https://x.com/nbergwx/status/1843444771135861007?s=46&t=9FPxCfjU5uuRXH3QXtrs8w
From this tweet. Additional, how would we know, and how would this be a stationary target given global warming or general changes?
In: Planetary Science
So, the tweet in question is likely referring to the conceptual model of hurricanes as being [a Carnot heat engine.](https://en.wikipedia.org/wiki/Carnot_heat_engine) Basically, this is a system that converts thermal energy into kinetic energy. In a hurricane, this translates to the conversion of heat from warm, moist tropical oceanic air into kinetic energy (wind). So, you start out by having a weak low pressure center. Air spirals inward from high pressure to low pressure. As air spirals inward towards a central point, it converges and mechanically forces upward motion. Warm, moist air is less dense than cool, dry air, so the warmer & moister the converging air, the more likely it vigorously rises. As air rises, it leaves behind a “hole” that needs to be filled, causing more air to rush in to fill that gap and spiral inward. A lower pressure at the center, caused by this vigorous upward displacement of air, results in a stronger gradient of pressure between the center and the surrounding environment which in turn drives more vigorous winds.
Tying it all together, we can see that the temperature and moisture content of air is related to its ability to be displaced upward. The warmer and moister the air, the more likely it is to rise without facing any strong opposition. As upward motion is enhanced, it leaves behind a bigger “hole” that needs to be filled (low pressure center). The bigger the hole, the faster air will rush in to fill it (winds). Some of the energy associated with winds are dissipated by surface friction. So long as there isn’t anything to mess with this process, the hurricane will continue to intensify until it reaches a point where the conversion of thermal energy into kinetic energy is balanced by the dissipation of kinetic energy by surface friction. This point would be the theoretical maximum strength of a hurricane. Warmer sea surface temperatures allow for warmer, moister near-surface air. So the warmer the ocean, the higher the theoretical maximum strength of a hurricane.
However, I have to stress that this assumes nothing exists to mess with the process. Within the context of climate change, there are external factors (such as an increase in wind shear – how the speed and direction of winds change with height) that could act opposite increasing ocean temperatures. This means that even though ocean temperatures are warmer, which would theoretically act to support stronger hurricanes, if wind shear were to increase as well it would inhibit this potential for hurricanes to become stronger. We don’t understand very well how hurricane frequency will change in a warmer climate because of this. We do have moderate confidence that they will likely become more intense.
A more involved explanation of this process (and an equation that estimates the theoretical upper bounds of a hurricane’s strength) can be found in this [great writeup](https://pubs.aip.org/physicstoday/online/12200/How-strong-can-a-hurricane-get) by Falko Judt at the National Center for Atmospheric Research.
EDIT: u/onewhitelight provided an actual ELI5 explanation that is worth repeating here: The low pressure center of a hurricane is like a valley. The lower the pressure, the “steeper” the valley. A steeper valley means objects can more quickly roll down the slope towards the bottom (think of this as the wind). When the ocean is warmer, it acts to support a “steeper” valley. However, the rate at which you descend into the valley eventually evens out. You can keep going faster and faster but eventually your acceleration will be balanced by restoring forces (friction) and you’ll reach a constant velocity.
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