Sensitivity of contemporary sea level trends in a global ocean state estimate to effects of geothermal fluxes

Abstract

Geothermal fluxes constitute a sizable fraction of the present-day Earth net radiative imbalance and corresponding ocean heat uptake. Model simulations of contemporary sea level that impose a geothermal flux boundary condition are becoming increasingly common. To quantify the impact of geothermal fluxes on model estimates of contemporary (1993–2010) sea level changes, two ocean circulation model experiments are compared. The two simulations are based on a global ocean state estimate, produced by the Estimating the Circulation and Climate of the Ocean (ECCO) consortium, and differ only with regard to whether geothermal forcing is applied as a boundary condition. Geothermal forcing raises the global-mean sea level trend by 0.11 mm yr−1 in the perturbation experiment by suppressing a cooling trend present in the baseline solution below 2000 m. The imposed forcing also affects regional sea level trends. The Southern Ocean is particularly sensitive. In this region, anomalous heat redistribution due to geothermal fluxes results in steric height trends of up to ± 1 mm yr−1 in the perturbation experiment relative to the baseline simulation. Analysis of a passive tracer experiment suggests that the geothermal input itself is transported by horizontal diffusion, resulting in more thermal expansion over deeper ocean basins. Thermal expansion in the perturbation simulation gives rise to bottom pressure increase over shallower regions and decrease over deeper areas relative to the baseline run, consistent with mass redistribution expected for deep ocean warming. These results elucidate the influence of geothermal fluxes on sea level rise and global heat budgets in model simulations of contemporary ocean circulation and climate.