Cross-Equatorial and Subtropical Cells

The Cross Equatorial (CEC) and Subtropical (STC) Cells are the major shallow-overturning cells of the Indian Ocean, corresponding to the North and South STCs in the Pacific and Atlantic Oceans. Their meridional (overturning) structure is illustrated by two-dimensional (y, z) streamfunction plots from two solutions to ocean general circulation models (OGCMs). The plots show that the cells’ sinking branches are located in the southern hemisphere, whereas their upwelling branches are located in the southern hemisphere along the South Equatorial Thermocline Ridge for the STC, and in the northern hemisphere for the CEC. The cells’ three-dimensional (x, y, z) structures are determined by following pathways of model drifters in solutions to two layer models and an OGCM. Transports of the cell branches in each solution are compared to each other and to available observations. A suite of solutions to an idealized two-layer, reduced-gravity model (a 2 $$\frac{1}{2}$$ -layer model) is used to isolate the basic processes (wind forcing, upwelling, and detrainment) that generate the cells. In addition to the CEC and STC, the solutions have an equatorial “roll,” a small-scale overturning feature confined to the upper 50–100 m within a few degrees of the equator; its dynamics are discussed, and observational support for its existence noted.