26th Jul – Whirlpools

SST update

SST Anomaly for Australia on 24th Jul 2019 – RAMSSA

Warm anomalies have intensified off the NSW coast. There are large areas at 2 deg C above normal.

SST anomaly for NSW – RAMSSA

Warm water continues to flow close to the coast. The pattern is driven by a strong warm eddy 35 S 154 E which is pulling EAC water down its western side.

SST (left) and SLA and surface currents (right) for 25th July 2019 – OceanMAPS
SST (left) and SLA and surface currents (right) forecast for 31st July 2019. Water mass boundaries drawn in pen – OceanMAPS

By mid next week OceanMAPS is forecasting a strong temperature gradient between Gabo Island with 13 deg C and offshore from Sydney with 21 deg C. This is likely to encourage atmospheric convection such as thunderstorms in the event of any cold air outbreaks.

Wintertime blues

If you think Gabo Island looks cold, then have a look at Port Phillip Bay temperatures. Temperatures inside the Bay are now at 10 to 11 deg C. The annual minimum of 10 deg C is usually reached in August, but the mid-bay measurements have already dropped to that number a few weeks early.

Water temperature measurements in Port Phillip Bay, VIC – BayWx

And that’s in the deeper water.

In the shallow water along the Bayside beaches it is likely to be even colder. Fancy a swim?

The Brighton Icebergs’s website says they swim in 7 to 12 deg C water during winter. They are a hardy bunch. But they do take precautions.

If you feel like swimming in the Bay this month, here are the tips on preventing hypothermia from the Brighton Icebergers:

How to prevent hypothermia – Source: Brighton Icebergers

A quasi-stationary eddy

A warm-core eddy has been hanging around Gabo Island in far eastern Victoria for months now.

SSH ensemble mean for the 26th of each month March to July 2019 – OceanMAPS

Quasi-stationary eddies have been monitored and studied all over the world, including in the northwest North Atlantic, the northeast Indian Ocean, California, and the Bay of Biscay. Some of these eddies were stationary for up to 7 months.

Our ‘Gabo Eddy’ is now touching 5 months.

Why is it still there? Theories say that eddies can get trapped by topography. That seems to ring true in this case. We would expect an eddy to travel west, but the Gabo Eddy is already up against steep bathymetry.

So why doesn’t it travel south? As we know the EAC is strongest during summer. The Gabo Eddy didn’t appear until the end of summer. Perhaps there isn’t enough southward ‘push’ from the EAC during the winter. Perhaps there are other dynamics from the Bass Strait and Southern Ocean that are counteracting any southward tendency.

The currents around the Gabo Eddy are strong. Is this Australia’s wintertime quasi-stationary whirlpool?

Surface currents for 26th July 2019 – OceanMAPS

The Great Whirl

The western Indian Ocean has more interest to oceanographers than just being the western box of the IOD.

It is the home of the Great Whirl. The whirlpool has been known for centuries amongst trading ships, but was first officially described in 1866 by British geographer Alexander Findley who wrote a book about navigating the Indian Ocean.

OceanMAPS surface currents for 25th July – BOM

Modern attempts to study the feature have been hampered by pirate activity.

In 2001 a research ship off Somalia was attacked with rocket propelled grenades.  Climate measurements in the region were halted. Ships of Opportunity (SOOP) also stopped reporting surface observations as they would turn of their AIS positioning data and ‘go dark’ to avoid being targeted. Ten years after that in 2011 it was still too dangerous to transit through, and CSIRO had to ask the US Navy to deploy their Argo profilers.

Pirate activity is now on the decline but observation coverage in the area continues to be poor.

Luckily the whirl can be tracked using satellite observations. Here is the OceanMAPS field for today.

SSH field for 24th July – OceanMAPS

Previous studies found that the Whirl peaks during July, August and September and then dissipates a month after the end of the monsoon (Beal and Donahue 2013).

However a recent study has found that it is bigger and longer-lasting than first thought. Melzer et al (2019) used satellite data to show that the whirlpool starts with the onset of the Indian Southwest Monsoon but doesn’t peak until early September. It lasts 198 days on average.

temperature and salinity

The Great Whirl is an important feature for the region as it recirculates the coastal current, effectively preventing the fresher water from the Indian Ocean entering the highly saline Arabian Gulf.

Images from OceanMAPS of the surface temperature and salinity (below) show that the water in the Whirl is indeed relatively fresh. Also, a region of upwelling driven by the monsoon winds and currents can be seen off northeast Somalia.

Temperature and salinity at the surface for 25th July 2019 – OceanMAPS

Depth profile

How do the whirlpool currents track with depth? The currents on the western side of the whirl seem strongest at the surface, but the eastern side has the maximum velocity at 75 m depth. In today’s OceanMAPS forecast the feature has structure until 200 to 300 m depth.

Studies show that the feature can deepen from 200 m to over 1000 m by September (Beal and Donahue 2013).

Velocity at depths of 75 m (left) and 105 m (right). Vertical profile of current speed at two locations shown. – click to expand

A last thought to close this story: is the decline in pirate activity really to do with the international naval task force, or did the pirates not survive the perils of the Great Whirl?


One thought on “26th Jul – Whirlpools”

  1. Thanks Jess for most interesting information about the East Australian stationary eddy, and Western Indian Ocean “Great Whirl”. Does anyone know if BRAN or other data sets indicate a stationary “Gabo Eddy” in past years? I had a quick look at IMOS Ocean Current (e.g. http://oceancurrent.imos.org.au/monthlymeans.php?link=30d_MEAN/SE_mm/20190615.html) and the monthly mean Sea Level Anomaly plots do not appear to indicate a stationary eddy around Gabo Island. Any comments anyone?

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