

Visualization of Hurricane Sandy (NASA)

As the Earth’s oceans have warmed over the last three decades, the grounds for destructive hurricanes have crept northward finds a new study, published by three of the world’s leading tropical weather experts. This observed poleward jog could reflect the first robust signal that the build-up of greenhouse gases from human activity is influencing tropical storms, the authors say.

Identifying a firm connection between hurricane activity and manmade global warming has proven elusive. The 2013 Intergovernmental Panel on Climate Change report stated there was “low confidence” in any link between observed changes in tropical storms and human activity.

But this poleward shift in the peak strength of tropical storms could be the smoking gun linking human-induced climate change and hurricane behavior.

The study, to be published in the journal Nature Thursday, analyzed where tropical storms around the world reached peak intensity between 1982 and 2012. It finds this location of maximum storm strength leaped poleward about 33 miles per decade in the Northern Hemisphere and 38 miles per decade in the Southern Hemisphere.

“The absolute value of the latitudes at which these storms reach their maximum intensity seems to be increasing over time, in most places,” says Kerry Emanuel, an MIT professor and co-author of the new paper. “The trend is statistically significant at a pretty high level.”

The author team of James Kossin of the National Oceanic and Atmospheric Administration (NOAA), MIT’s Emanuel, and Gabriel Vecchi of NOAA links the storm shift to an expansion of the Hadley Cell – a massive atmospheric circulation that transports heat from the tropics to the mid-latitudes and drives the easterly trade winds. In other words, the tropics are occupying a larger area. The expansion “is likely due largely to human influences” or the heating effect of increasing greenhouse gas concentrations, the study says.

As the subtropics are now becoming more tropical, this is reflected in an observed increase in the ingredients required for hurricanes to flourish the study finds. To intensify, hurricanes need warm ocean waters as fuel and a lack of vertical wind shear – or shifting winds with altitude – which otherwise can rip apart their thunderstorm complexes. Toasty water and weak wind shear have become more common in the subtropics over time the authors say, which they analyze by investigating changes in a metric called the genesis potential index – trends shown below.



Changes in the hurricane “genesis potential index”. Increases are shown in red, decreases in blue. (Kossin et al., 2014)

This “genesis potential index” – a proxy for favorable storm environments – has ticked up in the subtropics (red shades) while declining some in the equatorward extremes (southern section in Northern Hemisphere) of the tropics (blue shades).

The authors warn the shift in storms may have significant implications for major coastal regions in the mid-latitudes, including the Mid-Atlantic and Northeast U.S.

“[It would have] potentially profound consequences to life and property,” the study says. “Any related changes to positions where storms make landfall will have obvious effects on coastal residents and infrastructure. Increasing hazard exposure and mortality risk from tropical cyclones may be compounded in coastal cities outside of the tropics, while possibly being offset at lower latitudes.”

Tom Knutson, a NOAA scientist not involved in the study, says the study is interesting but that verdict is still out as to whether this northward shift continues.

“We can’t regard it as a prediction of what will happen in coming decades,” Knutson says. “It is more a call for further study to understand the changes to date.”

Ryan Maue, a meteorologist at WeatherBell who has published work on global hurricane trends, says the research is “consistent with other climate research” suggesting an “expansion” of the tropics. But Maue also cautioned “our understanding of the behavior of tropical cyclones within our current climate is still incomplete”.

Hurricane activity varies significantly year-to-year owing to a complex array of atmospheric cycles. Human influences can only be detected over long time horizons of at least a few decades.

This year’s Atlantic hurricane season begins June 1. A fairly quiet season is predicted due to the likely development of El Niño, which generates hostile wind shear in the tropical Atlantic. Today, AccuWeather released its hurricane season outlook projecting 10 named storms and four hurricanes, compared to the recent average of 12 and 6, respectively.

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