Improving the Representation of Estuarine and Shelf Processes in Earth System Models

<spanine-height: 'arial','sans-serif';="'Arial','sans-serif';" font-size:="font-size:" 115%;="115%;" 10pt;"="10pt;"">The fluvial discharges of freshwater have much larger impacts on oceanic dynamics than would be guessed based on their total volume flux alone. Model studies have shown that stronger stratification due to increasing fluvial discharge could substantially slow and even shut down meridional overturning in the North Atlantic (Rahmstorf, 1995; Marzeion, et. al. 2007). Indeed, not only the amount of riverine freshwater, but also the way it is injected into ocean affects ocean stratification (e.g Hordoir, et. al. 2008<spanine-height: 'arial','sans-serif';="'Arial','sans-serif';" font-size:="font-size:" 115%;="115%;" 10pt;"="10pt;"">). In most earth system models, the fluvial discharge is imported into the ocean model with zero salinity. This method omits important natural physical processes in estuaries and on continental shelves that pre-mix the riverine water with oceanic water and can change the location and timing of freshwater delivery to the ocean. However, computational time constraints limit global ocean models to coarse horizontal resolution (e.g. 1 degree latitude), so the estuarine and shelf processes cannot be resolved. In the current study, two adjacent box models are employed to represent the unresolved mixing processes in the estuary and shelf. The Estuary Box Model (EBM) with two-layer structure represents the mixing processes driven by tides and bottom friction in the estuaries and creates an exchange flow that introduces saltier lower-layer water into the fresher surface layer. The adjacent Shelf Plume Box Model (SPBM) treats the released water mass from EBM as a collection of plume boxes that can propagate along the coast and undergo shear-driven entrainment during light winds or rapidly mix and advected offshore during upwelling favorable winds. The EBM and SPBM are compared to observations and regional simulation data for the Columbia River. The EBM is globally implemented within the ocean model of the Community Earth System Model (CESM). The performance of SPBM is tested in CESM at Columbia River. Comparisons of runs with and without the box models show pronounce sensitivity for Sea Surface Salinity near river mouths and their nearby regions.