Representations of the Nordic Seas overflows and their large scale climate impact in coupled models

- We perform sensitivity experiments using two climate models: CM2G and CM2M.
- Two differences are identified in overflow representation which impacts climates.
- Inclusion of the Iceland-Scotland overflow modifies the North Atlantic horizontal circulation.
- Diapycnal diffusivity impacts the vertical structure of the Atlantic meridional overturning circulation.

The sensitivity of large scale ocean circulation and climate to overflow representation is studied using coupled climate models, motivated by the differences between two models differing only in their ocean components: CM2G (which uses an isopycnal-coordinate ocean model) and CM2M (which uses a z-coordinate ocean model). Analysis of the control simulations of the two models shows that the Atlantic Meridional Overturning Circulation (AMOC) and the North Atlantic climate have some differences, which may be related to the representation of overflow processes. Firstly, in CM2G, as in the real world, overflows have two branches flowing out of the Nordic Seas, to the east and west of Iceland, respectively, while only the western branch is present in CM2M. This difference in overflow location results in different horizontal circulation in the North Atlantic. Secondly, the diapycnal mixing in the overflow downstream region is much larger in CM2M than in CM2G, which affects the entrainment and product water properties. Two sensitivity experiments are conducted in CM2G to isolate the effect of these two model differences: in the first experiment, the outlet of the eastern branch of the overflow is blocked, and the North Atlantic horizontal circulation is modified due to the absence of the eastern branch of the overflow, although the AMOC has little change; in the second experiment, the diapycnal mixing downstream of the overflow is enhanced, resulting in changes in the structure and magnitude of the AMOC.