Background

1. Storm-Induced Circulation on the Meso-American Barrier Reef System during Hurricane Mitch: Coupling Remote Sensing Data and a Nested-Grid Ocean Circulation Modeling System Liang Wang, Chuanmin Hu and Frank Muller-Karger, IMARS, College of Marine Science, University of South Florida, St. Petersburg, FL 33701, USA Jinyu Sheng, Department of Oceanography, Dalhousie University, Halifax, NS B3H 4J1, Canada Serge Andrefouet, Institute de Researche pour le Developpement, BP A5 – 98848 Noumea cedex, Nouvelle Caledonie Bruce Hatcher, Marine Ecosystem Research, Cape Breton University, Sydney, NS B1P 6L2, Canada Fig.1 MBRS Background A triply nested-grid modeling system based on CANDIE Simulated Sea Surface Temperature (SST) and storm-induced SST(T(Clim+Storm) – T(Clim) ) • The Meso-American Barrier Reef System (MBRS) in the northwestern Caribbean Sea is one of large barrier reef ecosystems in the world. • Coral reefs and associated ecosystems in the MBRS provide important habitat, breeding and feeding grounds for a great diversity of marine invertebrates, fish, reptiles and mammals (Kramer and Kramer, 2002). Reefs of the MBRS have been negatively affected by various disturbances and stresses resulting from human activities and natural events in the region over the last 30 years. The outcomes include epidemics of coral disease, coral bleaching, overgrowth by macro-algae, decimation of fish abundances and intense as well as widespread physical destruction. • Understanding cause and effect and predicting impacts demands better knowledge of the interacting physical, ecological and biological processes that connect and sustain the marine ecosystems of the MBRS. • CANDIE stands for Canadian version of Diecast(Sheng et al. , 1998; Lu et al., 2001; Sheng et al., 2001; Zhang et al., 2001; Sheng and Tang, 2004; Sheng and Wang, 2004; Wang et al., 2005): 3D,  z-level model,  implicit free-surface,  fourth-order numerics. • The nested modeling system has three sub-components with different horizontal resolutions: an outer model (20 km), a middle model(6 km), and an inner model(2 km). 28 z-levels: 2 m for top ten levels and gradually increased level thickness to 500 m near bottom. • The semi-prognostic method (Sheng et al., 2001) is used in this study to reduce model drift. • The newly developed nesting technique based on the semi-prognostic methodwas used. • The system is forced by the NCEP wind and idealized wind forcing associated with a moving storm. Fig. 6 Fig. 7 Simulated sea surface salinity and satellite-derived ocean color along the north coast of Honduras The nested modeling system reproduces reasonably well the river plumes due to abnormal precipitation along the north coast of Honduras During Hurricane Mitch. The simulated pattern of river plume characterized by the lower salinity water are very comparable to the pattern characterized by ocean color in SeaWIFS images. Fig.1 Characteristics of the Caribbean Sea • The largest marginal sea of the Atlantic Ocean, separated from the Atlantic basin by an island-studded enclosure (Fig. 1). • Little seasonal variation in surface water temperatures (25 to 28o C). Stratified in the upper 1200 m and vertically uniform below 2000 m. • Under the influence of NA trade winds (westward). • Dominated by a through-flow known as the Caribbean Current in the upper ocean. Nested-grid modeling system Wind forcing associated with a moving storm Fig. 8 Fig. 9 Dispersion of near-surface particles during Hurricane Mitch The particle tracers are seeded initially in four areas near the sea surface of the MBRS before Hurricane Mitch significantly affects the region. The dispersion of particle tracers during Hurricane Mitch demonstrates the important hydrodynamic connectivity of surface waters in the MBRS. Fig. 4 Simulated Currents during Hurricane Mitch MAIN OBJECTIVES of this study are: (a) to develop a nested-grid circulation model for the MBRS; (b) to use the model to predict the effect of a major weather disturbance (Hurricane Mitch) on the circulation and density field in the region; and (c) to compare the predicted outcomes with remotely sensed data for model validation. Fig. 10a Fig. 10b Fig. 10c Fig. 10d Conclusions At 1 m Hurricane Mitch (October – November, 1998) The triply-nested grid model simulation resolves key aspects of circulation of the upper ocean in the west Caribbean Sea in response to Hurricane Mitch in 1998. The nested-grid modeling system generates strong currents on the upper ocean (Fig. 5) and significant sea surface temperature cooling (Fig. 6 and 7) biased to the right of the storm track. The simulated river plume transport characterized by the lower salinity water along the north coast of Honduras (Figs. 8 and 9) agrees well with a satellite-derived analysis by Andrefouet et al. (2002). Dispersion of particle tracers (Fig. 10) demonstrates the important hydrodynamic connectivity of coral reef ecosystems in MBRS during Hurricane Mitch.  Mitch started as a tropic depression on 22 October, 1998 and strengthened to a Category 5 storm by 26 October with the maximum sustained wind speeds of 155 knots (Figs. 2 and 3).  Mitch skirted the coasts of Nicaragua and Honduras, making landfall on 29th October, 1998.  Mitch is among the five strongest storms on record in the Atlantic Basin and generated significant coastal flooding and landslides. Fig. 5b Fig. 5a Fig. 5b Fig. 2 Bibliography At 75 m Andrefouet et al., 2002, Revising coral reef connectivity, Coral Reefs, DOI 10.1007/s00338-001-0199-0 21, 43- 48.  Sheng, J., and L. Tang, 2003, A numerical study of circulation in the western Caribbean Sea, J. Phys. Oceanogr., 33, 2049-2069. • Sheng, J., and L. Tang, 2004, A two-way nested-grid ocean-circulation model for the Meso-American Barrier Reef System, Ocean Dynamics, 54, 232-242. • Tang, L., J. Sheng, B. G. Hatcher, and P. F. Sale, 2006, Numerical study of circulation, dispersion and hydrodynamic connectivity of surface waters on the Belize shelf, J. Geophys. Res., in press. Fig. 5c Fig. 5d Fig. 5c Fig. 3