Improved Surface Mass Balance Closure in Ocean Hindcast Simulations

Forced global ocean/sea-ice hindcast simulations are subject to persistent surface mass flux estimation biases, for example, configurations with an explicit-free surface may not take into account the seasonal storage of water on land when constraining sea level. We present a physically motivated surface mass flux closure, that results in: reduced watermass drift from initialization; improved Atlantic meridional overturning cirulation intensity; and more realistic rates of ocean heat uptake, in simulations using global ocean/sea-ice/land (MOM6/SIS2/LM3) model configurations, forced with atmospheric reanalysis data. In addition to accounting for the land storage, the area-integrated subpolar-to-polar (40°–90°N/S) surface mass fluxes are constrained, using a climatological estimate derived from the the CMIP6 historical ensemble, which helps to further improve hindcast performance. Simulations using MERRA-2 and JRA55-do forcing, subject to identical hydrologic constraints, exhibit similar reductions in drift.