Pezzi, L. P., M. F. L. Quadro, J. A. Lorenzzetti, A. J. Miller, E. B. Rosa,
L. N. Lima and U. A. Sutil, 2022:
The effect of Oceanic South Atlantic Convergence Zone episodes on regional SST anomalies: The roles of heat fluxes and upper-ocean dynamics
Climate Dynamics, 59, 2041-2065.
The South Atlantic Convergence Zone (SACZ) is an atmospheric system occurring in austral summer
on the South America continent and sometimes extending over the adjacent South Atlantic.
It is characterized by a persistent and very large, northwest-southeast-oriented, cloud band.
Its presence over the ocean causes sea surface cooling that some past studies indicated as being
produced by a decrease of incoming solar heat flux induced by the extensive cloud cover.
Here we investigate ocean-atmosphere interaction processes in the Southwestern Atlantic Ocean (SWA)
during SACZ oceanic episodes, as well as the resulting modulations occurring in the
oceanic mixed layer and their possible feedbacks on the marine atmospheric boundary layer. Our
main interests and novel results are on verifying how the oceanic SACZ acts
on dynamic and thermodynamic mechanisms and contributes to the sea surface thermal
balance in that region. In our oceanic SACZ episodes simulations we confirm an
ocean surface cooling. Model results indicate that surface atmospheric circulation and the
presence of an extensive cloud cover band over the SWA promote sea surface cooling via
a combined effect of dynamic and thermodynamic mechanisms, which are of the same order of magnitude.
The sea surface temperature (SST) decreases in regions underneath oceanic SACZ positions,
near Southeast Brazilian coast, in the South Brazil Bight (SBB) and offshore.
This cooling is the result of a complex combination of factors caused by the decrease of solar
shortwave radiation reaching the sea surface and the reduction of horizontal heat advection in
the Brazil Current (BC) region. The weakened southward BC and adjacent offshore
region heat advection seems to be associated with the surface atmospheric circulation caused by
oceanic SACZ episodes, which rotate the surface wind and strengthen cyclonic oceanic mesoscale eddy.
Another singular feature found in this study is the presence of an atmospheric cyclonic vortex Southwest
of the SACZ (CVSS), both at the surface and aloft at 850 hPa near 24S and 45W.
The CVSS induces an SST decrease southwestward from the SACZ position by inducing divergent
Ekman transport and consequent offshore upwelling. This shows that the dynamical effects of
atmospheric surface circulation associated with the oceanic SACZ are not restricted only
to the region underneath the cloud band, but that they extend southwestward where
the CVSS presence supports the oceanic SACZ convective activity and concomitantly
modifies the ocean dynamics. Therefore, the changes produced in the oceanic
dynamics by these SACZ events may be important to many areas of scientific and
applied climate research. For example, episodes of oceanic SACZ may influence the pathways of
pollutants as well as fish larvae dispersion in the region.