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The key factor behind the relentless NSW rain

New South Wales has been relentlessly wet this week, continuing a soggy August where Sydney has seen rain on all but five days to date – including 79.9mm in the 24 hours to 9am Thursday, a total which exceeded the city's monthly average in a day.

But why?

As discussed in recent Weatherzone stories, the primary driver of the current wet spell is the large high pressure system centred over the Tasman Sea, which is positioned much further south than usual for late winter.

Air moves anti-clockwise around high pressure systems, pushing moisture-laden air onshore from the east. That saturated airmass is interacting with a persistent coastal trough to generate frequent heavy showers and periods of rain.

Image: Synoptic chart for Thursday, August 21, 2025, showing air circulating anti-clockwise around the high.

But if one factor has been the common denominator fuelling the back-to-back heavy rainfall events along the NSW coastline and adjacent inland areas in recent weeks and months, it’s the unusually warm sea surface temperatures off the coast.

Image: Sea surface temperature anomalies off the NSW coast, with the darkest red areas showing water that is up to 4°C warmer than usual for this time of year.

The map above shows sea surface temperature anomalies (temperature difference from the long-term average) for the NSW coastline on August 19 (the most recent reading).

A large red blob of extremely warm water for winter lies just southeast of Sydney. And if you look at the video loop below, you can see that sea surface temperatures have been relatively warm all winter, with the warmth anomaly increasing in recent weeks.

These NSW sea surface temperatures are in the top 10% of historical records.

Image: Animated loop showing sea surface temperature anomalies off the NSW coast from the start of winter 2025 to August 19.

How do warm sea surface temperatures increase rainfall?

As Weatherzone meteorologist Felix Levesque explains, abnormally warm sea surface temperatures help provide moisture in the atmosphere, because the rate of evaporation is higher with warmer water.

The East Australia Current – which feeds water in a general southwards direction along Australia’s east coast – has been warmer than average for much of 2024 and 2025. While the current runs less strongly in winter than summer, it is still largely responsible for the warm water off the NSW coast.

Looking at the bigger picture, the underlying effects of climate change are a key factor behind warmer sea surface temperatures across much of the globe, with the oceans absorbing up to 90% of the heat generated by increasing greenhouse gases.

Image: Tasman Sea sea surface temperature anomalies from 1900 to 2024. Source: BoM.

As the image above shows, the Tasman Sea has been considerably warmer than the long-term average most years since the turn of the millennium.