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

Product Code: IDCKGEWW00

CURRENT STATUS as at 7^th May 2009
Next update expected by 21^st May 2009 (two weeks after this update).

Summary: Tropical Pacific neutral; ocean warms slowly.

Climate indicators across the equatorial Pacific are currently neutral. Both the equatorial Pacific Ocean surface and sub-surface have continued to warm over the past few weeks, resulting in near average ocean temperatures across most of the tropical Pacific, but slightly warmer than normal sea surface temperatures (SSTs) in the east. Trade winds have weakened further, and are now widely weaker than average. The SOI rose from zero in March to +9 in April, however the current (5 May) 30-day value has fallen slightly to +7 and remains in the neutral range.

Most international coupled climate models predict further warming of Pacific Ocean sea surface temperatures, with SSTs remaining in the ENSO-neutral range until at least mid-winter. About half of the models surveyed are predicting that this warming will be sufficient to see the development of El Niño conditions later in 2009, but the southern autumn is the "predictability barrier" for model predictions, with skill levels at their minimum. As the period from March-June is the preferred El Niño genesis period, Pacific conditions and model predictions will continue to be monitored closely for any indications of an event.

The Indian Ocean Dipole (IOD) is currently neutral. The typical development period for an IOD event is late autumn to early winter, therefore model predictions will continue to be monitored closely for any signs of an emerging event. About IOD.

From the NOAA/PMEL/TAO website.

In Brief

• SSTs continue to warm across the central and eastern equatorial Pacific. SST values are now close to normal across most of the central equatorial Pacific, but above normal in the east.
• The equatorial sub-surface has also continued to warm, removing most of the areas of previously cool water.
• The SOI rebounded in April after falling in March, but the most recent values indicate a renewed falling trend. The latest 30-day SOI value for 5 May is +7. The monthly value for April was +9.
• Trade winds have weakened further. Anomalous westerly flow currently covers much of the equatorial Pacific.
• Cloudiness near the date-line increased, but remains below average.
• The majority of dynamic computer models predict neutral conditions to continue until at least mid-winter.

Details

SSTs across the central and eastern tropical Pacific continue to warm slowly, as they have done since February. When averaged over the month of April this warming has resulted in the disappearance of cool SST anomalies from the central and eastern equatorial Pacific. SSTs were close to normal everywhere with the exception of the far eastern regions, where they are now warmer than normal. The monthly NINO indices for April were +0.3°C, 0°C and +0.1°C for NINO3, NINO3.4 and NINO4 and respectively.

In terms of weekly data, the most recent NINO indices are +0.6°C, +0.3°C and 0.3°C for NINO3, NINO3.4 and NINO4 respectively. Over the past three weeks all three NINO regions have warmed. NINO3 and NINO3.4 warmed the most with an increase of approximately 0.3°C, while NINO4 warmed by approximately 0.2°C. The 7-day SST anomaly map map shows that SSTs across the central equatorial Pacific are near normal, while warm anomalies are now evident in regions further east. When compared with three weeks ago, positive anomalies in the far east of the Pacific, near South America, have strengthened. An animation of recent SST changes is available.

The sub-surface of the equatorial Pacific continued to warm through April and early May, effectively replacing most of the anomalously cool sub-surface water in the central and eastern tropical Pacific that had persisted since September 2008. A recent map for the 5 days ending 4 May shows warm sub-surface anomalies above +1.0°C evident both in the western and eastern Pacific. An animation of recent sub-surface changes is available.

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

Trade winds weakened right across the equatorial Pacific, most notably in late April and early May following a westerly wind burst in the western Pacific. The latest weekly wind anomalies, shown in the TAO/TRITON map (small image above) for the five days ending 4 May, indicate weaker than normal Trades across much of the near-equatorial Pacific.

The Southern Oscillation Index (SOI) rose slowly during much of April, though started to fall once more late in the month and into early May. The SOI rose from a March value of zero to an April value of +9, but the current approximate 30-day value has fallen back to +7 on 5 May. The Tahiti mean sea level pressure (MSLP) anomaly rose once more during April. Prior to March 2009, the Tahiti MSLP had generally been above average since August 2008. The Darwin MSLP fell to slightly below normal by late April. (SOI graph, SOI table).

Cloudiness near the date-line over the central to western Pacific is another important indicator of warm/cool ENSO conditions, as it normally increases/decreases (negative OLR/positive OLR anomalies) during these episodes. Cloudiness near the date-line remained below average in the first half of April, but it has increased to near-normal levels during the past three weeks.

The majority of international dynamic computer models surveyed by the Bureau of Meteorology predict a continuation of neutral conditions for the tropical Pacific until at least mid-winter. Three of the six models are suggesting ENSO neutral conditions for winter and spring, while three models have outlooks for El Niño conditions to develop. However, models are still having to contend with the March to June "predictability barrier". Given this uncertainty and the wide range of predicted outlooks, continuing neutral conditions is the most likely outcome for the period through winter. Models suggest a return to La Niña conditions is the least likely outcome. Recent forecasts from the POAMA model, run daily at the Bureau of Meteorology, show a steady warming, with ENSO-neutral conditions likely to persist through to at least mid-winter, but with the possibility of El Niño conditions into the spring. Pacific conditions and model predictions will continue to be monitored closely for any strengthening indications of an event. POAMA outlooks for the IOD suggest values will reduce slightly, but remain within the neutral range during winter and spring.

THE NEXT UPDATE OF THE DETAILED SECTION ABOVE IS EXPECTED BY 21ST MAY 2009

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.