06 Apr – Tasman warmth below surface


Along the southwest coast of WA, the Leeuwin current has pushed south, perhaps assisted by ex-TC Marcus bringing heat from northern parts last week. Sea surface temperatures are around 22 deg C which is 1 to 1.5 degrees above normal off Cape Leeuwin.

The very warm extension of the East Australia Current continues down the northern NSW coast. It seperates at Port Macquarie but continues due south until 35 S when it curves eastward and then retrogrades back northward. Surface temperatures in this lobe of water are 25 to 26 deg C.

The Tasman Sea has cooled down at the surface and is no longer showing the large anomalies that prompted a recent Special Climate Statement issued between BoM and NIWA about record warmth in the Tasman Sea, New Zealand and Tasmania.

However, a cross section of temperature anomalies shows that significant heat remains at depth. The image below shows the degrees above the 95 percentile values for the top 300 m of the Tasman Sea in a cross section between Hobart and Christchurch. The anomalous heat is sitting at approximately 50 m depth.

Sea Level Anomalies and Ocean Currents

The reformed TC Iris will continue to push up sea level along the QLD coast, as shown in the anomaly plot below. The OceanMAPS output shows decreased sea level underneath the cyclone itself due to Ekman transport away from the centre of the vortex. The model does not account for the inverse barometer effect, so we can expect that in reality the sea level is more elevated and perhaps close to neutral height under the cyclone.

As mentioned above, the EAC retrogrades off NSW and turns northward, forming a giant loop of warm water and an enlarged area of positive sea level anomaly.

Further south, the small warm eddy off eastern Victoria persists, despite cooling at the surface. A cross section of the 95 percentile values reveals that warm anomalies of 1 to 3 degrees C continue at depth, between 300 and 1000 m deep. This is enough to expand the water column and continue to generate the geostrophic anti-cyclonic currents.

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