A new law of hurricane formation

Published 24 February 2010

Robert Ehrlich, a physicist at George Mason University, offers a new mathematical model of hurricane formation which appears finally to solve one of the outstanding puzzles of climate change; the model also predicts dramatic increases in the number of storms as the world warms

 Meteorologists have long known that two factors play crucial roles. First, the temperature of the sea determines the updraft of air that leads to a storm. Second, the latitude governs the strength of the Coriolis Force which triggers the initial vorticity of the storm (which is why hurricanes do not form at the equator and rotate in opposite senses in each hemisphere).

Technology Review reports that Ehrlich shows how these two variables alone can account for the probability density of a hurricane or tropical storm forming. No other factors need be taken into account.
Ehrlich creates an elegant mathematical model of the system that relies on only two variables: the temperature of the sea above a threshold of 25.5 degrees C and the latitude of the ocean at that point. He then fits the function to the data from real hurricanes, that is, sea surface temperatures and latitude data from satellite images from 1960 until 2007. This determines that the power law has an exponent of 3.5 for most parts of the globe.
Technology Review notes that fitting the data to a curve by no means proves that a model is correct, but Ehrlich is able to make some interesting observations using it. One problem that climatologists have puzzled over in recent years is that the number of hurricanes have increased in the north Atlantic but not in the Pacific, despite similar temperature increases. Many say that this is proof that other factors must influence hurricane formation.
There is, however, an important difference between these regions: in the Atlantic, the water tends to be cooler to start with and the hurricanes tend to form at a slightly higher latitude. “When you take this into account, the difference in the number of hurricanes is exactly what Ehrlich’s model predicts, Technology Review comments.
Ehrlich says the specific form of his mathematical model “yields larger percentage increases when a fixed increase in sea surface temperature occurs at higher latitudes and lower temperatures.” This could help to solve an important climate change puzzle — but before greater reliance can be placed on Ehrlich’s, we should remind ourselves that scientific theories should do three things: describe, explain, and predict. Ehrlich’s elegant theory needs to show its utility by accurately forecasting the numbers of hurricanes in the next few years.
The model’s predictions make for disturbing reading. Recall that the exponent in Ehrlich’s power law is of 3.5. This means that the number of hurricanes should increase sharply as the climate warms, and much more dramatically than climatologists have been expecting. How dramatic? Ehrlich predicts that a 2 degree C increase in average temperature will lead to an 11-fold increase in the number of hurricanes.
The increase in numbers of hurricanes is only part of the story, he says. “An eleven-fold increase in hurricanes at a particular location would just be one part of the story, which would include (1) a potentially larger increase in the total number of hurricanes given the increase in the size of the basin as temperatures rise, (2) an increase in the destructive potential of each hurricane, and (3) an increase in the height of the storm surge due to rising sea levels that would invariably occur in a warmer world.”
-read more in Robert Ehrlich, “A Universal Hurricane Frequency Function,” arXiv:1002.3291v1 (17 February 2010)