Subthermocline tropical cells and equatorial subsurface countercurrents

Eastward subsurface countercurrents (SSCCs) are observed in both the Pacific and Atlantic Oceans on either side of the equator, and are associated with a poleward shoaling of subthermocline isotherms at the poleward flanks of the equatorial 13°C thermostad. Observations suggest that eastward SSCCs correspond to a meridional circulation of subthermocline tropical cells (STTCs). The STTCs are below and weaker than the tropical cells, being characterized by an equatorward flow in the thermocline, an equatorial downwelling, a poleward flow in the subthermocline, and an upwelling about 3–4° from the equator. The essential dynamics of the SSCCs can be explained in terms of the conservation of absolute vorticity in the poleward flow of the lower branch of the STTCs. As a parcel within the subthermocline moves poleward, its gain of planetary vorticity is compensated by a loss of relative vorticity, resulting in the eastward SSCCs. By applying the conservation of potential vorticity, the paper also shows that the poleward shoaling of subthermocline isotherms at the poleward flanks of the thermostad can contribute to the eastward SSCCs.