Impact of Fully Time‐Varying Freshwater Fluxes From Greenland and Rivers on Externally Forced and Internal Sea Level Variability

In forced ocean general circulation models (OGCMs), regional sea‐level interannual variability arises from internal ocean dynamics and external forcings, among which freshwater fluxes from Greenland and rivers are typically prescribed as a seasonal climatology rather than with their full variability. This study examines the impact of fully time‐varying rivers and Greenland fluxes on forced and internal sea‐level variability over 1980–2018, using ensemble‐based sensitivity experiments of an eddy‐permitting OGCM. We introduce an improved, closed ensemble variance decomposition by adding a biased variance term which accounts for ensemble variance in the long‐term time mean. The impact of continental freshwater fluxes variability on sea‐level variance is highest in the tropical South Atlantic, the maritime continent, and the Kuroshio region. The first two regions are mainly impacted by rivers, primarily through a forced halosteric response, occurring over the full ocean depth in the tropical South Atlantic and mostly over the first 50 m in the maritime continent. In the Kuroshio region, sea‐level variance is impacted by both rivers and Greenland through an internally driven response of thermosteric origin. The biased variance enables to correctly evaluate the sources of halosteric sea‐level variability in that region as it reveals that its long‐term variability is not well resolved by our experiment length. These results emphasize the need for accurately representing time‐varying freshwater fluxes in OGCMs to better capture sea‐level variability, and the value of the newly closed decomposition for correctly attributing changes in internal variance of any variable, especially in regions of high variability and for short‐timescale experiments.