If we could somehow transform the earth’s atmosphere and oceans from 1957 to 2017, there still would have been a Hurricane Harvey. It still would have dumped tremendous amounts of rain on Texas, and caused disastrous flooding. But……
1. Would it have been as intense a storm as the 2017 version?
2. Would it have moved as slowly, leading to much more rain?
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3. Would it have had produced as much rain each day?
A new study suggests the answer to all three questions is “NO”.
It is surely no coincidence that Harvey hit at the same time that part of the Gulf of Mexico measured its’ warmest water temperatures ever recorded. The connection between warmer water and more moisture to produce rainfall is clearly established. It’s simple physics.
This new study shows the track of Harvey plotted over the map of Ocean Heat Content (OHC). This is eerily similar to what happened to Hurricanes Katrina and Rita in 2005:
The prime author in the AGU paper, Kevin Trenberth, says:
"Record high ocean heat values not only increased the fuel available to sustain and intensify Harvey," he says, "but also increased its flooding rains on land."
A more intense hurricane means stronger winds. When stronger winds collide (or “converge” in meteorological terms), it leads to more upward motion, which in turn leads to heavier rain.
A hurricane moving over extra warm water has more overall moisture than if the same storm moved over colder water. More potential moisture (“precipitable water”) leads to heavier rain.
And when hurricanes move more slowly, it means more hours of rain, which increases the total. (There is increasing evidence that hurricanes that do develop in the future will have weaker steering currents, meaning slower movement). Even though several studies show fewer hurricanes overall, there would be an increase in the most intense hurricanes. Stronger. Slower. Wetter. A bad combination for future hurricane seasons.