Hurricanes can get much, much bigger and stronger than we have so far seen in the Atlantic. The most intense Pacific storm on record was Super Typhoon Tip in 1979, which reached maximum sustained winds of 190 mph near the center. On its wide rim, gale-force winds (39 mph) extended over a diameter of an astonishing 1350 miles. It would have covered nearly half the continental United States.

“More than half the total hurricane damage in the U.S. (normalized for inflation and populations trends) was caused by just five events,” explained MIT hurricane expert Kerry Emanuel in an email. Storms that are Category 4 and 5 at landfall (or just before) are what destroy major cities like New Orleans and Galveston with devastating winds, rains, and storm surges.

In Part 2, we’ll look a little more in detail at Katrina (and Gustav), and why they weren’t (and probably won’t be) as strong and hence as devastating at landfall as they could have been.

But let’s first ask, How did Katrina turn into a powerful Category 5 hurricane ? The National Climatic Data Center 2006 report on Katrina begins its explanation by noting that the surface temperatures (SSTs) in the Gulf of Mexico during the last week in August 2005 “were one to two degrees Celsius above normal, and the warm temperatures extended to a considerable depth through the upper ocean layer.” The report continues, “Also, Katrina crossed the ‘loop current‘ (belt of even warmer water), during which time explosive intensification occurred. The temperature of the ocean surface is a critical element in the formation and strength of hurricanes.”

An important factor was that the ocean warming had penetrated to a considerable depth. One of the ways that hurricanes are weakened is the upwelling of colder, deeper water due to the hurricane’s own violent action. But if the deeper water is also warm, it doesn’t weaken the hurricane. In fact, it may continue to intensify. Global warming heats both the sea surface and the deep water, thus creating ideal conditions for a hurricane to survive and thrive in its long journey from tropical depression to Category Four or Five superstorm.

A 2005 study, “Penetration of Human-Induced Warming into the World’s Oceans,” led by Scripps Institution of Oceanography compared actual ocean temperature data from the surface down to hundreds of meters (in the Atlantic, Pacific, and Indian oceans) with climate models and concluded:

A warming signal has penetrated into the world’s oceans over the past 40 years. The signal is complex, with a vertical structure that varies widely by ocean; it cannot be explained by natural internal climate variability or solar and volcanic forcing, but is well simulated by two anthropogenically forced climate models. We conclude that it is of human origin, a conclusion robust to observational sampling and model differences.

This figure shows what they found:

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