Effects of model resolution on salt transport through northern high-latitude passages and Atlantic meridional overturning circulation

We investigate the formation of dense water in the Greenland, Iceland, and Norwegian (GIN) Seas by focusing on salt balance between oceanic salt transport and sea surface flux. We conduct ocean general circulation model (OGCM) experiments under a sea surface salinity (SSS) restoring boundary condition with a horizontal resolution of 2° and those where the resolution is increased for northern high latitudes. A comparison of the salt transport at each passage and passive tracer distribution between the experiments reveals the following features: (1) in higher-resolution experiments, a stronger current at the Iceland–Scotland passage can carry saline water from the Atlantic Ocean to the GIN Seas and (2) higher resolution can represent the stronger West Spitsbergen Current that confines fresher water from the Arctic Ocean to the east coast of Greenland, thereby allowing saline water from the Atlantic Ocean to flow toward the Greenland Sea more efficiently. As the resolution increases, they contribute to a more realistic simulation of salinity distribution in the GIN Seas. These improvements are critically important for a realistic simulation of the Atlantic meridional overturning circulation (AMOC) when the model is forced without the SSS restoring condition. In OGCM experiments forced by climatological freshwater flux, the modeled AMOC in the experiment with the resolution of 2° is unrealistically weak because the inability to simulate the salt transport into the GIN Seas dramatically reduces the salinity in the GIN Seas and weakens deep water formation there. As the resolution increases, the AMOC becomes stronger due to a more realistic representation of ocean salt transport into the GIN Seas. These results demonstrate that the adoption of high resolution at least for the northern high latitudes facilitates realistic simulation of deep water formation in the GIN Seas and AMOC.