Collaborative Research: U.S. Crossroads—Connectivity of the North Atlantic Deep Western Boundary Current through the Subpolar-Subtropical Transition Zone

The subpolar-subtropical transition zone in the western North Atlantic has been identified as potentially key to Atlantic Meridional Overturning Circulation (AMOC) variability on decadal time scales. At this crossroads of the AMOC, the southbound Deep Western Boundary Current (DWBC) meets the northbound North Atlantic Current (NAC), and their interaction sets the properties of the deep waters exported to the rest of the Atlantic Ocean and beyond. The global overturning circulation—of which the AMOC is an important component—has proven to be a far more complex system than perhaps imagined when it was first likened to a great ocean conveyer in the 1980s. Progress in developing a mechanistic understanding of the response of the AMOC to various forcings is accelerating as direct continuous observations of its structure and variability are being sustained at a few latitudes for multiple years and even decades. Of interest here is the southward progression of water mass anomalies, generated at high North Atlantic latitudes, along the western boundary, particularly for overflow waters crossing the subpolar-subtropical transition zone. Recent observations of a decline in AMOC strength at 26°N have been attributed in part to overflow water density anomalies at the western boundary. Tracking down the upstream origin and pathways of these anomalies, and having them realistically represented in models, is key to being able to predict future AMOC changes. This project will measure the pathways of overflow waters with the deployment of 80 acoustically tracked floats and virtual floats in a high-resolution model. The float measurements will fill a critical gap in observations needed for model evaluation and contribute new understanding of the pathways and processes impacting the properties of overflow waters as they transit from the subpolar to subtropical North Atlantic. AMOC variability has been associated with a long list of climate impacts with societal relevance. U.S. Crossroads is focused on a region that has been proposed to "set the variability of the AMOC on decadal time scales," and therefore has implications beyond a regional process study. U.S. Crossroads is highly synergistic with two concurrent European-led programs with similar aims and complementary tools, namely "Explaining and Predicting the Overturning Circulation" (EPOC) and "French Crossroads." The partnerships with European programs will foster international resource sharing as well as exchange of ideas. The project will also support training and career development for a post-doc, who will spend time at both WHOI and FSU to gain experience in the analysis of both observations and model output. The project will also extend OceanInsight, a long-standing outreach program, with support for development of a "remote field trip kit" to help foster interest and excitement in ocean science for all learners. Most attention has to date been focused on Labrador Sea Water (LSW) export and its impact on AMOC variability, revealing weak connectivity across the subpolar-subtropical boundary. Modeled particle trajectories suggest a greater connectivity for the deeper overflow waters (lower NADW) passing through the transition zone via the DWBC compared to LSW, but equivalent Lagrangian observations in overflow waters are lacking. This project will build on previous observational and modeling work with new float observations of overflow water pathways and new particle simulations using state-of-the-art, high-resolution, multi-decade North Atlantic simulations. A total of 80 acoustically tracked floats will be released in overflow waters in the DWBC to measure their pathways through the transition zone and identify processes that lead to boundary-interior exchange. Orders of magnitude more modeled particle trajectories will be generated using 1/12° (6 km) and 1/50° (1.5 km) configurations of HYCOM (Hybrid Coordinate Ocean Model) North Atlantic simulations to amplify the necessarily limited float observations, test the sensitivity of overflow water pathways to model resolution and investigate decadal variability in those pathways related to NAC and AMOC variability. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.