Subtropical countercurrent variations in cooling climates induced by freshwater forcing over the subarctic North Atlantic

Abstract

The subtropical countercurrent (STCC) could affect ocean eddy activity and the atmosphere above. How the STCC varied in a cooling scenario, such as the stadials during the last deglaciation (19–11 ka; ka = 1000 years ago), is still unclear. By 1-Sv (1 Sv = 106 m3 s− 1) freshwater hosing over the subpolar North Atlantic to suppress the Atlantic Meridional Overturning Circulation (AMOC), the STCC variations under pre-industrial (PI) and last glacial maximum (LGM, ~ 21 ka) conditions are analyzed. In both hosing experiments, the northern hemisphere cools. The upper ocean becomes less stratified, and the wintertime mixed layer deepens in the Kuroshio–Oyashio extension (KOE) region. As a result, more mode waters (~ 20%) are formed, and they become more vertically uniform. The enhanced mode waters induce a stronger vertical shear and accelerate the STCC based on thermal wind relationship. The strengthened STCC further induces warm ocean temperature and upward air-sea heat flux anomalies along it. While the STCC strengthens in both hosing experiments, the location variations are different. In the PI hosing experiment, the Bering Strait is open. Fresh water enters the North Pacific through the Bering Strait and increases the stratification in the downstream of the KOE region. The mixed layer front shifts westward and induces a robust westward shift of mode waters and the STCC. In the LGM hosing experiment, on the contrast, there is no obvious location shift because the Bering Strait is closed.