Abstract: The water exchange and its spatial and temporal variation in the Xiangshangang Bay were studied with an advection-diffusion water exchange numerical model. The model is based on the well-verified three-dimensional baroclinic model for hydrodynamic process and dissolved conservative material was used as the tracer. The initial tracer concentration was set to 1 mg/L inside the bay and 0 mg/L outside the bay and at the open boundaries inflow. According to the tracer concentration, half-life time and mean residence time of the water mass at different points of the bay were calculated, and the contribution of baroclinic power to the water exchange was studied by comparison with that at barotropic model. The results show that the water exchange, to the east of the Xize section, is fast with a half-life time of 5 days and a mean residence time of about 15 days. While the half-life time and mean residence time to the east of the Xihugang inlet are 15~20 days and 25 days separately. And the section near the Wusha Mountain has a half-life time of about 25 days and a mean residence time of about 30 days. Moreover, water exchange in the top of the bay is even slower, with a half-life time of 30~ 35 days and a mean residence time of 35~ 40 days in Tiegang harbor and Huangdungang harbor. The result shows that, both the half-life time and mean residence time increase gradually from the mouth to the top of the Xiangshangang Bay, suggesting the water exchange rate and governing mechanism differ spatially in the Bay. The region between the mouth of the bay and Xize is an area of strong tidal current, where the water flows out with the ebb current and mixes with the water outside, then flows back into the bay with flood current. So the exchange rate here is quite high, and the water closer to the mouth is mixed more fully. However, the inner part of the Xiangshangang Bay is dominated by gravitational circulation and tidal oscillation, where the mixing is much weaker and the period of water exchange is much longer than that in the outer part of the bay. The comparison of the water exchange in barotropic model with that in baroclinic model shows that the baroclinic hydrodynamic power has; considerable influence on the water exchange process inside the Xiangshangang Bay, and that the gravitational circulation and water stratification increase obviously the exchange rate, while, it has little effect on the outside part of the bay where it is controlled by advection and tidal diffusion.

Key words: Xiangshangang Bay, water exchange, baroclinic, half-life time, mean residence time.