Response of regional sea level to atmospheric pressure loading in a climate change scenario

The regional response of the global ocean to low-frequency changes in atmospheric pressure loading, η_ib, is analyzed as it occurs in the Max Planck Institute for Meteorology (MPI-M) coupled ocean–atmosphere climate model in response to increased atmospheric CO₂ concentrations. Results suggest that long-term changes in η_ib can lead to increases in high-latitude sea level by up to 5 and 10 cm, respectively, after doubling and quadrupling the atmospheric CO₂ content. At low latitudes, sea level will decrease simultaneously between 2 and 4 cm through the combined effects of changes in the atmospheric circulation and through the increase of its moisture content. In subpolar regions, associated rates of sea level increase are of the order of 0.4–0.6 mm yr⁻¹ for quadrupled atmospheric CO₂ concentrations, while in mid- and low latitudes, sea level will decrease at a rate of 0.2 mm yr⁻¹. Differences between doubling and quadrupling CO₂ concentrations indicate regionally dependent nonlinearities in the changing climate system. The analysis suggests that in some regions (including the coasts of northern Europe) low-frequency η_ib changes could be as large as 10%–20% of a global sea level increase anticipated over the next 100 yr. While not being a dominant effect, amplitudes of long-period η_ib changes are large enough to be included in future estimates of climate-related regional sea level change. Increasing the vertically integrated atmospheric CO₂ content by 4 and 12 kg m⁻² (on global average), in response to doubling and quadrupling atmospheric CO₂ concentrations, suggests associated reductions of global sea level by 0.6 and 1.7 cm, respectively. The differences between two different model solutions are significant, especially in the Southern Ocean, where they show significantly different atmospheric mass and pressure distributions, and at low latitudes, where differences resemble the contribution of increased moisture content added to the inverted-barometer (IB) effect in the MPI-M solution.