NIWA’s Sea Surface Temperature Update

Wednesday, 30 January 2019

A fortnightly report, updated over summer, covering sea surface temperatures and anomalies (differences from average) around New Zealand.

Following the Tasman Sea marine heatwave event of 2017-18, this report will help users understand the latest conditions in the ocean and their expected changes over the upcoming weeks.

Over the past two weeks, sea surface temperatures (SSTs) have remained the same or increased slightly in the New Zealand region. As a whole, Tasman Sea surface temperatures are +1.3C above average for the time of year, although localized anomalies of +2.0 to +3.0C exist in the central and west. New Zealand coastal waters are generally 0.5C to 1.5C above average, a decline from two weeks ago.

Marine heatwave conditions are likely still occurring in parts of the central and western Tasman Sea and in coastal waters east of the North Island, according to the definition proposed by Australian research in 2016 (see background below).

Compared to the same 2-week period in 2018, New Zealand coastal waters are 1.0 to 3.0C cooler, although the western Tasman Sea is 1.0 to 3.0C warmer.

Expected Conditions over the Next Several Weeks

A hot airmass over New Zealand has led to SST warming of 1.0 to 2.0C over the last several days. A southerly change is forecast to occur from Friday into Saturday, ending the spell of unusual warmth. Heading through the first half of February, weather patterns are forecast to have more variability than what was observed during January and while sea surface temperatures are expected to remain above average for the time of year, marine heatwave conditions are unlikely to develop in new areas.

Background:

Sea Surface Temperature Update is a fortnightly report for New Zealand media. It provides measurements of sea surface temperature and anomalies (differences from average) measurements for New Zealand coastal and Tasman Sea waters.

Sea surface temperature anomaly: the difference between the long-term (1981-2010) average sea surface temperature and current sea surface temperature for a given time of year.

New Zealand region: within this document, any quoted anomalies refer to spatial averages over the ocean region bounded by the coordinates (145°E-175°W, 25-55°S)

SST: abbreviation for sea surface temperature.

Marine heatwave: periods of extremely warm sea surface temperature that persists for a prolonged period of time and can extend up to thousands of kilometres. According to Australian research (Hobday et al. 2016), warm sea surface temperature events are considered marine heatwaves (MHWs) if they last for five or more days with temperatures warmer than the 90th percentile based on a 30-year historical baseline period.

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