The Tasman Sea heat wave


A major heatwave has arrived in New Zealand, and its signature is big enough to be picked up on satellites.

But this heatwave is different, in that it is not happening in the air. Whilst it is certainly true that parts of the South Island are experiencing very warm and in some cases, record breaking, temperatures, the source of heat is different. A muggy warm northerly airflow over New Zealand is dragging tropical air down from the Coral Sea.

The heatwave I am talking about is in the Tasman Sea. At the Port of Lyttelton on the east coast of the South Island it was 17ºC in January 2017. A year later it is 22ºC. Atmospheric imaging of the Tasman Sea shows that much of its area as well as the seas around the rest of New Zealand are generally warmer than they were in 2017.

Currently sea temperatures in the Tasman Sea are about 6ºC above normal. National Institute of Weather and Atmospherics (N.I.W.A.)data shows that not since before 1900 has there been a year when sea water temperatures in the Tasman Sea were this high. Likewise, the American National Oceanic and Atmospheric Administration (N.O.A.A.) thermal imagery shows a large red blob in the Tasman basin and the rest of New Zealand surrounded by yellow.

The same thermal anomalies in the sea that are causing this heat wave are also the same ones that enable low pressure systems to suddenly deepen rapidly or even explosively and throw up strong storm like conditions. It is over these seas that features such as the January 2017 weather bomb event, where a low pressure system over the Tasman Sea suddenly became a significant storm with 300 millimetres of rain at Arthurs Pass and various locations along the Southern Alps and West Coast in 24 hours and storm force winds in many locations.

The heatwave might be good for people seeking some nice warm seawater to swim in. However it is potentially stressful for marine life and there are concerns that it might be linked to any near future die offs, of shell and kelp forests. The warm sea water has also been linked to the earlier than normal arrival of blue bottle jellyfish.

How long this warm sea water lingers for is unknown. An approaching low pressure system in the Tasman Sea is expected to deliver heavy rain to the West Coast on Wednesday with showery conditions in its wake across New Zealand.

The joys of living in a maritime climate.

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