Storm

Storm. (photo credit:ILAN SHOCHAT)

Under global climate change, the dry, semi-arid regions are expanding into higher latitudes while the temperate, rainy regions are migrating towards the North and South Poles.

For decades, the Earth’s climate zones have shifted toward the poles due to global warming – and they will continue to do so.

This according to researchers at Rehovot’s Weizmann Institute of Science, who recently published their findings in Nature Geoscience.

Under global climate change, the dry, semi-arid regions are expanding into higher latitudes while the temperate, rainy regions are migrating towards the North and South Poles.

By discovering that mid-latitude storms are steered further toward the poles in a warmer climate, the researchers have provided new insight into this phenomenon.

The teams’ models of climate change predict that if average global temperatures rise by four degrees over the next 100 years, while storms will deviate poleward from their present tracks by two degrees of latitude.

Prof. Yohai Kaspi of the institute’s Earth and Planetary Sciences Department said: “Although two degrees may not sound like a lot, the resulting deviation in temperature and rain patterns will have a significant effect on climate zones.”

The teams’ analysis, which also revealed the physical mechanisms that control this phenomenon, involved a unique approach of analyzing the storms’ dynamics that traced the progression of low-pressure weather systems both from the outside – in their movement around the globe – and from the inside.

Kaspi explained that the Earth’s climate zones roughly follow latitudinal bands. “Storms usually move around the globe in preferred regions called ‘storm tracks,’ forming over the ocean and generally traveling eastward and somewhat poleward along these paths.

“Thus, a storm that forms in the Atlantic off the East Coast of the US at about Lat. 40 degrees N. will reach Europe in the region of Lat. 50 degrees N.,” he said.

Until recently this phenomenon was not really understood.

However, Dr. Talia Tamarin from Kaspi’s group has solved this fundamental question in her doctoral research.

“From the existing climate models, one can observe the average storm tracks, but it is hard to prove cause and effect from these,” explained Kaspi. “They show us only where there are relatively more or fewer storms.

Another approach is following individual storms; but we must deal with chaotic, noisy systems that are heavily dependent on the initial conditions, meaning no storm is exactly like another.”

Tamarin developed a method that combines these two approaches. She applied a storm-tracking algorithm to simplify atmospheric circulation models in which thousands of storms are generated. This eliminated the dependence on initial conditions.

It also allowed her to understand how such storms develop over time, space and what controls their movement.

In the present study, to understand how the movement of storms may change in a warmer world, Tamarin and Kaspi applied the same method to full-complexity simulations of climate change predictions.

Their analysis showed that the tendency for tracked storms to veer in the direction of the poles intensified in warmer conditions.

They discovered that two processes are responsible for this phenomenon. One is connected to the vertical structure and circulation near the tops of these weather systems, while the second process is connected to the energy tied up in the water vapor in such storms.

Global warming studies have shown that hotter air will contain more water vapor, and thus more energy will be released when the vapor condenses to drops.

“The hottest, wettest air is circulating up the eastern flank of the storm – to the northern side – and releasing energy there,” said Tamarin. “This process pushes the storm northward (or southward in the southern hemisphere), and this effect will also be stronger in a warmer climate.”

The institute’s research shows that part of this shift will be due to the mechanism demonstrated, while the other part is tied to the fact that storms are born at a higher latitude in a warmer world.