Tropical cyclones
April was a ‘quiet’ month
There were no further tropical cyclones in the Southwest Pacific during April. Five tropical cyclones have occurred so far this season, which is the same number as last season and well below average. There is a 50 percent chance of another tropical cyclone occurring in May.
ENSO update
No further progress towards an El Niño in the last month
There is still about a 60% chance of an El Niño event affecting the Southwest Pacific climate by September this year. However, most key indicators remain in neutral values with little change in conditions in the tropical Pacific since March. Although outlook guidance for the Southwest Pacific doesn’t indicate anything major at this stage, significant anomalies are still possible should an El Niño evolve.
Present situation and outlook
Historically, we are still in the period of year when transitions to an El Niño are most likely to occur, provided conditions are suitably favourable.
At present, Equatorial Pacific Ocean temperatures continue warmer than average (US analysis, +1.0°C) around the date line, and exceed +2°C in a small region near the South American coast. The NINO3 and NINO4 region SST anomalies showed little change from March, but overall surface warming is still occurring, although it has been slow. Bands of previously cooler than normal water within 10° of the equator near 120°W have now all but disappeared. Previously warmer than average equatorial subsurface temperatures in the upper 100m west of South America have returned to normal.
The April OLR data showed positive anomalies (indicating decreased convection) over northern Australia and Queensland, with areas of enhanced convection observed over the anomalously warm waters off the west coast of South America near Ecuador. We would expect to see enhanced cloudiness developing over the central equatorial Pacific region (Kiribati) with the establishment of an El Niño.
The Southern Oscillation Index (SOI) was weakly negative in April (-0.5), but showed little change from March. The three-month SOI is still near neutral (-0.2).
About 60-70% of global ENSO forecast models continue to show that a warm event (El Niño) may occur at some time in the tropical Pacific over the next 6 to 9 months. However, about 30% of the models are consistently forecasting neutral conditions. Overall, although atmospheric and oceanic signals are present for the possible development of an El Niño later this year, recent progress has been slow and we are not in an El Niño yet.
An important ‘ingredient’ still required for the further development of an El Niño would be the onset of a significant westerly wind burst (like that of December 2001) in the Equatorial western Pacific, along with a weakening of the easterly trade winds in the central and eastern Pacific.
The next 4-6 weeks remain critical to any further development and establishment of an El Niño episode this year.