Evolution of trends in North Atlantic dynamic sea level in the twenty-first century

This study looks at the evolution of trends in the dynamic sea level (DSL) component of sea-level rise for the North Atlantic (18°–67° N), primarily over the twenty-first century but with some analysis extending to 2300. We use an 18-member climate model ensemble of projections under medium and high greenhouse gas scenarios. We find that the long-term trend pattern (1950–2099) tends to be indicated in the early twenty-first century: 14 (11) of the 18 models have pattern correlations between this period and 1993–2022 of > 0.5 under the medium (high) greenhouse gas scenario. Whether a particular 1993–2022 DSL trend indicates the long-term trend in our ensemble can be assessed from concurrent changes in the Atlantic Meridional Overturning Circulation (AMOC) at 30° N: those cases with larger AMOC weakening trends have high DSL-trend pattern correlations. This suggests that the observed AMOC (or a proxy for that) could indicate whether observed DSL trends are likely to be a constraint on projections, provided suitable AMOC trend indicators can be determined. More generally, North Atlantic DSL and the AMOC show a similar evolution of measures of their successive 30-year trends (with these being projection-onto-a-pattern and the ratio with the long-term trend, respectively), both in the ensemble mean and for individual models. Model-to-model differences in this evolution indicate that the changing DSL and AMOC trends are more interconnected than solely the common influence of external forcings associated with the climate-change scenario. Model simulations extending to 2300 indicate that the twenty-first century trend patterns do not generally persist into subsequent centuries, as has been shown in previous studies.