Saturday 30 May 2015

A warming planet increasing the intensity of tropical cyclones

Climate change makes a rare appearance in the NZ media

Super typhoons to increase in strength with climate change

Typhoon Maysak as it strengthened into a Category 5 hurricane in March 2015.
Typhoon Maysak as it strengthened into a Category 5 hurricane in March 2015.
30 May, 2015

A warming planet is already stoking the intensity of tropical cyclones in the north-west Pacific and their ferocity will continue to increase even with moderate climate change over this century, an international research team has found.

A study covering 850 typhoons in the region found the intensity of the damaging storms has increased by about 10 per cent since the 1970s, said Wei Mei, a climate scientist at the Scripps Institution of Oceanography at the University of California, San Diego, and a co-author of the study published in the journal Science Advances.

Using 20 models and a mid-range projection of carbon dioxide emissions, the researchers found the peak intensity of storms such as super Typhoon Haiyan, which tore through the Philippines in November 2013, will become even stronger and more common.
Super Typhoon Haiyan as it approached the Philippines in November 2013.
NASA   Super Typhoon Haiyan as it approached the Philippines in November 2013.

Such storms will be 14 per cent stronger by 2100, equivalent to adding another category to the current top severity rating of 5, the study found.

Research on tropical cyclones – known as hurricanes in the Atlantic basin – has sought to identify whether factors contributing to more powerful events such as warmer sea surface temperatures might be countered by changes to ocean or atmospheric circulation that may hinder the storms' genesis or force. Warming in the top 75 metres of the oceans will dominate other influences, the researchers found.

"This projected increase in typhoon intensity is largely due to [sea surface temperatures] warming," the study found, adding that the findings are "at the high end" of previous projections.
This tanker ran aground, landing in the middle of houses, as typhoon Haiyan ripped through Philippines.


More storms such as Haiyan could have significant impacts on regions in their path. Haiyan left more than 6200 people dead and 1785 missing in the Philippines alone, clocking sustained wind speeds of 315 kmh, the fastest ever recorded.

"The strengthened typhoon intensity poses heightened threats to human society and marine/terrestrial ecosystems," the paper said.

"Meanwhile, the intensification of these powerful storms may accelerate ocean warming and affect heat transport in both the ocean and the atmosphere," the researchers said.

That accelerated warming is because ocean mixing is increased, Mei said.

"Strong storms generate intense mixing in the upper ocean, cooling the surface layer while warming the subsurface," he said. "Thus the net effect of the storm passage is to pump heat downward from the surface to the subsurface ocean."

"This warming effect becomes stronger as the storm becomes stronger, because stronger storms can generate more vigorous mixing and thereby pump more heat into the ocean and to a greater depth."


Mei said the increase may be more than 14 per cent if greenhouse gases rise faster than the RCP (representative concentration pathways) 4.5 climate model used for the study. The model projects carbon dioxide-equivalent levels will reach 550 parts per million by 2060, up from about 404 ppm now.

"We believe the intensity will be even stronger" with a higher CO2 concentration, he said.

Mei said the study did not examine other basins, such as the south-west Pacific.


El Nino years are likely to foster stronger typhoons in the north-west Pacific because the region they form in tends to shift - along with the warming surface waters - further southeast towards the central Pacific.

That move gives storms a longer period to intensify before they make landfall or reach cooler waters, Mei said.

Australia's Bureau of Meteorology earlier this month declared an El Nino to be under way and this week said the event was intensifying.

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