Hurricane Harvey & The New Normal

There’s a new study up which does something that I hadn’t realized atmospheric physicists had not yet accomplished: equate the energy deposited over land by a hurricane with the energy removed from the ocean by that same hurricane. It seems that one of the reasons that hadn’t yet been done is the need for relatively calm seas in order to get sufficiently accurate measurements of the oceanic heat energy transferred.

That’s always going to be easier in the Caribbean and the Gulf of Mexico than in the open Atlantic, so there aren’t as many storms to try this on as one might think.

You gain a large advantage in your attempt if the storm has enough time in the Caribbean to dump any energy from the Atlantic and make landfall with only energy derived from the Caribbean/GoM. Harvey crossed the entire area on its way to Texas, which takes some time, and the Caribbean waters were not only calmer than the open Atlantic, but happened to be calmer than even the average for the Caribbean. That enabled good measurements of sea temperatures before Harvey passed and after (in order to measure the amount of energy removed you have to know not only the temp at the beginning of the transfer process, but at the end as well).

Scientific American has an interview with Kevin Trenberth, leader of the team that did this study. While the physics is interesting on its own and can probably serve to help refine atmospheric modeling, the sexxxiest finding of the team is that it was able to show that

climate change caused Harvey’s rainfall to be 15 to 38 percent greater than it would have been otherwise.

This dovetails with a great deal of research that predicts that hurricanes may be stronger more often in the future. Specifically, the researchers report that the energy equivalence provides additional support to the idea that warmer waters will fuel more powerful storms, and thus that

climate change is likely to make Atlantic hurricanes bigger, more intense and longer-lasting than in the past.

We’ve long known, of course, that storms generally are mechanisms by which geologically significant heat wells shunt energy to geologically significant heat drains. We’ve further known that one significant aspect of this is related to how earth, rock, and water all transfer heat more slowly than air, which (under the right conditions) can directly mix high energy molecules with low-energy molecules creating much more rapid heat diffusion than is possible for solids or for mixable but slower-moving liquids like water. Since warmer air can hold more water than colder, mixing can easily bring humid air below the temperature where it can hold all the water vapor it contains, resulting in precipitation.

Given all this obvious physics and the basic knowledge – also obvious – that the tropical Atlantic is a geologically significant heat well, it might seem trivial to conclude that higher average global temperatures = > higher average tropical Atlantic temperatures = > higher energy storms. But this isn’t obvious, for a number of reasons.

First, the heat well which initially forms most Atlantic hurricanes is off the western coast of Africa, as far east as the northern end of the Cape Verde Islands, and including ocean a bit north and west from there, though they can also form much farther west*1. If the storms have to pass over enough cold water before hitting the US, any additional heat energy from a storm’s origin point would have been spent over the ocean. In that sense, the storm may become more severe, but it wouldn’t make landfall as more severe, which is the severity that primarily affects humans. This study shows that the Caribbean provides enough energy to storms that warming of Caribbean waters can provide a meaningful enhancement to tropical storms and hurricanes that pass through them.

Second, if the energy removed from the water wasn’t equal to that deposited on land, that might mean that energy was escaping from the storm, perhaps via high-altitude winds. Powerful enough storms might lose more energy through those winds or the atmosphere might change its background conditions in response to climate change in ways that facilitate the escape of energy from hurricanes more easily.

If this had been true, then a hurricane might whip hot water vapor up out of tropical oceans, lose some of that to high-altitude winds, and then hit land with the same force it would have absent systemic changes to climate. Of course, the escaping wind and vapor would have carried a larger amount of precipitation somewhere else, but late summer rains in Mexico or the Plains states (two obvious possible targets) might be welcome, and in any case would no longer be part of the direct effects of the hurricane.

There are any number of other ways in which the simple initial idea, that heat energy from the ocean is distributed in part through hurricanes, can be consistent with an atmosphere that does not produce harder-hitting hurricanes when ocean waters warm under the particular conditions of anthropogenic, fossil fuel sourced, geographically uneven CO2 emissions. But proving that significant amounts of energy aren’t escaping these storms is an important part of increasing our certainty in our predictions that these storms really will grow worse over time. And improvements in both our severity estimates and our certainty in them are vital to properly preparing for our warmer future, given that many buildings designed today will still be standing 50 years in the future.


1: I didn’t know until I was fact-checking myself for this post, but there’s actually a thing called a “Cape Verde Hurricane” in which the initial impetus for a storm forms over Africa and thus is able to start spinning up into a tropical storm and baby hurricane almost immediately after moving over water, thus creating stormy (though not hurricane) conditions directly above the Cape Verde Islands. Though these can be among the most powerful of hurricanes and among the longest lasting, most systems that eventually become hurricanes do not begin over the African landmass and are not this type of “Cape Verde Hurricane”.

Additionally, it should be noted that it’s actually possible for a hurricane to start in the Caribbean, or at least for a previous storm to become a hurricane only after entering the Caribbean, but the region west and over or a bit north of the Cape Verde Islands is the more typical origin point.

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