Changes in the geometry of ocean basins have been influential in driving climate change throughout Earth’s history. Here we focus on the appearance of the Greenland-Scotland Ridge (GSR) and try to understand its impact on the ocean state, including global circulation, heat transport, T and S properties and ventilation timescales, which will be useful for interpreting paleoproxies. To this end, we use a coupled atmosphere-ocean-sea ice model with idealized geometry and consider two geometrical configurations. The reference configuration (noridge) comprises two wide strips of land set 90° apart extending from the North Pole to 40°S, separating the Northern Hemisphere ocean into a small and a large basin. In the ridge configuration a zonally symmetric oceanic ridge, that extends across the Atlantic-like basin at 60°N, mimicking the GSR, is added. In addition, we consider two climatic limits of noridge: a warm case where the northern high latitudes are seasonally ice-free and a cold case where a perennial sea ice cover is present. In both cases of noridge deep-water formation occurs at the North Pole in the Atlantic-like basin. When the ridge is introduced, the flow of warm Atlantic water to the high latitudes is hampered and the ocean heat transport across 70°N decreases by ~60% which causes cooling and freshening north of the ridge. Downwelling shifts south of the ridge, thereby altering the structure of the upper overturning cell dramatically. Despite these changes, the Northern Hemisphere surface climate response is surprisingly small for the warm climate case. This is because the subpolar gyre circulation continues to transport warm water across the ridge, keeping the northern North Atlantic relatively warm and ice-free. In the colder climate case, however, the presence of sea ice provides a strong non-linear feedback, which amplifies the cooling induced by the ridge, and causes sea ice to expand. Our results highlight the possible disconnect between changes in the localization of deep-water formation, the structure of the AMOC and the properties of water masses and changes in Northern Hemisphere climate. Implications for the interpretation of paleoproxy records from the North Atlantic region will be discussed.

Philip Craig

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The net surface water flux (evaporation minus precipitation minus runoff, E-P-R) of the Atlantic Ocean is approximately 0.4 – 0.6 Sv (1 Sv = 10^9 kg s-1) larger than that of the Pacific Ocean, as shown in atmospheric and oceanic reanalyses and by oceanographic estimates. This asymmetry is linked to the asymmetry in sea surface salinity and the existence of the Atlantic Meridional Overturning Circulation. It is shown that the reason for the asymmetry in E-P-R is greater precipitation per unit area over the Pacific south of 30N, while evaporation rates are similar over both basins. It is further argued that the Pacific Ocean is anomalous compared to the Atlantic and Indian Oceans in terms of atmospheric moisture flux convergence and precipitation across the tropics and subtropics. To clarify the mechanism by which water vapour is exported out of the Atlantic basin and imported into the Pacific, we use an air mass trajectory model driven by ERA-Interim reanalysis. Using 12-hourly releases of 14-day back trajectories on the boundaries of ocean drainage basins over the period 2010-2014, we are able to partition the atmospheric moisture fluxes between basins according to their origins (i.e. last contact with the boundary layer). We show that at most a quarter of the E-P-R asymmetry is explained by higher moisture export to the Arctic and Southern basins from the Atlantic than from the Pacific. The main contributions come from differences in the longitudinal atmospheric transport of moisture between the Atlantic, Indian and Pacific basins. In particular, during the Asian summer monsoon the recurvature of the low level flow in the Somali Jet results in a much weaker westward moisture transport from the Indian into the Atlantic basin than across Central America (where it is similar to the zonal average) while there is stronger eastward transport from the Indian to Pacific basins. The net effect is stronger moisture convergence into the Pacific, but weaker into the Atlantic. In contrast to previous thinking, the role of the moisture flux across Central America in the asymmetry, albeit significant, is not dominant.