Special Climate Statement - record warmth in the Tasman Sea, New Zealand and Tasmania

Sea surface temperatures in the southern Tasman Sea rose to exceptionally high levels in late 2017 and early 2018. These temperatures were far above any others previously observed at that time of year in the region, and extended west from New Zealand to Tasmania and mainland southeast Australia. In parallel with this, land temperatures were well above average in areas near the Tasman Sea warm anomalies, with many records set both in New Zealand and in southeast Australia (especially Tasmania).

Sea surface temperatures in the southern Tasman Sea were above average throughout 2017 but rose sharply from November, reaching 2 °C or more above average over most of the Tasman Sea south of 35 °S. In November, they were at record-high levels over a region extending from New Zealand to Tasmania. This warming coincided with an extended period of blocking high pressure over the Tasman Sea sector, with monthly mean sea level pressure (MSLP) for November as much as 10 hPa above average in places, and 7.5 hPa or more above average over a belt extending from the South Island of New Zealand to west of Tasmania. Sea surface temperatures persisted at record-high levels through December 2017 and January 2018. The peak of the event had passed by February 2018, with sea surface temperatures falling below record levels but remaining well above average.

Abnormally warm conditions developed on land on both sides of the Tasman in relation to the Tasman Sea marine heat wave. In New Zealand, very warm conditions began to develop in the interior South Island from October onwards, before extending nationwide during the summer. The summer of 2017–18 (December 2017 to February 2018) was the hottest on record for New Zealand, and January 2018 was the hottest month recorded. Atmospheric moisture over much of New Zealand was also notably high during this time, and February 2018 saw several extreme rainfall events, two of which were associated with former tropical cyclones.

Tasmania also warmed markedly from October 2017 onwards. In Tasmania, November was the most extreme month of the period, with the largest mean temperature anomaly on record for any month in the State. October 2017, December 2017 and January 2018 were all the second-warmest on record for the respective month, and the October–January and November–January periods were both the warmest on record by substantial margins. Tasmania cooled substantially in February 2018 with temperatures in that month close to average, resulting in summer mean temperatures falling below record levels. It was also a very warm period in coastal and near-coastal areas of mainland southeast Australia, particularly southern Victoria and adjacent areas of southeast South Australia and the southern New South Wales coast, although conditions in these regions were generally less exceptional than those in Tasmania.

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