Ag Weather Forum

Australia La Nina Comment

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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Pacific Ocean temperature forecast models indicate a trend toward average values in the February-to-April time frame of 2021. (Australia BOM graphic)

Following is the latest discussion on La Nina in the Pacific Ocean from the Australia Bureau of Meteorology (BOM). The Australia BOM keeps track of the Southern Oscillation Index (SOI), which is the atmospheric indicator of either El Nino or La Nina.

As of Dec. 17, the SOI indicates a moderate to strong La Nina event in the Pacific: the 30-day average is plus 12.22; 90-day average plus 9.08; and the daily contribution to the SOI data set is plus 34.10. Here is the text of the Australia BOM discussion dated Dec. 8, 2020:


La Nina continues in the tropical Pacific.

Oceanic and atmospheric indicators reflect a mature La Nina with little variation over last fortnight. Model outlooks suggest the event will peak at moderate levels during December, returning to a neutral phase during the late summer or autumn.

The Southern Annular Mode (SAM) is expected to increase to strongly positive values over the coming week. This is driven in part by the La Nina influence, and in part by a stronger-than-average polar vortex over Antarctica. Positive values are expected at least into early 2021, and typically increase the chance of rainfall in eastern Australia.

The Madden-Julian Oscillation (MJO) is relatively weak and is currently over the Maritime Continent (Indonesia). It is forecast to move east and across Australian longitudes over the next fortnight. The MJO, in conjunction with other tropical influences, is looking favorable for monsoon onset and producing above-average rainfall over northern Australia.

Climate change is also influencing the Australian climate. Rainfall across northern Australia during its wet season (October-to-April) has increased since the late 1990s, with a greater proportion of high intensity short-duration rainfall events.

Climate outlooks, which include all climate drivers, indicate rainfall during December 2020 is likely to be above average over most of northern Australia while for January to March 2021, rainfall is likely to be above average for most of the northeastern half of the country.

Bryce Anderson can be reached at

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