Where are hurricanes likely to increase in coming years?

On May 4, 2020, acclaimed climate researcher Hiroyuki Murakami (Associate Research Scholar from Princeton University and a researcher at the Geophysical Fluid Dynamics Laboratory) released a new study in the Proceedings of the National Academy of Sciences that indicates that since 1980 tropical cyclones (aka, hurricanes) have been increasing in the North Atlantic and Central Pacific, while decreasing in the South Indian Ocean and the Western Pacific. Not only that, but his latest research shows that this shift is not explained by natural variability. The total number of storms does not appear to have changed, but severity has increased.

The National Oceanic and Atmospheric Administration reports that Murakami gives some guidance on what may be influencing these shifts in storm location, such as particulate pollution and volcanic eruptions. For example, he asserts that a large eruption in Mexico in 1982 and one in the Phillippines in 1991 both dramatically cooled the northern hemisphere and shifted the storms towards the southern hemisphere for a period of time. A similar effect is created when increases in particulate pollution are found in upper altitudes.

WHY
Tropical storms are highly influenced by convection air currents, where increased surface temperatures of the ocean cause air to expand, becoming less dense than the surrounding air, and droplets from the ocean surface begin rising rapidly (just as steam rises from a boiling pot). That heated water vapor rises, and then collides with dramatically cooler air at altitude. This lowers the temperature of the droplets at altitude, making them colder and more dense, and causing them to fall as heavy rain. Any paragliding pilot knows the concept of cloud suck, where the strong upward drafts found around cumulus clouds operate in a similar convection manner, and can have deadly results for paragliders. When there is dense particulate matter in the higher altitudes, whether from wildfires or pollution or volcanic ash, it tends to have a heating effect on the higher altitudes and a cooling (i.e., shading effect) on surface ocean temperatures under it. Because particulate matter in the upper altitudes absorbs solar radiation, it reduces the likelihood of dramatic temperature differentials between the cool upper atmosphere and the hot surface ocean temperature. However, because gases in the atmosphere such as CO2 and methane also absorb infrared thermal radiation being released from the earth and prevent its departure from the atmosphere, then those locations that do not have the shading effect of particulate matter in the air end up with even more dramatic temperature differentials and thus more powerful tropical storms. Obviously, as these storms then begin to pass over land they have less hot ocean surface to pull upward-drafting droplets, and thus the differential is reduced and the storm dies out.

THE TAKEAWAY
Nearly 30% of the radiation from the sun is reflected back by ice and clouds, and the remaining 70% is absorbed by oceans and the earth. When that heat tries to release back out of the atmosphere in the form of IR thermal radiation, it is trapped by gases such as CO2 and methane, thus overheating surface ocean temperatures. In areas where there is particulate matter in upper altitudes, convection air currents will be reduced (and thus, tropical cyclones will be reduced). However, that means that increasingly powerful tropical cyclones may be found in zones that do have strong convection air currents fueled by ever-increasing greenhouse gases. With COVID19 shelter-in-place practices, we have seen dramatically decreased particulate matter over China, Los Angeles, Italy, and other zones in the northern hemisphere. Air that is clearer from pollution and particulate matter is undoubtedly a positive thing for general health. And, reducing the gases that typically accompany industrial pollution can allow more IR thermal radiation to escape the "greenhouse" of our atmosphere. Your humble researcher believes this increased release of IR thermal radiation might begin to have a positive net effect on reducing overall average ocean surface temperatures, thus reducing the severity of storms. However, as a side effect, is it possible that with the dramatically reduced particulate matter in some zones, we might actually see increased air convection currents over the oceans of those areas, and thus expect some degree of increase in the number (if not severity) of tropical storms in those zones?

• NOAA.gov
• PNAS
• GFDL
• UIUC
• USHPA
• Science Daily