GFDL - Geophysical Fluid Dynamics Laboratory

Dominant Effect of Relative Tropical Atlantic Warming on Major Hurricane Occurrence

September 27th, 2018

Hiroyuki Murakami, Emma Levin, Thomas L. Delworth, Richard Gudgel, and Pang-Chi Hsu. Science. DOI: 10.1126/science.aat6711

The 2017 hurricane season in the North Atlantic Ocean was highly active, with six major hurricanes (MH). Three storms made landfall (Hurricanes Harvey, Irma, and Maria) – causing widespread damage over the Gulf Coast and the Caribbean.

A number of factors might be linked to this enhanced MH activity in 2017, including moderate La Niña conditions in the Pacific. Unusually warm sea surface conditions over the tropical Atlantic and off the coast of North America were also observed. GFDL’s global coupled model HiFLOR successfully predicted the very active MH season in real-time seasonal predictions. Results of additional experiments using HiFLOR determined that the unusual warmth in the tropical Atlantic was the main cause of the active season. The key factor controlling MH activity appears to be how much the tropical Atlantic warms relative to the rest of the global ocean, rather than absolute warming in the North Atlantic alone.

The authors also used these model experiments to determine possible future changes in MH activity given similar summer conditions, but with increased anthropogenic forcing. In the future, a similar pattern of the 2017 global surface temperature anomalies, superimposed upon long-term increasing sea surface temperatures from increases in greenhouse gas concentrations and decreases in aerosol loading, will likely lead to even higher numbers of MHs. Thus, continued anthropogenic forcing has the potential to further amplify the risk of MHs in the North Atlantic, with corresponding socio-economic implications.

Major hurricanes are one of the most damaging and deadly natural disasters, and are an energetic element of the climate system. Understanding the character and causes of variations and changes, including potential influences of anthropogenic climate changes, is central to NOAA’s mission and highly relevant to society.

Left: Prescribed SST anomalies in the idealized model including 2017 SST anomalies but with the Pacific SST anomalies removed. Right: Predicted major hurricane anomalies using the modified SST as shown in left, showing active major hurricanes even after removing moderate La Niña conditions in the Pacific Ocean. This result highlights that the moderate 2017 La Niña conditions in Pacific does not play an important role for the active major hurricanes in Atlantic. Units: number per 2.5°x2.5° grid cell per season. Dots indicate statistically significant changes from climatological mean.
Left: Prescribed SST anomalies in the idealized model including 2017 SST anomalies but with the tropical Atlantic SST anomalies removed. Right: Predicted major hurricane anomalies using the modified SST as shown in left, showing inactive major hurricanes after removing the tropical Atlantic SST anomalies. This result highlights that the warmer tropical Atlantic does play an important role for the active major hurricanes in the Atlantic Ocean. Units: number per 2.5°x2.5° grid cell per season. Dots indicate statistically significant changes from climatological mean.
Left: Prescribed major hurricane density anomaly in the future RCP4.5 experiment given the similar 2017 SST anomaly pattern in addition to the mean future changes in SSTs. Rights: As in left, but for RCP8.5 experiment. These results indicate more active major hurricanes in the future even with the similar 2017 SST anomaly pattern. Units: number per 2.5°x2.5° grid cell per season. Dots indicate statistically significant changes from climatological mean in the future.