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Title
Upper ocean fresh biases in low-resolution E3SMv1 simulations: diagnostics and possible remedies
Authors
Milena Veneziani, Luke Van Roekel, Jon Wolfe, Mark Petersen, Darin Comeau, Hyein Jeong, Mat Maltrud, Xylar Asay-Davis, Matt Hoffman, Steve Price (all at LANL), Chris Golaz (LLNL)
Abstract
A common problem of low-resolution E3SMv1 simulations is the occurrence of high upper ocean salinity biases. These fresh biases develop relatively quickly (over the first 10-20 years in fully-coupled simulations) and have reached maxima of up to 5 psu over the last 50 years of the DECK control simulation, for example. They manifest themselves almost globally, but they are particularly evident in the North Atlantic, especially in the subpolar gyre, and in the southern hemisphere, both in the mid-latitudes and in the Southern Ocean. Fresh biases of this magnitude are concerning at all latitudes, but they can substantially change the ocean stratification and circulation at high-latitudes, where salinity often plays a more important role than temperature on density. Here, we first diagnose the problem considering the sources of freshwater in the upper 100 m of the water column, through atmosphere-ocean, land-ocean, and sea-ice formation and melting processes. We then report on various remedies being considered, mostly in MPAS-ocean, to allow the ocean model component to better represent the upper ocean stratification at low-resolution. This investigation has helped identifying improvements in the eddy parameterization and vertical mixing schemes, which will be helpful for the next phase of E3SM.