2005 LAYERED OCEAN MODEL WORKSHOP

Hurricane passage over ocean represents one of the most extreme atmospheric forcing events of the upper ocean. Upper ocean response to a single hurricane over the open oceans has been studied previously using both observations and numerical methods. However, the hurricane forcing and its interaction with oceanic features near the coastal and marginal seas are not well understood. During the 2002 hurricane season, Hurricanes Isidore and Lili propagated through the Gulf of Mexico following each other in a similar track only a week apart. This tandem hurricane event provides a rare opportunity to study the upper ocean response to the complex atmospheric forcing and the interactions of the two hurricanes with the loop current and warm eddies in the Gulf of Mexico. The objective of this study is to examine the upper ocean circulation in response to Hurricanes Isidore and Lili in tandem and to assess potential feedback to the atmosphere by the complex oceanic features in the Gulf of Mexico. In this study, the full physics Hybrid Coordinate Ocean Model (HYCOM) is used to examine the upper ocean response to the hurricane forcing. The HYCOM domain is a sub-region of the Atlantic basin-scale HYCOM with data assimilation of SST and SSH fields. It is forced by a very high resolution (~1.67 km) atmospheric model simulations of Hurricanes Isidore and Lili, in which the extreme high winds and strong pressure gradients associated with the hurricane eyewall are explicitly resolved. The HYCOM simulated SST is compared with satellite observations from TRMM/TMI and AMSR-E data and in situ measurements from the NDBC buoys. The comparisons show that HYCOM is capable of producing observed fine upperocean features in SST and responses to the hurricane forcing. The upper ocean was cooled significantly by the passage of Isidore and, therefore, the heat content loss was much larger during Isidore than Lili. The Loop Current and warm core rings continue to provide large amount heat during Lili while the Gulf common water heat content loss was close to zero. Although satellite observed SST shows a recovery in a few days after each hurricane, full recovery of the upper ocean in HYCOM takes much longer, which is largely affected by local circulation. One of the uncertainties in the model simulated upper ocean response to hurricane forcing is the representation of the physical processes in the upper ocean. In this study, we examine the performance of HYCOM using different vertical mixing parameterizations in several numerical experiments, including the K-Profile Parameterization, Karus-Turner slab mixed layer model, NASA GISS level 2 turbulence closure, Mellor-Yamada level 2.5 turbulence closure and Price-Weller-Pinkel dynamical instability model. The upper ocean current, SST, and heat content fields are compared with the available, but limited observations under the complex atmospheric forcing.