Estuaries and coastal oceans are of immediate concern to us but are also most challenging places to make model predictions. They are constrained by irregular coastlines and variable bathymetry, and forced by a complex array of tidal, wind and buoyancy forces on a broad range of space and time scales. Inhabited in these dynamic ocean regions are complex and diverse marine ecosystems that are being threatened by human activities. Given the complex nature of the coastal ocean, we are developing a hierarchy of numerical models, ranging from simple box models to full-blown three-dimensional hydrodynamic models. 

(1) The Chesapeake Bay estuary

We are applying Regional Ocean Modeling System (ROMS) to the Chesapeake Bay and coupling it to a simple marine ecosystem/water quality model. The Chesapeake is made of a mainstem and a number of tributaries. The mainstem is wide and shallow, with a mean water depth of 6.5 m. However, a deep channel running in the north-south direction dominates the bathymetry in the middle reaches of the main Bay. Eight major tributaries (Susquehanna, Patapsco, Patuxent, Potomac, Rappahannock, York, James and Choptank) contribute to most of the river input to the Bay estuary. Tidal forcing is modest inside the Bay with tidal range rarely exceeding 1 m. Winds are episodic but an important mixing mechanism. The Chesapeake Bay is a partially-mixed estuary. As fresh water in the upper layer flows seaward, saline water in the lower layer flows landward. In particular, there is a persistent, landward return flow in the deep channel. ROMS is well suited as a hydrodynamic model for the Chesapeake Bay. It allows high resolution in the surface and bottom boundary layers and incorporates versatile turbulent mixing schemes. 

In this project we are collaborating with Drs. Raleigh Hood and Bill Boicourt at Horn Point Lab, scientists and managers at NOAA NERR sites.

 

 

(2) Georgia-Fuca estuary

The Strait of Georgia and Juan de Fuca Strait are two coastal ocean basins situated between Vancouver Island and the mainland coasts of British Columbia and Washington State. The two straits are connected through Haro Strait-- a narrow channel with sills at its southern and northern ends. A primary forcing of this system is the fresh water runoff from the Fraser River, which peaks during the summer freshet. A plume of brackish, silt-laden water is often seen to spread over the oceanic water in the southern part of the Strait of Georgia. The estuarine flow is characterized by a seaward outflow in the upper portion of the water column and a landward inflow below.

 

                       

 

We have developed a box model for the Georgia-Fuca estuarine system. We divide this system into three basins: Georgia basin, Haro Strait and Fuca basin, each of which consisting of an upper box and a lower box.  The main exchange between the estuary and the Pacific Ocean is at the western end of the Juan de Fuca Strait.

 

 

 

The following figure gives an example of multi-year simulations of the box model. Both the river off (top panel) and the salinity in the Pacific Ocean (second panel) exhibit year-to-year fluctuations. These fluctuations transmit to the Georgia-Fuca estuary, as shown in the time series of box salinities (third panel, green lines for Georgia boxes, red lines for Haro boxes and blue lines for Fuca boxes). The bottom panel simply shows that the salinity budget in the estuary remains balanced every year. Our model reveals a rapid response of the estuarine circulation to interannual variability in the fresh water forcing and in the properties of the continental shelf water. Hence the Georgia-Fuca estuary will respond passively to the large-scale climate variability in the North Pacific. 

 

(3) South-China Sea

The South China Sea (SCS) is a semi-enclosed marginal sea in the western North Pacific Ocean. The circulation of the northern SCS is strongly influenced by the Kuroshio, the western boundary current of the North Pacific, which frequently intrudes into the northern SCS through the Luzon Strait, the only deep passage connecting SCS to the open Pacific. The intruded Kuroshio moves westward and collides with the Dongsha Islands off the China coast. The collision forces most of the flow to return as a strong return flow to the north and a splinter current to the southwest along the upper continental slope. The return loop occasionally spawns anti-cyclonic rings which are instrumental in transporting tropical heat and salt into the northern SCS. The splinter current called South China Sea Branch of the Kuroshio (SCSBK), on the other hand, feeds a northeastward shelf-break current that arises due to the alongshore pressure variation imposed offshore by the collision action, at the Dongsha Islands, of the intruded Kuroshio. 

 

The GFDL ocean model is applied to study the circulation in the northern South China Sea and it reproduces the main features. Model output shows the existence of step-like pressure distribution along the shelf break. In a reduced-gravity inviscid model ocean, the step-like pressure distribution results from the deflection of a constant-potential vorticity current at a step-shelf fronted coast. This step-like pressure distribution accounts for the year-round existence of the northeastward shelf-break current (the so-called South China Sea Warm Current). A SCRUM model demonstrates that SCSBK feeds the northeastward shelf-break current all along the shelf-break, making the latter a warm current.