Odds increasing for La Nina in late spring

Odds increasing for La Nina in late spring | The Outlook


Spring is expected to be warmer than normal, but not as hot as last spring.


ONCE again there has been little change in the medium and long term climate indicators in the past couple of weeks. However, the Pacific continues to be of interest.

Currently the El Nino-Southern Oscillation is still neutral, but there has been continued cooling of the sea surface temperatures of the tropical Pacific Ocean east of the International Date Line as well as changes in tropical weather patterns, so climate model outlooks are shifting slightly more strongly towards La Nina developing by the second half of spring.

In its analysis, the Bureau of Meteorology now puts the development of a La Nina by later in spring at around 70 per cent, which is a fairly strong indicator of such things.

The increased chance has been due to the sub surface waters in the tropical Pacific cooling further and in the atmosphere, the south-east trades have become a little stronger than normal.

The Southern Oscillation Index has been considered to be in the neutral range when it is in between -7 and +7.

Although still in that range at the moment, the current 30-day running mean is around +6 after being +4 in July and negative in June so it too is trending in the direction that supports a La Nina development.

Past La Nina events have meant an increased chance of above average rainfall in eastern Australia, especially east of a line joining Albury to Mt Isa.

It must be emphasised that this is an increased chance but not certain because in a few such events in the past, rainfall has been a little disappointing.

However, on this occasion, the Indian Ocean Dipole is favoured to be slightly supportive or at least neutral and that will further increase the chance of rain events in the south-east states, especially in spring.

As I have said previously on a few occasions, rainfall in the coming months will be dependent on the occasional events with useful falls interspersed by periods of stability.

We are currently in one of those periods of stable weather when the local synoptic weather patterns take control for a period.

At least in the early parts of spring, some of these stable periods could be quite long.

Although cool winter weather has dominated in recent weeks in the south-east states, a couple of sites in north-west Australia have already reached over 40 degrees, an Australian record August temperature and the first +40 degree temperature in Australia for 50 years, to remind us that all it takes is a few days of north-west winds and our temperatures will rise quickly.

Overall, spring is expected to be warmer than normal but not as hot as last spring.

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