ECOM-si was first applied to study the cross-frontal water exchange in the inner shelf of the SAB by Dr. Changsheng Chen in 1996. At that time, ECOM-si was the most advanced coastal ocean model that was widely used by scientists in the coastal community. The objective of that modeling work was to study numerically the physical processes associated with the low-salinity front over the inner shelf of the SAB, with particular interests in (1) physical mechanisms responsible for the formation, evolution, and perturbation of the front and (2) cross-frontal water exchanges.
We did experienced difficulties in generating the orthogonal grid to provide a moderate fitting of the coastline and making the high resolution along the coastal region. We have changed several times of our model grids. One shown here is the high resolution one we used in the cross-frontal water exchange study. The horizontal resolution in this grid was about 1 km in the inner shelf and 10 km near the open boundary.
The ECOM-si was forced by semidiurnal tides (M2, S2, and N2), multiple river discharges (10 rivers), and winds. This model has provided a reasonable simulation for the M2 tides and also fortnightly/monthly variation of semidiurnal tides along the coast. The tidal model was only validated using the 3 tidal gage data and a few previous current records in the inner shelf. The good tidal simulation was approached by tuning the tidal forcing at the open boundary.
We examined the physical mechanism responsible for the offshore cross-frontal detachment of low-salinity water in the inner shelf. The offshore detachment of the low-salinity water was well-recognized SAB shelf salinity feature, which was often mis-interpreted to the groundwater by some non-physical oceanographers. Our numerical experiments clearly show that the low-salinity water found in the mid-shelf of SAB comes from the inner shelf due to the detachment of the low-salinity water from the inner shelf low-salinity front. Physical mechanisms controlling this detachment is very complex, which is related to (1) wind speed, (2) amount of river discharges, and (3) tides.
For the case with small river discharges, isolated low-salinity lenses formed episodically in two step. At first, a geometrically controlling wave-like frontal shape developed at the outer dege of the frontal zone as a result of the interaction between tides, multiple river discharges, and upwelling-favorable wind. Then, isolated low-salinity lenses fromed at the crest when water on the shoreward side of the crest is displaced by relatively high salinity water advected from the upstream trough of the crest and diffused upward from the deeper region. In this case, wind-induced upwelling is noticeable to compensate for the water loss due to the near-surface offshore Ekman transport, but it doesn't play critical role in the formation of the lenses.
For the case with large river discharges, stronger wind was required to form the isolated lenses and also tidal mixing acted like a drag forcing to delay the occurrence of isolated lenses. In this case, isolated low-salinity lenses formed due to upwelling and vertical diffusions. |
