What Controls the Estuarine Exchange Flow of the Salish Sea?

Speaker: Parker MacCready
Institution: UCLA
Location: MS 7124
Date: February 8, 2023
Time: 3:30 pm to 4:30 pm


The estuarine exchange flow brings ocean water, and chemical properties such as high Nitrate and low Dissolved Oxygen, into the Salish Sea, a large, fjordal estuarine system on the US west coast. It is well-established from observations and modeling studies that this exchange flow is large, typically an order of magnitude or more greater than the net river flow entering the system. As a result, the ocean exerts a strong influence on the system and its biogeochemistry. The extreme bathymetric complexity of the Salish Sea has made it difficult to apply existing dynamical theories to understanding the physical controls on the strength of this exchange flow.

In a new analysis of a realistic model simulation (MacCready et al., 2021) , including runs with increased and decreased tides, we find that the exchange flow is highly correlated with the tidal transport at sections throughout the system. Surprisingly, the general trend is for the exchange flow to increase with tidal transport, contrary to the expected result from gravitational circulation theory in which decreased turbulence during neaps increases the exchange flow. Similar dynamics have been reported in other tidally energetic systems, albeit much smaller ones (Conroy et al., 2020; Chen et al., 2012; MacCready, 2011).

Nonetheless, the Salish Sea overall still grows fresher with stronger tides, indicating that the salt transport due to the exchange flow decreases with stronger tides, even as the exchange flow volume transport increases. These results highlight the importance of sills where flow separation and other processes that lead to tidal pumping appear to govern the exchange flow, while at the same time the turbulent mixing at these sill plays a crucial role in governing the salt transport due to the exchange flow.

These results will be considered in context with biogeochemical budgets as well.