Slow trend towards weak La Nina continues

Slow trend towards weak La Nina continues | The Outlook


In the longer term the IOD is favoured to stay neutral for the rest of the year.


THE main climate indicator features of the past week have been a continued slow trend towards a weak La Nina while remaining neutral for the present (although this trend has slowed slightly in the past week or two).

Also the Indian Ocean Dipole and the Southern Annular Mode to the south are still neutral.

Unfortunately the SAM is still slightly positive thus keeping active Southern Ocean fronts a little further south than would normally be the case at this time of year and decreasing the chance of pre-frontal rain in the south of NSW and over Victoria.

In the Indian Ocean, there has been no change to the neutral set up there, although much of the eastern Indian Ocean remains a little warmer than normal, which is preferable to the alternative.

All this means that in the short term (up to one month) little change is expected to the previous prognosis of near normal winter rainfall, but still dependent on the occasional one-off events, which have appeared to have "dried up" a little.

In the longer term the IOD is favoured to stay neutral for the rest of the year, possibly edging into the negative by late spring/early summer, which is potentially good news, even though its effects on eastern Australian rainfall usually become minimal by summer.

However, also in the longer term the Pacific La Nina holds an important key. Its continued trend is a positive sign and if it continues to develop that way - and there is now around a 60 to 70 per cent chance of that happening - then spring rainfall potential in eastern Australia will improve. So, this remains a positive in the long-term outlook.

Forecasting future trends for the SAM are more challenging. There is considerable variation with its future trends and it can change more quickly than the other indicators.

Even if the SAM went negative for a while in August, the effect would be mainly a minor increase in rain in Victoria unless it was matched by a negative phase in the IOD which would increase the chances of pre-frontal rain.

Overall, the prospects for the rest of the year remain reasonable good. The next El Nino is at least a year away (hopefully longer) and the chances of reasonable summer rains in 2020-21 are better than they have been for a few years.

Although the brief cold outbreaks are stronger than normal possibility in the next two months, overall temperatures are likely to remain a little up on normal for the rest of the year and early next year across all of eastern Australia.

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