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Why October has been wet in NSW

Ben Domensino, Wednesday October 24, 2018 - 11:00 AEDT

October has been a surprisingly good month in terms of rainfall for much of NSW. By the middle of this week, large swathes of the state's west, north and east had received one to three times their average October rainfall.

This month's wet weather has been a rapid shift from the dry conditions that have gripped NSW during the rest of 2018. Below-average rain between January and September broke multi-month dry-spell records in some areas and lead to drought conditions across much of the state.

But while this month's rain has certainly been welcome news across the drought-affected state, it may have come as a surprise considering the looming threats of El Nino and a positive Indian Ocean Dipole. So, what's going on?

As of this month, Australia is on El Nino Alert, meaning the Pacific Ocean is showing strong signs of a developing El Nino pattern and that there is a 70 percent chance of an El Nino event occurring in the coming months. Furthermore, the Indian Ocean appears to be in the early stages of a positive Indian Ocean Dipole event.

Both El Nino and a positive Indian Ocean Dipole increase the likelihood of below average rain for most of NSW, particularly west of the ranges. However, they don't guarantee dry weather each time they occur.

Local features, such as warm water near the Australian coastline, or a processions of unusually strong cold fronts, can cause periods of unexpectedly wet weather during El Nino and positive Indian Ocean Dipole events.

This month's rainfall in NSW was enhanced by a tongue of abnormally warm water in the western Tasman Sea, which was transported southwards along the state's coastline by the East Australian Current. As of this week, sea surface temperatures off parts of the NSW coast are around 3-4 degrees above average for this time of year.

Warmer water in the Tasman Sea leads to more evaporation, which puts more moisture in the atmosphere. Another notable feature this month has been stubborn high pressure systems centred near New Zealand. These highs have maintained unusually persistent easterly winds across the Tasman Sea, which have carried the moisture-laden air across NSW.

A wet October does not guarantee more rainy weather during the months ahead. With the prospect of El Nino and a positive Indian Ocean Dipole still on the table later this year, the odds still favour drier than usual weather in late spring and early summer for much of the country.

- Weatherzone

© Weatherzone 2018

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