Oceanography

to the south then westward over the mountainous regions of Honduras and Guatemala. During its passage, Mitch generated significant precipitation over much of Nicaragua, Honduras, and Guatemala, which in turn caused flooding and land slides and massive river discharge to the adjacent coast .

1] The passage of category-5 Hurricane Mitch through the Mesoamerican Barrier Reef System (MBRS) in October 1998 was an extreme event with the potential to create unusual patterns of reef connectivity. The impact of this hurricane on the upper ocean of the MBRS is investigated using a triply nested grid ocean circulation modeling system. The model results are validated with contemporaneous ocean color data from the Seaviewing Wide Field-of-view Sensor (SeaWiFS) satellite and oceanographic measurements in the MBRS. The nested grid system is forced by 6-hourly National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) winds for the first 294 days prior to the arrival of the hurricane in the MBRS, and then by the combination of the NCEP/NCAR wind-forcing and an idealized vortex representative of Mitch for the following 20 days. The system is also forced by the monthly mean sea surface heat and freshwater fluxes and buoyancy forcing associated with major river discharges and storm-induced precipitation in the western Caribbean Sea. The simulated upper ocean circulation during Mitch is characterized by strong and divergent currents under the storm and intense near-inertial currents and sea surface temperature cooling behind the storm. The nested grid system also reproduces the buoyant estuarine plumes extending from the coast off Honduras as inferred from SeaWiFS satellite data and detected in field measurements at Gladden Spit in Belize shortly after the passage of Hurricane Mitch. The present model results suggest that populations of site-attached organisms associated with nearshore and offshore reef features that are dynamically isolated in normal conditions experienced greater potential for ecological connection under Mitch's extreme conditions.

The Bras d’Or Lakes (BdOL) are a large, complex and virtually land-locked estuary in central Cape Breton Island of Nova Scotia and one of Canada’s charismatic ecosystems, sustaining ecological and cultural communities unique in many aspects. The BdOL comprise two major basins, many deep and shallow bays, several narrow channels and straits and a large, geologically complex watershed. Predictive knowledge of the water movement within the estuary is a key requirement for effective management and sustainable development of the BdOL ecosystem. A three-dimensional (3D) primitive-equation ocean circulation model is used to examine the estuary’s response to tides, winds and buoyancy forcing associated with freshwater runoff in a series of numerical experiments validated with empirical data. The model results generate intense, jet-like tidal flows of about 1 m s−1 in the channels between the basins and connecting them to the ocean and relatively weak tidal currents in other regions, which agrees well with previous observations and numerical results. Wind forcing and buoyancy forcing associated with river runoff play important roles in generating the significant sub-tidal circulations in the estuary, including narrow channels, deep basins and shallow bays. The circulation model is also used to reconstruct the 3D circulation and temperature-salinity distributions in the summer months of 1974, when current and hydrographic measurements were made at several locations. The sub-tidal circulation in the estuary produced by the model is characterised by wind and barometric set-up and set-down in different sections of the system, and a classic two-layer estuarine circulation in which brackish, near-surface waters flow seaward from the estuary into the Atlantic Ocean, and deep salty waters flow landward through the major channel. The model results reproduce reasonably well the overall features of observed circulation and temperature-salinity fields made in the BdOL in 1974 but generally underestimate the observed currents and density stratification. The model discrepancies reflect the use of spatially mean wind forcing and spatially and monthly mean surface heat flux and the inability of the coarse model horizontal resolution (∼500 m) to resolve narrow channels and straits.

The Bras d'Or Lakes (BdOL) is a land-locked estuarine system in central Cape Breton Island. Nova Scotia, Canada. Recent outbreaks of waterborne diseases at several sites in the BdOL have decimated shellfish populations and threaten its unusual biodiversity. Assessing the potential for spread of invasive pathogens is a priority for management. A nested-grid hydrodynamic modeling system is used to simulate the three-dimensional (3D) circulation and temperature-salinity distributions for summer 1974, when currents and hydrographic measurements suitable for model calibration were made at several locations. The modeled 3D velocity fields are then used in Eulerian-Lagrangian transform to study the spatial patterns of retention and dispersion of passive particles, as a proxy for the spread of disease by water flow in the estuary. Probabilities of transfers (connectivity) among 10 sub-domains of the BdOL are calculated from the statistics of the 3D particle trajectories and portrayed in a transition matrix. The model results demonstrate that the hydrodynamic connectivity among the several small bays in the western portion of the BdOL (ranging from 0.1% to 13.5%) is much weaker than those among the main basins (16% to 62.4%) at monthly time scales. Restriction by narrow passages between the small bays and the main basins reduce connectivity among populations and their habitats, but the effect is partially offset by density-driven flows associated with freshwater inputs at the land boundaries. The model can be used to guide epidemiological surveys and generate testable hypotheses of disease spread and species invasions within the ecosystem.