Presenter: Tsubasa Kohyama (Ochanomizu University)

Description:
The Gulf Stream and the Kuroshio Current transport heat from the tropics to the extratropics, so their temperature variations affect densely-populated areas in the northern hemisphere through extreme weather and fisheries production. The two ocean currents are separated by the North American continent, and thus, they cannot exchange heat by oceanic processes within a few years. Based on data analyses of satellite observations and high-resolution global climate models (GCMs), we show that sea surface temperatures of the Gulf Stream and the Kuroshio are synchronized at the inter annual to decadal time scales. This synchronization, which we refer to as the Boundary Current Synchronization (BCS), is reproduced only in global climate models with high spatial resolution, including GFDL-CM4C192 and MIROC6subhires, and not in lower resolution counterparts.  Both in observations and model simulations, BCS is associated with meridional migrations of the atmospheric jet stream. Changes in the strength and path of the ocean currents associated with the jet shifts lead to the synchronicity of sea surface temperatures. These mesoscale air-sea interactions in both the atmosphere and the ocean have to be resolved to reproduce BCS realistically. Numerical simulations using a conceptual model and an atmospheric general circulation model are consistent with a notion that BCS is an interbasin air-sea coupled mode. The conceptual model predicts that the warm phase of BCS are associated with a northward shift of jet stream and vice versa. Atmospheric GCM experiments show that SST variability of the MIROC6subhires model actively modulates the position of the westerly jet stream. On the contrary, as suggested by the conventional view, SST variability of the low-resolution version of MIROC6 has less influence on the midlatitude atmosphere. Air temperature patterns similar to the one associated with BCS have been repeatedly observed, including in July 1994 and 2018.