The Outlook | Neutral pattern by autumn

Neutral pattern by autumn


Weather
With a neutral pattern likely to return from autumn, near average rainfall is favoured for much of 2019 but above average temperatures will persist and so will evaporation levels. Photo by Rachael Webb.

With a neutral pattern likely to return from autumn, near average rainfall is favoured for much of 2019 but above average temperatures will persist and so will evaporation levels. Photo by Rachael Webb.

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A dry year, but one day closer to rain

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The sea surface temperature patterns across the tropical eastern Pacific indicate we are now in a weak to moderate El Nino event.

The sub surface temperature in these areas confirms that situation.

However, SSTs in the tropical western Pacific and in Coral and Tasman Seas would normally be decreasing with the onset of El Nino.

This is not happening and SSTs in these regions remain warmer than normal.

Also, the atmospheric structure across the Pacific is not reflecting this.

Such a conflict is rare and any prognosis for the future has a lower level of confidence associated with it.

The atmospheric indicators include the Southern Oscillation Index (SOI) which continues to increase.

The 30-day running mean remains strongly positive - well within the neutral range and nowhere near the expected El Nino values.

In addition, the trade winds remain near normal across the tropical western Pacific although showing some variability at times due to local synoptic factors.

During normal El Ninos, there is usually a sustained weakening of trade winds across the tropical Pacific.

Finally, cloud cover remains near normal especially near the Date Line in the central Pacific.

Also atmospheric convection remains close to average near the Date Line and suppressed over Indonesia.

Typically, cloud cover would be above average in an El Nino. So an El Nino without normal atmospheric indicators probably means an El Nino without the usual effect on rain patterns.

The majority of the international models continue to favour at least a weak El Nino persisting through summer, with the expectation that the atmospheric circulation will eventually couple to the anomalous equatorial Pacific warmth, but there are no signs of this happening in the immediate future.

So the most likely scenario for eastern Australia is a continuation of occasional “one off” rain events in eastern Australia but such events will continue to be patchy and therefore rainfall will be variable with some areas missing out and others getting useful falls.

Such events are probably going to become less frequent in the coming weeks but may increase again by February especially in eastern and northern NSW and eastern Queensland.

During this time the frequency of high temperatures (those when tops are at least five degrees above the summer norms) are likely to increase in frequency, especially over south-eastern South Australia, Victoria and inland NSW.

With a neutral pattern likely to return from autumn, near average rainfall is favoured for much of 2019 but above average temperatures will persist and so will evaporation levels.

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