ENSO Wrap-Up
A regular commentary on the El Niño-Southern Oscillation

Product Code: IDCKGEWW00

CURRENT STATUS as at 30^th April 2008
Next update expected by 21^st May 2008 (three weeks after this update).

Summary: La Niña pattern fades; Pacific now generally neutral

Pacific climate patterns are now generally neutral. The 2007/08 La Niña event continued to fade during the past two weeks, with the majority of indicators returning to near-normal levels. Only the western to central Pacific displays any features typical of a La Niña event, with cooler than normal ocean temperatures, enhanced Trade Winds and reduced cloud amounts.

Elsewhere, ocean surface and sub-surface temperatures continued to warm, and as a result, are no longer at levels typical of a La Niña event. Furthermore, the Trade Winds across the eastern half of the Pacific are close-to or weaker than average, and the SOI has continued to retreat from the strong positive values seen earlier in the year and is now in the neutral range. The decline in La Niña has seen below average rainfall return to large parts of eastern and northern Australia during March and April.

Computer model predictions show Pacific temperatures gradually increasing over the next two seasons, but remaining near-average. The models indicate a low chance of either a stronger warming to El Niño levels or a re-intensification of La Niña conditions during 2008.

From the NOAA/PMEL/TAO website.

In Brief

• The La Niña has weakened to neutral conditions.
• Weak negative SST anomalies remain in the central to west Pacific. There are weak positive anomalies in the eastern Pacific.
• Warm sub-surface anomalies lie along much of the thermocline on the equator.
• The SOI is on a downward trend but remains positive at +5.
• Trade Winds remain stronger than average across the central to western equatorial Pacific, but are weaker in the east.
• Cloudiness near the date-line has been very much below average in recent months.
• All the dynamic computer models predict neutral conditions to remain in the eastern to central Pacific in the coming months.

Details

Preliminary sea surface temperature data for April show that the La Niña pattern of cool anomalies has weakened further since March, continuing its steady weakening since peaking around February. Though negative anomalies remain in the central to western Pacific, especially between 170°E and 140°W, these are weaker than the previous month and have shifted further to the west. Further east near the South American coast there has been some further weak warming, with a small area of positive anomalies greater than 1°C evident at the monthly timescale. The central Pacific NINO indices, NINO3.4 and NINO4, both reflect the weakening of the La Niña signal with their preliminary monthly values rising to −0.7°C and −0.9°C respectively in April. At their peak in February they were −1.7°C and −1.5°C respectively. The NINO3 index is now only very weakly negative at the monthly time scale, being −0.02°C for the month.

In terms of weekly data, all indices have shown weak warming over the past fortnight. The NINO3, NINO3.4 and NINO4 indices are now on 0.2°C, −0.5°C and −0.7°C respectively. When compared with two weeks ago, the 7-day SST anomaly map shows a continuance and slight strengthening of positive anomalies next to the South American coast, and a further weakening of negative anomalies in the eastern to central Pacific. There are now no anomalies below −0.5°C east of 130°W, whilst anomalies below −1°C have further decreased in area. The appearance of positive anomalies near South America is not a sign of an imminent El Niño, though they will be monitored for any indications of further strengthening. An animation of recent SST changes is available.

The La Niña signal in the sub-surface has weakened further in April. Negative anomalies in the eastern Pacific have weakened considerably, especially along the thermocline. Positive anomalies in the western Pacific remain strong, though have weakened somewhat in the area to the west of 160°E. There has been little change in the subsurface in the past fortnight, with the positive anomalies now nearly covering the entire thermocline, except for one small area of negative anomalies around 130°W. However, positive anomalies in the far-western Pacific have shown some weak cooling. This is confirmed by a recent map for the 5 days ending 28th April which shows anomalies greater than +4°C in the sub-surface in the western Pacific, down from the +5°C of a fortnight ago, with positive anomalies now spread along the thermocline to around 140°W. In the eastern Pacific only a small area of negative anomalies now remains on the thermocline at about 130°W, and weak warm sub-surface anomalies in the eastern Pacific have shown further signs of strengthening. An animation of recent sub-surface changes is available.

An archive of past SST and sub-surface temperature charts is available.

The Trade Winds have been stronger than normal over the western to central Pacific since about August or September 2007 - one of the key features of La Niña. However, in the eastern Pacific the Trades have been much weaker than average since early to mid-February, and this has contributed to the surface warming in the same region. The TAO/TRITON map (small image above), for the five days ending 28th April is similar to the pattern described, with stronger than average Trade Winds west of 170°W and weaker than normal winds east of around 150°W.

Though remaining positive, the Southern Oscillation Index (SOI), is on a downward trend (see SOI graph, SOI table). After a record February value of +21.3, which was the highest value for any month since +22.4 in November 2000, the SOI fell to a March value of +12. The current 30-day value is +5 as of 28th April (see graph above). The cause of this remains continued anomalously high pressures at Tahiti rather than low pressures at Darwin.

Cloudiness near the date-line in the central to west Pacific is another important indicator of warm/cool ENSO conditions, as it normally increases/decreases (negative OLR/positive OLR anomalies) during these episodes. Cloudiness around the equatorial date-line has mostly been below average since late May 2007, particularly so from December. It currently remains below average in keeping with the negative SST anomalies in the region.

The latest survey (late Apr) of six international dynamic computer models, was unanimous in predicting neutral temperatures in the central to eastern Pacific over the southern winter. Of the five that predict beyond four months, all predict the neutral conditions to continue through the southern spring of 2008. Recent forecasts from the POAMA model, run daily at the Bureau of Meteorology, predict neutral conditions to remain in the eastern to central Pacific through to November.

THE NEXT UPDATE OF THE DETAILED SECTION ABOVE IS EXPECTED BY 21ST MAY

The links below can be used to keep track of important developments across the Pacific Basin.

Other Useful Links

The Weekly Tropical Climate Note issued by the Darwin office of the Bureau of Meteorology discusses the main features of the tropical atmosphere and ocean, including the intra-seasonal oscillation or 30-60 day wave which is thought to sometimes impact on the development of El Niño events.

The Bureau of Meteorology Research Centre (BMRC) has recently developed maps of Out-going Longwave Radiation (OLR), a useful El Niño monitoring tool. Negative anomalies show areas which, in general, have been cloudier (and potentially wetter) than normal.

The TAO / TRITON data display page is excellent for creating your own plots of numerous variables that are relevant to El Niño.

Note however that information coming from other countries is likely to describe timing and impacts relevant to those countries, which will not be the same as those in Australia.