Weekly Tropical Climate Note

Issued on Tuesday 31 January 2012

La Niña shows some weakening

Most indicators of La Niña showed small changes over the past week. The Southern Oscillation Index (SOI) and trade winds, continue to exceed La Niña thresholds with a 30-day SOI value of +11.0 to 29 January. The contributing atmospheric pressures were 1.9 hPa above normal at Tahiti and 0.4 hPa below normal at Darwin. The monthly SOI for December was +23.0, the highest monthly value since April 2011. The 5-month running mean (centred on October) was +11.6.

Below average sea surface temperatures (SSTs) and sub-surface temperatures persist across the central and eastern tropical Pacific Ocean, with the NINO3.4 value currently at -1.0. Cloudiness near the Date Line continues to be below average, particularly in areas south of the equator. Enhanced convection has been dominant across the western parts of south-east Asia and northern Australia, and has intensified within the southern flank of the South Pacific Convergence Zone (SPCZ).

Forecast models surveyed by the Bureau suggest this La Niña event will gradually decline over the coming months. However, it is likely to remain a dominant climate driver for much of the remaining northern Australian wet season.

La Niña periods are typically associated with above average rainfall across northern Australia during the wet season. Daytime temperatures are typically cooler than normal from December onwards, and tropical cyclone activity is usually above average between November and April.

See the Bureau's ENSO Wrap-Up which includes a compilation of ENSO computer model predictions.

Monsoon returns to northern Australia

A deep and well organized trough formed last week over the Arafura and Timor seas and gradually migrated southwards towards central Australia, bringing an active monsoon to our latitudes. At the same time, the Madden-Julian Oscillation (MJO) gained strength and propagated east across the southern Maritime Continent. The MJO is currently positioned over the western Pacific Ocean. There is some degree of uncertainty on the positioning and amplitude of the MJO in the forthcoming weeks due to the influence of La Niña, but it is likely to contribute to enhanced storm activity within the SPCZ including the Coral Sea.

Over the next week, monsoonal conditions will prevail over northern Australia with above average rainfall and enhanced low-level westerly winds. The Monsoon trough is expected to migrate north and reach the base of Australia’s Top End later this week. Off the Western Australia coast Tropical Cyclone Iggy is expected to continue on a southwest track. Over Australia’s eastern seaboard, a low is likely to develop over the Coral Sea by the end of the week although at this point it is too early to predict its strength or possible track.

See the Bureau's MJO Monitoring for more information, including rainfall maps.

Next update expected by 7 February 2012 | Product Code IDCKGEWOOO