Modeled Ocean Response to Hurricane Katrina

A versatile full MEDiterranean Sea and North Atlantic (MEDiNA) model that uses six nearly seamlessly coupled grids is used to model ocean response to Hurricane Katrina. Grid resolutions vary from 1/24° in the Strait of Gibraltar region to 1/4° in the central North Atlantic. The western North Atlantic, Gulf of Mexico and Caribbean Sea use a 1/8° resolution grid. A very intense nonlinear eddy with Rossby number O(1) is generated. Its scale is 50-100 km and sustained top layer currents as big as 5 m/sec occur. As Katrina approaches the north coast of the Gulf of Mexico, eddy currents more than 4 m/sec extend down to about 50 m depth; currents nearly 2 m/sec extend down to almost 100 m depth; and currents over 1 m/sec extend down to nearly 200m depth. With higher resolution and better air-sea interface parameterization these currents may have been even bigger, but they were strong enough to destroy oil rigs, which is what happened. The strong response to Katrina winds is not surprising in that the currents generated and Gulf of Mexico internal wave speeds are comparable to the Katrina translation speed, which may lead to big energy absorption by a given material element or by a propagating solitary internal wave, somewhat analogous to nonlinear critical layer dynamics. Vertical mode structures having velocity or internal wave phase velocity close to Katrina's translation velocity are especially likely to amplify. However, the flow is very nonlinear and time dependent. The key points are that the response can be huge and nonlinearities can further focus and intensify the response.

Acknowledgement: The support from the Marcelino Botin Foundation, Spain, AcuSea, Inc. and National Science Council, Taiwan (grant number NSC952119M002048) is acknowledged.