Mean state and seasonal to interannual variability of the Atlantic Subtropical Cells inferred from recent observations

The Atlantic Subtropical Cells (STCs) are shallow wind-driven overturning circulations connecting the tropical upwelling areas with the subtropical subduction regions. In both hemispheres they are characterized by equatorward transports at thermocline level, upwelling both at the equator and the eastern boundary and poleward Ekman transport in the surface layer. STCs are suggested to impact sea surface temperature variability in tropical upwelling regions on interannual to decadal time scales. Therefore, STC variability can play a role in tropical Atlantic climate variability.

Based on Argo float measurements, repeated ship sections and several wind reanalysis products, a 21^st century mean state of the individual branches of the Atlantic STCs is presented. The observations show that the Atlantic STCs are characterized by a lower boundary at the 26.0 kg m^-3 isopycnal and an upper boundary at the seasonally varying Ekman depth (30-70 m). Transport estimates within this layer at 10°N and 10°S reveal that the southern hemisphere contributes about 3 times more to the equatorial transport convergence than the northern hemisphere. In contrast, poleward transports in the surface layer are rather symmetric. Overall, a residual transport of about 4 Sv remains. This diapycnal transport across the 26.0 kg m^-3 isopycnal is most likely associated with the Atlantic Meridional Overturning Circulation which partly upwells in the tropics.

Based on the definition of the Atlantic STCs described above further analysis is carried out on the seasonal to interannual variability of hydrographic properties and transports associated with the STCs.