By GCOOS Members Nan Walker and Alaric Haag (LSU Earth Scan Laboratory, Department of Oceanography and Coastal Sciences, College of the Coast and Environment) and Bob Leben (Colorado Center for Astrodynamics Research, University of Colorado Boulder)

On Aug. 23, Harvey moved over waters of the southwestern Gulf of Mexico as a tropical depression with a maximum sustained wind speed of 30 knots at 10 p.m. CDT (03 Zulu). Just 24 hours later, Harvey’s winds were 75 knots — a Category 1 hurricane. Another 24 hours later — just before it hit the south Texas coast — Harvey’s maximum wind speed reached Category 4 status with 115-knot winds.

SeaSurfaceTemp-HurricaneHarvey

Sea surface temperatures during Hurricane Harvey.

Satellite data provide valuable information to help understand Harvey’s rapid strengthening. These graphics show Harvey’s track, 6-hourly positions and maximum sustained wind speeds in knots superimposed on satellite imagery of sea surface temperature (SST) and sea surface height (SSH).

 

SeaSurfaceHeight-HurricaneHarvey

Sea surface height during Hurricane Harvey.

As Harvey crossed the Gulf of Mexico from Mexico to south Texas, it spent most of its time over a very large area of relatively high SSH (above 0 cm) and a warm core eddy (> 10 cm) in the western Gulf of Mexico. Harvey’s wind speeds began to increase soon after it crossed over the southern boundary of positive SSH. Harvey’s strength climbed steadily over the large area of positive SSH.

In these areas, we can expect abnormally high heat content as the warm water extends much deeper in the water column than in the surrounding areas. Tropical storms and hurricanes are usually impacted by the upper ~50-100 m of the water column. The warmer the oceanic heat content under its track, the more likely a storm will intensify.

The atmosphere also plays a critical role, and conditions of low vertical wind shear and high water vapor enhance intensification. The satellite-derived SST within the eddy ranged from 28.5-29.5 C (83-85 F). You may notice that most of the Gulf of Mexico exhibited a similarly high SST. Notice also that there was another warm eddy (see 10 cm line to the east of track) in the central Gulf, which may have had an additional impact since these storms extract heat from a very large area.

In the eastern Gulf (too far to be of major impact to Harvey), the Loop Current exhibited extremely high SSH and, thus, high oceanic heat content. Storms and hurricanes tracking over the Loop Current often intensify rapidly and both Hurricanes Katrina and Rita in 2005 intensified over it.

Our preliminary assessment from these oceanic datasets is that Harvey tracked over a large area of positive SSH and a warm core eddy as well as high SST, which provided an extreme amount of heat through evaporation and later condensation that allowed the rapid intensification of Harvey from a tropical storm to a Category 4 hurricane in less than 48 hours.