The shallow limb of the Atlantic meridional overturning circulation: variability and impacts on the ocean's carbon and nutrient reservoirs

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The subpolar North Atlantic is a region where intense air-sea interactions cause the transformation of thermocline waters into of a collection of water masses known as North Atlantic Deep Water (NADW). The NADW floods the deep and intermediate depths of the basin and provides a pathway for global change to find its oceanic expression. The precursor waters that are transformed to become NADW stem partly from subtropical water masses, whose penetration into the subpolar gyre is thought to vary temporally. The basic mechanisms that govern the overturning transport and its temporal variability remain poorly understood, as do the implications for the ocean's interior heat, carbon, and nutrient reservoirs. In order to detect anthropogenic change in the ocean's circulation and biogeochemical properties, we must enhance our understanding of the mechanisms causing this variability. Therefore, the fundamental questions to be addressed by the proposed work are: What are the dominant mechanisms setting the temporal variability of the Atlantic overturning transport? What are the consequences of such variability on the biogeochemical properties of the NADW? The work will be organized around 3 objectives: 1. Characterize mechanisms governing variability in the MOC, its vulnerability to anthropogenic change, and its impacts on the ocean's carbon and nutrient reservoirs in a General Circulation model. 2. Investigate the variability and trends emerging in observational data from hydrographic missions, satellite products, and nearly two decades of data collected by profiling floats in the North Atlantic. 3. Build a program at McGill that will collect direct observations of the shallow limb of the Atlantic overturning circulation. This goal includes the mounting of a lab for the autonomous collection of observations in the harsh ocean environments in which water masses are formed.