At 11:45 AM EST today Tropical Storm Bill slugged its way over the Texas Coastline near Matagorda Island. The storm, packing sustained winds of 60 miles per hour and a minimum central pressure near 997 mb was relatively mild as Tropical Cyclones go. But Bill is heavily entrenched in a long train of tropical moisture straddling the Gulf of Mexico and flooding up from an intensifying Pacific El Nino. It therefore represents an extreme flood risk for a massive region stretching from Texas through a good chunk of the Central US.
(Bill makes landfall along Texas’s Central Gulf Coast dragging a huge train of thunderstorms along with it. Recent extreme floods have saturated the lands of Texas, Oklahoma and the Central US creating a condition that NASA researchers now call a Brown Ocean. The water saturation of the land mass due to extreme rainfall events and increased atmospheric moisture loading associated with climate change is a condition that some scientists believe may increase the likelihood of tropical storms, like Bill, intensifying over land. Image source: NOAA.)
As Bill moves northward, it is expected to pull this massive band of moisture behind it. The result is that areas of Texas already saturated with moisture from last month’s heavy rains could see 6-10 more inches in a broad band and greater than 12 inches locally near the San Antonio and Dallas region. Bill is projected to then sweep northward through Oklahoma and on through a wide crescent of the Central US — dumping 2-6 inches of rain with locally as much as 8 inches directly along its path.
Such heavy rainfall and thunderstorms associated with Bill have the potential to set off a repeat of the kind of epic deluges this same region witnessed over Memorial Day. And due to the fact that grounds are already saturated and many streams remain near flood stage, this particular event has a high risk of producing even more extreme flooding.
(Bill shows extreme rainfall potential over areas still recovering from record flooding late last month. Image source: National Hurricane Center.)
Bill and Global Warming’s Brown Ocean
This extreme rainfall potential arises from a combination of factors. The first is the added moisture loading over the region due to El Nino combined with the record high global temperatures of human caused climate change — which increases the atmosphere’s ability to carry water vapor and accelerates the hydrological cycle. The second is a related potential feature likely linked to this extra moisture — a circumstance that scientists have called ‘the Brown Ocean.’
The 2013 NASA Brown Ocean study showed that:
A Brown Ocean environment consists of three observable conditions. First, the lower level of the atmosphere mimics a tropical atmosphere with minimal variation in temperature. Second, soils in the vicinity of the storms need to contain ample moisture. Finally, evaporation of the soil moisture releases latent heat, which the team found must measure at least 70 watts averaged per square meter. For comparison, the latent heat flux from the ocean averages about 200 watts per square meter.
(Since 1979 16 Tropical Cyclones have maintained TC characteristics while intensifying or keeping a steady strength over land. Bill has a potential to become one of these freakish systems. Image source: NASA.)
Brown Oceans can thus form over areas that have received extremely heavy rainfall and are experiencing hot, moist tropical conditions. The result is increased evaporation that mimics features similar to those of a warm sea surface. In such cases, Tropical Cyclones can intensify over land due to the effect of the extra moisture bleed-off. And it is these conditions that atmospheric scientists are warning now predominate over Texas:
“All the things a hurricane likes over the ocean is what we have over land right now,” said Marshall Shepherd, director of atmospheric sciences at the University of Georgia and one of the leads of a NASA-funded Brown Ocean study.
It’s worth noting that Brown Oceans have not been officially linked to human caused climate change. But the factors that feed Brown Oceans — high heat and humidity that is the upshot of very extreme rainfall events — are multiplied in a warming world. And it’s this kind of moist hot zone that Bill is now barreling toward.