Climate impacts of intermittent upper ocean mixing induced by tropical cyclones

Tropical cyclones (TC) represent a powerful, albeit highly transient forcing able to redistribute ocean heat content locally. Recent studies suggest that TC-induced ocean mixing can have global climate impacts as well, including changes in poleward heat transport, ocean circulation, and thermal structure. In several previous modeling studies devoted to this problem, the TC mixing was treated as a permanent (constant in time) source of additional vertical diffusion in the upper ocean. In contrast, this study aims to explore the highly intermittent character of the mixing. We present results from a series of coupled climate experiments with different durations of the imposed intermittent mixing but where each has the same annual mean diffusivity. All simulations show robust changes in sea surface temperature and ocean subsurface temperature, independent of the duration of the mixing that varies between the experiments from a few days to a full year. Simulated temperature anomalies are characterized by a cooling in the subtropics, a moderate warming in middle to high latitudes, a pronounced warming of the equatorial cold tongue, and a deepening of the tropical thermocline. These effects are paralleled by substantial changes in ocean and atmosphere circulation and heat transports. While the general patterns of changes remain the same from one experiment to the next, their magnitude depends on the relative duration of the mixing. Stronger mixing, but of a shorter duration, has less of an impact. These results agree with a simple model of heat transfer for the upper ocean with a time-dependent vertical diffusivity.