Observation-based quantification of physical processes that impact sea level

Abstract

This study provides observationally based estimates of the contributions to sea level rise from individual physical oceanographic processes. The kinematic equation for sea level evolution is used to calculate the spatial distribution of the evolution of sea level rise, and its global integral. Results are separated into impacts from boundary mass fluxes and from non-Boussinesq steric effects. The non-Boussinesq steric effect itself is further decomposed into contributions from boundary buoyancy fluxes and interior buoyancy changes driven by mixing processes. It is neither the intention nor currently possible to close the global mean sea level (GMSL) budget using this approach. Instead, the results quantify the magnitude and uncertainty of the physical oceanographic processes and their relative importance in shaping GMSL rise. This allows for a comparison of the impact on GMSL by single processes or parameterizations. Results indicate large uncertainties associated with boundary heat, mass, and freshwater fluxes and highlight the importance of ocean mixing for GMSL rise. Additionally, GMSL rise is substantially affected by how shortwave radiation is redistributed with depth and by the choice of neutral physics calculation method. This study also finds that nonlinear thermal expansion is offset by mixing-induced densification, and that both processes strongly influence sea level as consequences of the nonlinear equation of state. Many of the results are relevant for observationally based calculations and for modelers making decisions about which methods or parameterizations to use. Understanding the impact of these processes on GMSL rise, and how they change in a transient ocean and climate system, is important because these choices influence projections of future sea level rise, which inform policy decisions.