November 2021 was one for the record books.

Across Australia it was the wettest November on record, and it was also the coolest November in 21 years.

To find out why we have to look inland, as it's there - in Australia's interior - where our warmth originates. This year, however, the inland failed to heat up through spring owing to the constant in-feed of moisture and cloud from the north-west of the country (thanks to the negative Indian Ocean Dipole). We also saw similar instability and moisture feeding in from the Pacific Ocean to the north-east thanks to the developing La Niña.

The formation of coastal lows and troughs blanketed the coast in cloud while ocean temperatures remained stubbornly cool.

Switch to the start of December, and while the cloudy, cooler weather and intermittent rain storms are persisting thanks to La Niña, we've seen the water temperature rocket upwards in NSW - at least as far south as Wollongong.

Where has this warm water come from and is it normal?

Under a La Niña setup (with tropical winds blowing towards the west) we get warm water piling up across the western Pacific Ocean and this then starts flowing north and south away from the equator, under the influence of the Coriolis Force due to the Earth's rotation. These currents flowing on the western side of ocean basins are known as Western Boundary Currents and transport warmer tropical waters towards the poles. A return flow along the eastern sides brings cooler water towards the tropics, with the complete circulation known as an ocean gyre.

For the Northern Hemisphere we have the Kuroshio Current and Gulf Stream flowing on the western side of the north Pacific Ocean and north Atlantic Oceans respectively, providing warm water to Japan and north-east America.

For the south Pacific Ocean we have the East Australian Current (EAC) and the Agulhas Current for the Indian Ocean.

The EAC originates in the Coral Sea where sea surface temperatures are currently warmer than normal (1.5-2.5°) thanks to the current La Niña setup. It flows southwards down the Qld and NSW coasts, sometimes reaching as far as Tasmania. 

The EAC flows all year but is strongest during summer and usually separates around Seal Rocks, travelling east towards New Zealand. This is known as the Tasman Front.

The East Australian Current flowing south down the East Coast from the Coral Sea

The remaining current pushes south in fits and bursts, spawning off tongues and eddies which, while inducing coastal upwelling, also provide warmer water to the southern NSW region. 

One such intrusion has recently made its way south over the past two weeks and has brought bath-like water to the Hunter, Sydney, and Illawarra regions. From my personal observations, it feels like it's sitting at 21-22° while the MHL wave buoy positioned 10km off Long Reef in Sydney is currently recording temperatures of 23°. Further up the coast temperatures are 25° off Coffs Harbour and 26° off Byron Bay.

What's helped bring this water in so early is the lack of north-east winds that usually plague the coast in summer. These persistent north-east winds promote upwelling of cooler water, but with back to back lows and troughs forming off the coast, bringing southerly winds, the EAC has been allowed to fill in the near-shore zone. The opposite occurs in El Niño events with strong episodes of north-east winds and frigidly cold water upwelled along the East Coast (think of those Mid North Coast Christmas surfs in winter wetsuits).

Sea Surface Temperature observations from the 13th of December. The warm water intrusion into the Sydney region can be clearly seen.

The southwards intrusion is shown clearly in the above image, but you can also see that the EAC hasn't quite reached the Shoalhaven region or south of Jervis Bay yet, with it sitting further offshore, while off the far South Coast the water is still quite cool and below average.

Now that the warm is in, it should be here to stay during summer, with more warm water poised to flow down from the north. This takes into account the current La Niña setup which is less conducive for strong north-east wind events to develop owing to the sub-tropical high sitting further south. The South Coast should start to see the warmer water filling in over the coming weeks as the tongue extends gradually further south.