A Wet Summer and Monsoon Season in Australia 2007/2008?

The 30-day running average for the southern oscillation index (SOI) has continued its current upwards surge. It reached +11.08 at the start of this week.

The Bureau of Meteorology (BOM) says a SOI consistently above +6 for two months indicates an above-average probability of above-normal rainfall during this summer and autumn, in most parts of eastern Australia. It confirms the arrival of a La Nina (wet) period for the year ahead.

The current SOI figure has been maintained above +6 since the last two weeks in October.
So it's now reaching the critical two-months qualification period, as we enter December and the start of summer. The SOI plunged below zero in April last year, signifying last year's temperate zone drought. It stayed there until the last week of May this year, when it rose back above zero.
But, after just a month in positive territory, it plunged below zero again - which coincided with the failure of follow-up rains this autumn and winter, after an encouraging start for this year's winter crop.

BOM says a consistently negative SOI pattern (less than about minus 6 over a two month period) is related to a high probability of below median rainfall for many areas of Australia at certain times of the year.

At the end of August, the SOI lifted back above zero again, indicating that the El Nino (dry) phase of the climate cycle might be ending. But first it spent two months timidly hovering between zero and +5, without going anywhere higher. Then, during the last half of October and throughout November, the SOI has accelerated confidently upwards.

The SOI is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin. This rising value for the SOI has contributed to the encouraging Bureau of Meteorology's (BOM) latest rainfall forecasts for summer and autumn in most parts of eastern Australia.

For the NT region, cloudiness over the past several months, around the near-equatorial dateline and further east in the Pacific has mostly been less than normal, corresponding to cooler than average sea-surface temperatures of that region. Cool anomalies in subsurface water of the near-equatorial eastern and central Pacific have also existed for much of the year. Trade-winds in the near-equatorial Pacific have generally been stronger than normal and the Maritime Continent region [including tropical north-Australia] and the north-Indian Ocean has seen above average cloudiness.

La Nina events are generally associated with increased rainfall over much of northern Australia, but the response to this event has not been entirely typical. This may in part be due to these sea-surface temperatures of seas about the north Australian coast, which have been cooler than would typically be expected in La Nina events.

How the tropical climate system responds in coming months may be determined by the evolution of this sea-surface temperature pattern, which has shown some warming in recent weeks.

The influence of the recent active rainfall phase seems to be waning over the longitudes of northern Australia and there is a reasonable expectation of a suppressing effect on tropical weather from late November to the middle part of December.

Note though that the uncertain prognosis regarding the broader scale influence of La Nina conditions to this region introduces uncertainty regarding such an outlook. However, active conditions would be seen developing over northern Australia some time during the second half of December and as early as the middle of December.

At this time of year, development of an active MJO phase often shortly precedes monsoon development.

So, in summary over the next few weeks, we could have less rain, followed up by a potential improvement in mid to late December.......maybe even some monsoon conditions!