Dynamics of Estuarine and Coastal Circulations

Estuaries and coastal oceans are of immediate concern to us but are also most challenging places to investigate and make 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) Chesapeake Bay estuary

Chesapeake Bay is a partially-mixed estuary, featuring a two-layer circulation with net seaward motion in a surface layer and net landward flow in a bottom layer. Typical vertical salinity differences of 2-8 stratify the water column, while salinity differences in the longitudinal direction lie between 20 and 30, depending on the fresh water input from rivers.  The tidally-averaged residual flows are of order of 0.1 ms-1. Moreover, the salinity distribution shows large cross-channel variations, especially in the broader reaches in the south end of the Bay. As fresh water plume moves seaward, tending to the Western Shore, high-salinity ocean water moves landward primarily through deep paleochannels. Both tidal and wind mixing play an important role in determining the salinity distribution and residual circulation. Compared with other estuaries, tidal forcing in the Bay is relatively modest with tidal range rarely exceeding 1 m. Winds are episodic with dominant periods of 2-7 days. Northwesterly winds dominate in winter months (November-February) whereas southerly winds of several days each are more frequent in the summer. During the fall transition period, strong wind storms can occasionally destratify the entire water column.

We have applied Regional Ocean Modeling System (ROMS) to the Chesapeake Bay and used it to investigate a number of dynamical processes in this estuary.  The following provides some examples.

a) Tidal dynamics

We have used a three-dimensional baroclinic model to investigate tidal energy flux and dissipation in the Chesapeake Bay. The model currents are validated by a comparison with observed tidal current ellipses collected during previous field surveys. The model elevations are validated against sea-level records collected at tidal gauges. The total amount of tidal energy flux entering the Bay mouth is found to be 177 W, 90% of which is associated with the M2 component. Dissipation of tidal energy is highly non-uniform in the Chesapeake Bay. Most energy dissipation occurs in four topographic hotspots: the Bay mouth region around the headland of Delmarva Peninsula, the region south of the Rappahannock sill, the constriction near the Bay Bridge and the constriction north of Baltimore.

Figure 1. Depth-integrated barotropic energy flux vectors for the M2 constituent and distribution of energy dissipation rate per unit area (b).

b) Hurricane-induced storm surges, currents and destratification

Semi-enclosed bays and estuaries are usually protected from hurricane-generated storm surges. When a hurricane travels on the land side, however, it may induce high storm surges, strong currents and destratification in the water column. Real-time observations and numerical model prediction both show a slab-like sloshing in Chesapeake Bay when it was hit by Hurricane Isabel in September 2003. Strong southeasterly winds in the right front quadrant of the storm forced water in Chesapeake Bay to move northward as a single layer, producing high sea levels and flooding in the northern Bay region including Baltimore and Annapolis. Furthermore, the strong landward winds erased water-column stratification and caused a strong intrusion of high-salinity shelf water into the Bay. After Isabel’s passage, the longitudinal salinity gradient produces restratification and two-layer circulation in the Bay.

Figure 2. Storm surges in Chesapeake Bay and its tributaries. ROMS-predicted sea-level distribution: (a) a 3D view showing high storm surges in the Potomac River and northern Bay at 0400 LST 19 September; (b) comparison of time series of the predicted (black) and observed (red) sea levels at a few selected tidal gauge stations.

Figure 3. Water-column destratification due to wind mixing. Predicted along-channel salinity distributions (a) before (0000 LST 16 September); (b) during (0000 LST 19 September); (c) after (0000 LST 22 September) Isabel’s passage, and (d) predicted non-tidal velocity after Isabel’s passage (0000 LST 22 September).

c) Dynamics of estuarine circulation

As a partially-mixed estuary, the Chesapeake Bay features a two-layer circulation. The ROMS model reproduces this circulation pattern. Figure 5 shows distributions of salinity and residual velocity along the axis of Chesapeake Bay (see Figure 1c for the geographic location of this longitudinal transect). Both salinity and longitudinal velocity are averaged over the month of April to filter out the tidal effects. Isohalines in the upper reaches of the estuary appear to be vertical, reflecting strong vertical mixing in this shallow region (see Figure 9 for model-inferred eddy diffusivity). South of 39.3 latitude, isohalines slope upward toward the sea, as is shown in Figure 5a. Due to strong tidal mixing in the bottom boundary layer, isohalines are nearly vertical near the bottom. Top-bottom salinity differences in the middle and lower reaches of the Bay vary between 5 and 8, while the head-mouth salinity difference lies between 20 and 25. The longitudinal salinity difference in the deep channel (between 37.7 and 39 latitudes) is around 10. Figure 5b shows the axial component of the residual velocity. Water moves seaward in the surface layers and landward in the bottom layers. Maximum residual velocity is about 0.2 ms-1. The level of no motion separating the two counter-flowing layers lies somewhere between 5 and 8 meters. The landward flow in the deep channel reaches the maximum speed at a depth of 15 meters, well above the bottom.

(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. 

Figure 8. A map of South China Sea.

Figure 9. Simulated sea level and currents in South China Sea.

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.

Publications:

Li, M., L. Zhong, W. C. Boicourt, S. Zhang and D. Zhang. 2007. Hurricane-induced destratification and destratification in a partially-mixed estuary. Journal of Marine Research, 65, 169-192.

Li, M., L. Zhong, W. C. Boicourt, S. Zhang and D. Zhang. 2006. Hurricane-induced storm surges, currents and destratification in a semi-enclosed bay. Geophysical Research Letters, 33, L02604, doi:10.1029/2005GL024992.

Li, M., L. Zhong, and W. C. Boicourt. 2005. Simulations of Chesapeake Bay estuary: Sensitivity to turbulence mixing parameterizations and comparison with observations, J. Geophys. Res., 110, C12004, doi:10.1029/2004JC002585.

Zhong, L. and M. Li. 2005. Tidal energy fluxes and dissipation in the Chesapeake Bay. Cont. Shelf Res., In review.

Kana, T. M., Cornwell, J. C, and L. Zhong. 2005. Determination of denitrification in the Chesapeake Bay from measurements of N2 accumulation in bottom water, accepted by Estuaires and coasts.

Hsueh, Y., and L. Zhong. 2004. A pressure-driven South China Sea Warm Current. J. Geophys. Res., 109, C09014, doi:10.1029/2004JC002374.

Hsueh, Y., and L. Zhong. 2003.  A note on the deflection of a baroclinic current by a continental shelf. Geophys. Astrophy. Fluid Dyn., 97(5), 393-415.

Li, M., A. Gargett and K.L. Denman. 1999. Seasonal and interannual variability of estuarine circulation in a box model of the Strait of Georgia and Juan de Fuca Strait. Atmos-Ocean, 37, 1-19.