Role of bottom pressure in the seasonal cycle of coastal sea level

Changes in sea level can arise from steric and manometric effects, with the partition between them depending on depth and other dynamical factors. In the shallow coastal oceans, the manometric term associated with bottom pressure variations is expected to become larger than the steric term, but a quantitative analysis of this breakdown remains missing. In this work, we use gridded monthly bottom pressure estimates from GRACE and GRACE-FO, together with temperature and salinity data compiled under the World Ocean Atlas 2023, to quantify manometric and steric contributions to the mean seasonal cycle in sea level across the coastal zone, as observed by tide gauges and satellite altimeters.

Focusing on depths shallower than 2000 m and global median values, the sum of the estimated steric and manometric terms can explain approximately 65% of the annual variance and 40% of the semiannual variance in the sea level observations. We identify several regions, e.g., the Australian seaboard, where the seasonal sea level budget is not closed, pointing to data noise, limited horizontal resolution, or residual leakage errors in the GRACE/-FO solutions. Indeed, comparisons of sea level, steric and manometric terms calculated based on different products suggest sizable uncertainties in the available estimates of the mean seasonal cycle in coastal bottom pressure. For most regions with a sufficiently tight budget closure, we find that whilst the importance of the manometric term generally increases with decreasing water depth, steric contributions are non-negligible near coastlines, especially at the annual frequency.