprehensive list of El Niño impacts on the United States, other countries, and globally, please visit NOAA's Pacific Marine Environmental Laboratory El Niño Impacts page.
SST anomalies in all Niño regions were above average for the fifth consecutive month. The September SST anomaly of 0.86°C (1.49°F) in the Niño 3.4 region was above the 0.5°C threshold for El Niño conditions for the fourth month in a row. The three–month (July – September) average SST anomaly (Oceanic Niño Index) of 0.8°C (1.4°F) was also above the 0.5°C threshold for the third consecutive three–month period. Please refer to NCDC's ENSO Indicators page for more details.
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 the Climate Prediction Center's Atmospheric and SST Index values page (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition).
September's sub surface oceanic heat content (right) for the equatorial Pacific Ocean displayed a slight warming throughout the month. Near–surface heat anomalies rose slightly in the eastern third of the basin, as well as at around 100 meters just to the east of the Date Line. Anomalies exceeding 3°C (5.4°F) were present at times through the month near the surface east of 120° W, and also developed near the Date Line at around 110 meters depth toward the end of the month. These are the warmest sub surface anomalies seen in the region since June. Conversely, mixed–layer waters below 150 meters exhibited an expanding region of below average temperature migrating across the equatorial Pacific from the west.
During an El Niño (warm phase) event, positive (warm) heat content anomalies are present in the mixed layer of the equatorial Pacific Ocean, with the most above–average heat occurring at shallow depths in the eastern portion of the basin. During La Niña (cool phase) events, the mixed layer heat content tends to fall below normal, especially east of the Date Line.
Equatorial Zonal Winds (U–Component Winds) and Sea–Level Topography:
Trade winds during September, as measured by the NOAA Tropical Atmosphere Ocean (TAO) project were generally near average, with amplified (i.e., 4 to 5 m s–1) westerly flow to the west of the Date Line. East of the Date Line, easterly flow was slightly below average, with anomalies between 0 and –2 m s–1. Below–normal trade winds in the eastern part of the basin are historically consistent with weak or developing warm phase events.
Sea level is another important indicator of the presence of an El Niño or La Niña event. Above average sea level occurs when warm ocean water thermally expands, while cool water will result in contraction and below average sea level. Sea level in the Pacific is measured by a sensor on the NASA/JPL Jason satellite. In contrast with August, when above average sea level was present across much of the equatorial Pacific basin east of the Date Line, September saw near average sea level across much of that same region. Smaller regions of above average sea level persisted near the Date Line and in the eastern tropical Pacific near 120° W. This return to near average sea level across much of the Niño regions is inconsistent with the development of a strong El Niño event, but can occur in connection with a weaker warm phase event.
The continued development of El Niño conditions was reflected in below average outgoing longwave radiation (OLR), and thus above average cloudiness and convection, near the Date Line in September. OLR near the Date Line over the past four months has been variable, but generally near average. However, uncharacteristic of El Niño conditions, September's OLR was also below average across much of the tropical Pacific west of the Date Line. This above average cloudiness and convection allowed for slightly more precipitation over the region than would be expected during a typical El Niño development period. For the month as a whole, the September OLR Index — based on monthly average OLR for the equatorial Pacific between 160° W – 160° E longitude — was –0.6 W m–2, or slightly below average. This OLR Index value is a decrease of 0.8 W m–2 from August, and is consistent with a decreasing trend in the OLR Index since December, 2008.
The map below on the left shows the spatial pattern of global OLR (W m–2) measured by satellite during the past month, while the image to the right is the average OLR over the previous three months. OLR in the equatorial Pacific near the Date Line is an indicator of relative cloudiness and precipitation — important variables tied to El Niño and La Niña events. During El Niño, cloudiness and convection tends to increase near the Date Line and decrease to the west. This is reflected by below average OLR in the vicinity of the equatorial Date Line, and above average OLR in the western Pacific.
The Southern Oscillation Index (SOI), a standardized measure of the surface pressure difference between Tahiti and Darwin (Australia), was 0.3 for September, an increase of 1.0 from August. Since March, 2009, SOI has oscillated near its long–term monthly values, which is common during ENSO–neutral conditions, but may also occur as an El Niño or La Niña becomes established.
Typically, prolonged periods of negative SOI values correspond to warmer–than–normal sea surface temperatures and El Niño episodes, while positive SOI values occur in conjunction with ENSO cold phases. However, slightly positive SOI values have occasionally occurred during ENSO warm phase events. The recent SOI values, which have fluctuated near the long–term average, but are predominantly slightly below normal, are historically consistent with a developing El Niño.