Fig. 21: Maximum Atlantic Meridional Overturning Circulation (AMOC) at 26°N (Sv) for piControl and the idealized simulations 1pctCO2 and abrupt-4xCO2. Please note that the branch-off point for both idealized simulations is year 250 of piControl simulation. The year on the x-axis represents the year after this branch-off point and therefore the year after the start of the idealized forcing.
Table 4 shows the ocean volume transports through some key ocean straits, averaged over all 5 ensemble members and the time period 1985–2014 for the historical runs and over the years 2071–2100 for the SPP370 runs. The historical runs show volume transports that are comparable to observed estimates for most of the ocean straits. For some straits, however, the volume transports are underestimated, including the export from the Arctic Ocean to the North Atlantic measured at the David Strait, the Indonesian Throughflow, and the transport in the Mozambique Channel. The main reason for this underestimation is due to the fact that the model resolution is not fine enough to resolve those narrow straits. In particular, the three main straits in the Canadian Arctic Archipelago (CAA) are only 10, 30 and 50 km wide at their narrowest locations, respectively, which cannot be well resolved with the mesh we used in the CMIP6 simulations. Improved representation of the CAA, and thus of the ocean transport through the Davis Strait, is expected for future coupled model configurations with higher ocean resolution, following promising results with high-resolution stand-alone configurations using FESOM (Wang et al., 2018; Wekerle et al., 2013).
For some ocean straits, the ocean volume transport shows a large response to the climate change in the SPP370 scenario. The Florida Current, for example, decreases by about 15% at the end of the 21st century in the SPP370 scenario, which is consistent with the weakening trend of the AMOC described above. The ocean volume transport in the Indonesian Throughflow and the Mozambique Channel also decreases in a warming climate (by about 20%). This implies that the exchange between the Pacific, Indian and Atlantic Oceans will become weaker. The oceanic linkage between the North Atlantic and the Arctic Ocean, however, is strengthened significantly in a warmer world, as shown by the increase in the volume transport through the Barents Sea Opening (increase by about 40%). Together with the temperature increase in the Atlantic Water, this implies that oceanic heat supply from the North Atlantic to the Arctic Ocean, and hence Atlantification of the Arctic Ocean, will increase in the future. As a consequence of ocean volume conservation, the excess ocean volume inflow through the Barents Sea Opening is balanced by an increased outflow from the Arctic through the Fram Strait.
In a warming climate the strength of the North Atlantic subpolar gyre (SPG) decreases, as shown by the increase in the sea surface height (SSH) in the SPG region (Fig. 22). The weakened SPG brings less Atlantic Water into the gyre circulation from the northeastern North Atlantic, which allows more Atlantic Water to continue to the north into the Nordic Seas. The enhanced northward flow is manifested by the increase in the SSH along the European coast. This can explain the stronger ocean volume transport through the Barents Sea Opening at the end of the 21st century in the SPP370 scenario (Table 4). The SSH on the northwestern side of the Gulf Stream increases in the warming scenario, which indicates a weakening of the Atlantic Current and is consistent with the weakening of the AMOC and the warming off the East Coast of the USA (Fig. 14a).
Table 4: Ensemble mean of ocean volume transport (Sv) through different straits for the historical runs and spp370 runs. (Positive values mean north or eastward flows)