 one-way nested Western Atlantic-Gulf of Mexico-Caribbean Sea regional domain (with data assimilation of SSH and SST prior to hurricane simulations)  0.08 degree grid spacing  28 Vertical layers, 7-9 layers in mixed layer with the 1st layer at 3 meter  MM5 hourly atmospheric forcing from 08 September – 17 October 2002 (pre-, during- and post- Isidore and Lili)  KPP vertical mixing scheme (KT, M-Y, and PWP for sensitivity experiments) Upper Ocean Response to 2002 Hurricanes Isidore and Lili in Tandem Using HYCOM Wei Zhao and Shuyi S. Chen RSMAS/University of Miami 2. HYCOM CONFIGURATION AND FORCING Storm tracks and intensity (maximum surface wind speed) for Hurricanes Isidore (left) and Lili (right) from MM5 simulation (blue) and best track (red) SEP18SEP08OCT04SEP27SEP30OCT17 Hurricane IsidoreHurricane Lili 3. SST COOLING Satellite observations from a blended TRMM TMI and AMSR-E product show that maximum SST cooling caused by Isidore is close to 4°C over the Yucatan Shelf, while SST cooling is small (~1°C) over the Loop Current because of the deep ocean mixed layer. SST cooling caused by Lili is mainly after passing the Loop Current. HYCOM (with KPP) reproduced the cooling patterns, but overestimated the magnitude of the SST cooling. SST before Hurricanes (left) and SST cooling after Hurricane Isidore (middle) and SST cooling after Hurricane Lili (right) from the blended TRMM TMI / AMSR-E (top) and HYCOM simulation (bottom). AB Satellite HYCOM B: Loop CurrentA: Gulf Common Water IsidoreLiliIsidoreLili 4. UPPER OCEAN THERMAL RESPONSE Time series of observed SST and model simulated upper ocean temperature profiles are selected to compare the different ocean response over the Gulf common water (point A) and the Loop Current (point B). SST cooling after Isidore and Lili shows similar magnitude (close to 2°C) at point A. Lili had very little effect on SST at point B. HYCOM simulated evolution of upper ocean temperature profiles show that the depth of the mixed layer is about 30m over the Gulf common water before the storms, whereas Loop Current is 60m. Isidore passed B as a much stronger storm than when over A, which explains the large cooling at B. Lili had similar intensity at A and B, but induced very little cooling at B because of the deeper mixed layer of ~120m. The high-frequency oscillation in temperature is due to the inertial currents induced by hurricanes. Time series SST from blended TRMM TMI / AMSR-E (top) and SST profile from HYCOM simulation (bottom) at 2 select points which represent Gulf Common Water (left) and Loop Current (right). 1. INTRODUCTION Hurricane passage over ocean represents one of the most extreme atmospheric forcing events of the upper ocean because of the large heat and momentum fluxes. Upper ocean response to a single hurricane had been studied previously using both observations (Shay et. al., 2000; Jacob et. al. 2000) and numerical model simulations (Price et. al., 1981, 1994). 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. The main objectives of this study are to 1) examine the upper ocean circulation in response to Hurricanes Isidore and Lili in tandem, 2) assess potential feedback to the atmosphere by the complex oceanic features in the Gulf of Mexico, and 3) examine the mixed layer physics in HYCOM with different mixing schemes under extreme and complex atmospheric forcing (including diurnal heating/cooling and wind gusts). Hurricanes Isidore and Lili (2002) in tandem 6. SUMMARY  HYCOM produced satellite observed upper ocean features (including the Loop Current and eddies) and oceanic responses to hurricanes reasonable well.  The second storm (Lili) in tandem induced relatively less SST cooling because of the deepened mixed layer by the first storm Isidore), especially over the Loop Current.  All mixing schemes, except KT, produce similar horizontal cooling patterns, but vary in magnitudes.  Different mixing schemes produced very different mixing depth and vertical structure of upper ocean circulation (mostly inertial currents and vertical shear). ACKNOWLEDGEMENTS We thank Drs. Alan Wallcraft and George Halliwell for help in the initial set up of the HYCOM simulation. This work is supported by a grant from the Office of Naval Research under the CBLAST grant N SENSITIVITY TO VARIOUS MIXING SCHEMES IN HYCOM KPP mixing scheme PWP - KPP MY - KPPKT - KPP Surface Temperature and Current Post-Isidore KPP mixing schemePWP - KPP MY - KPPKT - KPP Surface Temperature and Current Post-Lili KPP mixing schemePWP - KPP KT - KPPMY - KPP Temperature Profile at 88W 25N KPP mixing schemePWP - KPP KT - KPPMY - KPP Current Shear Profile at 88W 25N 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, the upper ocean circulation in HYCOM is examined using four different vertical mixing parameterizations in several numerical experiments, including the K-Profile Param., Karus-Turner slab mixed layer model, Mellor-Yamada level 2.5 turbulence closure and Price-Weller-Pinkel dynamical instability model. Results show that although all mixing schemes, except KT, produce similar horizontal cooling patterns, different schemes produces different cooling magnitude, mixing depth, and vertical structure of upper ocean circulation. Some of the differences may be related to sensitivities to shear profiles.