The Response of the Equatorial Ocean to Upper Ocean Mixing in the Subtropics

We explore the response of the equatorial ocean and atmosphere to enhanced upper ocean mixing in the subtropics. This study is motivated by the potential role of tropical cyclones in climate - by vigorously mixing the top 100-200m of the ocean, tropical cyclones can affect the ocean heat uptake and poleward heat transport, the ocean thermal structure, and hence global temperatures. We investigate these effects by imposing different forms of intermittent ocean mixing in the subtropical bands in a suite of calculations with a comprehensive coupled GCM. The imposed vertical diffusivity has the same annual mean value in all experiments (1 cm2/s), but its duration varies between different experiments from 1 day to a full year. All simulations show robust patterns of change for the ocean thermal structure, independent of the duration of the mixing. The immediate effect of the mixing is a roughly 0.5°C surface cooling in the subtropical bands due to the vertical entrainment of cold water and, at the same time, the generation of subsurface warm temperature anomalies due to the pumping of surface water down the water column. The subsequent ocean adjustment includes a deepening of the tropical thermocline and a 2°C warming of the equatorial cold tongue in the Pacific. The latter effect is amplified by the weakening of the atmospheric Walker cell and the ensuing ocean-atmosphere interactions that reduce the zonal SST gradient along the equator. In contrast, the simulated increase in the mean meridional SST gradient leads to the strengthening of the atmospheric Hadley cells, followed by the strengthening of the shallow subtropical cells (STC) in the ocean. We conclude that upper ocean mixing in the subtropics is an important modulator of the mean state of the tropical ocean-atmosphere system.