Warmer seas may point to potential for rain | The Outlook

Warmer seas may point to potential for rain

Weather
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The first significant Southern Ocean frontal system of the season swept across south-east Australia last week.

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THE first significant Southern Ocean frontal system of the season swept across south-east Australia last week, which was a little earlier than normal and continued the relatively cool weather that has dominated in the south-east states for much of the time since the start of March.

This has been accompanied also by a slight change in many of the long-term climate models, although such changes can be common and unreliable in autumn and we will need to wait and see what they do in the coming weeks before we can consider them in more detail.

However, the changes have - initially at least - been encouraging.

These include a slight swing to a "neutral-La Nina pattern" in the Pacific by winter (although many models still stick to just neutral) and consolidation of the prediction that the Indian Ocean Dipole will remain neutral and will not become positive at all this year.

If these features continue to develop or are reinforced by new model "runs" in the coming weeks, then the prognosis of slightly above average rainfall in the coming months gains a little in confidence.

The possibility of a weak La Nina in the Pacific remains the most interesting development and is a significant change from the so called 'warm neutral' pattern favoured a month or so ago.

Although it is too early to be confident about such a development, it is interesting to look at some aspects of the sea surface temperatures that have led to this change.

The western tropical Pacific Ocean remains slightly warmer than average, but this is offset at the moment by similar patterns in the far eastern Pacific Ocean.

Of more interest is an increasing cold pool of water developing 100-200m below the surface in the western to central Pacific Ocean.

Water at these levels tends to move eastwards and this could see cooler ocean temperature anomalies developing in the eastern Pacific in the next few months which would help at least generate a cool neutral or La Nina phase in the Pacific Ocean.

To our west in the Indian Ocean, above average SSTs are occurring across the entire region, which is very unusual. As long as ocean temperatures off the northwest Australian coast remain above average, this will offset the above average ocean temperatures in the western Indian Ocean and keep the IOD neutral.

In fact, above average SSTs exist around most of continental Australia and this is another factor that could assist in a slight increase in potential for rain events in the coming months.

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