News•October 10, 2012

Global Warming May Shift Summer Weather Patterns

By altering the heat balance between land and sea, manmade global warming may be altering summer weather patterns in the Northern Hemisphere, a new study found. The study, published on Sept. 30 in Nature Geoscience, shows that the sprawling high pressure areas that set up shop over the Western North Atlantic and North Pacific Oceans during the summer months have become larger and stronger during the past 40 years, and these trends are likely to continue during the next several decades as temperatures increase.

These changing weather patterns could have far-reaching impacts, from redirecting powerful hurricanes toward the East Coast, to making the Southeast and Central states see-saw more frequently between extremely hot and dry summers and cooler, wetter summers. In addition, a shift in the strength and shape of the North Pacific subtropical high could affect the South Asian Monsoon, which is already being altered by warming and increased regional pollution.

The Bermuda High helps steer tropical storms and hurricanes.
Credit: Penn State.

The study does not formally attribute the cause of the recent trends, but says that the future changes will most likely be driven by global warming.

Although highs (the big “H” symbols on your local TV weathercast) are typically associated with pleasant weather, the position and shape of these systems shape large-scale weather patterns, helping to determine the locations of subtropical deserts. More importantly for the U.S., they help steer the most powerful storms on Earth, and modulate rainfall amounts in the Central, Southeast, and Mid-Atlantic states.

The Atlantic subtropical high, more commonly known as the “Bermuda High” because of its semi-permanent location near that Western Atlantic island during the summer months, helps determine whether Atlantic hurricanes recurve harmlessly out to sea before reaching the East Coast, or make landfall with potentially devastating impacts.

Hurricanes tend to skirt around the edges of the high by catching a ride on the clockwise flow of air around the periphery.

The Bermuda High also helps draw warm and humid air up the Eastern seaboard, contributing to some of the most intense heatwaves on record.

The study, which relies on climate model simulations as well as weather data for the past 40 years, shows that the Bermuda High has already expanded westward, which could be making summertime rainfall in the Central and Southeast U.S. much more variable.

Summer rainfall in the Southeast U.S. has become more variable in recent years, as this graphic shows.
Credit: Dan Satterfield/AGU.

“The intensification and westward movement of the subtropical highs may cause more landfalling hurricanes/typhoons and cause more intense Southeast U.S. rainfall variability, leading to more extreme events in the[se] regions,” said coauthor Mingfang Ting of Columbia University in an email conversation.

A 2010 study published in the Journal of Climate found that a westward shift in the Bermuda High helped cause a marked increase in the frequency of summers with “strongly anomalous precipitation” in the Southeast. Recent summers have seen dramatic flips between punishing droughts and severe flooding in states such as Georgia, for example.

According to the research of Ting and her colleagues, the sharpened temperature contrast between land areas and the oceans, which is related to manmade global warming, is the main mechanism behind the intensifying and expanding Highs.

“... In the future warming scenario, we show that this pattern is intensifying, and land and ocean heat contrasts are intensifying. This leads to the intensification of the anticyclones,” Ting said.

In the Pacific, the consequences of the intensifying and expanding subtropical high could be just as serious, considering that the high helps regulate the South Asian Monsoon season, which provides vital water for irrigating crops.