Friday 09 December 2022

La Niña decline may be hastened by recent shift in trade winds

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MELBOURNE, Australia — La Niña decline may be hastened by recent shift in trade winds
The La Niña in the Pacific Ocean continues to decline. Sea surface temperatures in the central tropical Pacific have warmed steadily since late December, with most models forecasting La Niña will end early in the southern hemisphere autumn.

El Niño–Southern Oscillation (ENSO) indicators continue to show at least some La Niña characteristics. Sea surface temperatures indicate a weak La Niña pattern, with the coolest waters concentrated in the eastern Pacific Ocean, while the Southern Oscillation Index is neutral, but weakly positive.

However, a very strong pulse of the  Madden-Julian Oscillation (MJO), which drove a burst of monsoonal activity over northern Australia in late January, has caused the western Pacific trade winds to weaken considerably.

The rapid weakening of the trade winds may hasten the decline of La Niña. The MJO also led to above-average cloudiness at the Date Line for the first time since early September 2017.

In order for 2017–18 to be classed as a La Niña year, thresholds need to be exceeded for at least three months. Four of the eight climate models surveyed by the Bureau suggest this event is likely to last at least until late summer, while a few continue the event into the southern hemisphere autumn of 2018.

Typically, the strength of a La Niña event reflects the strength of its impact upon Australian climate.

The current event is weak, and hence climate patterns have been significantly different from those observed in the last strong La Nina of 2010–12.

Large parts of eastern Australia have been drier than average for the past two or three months, the opposite of what is typically expected during La Niña.

The Indian Ocean Dipole (IOD) is currently neutral. IOD events are unable to form between December and April.

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