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)