Atmospheric origins of variability in the South Atlantic meridional
overturning circulation
Abstract
Insights from the RAPID–MOCHA observation network in the North Atlantic
have motivated a recent focus on the South Atlantic, where water masses
are exchanged with the neighboring Indian and Pacific ocean basins.
Moreover, the South Atlantic meridional overturning circulation
basin-wide array (SAMBA) was recently launched to monitor variability in
the South Atlantic MOC (SAMOC) at 34.5ºS. In this study, we are
interested in understanding the processes which generate volume
transport variability that would be observed at this latitude band. To
perform this attribution, we compute sensitivities of the SAMOC at 34ºS
to atmospheric state variables (e.g. wind stress, precipitation) using
the adjoint of a global ocean model which is fit to a vast number of
ocean observations over the past 20 years. These sensitivities isolate
the impact from each atmospheric variable, and highlight the oceanic
mechanisms, such as Kelvin and Rossby waves, which carry atmospheric
forcing perturbations to the SAMOC. The domain of influence for the
SAMOC is shown to be quite broad, covering neighboring ocean basins even
on short time scales. This result differs from what has previously been
shown in the North Atlantic, where Atlantic meridional overturning
circulation (AMOC) variability is largely governed by dynamics confined
to that basin. We convolve historical forcing variability from
ERA-Interim with the computed sensitivities in order to attribute
seasonal to interannual SAMOC variability to each atmospheric component.
The seasonal cycle of the SAMOC is therefore shown to be largely driven
by local zonal wind forcing. Interannual variability, however, is shown
to have originated from remote locations across the globe, including a
nontrivial component originating from the tropical Pacific. We conclude
with preliminary results which employ both modeling results and an
analysis of modern altimetry observations to show how El Niño Southern
Oscillation variability might influence the South Atlantic.