Anomalously warm SSTs in the Gulf of Carpentaria are supporting the redevelopment of Tropical Cyclone Owen. Temperatures are 1 to 2 deg C above normal (see image below).
SSTs are also heating up in the semi-enclosed bays on the south coast. Port Phillip Bay is already approaching 20 deg C. Similarly, in the Spencer and St Vincent Gulfs in South Australia the shallow inshore waters have warmed rapidly in recent weeks.
Severe TC Owen
the zombie returns
Owen has come back alive! The decayed ex-TC has reinvigorated in the Gulf of Carpentaria into a Category 3 system, and is expected to intensify further to reach Category 4 by Friday.
The Northern Territory and Queensland are bracing for storm-force winds and heavy rainfall as the system moves across the York Peninsula and down the QLD coast.
So how is the ocean model reacting?
heat flux out
The tropical cyclone will feed on the heat of the shallow waters in the Gulf of Carpentaria. This is evident if we look at the total heat flux at the bottom boundary of ACCESS-G. These are the fluxes used to force OceanMAPS.
The images below show the 6-hourly average Total Heat Flux for 15:30 local time (when the sun is up in the sky) for the 13th and 14th of Dec. We can see that away from the cyclone are large positive numbers (short wave radiation from the sun is putting heat into the ocean), but near the cyclone there are large negative numbers (latent heat being lost to the atmosphere).
This has a corresponding effect on sea surface temperatures. On the 12th of Dec most of the Gulf was 31 to 32 deg C. In a couple of days’ time, OceanMAPS predicts SSTs will drop to a more consistent 30 deg C. In reality the effect may be even starker than that.
sea level anomalies
OceanMAPS SLA forecast show a decrease in sea level under TC Owen.
Hang on, don’t we expect the sea level to be elevated under a vortex?
Forecasters call it the ‘inverse barometer effect’ – when reduced atmospheric pressure means less downward pressure force on the ocean surface which allows the sea to expand a little vertically.
For frequent users of OceanMAPS this event provides a good reminder: there are no atmospheric pressure effects in the model.
Hence, the primary effect being shown in the above plot is Ekman transport of the water away from the TC centre which causes a reduction in sea surface height.
For an aggregate of all the sea level forces at key coastal locations, check out the BOM’s Sea Level Viewer (internal).
Storm tide monitoring sites in western QLD are already showing elevated sea levels due to surge. The most dramatic one is the Burketown tide gauge which is 30 km up a river.
Yesterday this gauge recorded an impressive 0.7 m tidal residual. This could be exceeded further in the next couple of days as the TC tracks almost directly over the town.
Heavy rain around TC Owen will reduce the surface salinity values in the Gulf of Carpentaria. The left hand image shows mass flux (precipitation minus evaporation). Peak values are close to 160 mm/day. The right hand image shows the response in the surface salinity, with much fresher values showing on the western coast of the York Peninsula.
Salinity forecasts for QLD can also be viewed on the BOM website.
Surface currents will be generated from the strong wind forcing. OceanMAPS predicts cyclonic currents of up to 1.5 knots around the feature. Higher-resolution modelling would probably show larger values still.
Wave heights are also expected to build. AUSWAVE-R is forecasting up to 4 m in significant wave height, while higher-resolution ensembles show that 7 m is possible (not shown).
NT authorities are warning all mariners to stay ashore, after three fishermen were rescued from South West Island yesterday afternoon.
Strong winds cause large wind stress on the ocean and this deepens the mixed layer. The below image of the 6-hourly average momentum flux shows a maximum flux on the eastern side of the cyclone near York Peninsula.
How will this affect the mixed layer depth? We can see this using the proxy variable ‘depth of maximum sound speed’ from OceanMAPS. As the cyclone moves into the central Gulf on Friday, the MLD deepens from 20-30 m to 50-75 m. Most of the Gulf is only 30-40 m deep!
With so much mechanically-driven mixing in this shallow basin, large amounts of sediment from the seabed are likely to be churned up to the surface. Some articles suggest that tropical cyclones are the dominant cause of sedimentary transport in the Gulf, and substantial areas of coastline may owe their origin to this process.
Sediment resuspension, decreased water temperatures, fresh water run-off and strong currents will change the make up of the Gulf waters and impact biological activity.
TC Owen may die and rise again on its way back to QLD, but the effects of such an intense system may continue to be felt in the Gulf for some time to come.