“Brown Ocean” fuels some hurricanes
If you only look at this radar pattern from Tropical Storm Erin in 2007, you might not think there’s anything unusual about it. It’s clearly a tropical storm, there’s even a well-developed eye.
But if you take a look at the labeled cities on the map, you might note that there’s something odd here indeed. This storm still has a well-developed eye and it is sitting in northern Oklahoma. That’s about 800 kilometers (500 miles) from where the storm made landfall and somehow there is still a developed eye and circulation pattern. That’s at the least odd, and also impressive.
This storm sparked research into exactly this phenomenon. Most people who follow tropical storms on the news are familiar with storms losing strength and collapsing when they hit land. The moisture continues migrating and can cause flooding but the wind speeds and intensity decrease due to the loss of energy supplied by the ocean.
Research by scientists at the University of Georgia published earlier this year might explain this phenomenon. They took a look at the 200+ storms that had made landfall over a 30 year period and found that about 20% actually strengthen after hitting land. Storm weakening is the norm, but there must be a common process allowing for some storms to intensify above land.
Some fraction of these intensifications occur when a storm runs into a different air-mass over continents, as happened in Hurricane Sandy, but that clearly can’t explain a tropical storm eyewall surviving into Oklahoma as the separate airmass tends to break the storm up.
These researchers instead identify another process; storms actually drawing energy from the land, a process they term intensification due to a “brown ocean”. The technical term is “tropical cyclone maintenance and intensification event” or TCMI.
For a storm to draw energy from the land, several criteria must be met. The lower levels of the atmosphere need to be stable, with little temperature variation (no major frontal boundaries coming through). The soil moisture content needs to be high, and the temperatures in the soil need to be elevated so that a lot of evaporation is happening.
When these conditions are met, it becomes possible for a storm to draw enough energy from the land to maintain itself and even intensify, leading to significant impacts and increased chances of flooding inland. It’s rare, happening in less than 10% of tropical storm landfalls, but this radar image testifies to the fact that it does happen.
Also it turns out that this phenomenon appears to be most common in Australia. There may be other factors in play such as the type of vegetation, but as a consequence of the conditions noted above and perhaps other variables; Australia is a hotspot for these TCMI events.
And just as a final weird note, this article about a previous Tropical Storm Erin is being written only a week after there was a 2013 version of a Tropical Storm Erin out in the Atlantic Ocean.
