Warming climate favors shallower cyclones, challenging current risk assessments

As tropical cyclones (TCs) are among the most destructive natural hazards worldwide, understanding how TCs change under climate warming is of critical importance. While substantial progress has been made in projecting changes in TC intensity and precipitation, much less is known about how their vertical structure will respond to a warmer climate.

A study published in Nature Communications offers a new perspective: under extreme warm climates, the proportion of shallow cyclones, characterized by convective updraft maxima and low-pressure anomalies confined to the lower troposphere, increases significantly in the tropics. Notably, the hazards associated with shallow cyclones differ from those of the deep cyclones.

Using a combination of climate model simulations and proxy records, a research team from the Institute of Atmospheric Physics, Chinese Academy of Sciences, finds that during the Early Eocene Climatic Optimum (EECO; 56-48 million years ago), the proportion of shallow cyclones rises to 51.83% in the tropics, outnumbering the typical deep cyclones. This shift is driven by the elevated CO₂ concentrations, which lead to a more stable atmosphere and stronger mid-level ventilation, together limiting cyclone development. 

Do shallower cyclones mean less risk? Not necessarily. While shallow cyclones generally produce weaker winds, their overall contribution to extreme rainfall during the EECO is comparable to that of deep cyclones. “The decoupling of rainfall from wind speed in shallow cyclones is probably driven by the intense warm-rain processes.” said first author Tingyu Zhang.

“This study highlights the necessity of reassessing future cyclone-related hydrological hazards” said corresponding author Tianjun Zhou. “Existing projections rely heavily on upper-atmospheric indicators, a criterion met by deep cyclones but not by shallow ones, so shallow systems are often overlooked. Meanwhile, wind speed is widely used as a measure of TC intensity in current risk assessments. Given decoupling between rainfall and wind speed in shallow cyclones, wind-based metrics tend to underestimate future cyclone-related hydrological hazards.”