Strong Indian monsoons are moving Atlantic hurricanes to Americas
Strong monsoons in the Indian Ocean can move Atlantic hurricanes further westward, towards land, increasing the likelihood of making landfall in the Americas, researchers from Pacific Northwest National Laboratory projected in a study.
The study finds that in years where the rains aren't as strong, hurricanes tend to curve northward and fizzle out in the north Atlantic Ocean. However, stronger summer rainstorms in India move Atlantic hurricanes further westward towards land.
This newly-discovered relationship could help scientists better predict the path of oncoming hurricanes, especially in late summer months like September, when Atlantic hurricane activity peaks, according to the study's authors.
"What amazes me is how rainfall near India can drive important changes to Atlantic hurricanes half a world away," said Patrick Kelly, an atmospheric researcher at the Department of Energy's Pacific Northwest National Laboratory, Washington. "This research is the first to draw the connection between Atlantic hurricanes and the Indian monsoon."
The Indian monsoon season typically wanes by September, but climate projections suggest that under future warming conditions, monsoon precipitation will increase, and the monsoon season could last unconventionally longer, having an increased influence on the Atlantic hurricanes.
Previous research has attributed changes in hurricane steering to El Niño-Southern Oscillation (ENSO), a periodic fluctuation in sea surface temperature and air pressure in the equatorial Pacific Ocean.
Scientists traditionally rely on the La Niña, cool phase of ENSO, to predict the strength of a particular Atlantic hurricane season, but have trouble forecasting the paths of individual hurricanes.
Since the Indian monsoon had not yet been investigated in the context of Atlantic hurricanes, Kelly and his colleagues wanted to find out how the Indian monsoon, a known source of climate variability, affected hurricane tracks. They conducted simulations of hurricane tracks with a model that incorporates observed variations of monsoon intensity and found that in response to strong monsoons, hurricanes shifted significantly westward.
Strong monsoons influence hurricane steering by enhancing the effects of the North Atlantic subtropical high, a centre of high atmospheric pressure in the Atlantic Ocean. When the subtropical high increases, stronger winds come from the east and push hurricanes westward.
"This work untangles La Niña's role on frequency, whether there are more or less hurricanes, from the steering impacts of winds, governed by the Indian monsoon," Kelly said. "La Niña events often happen during a strong monsoon, and they are correlated, but this work helps separate the independent influence of those two phenomena."
According to Kelly, La Niña and the Indian monsoon are correlated, but the strength of the monsoon influences the steering of hurricanes independently of La Niña fluctuations, which are responsible for changes in hurricane frequency. In other words, La Niña fluctuations may result in more Atlantic hurricanes, but strong Indian monsoons steer them further westward, making it more likely they will make landfall in the Americas.