Upper-ocean salinity variability in the northwestern tropical Atlantic and its interactions with SST and winds

Two important topics in need of further research are (1) the impact of upper-ocean salinity on stratification, mixing, and sea surface temperature (SST), and (2) interactions between ocean mesoscale eddies and the atmosphere. The northwestern tropical Atlantic (NTA) is an advantageous place to address these questions because it experiences strong eddy activity and pronounced surface freshening from Amazon River outflow. Furthermore, during boreal winter, shallow cumulus clouds are prevalent in the NTA and are affected by the underlying SST. The details of how these clouds form and interact with the ocean are not well known, contributing to significant uncertainty in climate models’ radiation budgets.
Previous research has shown that, on average, six large anticyclonic rings translate northward in the NTA each year after separating from the North Brazil Current (NBC) retroflection. Though the volume transport associated with the rings has been quantified, the upper-ocean temperature and salinity structures are not well known. It is also unclear how the rings affect near-surface winds and heat fluxes. It has been established that salinity contributes significantly to upper-ocean stratification in the NTA during boreal winter, but there is debate on the impact of salinity on SST in this region. The uncertainties are compounded by strong mesoscale salinity variability from NBC rings. These gaps in knowledge limit our understanding of ocean-atmosphere coupling and cloud variability in the NTA.
This proposal aims to improve our understanding of eddy variability and ocean atmosphere interactions in the NTA through the deployment of a set of unique surface drifting buoys, combined with analysis of historical in situ and satellite data and one-dimensional ocean model experiments. The drifting buoys will provide new and valuable information on the temperature and salinity structure in the upper 10 m of the ocean, including the diurnal cycle and the conditions within and outside of rings and eddies. Wind velocity measurements from the drifters will be used to diagnose changes in upper-ocean temperature and salinity structure and potential eddy-atmosphere coupling. Ocean model experiments will provide deeper process oriented insight into the impacts of salinity, winds, and surface heat fluxes on upper-ocean mixing and SST.
The proposed work is directly related to Competition 2 of NOAA/CPO’s CVP announcement, which seeks studies focused on observing, understanding, and/or process modeling of upper ocean processes and air-sea interactions in the Northwest Tropical Atlantic as part of the ATOMIC/EUREC4A-OA field campaigns. The proposed measurements and analysis will also advance our understanding of upper ocean processes (diurnal cycle and evolution of upper-ocean temperature and salinity structure), ocean boundary layers, and mesoscale ocean eddies, which is a specific goal of the proposal call. More broadly, our proposed work aligns well with the mission of the CVP program, which is to support research that enhances our process level understanding of the climate system through observation, modeling, analysis, and field studies.