Testing the Assumption of a Static Sea Level Response to Self-Attraction and Loading Effects

Understanding of the ocean response to surface loading is essential to interpret observations of sea level. Such loading can be related, for example, to barometric pressure, surface mass exchange due to precipitation and evaporation, gravitational tide potential or changes in gravity field caused by non-uniform distribution of mass within the land-atmosphere-ocean system --- an effect that is often referred to as self-attraction and loading (SAL). Recent studies highlighted the importance of SAL to the understanding of the variability of sea level and ocean mass on monthly and longer time scales. The SAL-induced adjustments, however, are typically derived under the assumption that the effects give rise to a static ocean response, for which the applied loading is balanced by adjustments in the sea-level gradients. To test the static assumption, we use a global ocean model and examine the sea level response caused by SAL-induced perturbations in surface loading on monthly and longer time scales, associated with mass redistribution due to land hydrology and atmospheric and oceanic circulations. Typical standard deviations of the dynamic response (i.e., departures of sea-level from equilibrium response) are below 1 mm, which is about 10% of loading signal, suggesting that overall static assumption is valid over most of the oceans, as long as one is focused on long time scales. A few exceptions are shallow Arctic and other coastal regions, and the Southern Ocean, where deviations can exceed 30% of the surface loading and reach 2 mm. To test the equilibrium assumption at shorter (sub-monthly) time scales, we estimate the response to high-frequency (hourly) SAL load due to ocean circulation effects. In this case, for simplicity the SAL perturbations are assumed to be simply proportional to the estimated oceanic mass anomalies. Preliminary analyses point to more important dynamic signals at these high frequencies and emphasize the need for explicit inclusion of the missing SAL physics in numerical ocean models.