The role of the wind-driven shallow overturning circulation in the heat budget of the tropical Pacific: Ocean’s response to surface buoyancy and momentum flux perturbations

This study aims to determine the role of the shallow and deep circulations in the Tropical Pacific Ocean heat budget, under both unperturbed and transient climate conditions. The transient response of the ocean is evaluated by performing explicit flux experiments, whereby perturbations to the surface fluxes of momentum and heat are applied to the surface of the ocean. In equilibrium conditions, the overall balance of the column is achieved as the residual between advective cooling and warming by vertical diffusive mixing. The main contribution to the balance is by far achieved by the shallow ocean, with nearly 85% and 72% of the balance in the Northern and Southern Hemisphere, respectively. Under wind stress perturbations, net imbalance in both hemispheres is due to enhanced advective cooling largely compensated by vertical diffusive warming (i.e. a strengthened upwelling-diffusive balance). In both hemispheres, imbalances are mainly confined within the shallow ocean (60% and 70% in the Northern and Southern Hemispheres respectively). The last is in accordance with the observed strengthening of the Shallow Overturning Cell and a significant weakening of higher latitudes dense water production. So strengthened shallow overturn in the North Pacific reduce northward transport out of the cell, reducing the deep ocean ventilation. The Wind stress perturbation enhances the dominant upwelling-diffusive balance with the cell cooling the shallow ocean. Under heat flux perturbations, total warming imbalance is set primarily by strengthened vertical diffusive warming in both hemispheres. However, while in the Northern Hemisphere the Shallow Ocean held nearly 70% (0.66 PW) of the total net warming imbalance, the Southern Hemisphere only accounts for 10% of the imbalance (1.20 PW). Observed warming tendencies are arguably explained in terms of mass and heat transport changes. Under the wind-stress perturbation changes in the Shallow Overturning Cell in both hemispheres are in agreement with the net heat imbalance resulting from intensified advective cooling compensated by vertical diffusive warming. In the case of the heat flux perturbation, the overall Shallow Overturning Cell circulation strengthening in both hemispheres, and the subsequent reduction in mass export by the cell towards deeper levels seem to be responsible for the shallow and deep ocean warming.