Mass and Heat Transports in the South Atlantic Derived from Historical Hydrographie Data

Abstract

Mass and heat transports in the South Atlantic as well as exchange flows with the South Pacific and the Indian Ocean are determined by driving a conservative, steady box-model towards the historical temperature (θ) and salinity (s) observations. The optimal model circulation searched for is required (a) to approximately preserve the vertical velocity shear as given by geostrophic calculations and (b) to correctly reproduce the measured distributions of 9 and s Information contained in the θ/s data on baroclinic flows is exploited through constraint (a) and the unknown reference velocities are determined by the model in a way such that the resulting absolute flow velocities produce realistic θ and s fields (constraint (b)). The model is mass, heat and salt conserving and has realistic topography. The adjoint method is applied as an efficient means for calculating cost function gradients needed during the optimization process.

Model experiments show that indeed realistic θ and s model distributions can be obtained with flows that are consistent with geostrophy. Moreover, close agreement between measurements and model is obtained for a variety of model velocity fields that differ considerably with respect to strength of the meridional overturning cell and magnitude of meridional heat transports. The maximal acceptable meridional heat transport across 30°S (based on an evaluation of θ/s misfits and deviations from geostrophic shear) amounts to 0.4 PW. Forcing the model to produce larger heat fluxes results in systematic property misfits in the upper layers of the South Atlantic. Contrary to most published heat transport estimates the model also accommodates poleward (southward) heat fluxes of up to -0.5 PW. The best model property fields are obtained for a heat transport across 30°S close to zero. All acceptable model solutions show a dominance of northward flow of Antarctic Intermediate Water (AAIW) over warmer, upper layer waters, and all model temperature and salinity fields.