La Niña Alert
We're not there yet, but the likelihood of a coming La Niña has greatly increased.
This week, the Bureau of Meteorology upgraded its ENSO outlook to La Niña Alert, which puts the chance of La Niña forming in the coming months at 70% - that's three times the normal likelihood.
Despite not having passed the threshhold yet, the developing La Niña has already been influencing the surf and weather. Historically, the emerging stage of a La Niña event sees increased rainfall across Eastern Australia, and this has been the case. For example, a year ago Warragamba Dam, Sydney's main water supply, was under 50% capacity but now the dam is overflowing.
An emerging La Niña will also increase the prominence of cut-off lows and the clustering of storms, which we've been analysing on Swellnet.
Following this week's announcement, Dr Andrew Watkins from the Bureau said:
"La Niña typically results in above-average winter-spring rainfall for Australia, particularly across eastern, central, and northern regions.
"It typically also brings cooler and cloudier days, more tropical cyclones, and an earlier onset of the first rains of the wet season across the north.
"The cooling of surface temperatures in the tropical Pacific Ocean and an increase in the strength of the Pacific Trade Winds indicates the chance of La Niña has risen. When these two changes occur at the same time, at this time of year, we see a greatly increased chance of a La Niña forming and persisting through spring.
"Climate models suggest that further ocean cooling and intensification of Trade Winds may occur over the coming months, which has triggered the Bureau to shift from a La Niña Watch, issued on 26 June, to a La Niña Alert."
Increased rainfall is one thing, but what does an emerging La Niña mean for surf?
La Niña's impacts will be felt mostly across the Eastern Seaboard with more tropical cyclones forecast for the coming season, but as is always the case with small-scale storms, unless there is a supporting ridge of high pressure the benefits are limited.
The long-range Mean Sea Level Pressure forecasting by the European Centre for Medium-Range Weather Forecasts (ECMWF) has such scenario with higher than normal pressure sitting just south of New Zealand and stretching below Tasmania.
With this setup, we'll continue to see swells coming more from the east rather than south into summer, and that bodes well for a reduction of the chances of those pesky northerly winds that have plagued Northern NSW and south-east Queensland the past couple of summers. This is due to the sub-tropical high sitting further south and lower than normal pressure forecast through the Coral Sea.
For the southern states, Victoria and Tasmania are expected to see a little less swell than average owing to higher than normal pressure, while Western Australian is a tricky one to forecast though likely to remain active with mid-latitude frontal systems. Swells across South Australia are likely to sneak in under Western Australia and arrive from the west.
This is a very broad outlook, however, and the regional Forecaster Notes will provide much greater detail and accuracy over the coming months.