A spring La Nina now more likely than not

A spring La Nina now more likely | The Outlook


An analysis of the major international models puts the chance of a La Nina in spring about twice the normal chance.


OVERALL there has been little change in the principal indicators of climate in the past few weeks.

The El Nino-Southern Oscillation and Indian Ocean Dipole are both still in the neutral range with a trend in the Pacific towards a La Nina still occurring.

An analysis of the major international models put the chance of a La Nina in spring about twice the normal chance - so not certain, but more likely than not.

This, of course, would be good news because with a spring La Nina in the Pacific there is a much greater chance of rainfall reaching or exceeding normal in eastern Australia.

In the Indian Ocean, the IOD is defined by the difference in the sea surface temperatures between the eastern and western tropical Indian Ocean.

Much of the eastern Indian Ocean is staying a little warmer than normal and while SSTs in the north-east tropical Indian Ocean are also above normal, the difference between the two areas is close to zero - hence the neutral IOD.

Although some signs persist that the IOD could edge towards the negative (good news for us), it is more likely that it will stay close to zero in the foreseeable future.

This would not necessarily reduce the chance of spring rains in south-east Australia but it wouldn't encourage it, like the developing patterns in the Pacific will do.

To the south the Southern Annular Mode continues to vary considerably. However it is favoured to be basically positive for the next couple of weeks.

Remember that a positive SAM at this time of year would typically see below average rainfall across far southern parts of the country, but can increase rainfall in northern NSW and southern Queensland.

However, it is the weakest of the major indicators and any assessment based solely on the SAM is speculative at the best.

So it is likely that the influence of the ENSO in the Pacific will dominate in the next couple of months and as indicated above, the signs are positive.

The Southern Oscillation Index is around +4 which is still in the neutral range but towards the positive end of it.

The south-east trades are close to average with signs of a slight strengthening - also indicative of a developing La Nina.

Finally, SSTs around continental Australia are also still a little up on normal.

This could have two effects - one in assisting rain events a little and two, to keep temperatures in spring a little up on normal.

Little change is expected in the SSTs over summer.

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