The Tasman is heating up again

Scientists watching rising Tasman Sea temperatures – again

Stuff,

10 December, 2018

The Tasman Sea is heating up again, a year after the rare "marine heatwave" which brought New Zealand's hottest summer on record.

The National Institute of Water and Atmospheric Research (Niwa) says it is "keeping a close eye" on the Tasman's rising temperature.

The marine heatwave that contributed to the recent scorching summer was rare. The previous event was more than 80 years earlier, during the 1934-35 summer.

Niwa principal climate scientist Dr Brett Mullan said on Monday that parts of the Tasman were already about 2 degrees Celsius warmer than average.

"Currently, sea-surface temperatures in the Tasman are much warmer than normal, and the region of warmth extends well south of Campbell Island.

"However, the warm anomalies are not as extreme as at the end of 2017."

In last summer's marine heatwave, a large area of the Tasman Sea immediately offshore of the West Coast was about 2C hotter than normal right through December, January and February.

Niwa reported in March that parts of the Tasman had been up to 6C warmer than average

NIWA Weather‏ @NiwaWeather

Sea temps this time in 2017 () vs 2018 (): quite a difference! The #marineheatwave event was well and truly underway this time last year, whereas climate patterns have been more variable this year.

12:52 PM - 10 Dec 2018

Mullan said the current warm pool was also not as extensive as this time last year.

The #marineheatwave event was well and truly underway this time last year, whereas climate patterns have been more variable this year.

"Niwa is keeping a close eye on this."

A similar pattern to the 1934-35 summer is shown in this map of the marine heatwave during the 2017-18 summer, with sea-surface temperatures up to 2C above normal.

A similar pattern to the 1934-35 summer is shown in this map of the marine heatwave during the 2017-18 summer, with sea-surface temperatures up to 2C above normal.

Last summer's heat was due to an unusual concatenation of phenomena – the marine heatwave, a La Nina event, and the warm northerly winds that predominated.

The nationwide average temperature last summer was 18.8C, 0.3C above the previous heatwave record of 18.5C in the 1934-35 summer and 2.1C above the 1981-2010 normals.

Fifty-four climate stations across the country had their hottest summer temperatures, with Alexandra reaching 38.7C on January 30 this year.

The 'marine heatwave' summers of 1934-35 and 2017-18 stand out in this graph of New Zealand's seven-station summer temperatures minus their 1981-2010 averages. The scale on the left runs up on 0.5 degree Celsius increments, from -2.5C on the bottom line to +2.5C at the top.

NIWA

The 'marine heatwave' summers of 1934-35 and 2017-18 stand out in this graph of New Zealand's seven-station summer temperatures minus their 1981-2010 averages. The scale on the left runs up on 0.5 degree Celsius increments, from -2.5C on the bottom line to +2.5C at the top.

Mullan said it was likely New Zealand would experience the same level of marine heatwave as last summer more frequently because of climate change.

However, it would probably remain rare to have such a severe event in terms of how far above the rising background average temperatures the extremes would reach.

Extreme fire warnings were in place for Canterbury and Marlborough last summer after days of hot weather.

In a talk at last week's joint Meteorological Society-Hydrological Society conference in Christchurch, Mullan said "summers more than 2C above the climatology of the [future] period will continue to be rare events".

"This is basically a 'perfect storm' type of situation, where everything has to align just right and persist long enough to produce these super extremes."

The 1934-35 summer was more extreme than last year's relative to the average temperatures of the time, with temperatures 2.7C higher than normal.