Warm Water Hidden In The Depths
It doesn't matter which coast of Australia you reside, November is a time of transition. A time when offshore winds will, more often than not, bring warm air compared to the bone-chilling breezes of winter, and longer, thicker wetsuits are bundled away until next year.
For those in more sub-tropical climes this means sessions in boardshorts, and all the perks of comfort and freedom.
This past winter, water temperatures around the country were milder than average. In Victoria, there was less need for booties and hoods, while up the East Coast there was only one spell of cooler water. That was in early winter before temperatures rose again, settling in above average (and making the below average surf season a little more bearable).
During last summer - which was the third in a three-peat La Niña - warmer than normal water pooled around northern Australia, then flowed south via the Leeuwin Current (off Western Australia) and East Australian Current, transporting the warm water to the southern regions.
Despite La Niña ending, this heat transport provided a buffer against air temperatures that were cooler than normal through autumn and early winter.
Looking around the grounds and sea surface temperatures surrounding Australia are still slightly above normal, and if not the temperatures are bang on average - nowhere is under.
However, there's one standout area and that's the East Coast, where a warm tongue of the East Australian Current is filtering down from Queensland - see graph below - while tepid waters also sit off Tasmania's East Coast.
The spring transition season can often be dramatic on these coasts, as persistent northerly winds cause cold water upwellings to occur. Yet despite bouts of strong nor'east winds, there haven't been any cold water upwelling events across the southern NSW region.
Not that they've been missed - we all want to shed the rubber - but it's worth asking what's happening.
The answer is hidden below the surface.
While sea surface temperatures are warm, as mentioned above, they're also also extending deeper into the ocean. To explain, imagine looking at a water column in profile. The 'sheet' of warmer water near the surface is called the mixed layer, while the depth to which it extends to before temperatures drop noticeably is know as the mixed layer depth.
Even if they don't know the name, freedivers and spearfishers are aware of the mixed layer, as descending below it requires them to wear a thicker wetsuit than they would if they were surfing.
The deeper the mixed layer is, the greater the volume of warm water, and the more heat energy that is stored in the water column. And as a side note: The more fuel there is for developing storms, be that deepening surface troughs/lows and tropical cyclones. We'll be watching this closely.
Yet for the sake of this article, the deeper the mixed layer is, the greater the buffer against upwelling events. Recent north-east episodes haven't been able to pierce the mixed layer and draw up the colder water from below, though something very significant might.
With this in mind, there's currently a large warm-core eddy (anti-cyclonic whirlpool) sitting south-east of Seal Rocks and due east of Sydney - see graph above. A recent survey by CSIRO's RV Investigator shows it's storing 30% more heat than normal for this time of the year.
Temperatures at the surface are 2°C above normal while buried within the eddy are 3°C above normal.
I've pulled up data from an Argo float that's been placed within the eddy. Argo floats descend and ascend the water column, taking measurements of temperature and salinity among other things. The most recent Argo float readings have the mixed layer being around 100m deep and at 21.4°C.
The chart above shows the water column, with the top line being the surface, water depth on the left (descending down to 400m) and temperature running along the bottom axis. The mixed layer is shown by the continuous temperature of 21°C + descending to 95m before the following the general cooling trend at greater depths.
Remarkably, even at 150m deep the temperatures are at 19.5°C - warmer than the surface temperatures of Victoria, Tasmania, and South Australia.
This points to an enduring event, with warmer than normal water hanging around for at least the next two months, preventing any major cold upwelling from happening. The warm water will also feed into any unstable weather, producing rain, as is due over the East Coast for the next fortnight.
As for swell: If the warm water combines with any cold air intrusions from the south then we have a combustible scenario, and that's something we'll continue to monitor in the regional Forecaster Notes.