Analysis of magnitude, drivers, and mechanisms of salinity variability in the tropical Atlantic

by Peter Brandt, Johannes Karstensen and Tim Fischer

TP1.3 aims to enhance the current understanding of the magnitude and spatial and temporal scales of salinity variability in the tropical Atlantic, its drivers and mediating processes. The tropical Atlantic in particular shows large imbalances in precipitation and evaporation, which are largely compensated by oceanic freshwater transport on the various scales.
The first project phase focused on the basinscale meridional freshwater transport and the regional freshwater budget in the central to eastern equatorial Atlantic ('cold tongue' region). At the western boundary at 11°S, interannual to decadal salinity changes were observed in the North Brazil Current that are associated with changes of the Agulhas leakage [Hummels et al., 2015]. The Ekman contribution to meridional transport across latitudes 11°S and 14.5°N can consistently be estimated from different independent data sources, given high temporal resolution (Fig. 1b): hydrographic data combined with velocity profiles, ship based wind, daily satellite wind, reanalysis model GECCO (TP3.1) daily current velocity. In the region of the Atlantic cold tongue, the seasonal variability of the mixed layer salinity budget has been investigated in a dedicated observational campaign and turns out to be dominated by precipitation and zonal advection, also meridional advection and meridional eddy transport. Unlike the corresponding heat budget, salinity exchange with underlying waters by diapycnal mixing is only of minor importance [Schlundt et al., 2014].

Plans for the second phase of this project are:

The second project phase continues and complements the work on the various scales, while shifting the regional focus towards the latitudes (~0 to 12°N) strongly influenced by precipitation beneath the ITCZ. Here phenomena range from short timescales associated with single rainfall events to interannual and decadal timescales associated with the meridional mode. The main topics are the analysis of large-scale long-term relation of sea surface salinity variability and oceanic circulation/climate modes, the mixed layer freshwater budget beneath the ITCZ and near-surface effects of rain and solar heating (Fig.2).