Recent Changes of Sea Level and Circulation around the Grand Banks: Local Dynamics and Broader Connections

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

This project will examine large anomalous sea surface height (SSH) seen in satellite altimetry between the U.S. northeast coast and the Gulf Stream, using analyses of observations, data-model results (reanalysis products), realistic and idealized model simulations, and a simplified 2-layer process model. For example, the sea-surface height seen over the Grand Banks was about 7-8 cm higher in 2008-2018 than in 1993-2007. But still unknown are its causes and potential effects on the regional water motions, including the continuity of the Labrador Current and shelf and slope currents. Sea-level changes have broad impacts on coastal habitats, coastal hazards, and wetland ecological systems. The identified sea-level anomaly is accompanied by a rapid rising of water temperature on the shelves, which is known to have adverse impacts on the fishing industry and aquaculture. The project would: 1) investigate forcing mechanisms of this large sea level anomaly in the shelf-slope region; 2) examine how sea-level change on the continental shelf is connected to that in the open ocean; and 3) investigate whether and how the sea level changes on the Newfoundland and Scotian shelves may have affected coastal circulations and sea level off the Northeast US. Project PIs will keep NOAA Northeast Fishery Sciences Center, the Commercial Fisheries Research Foundation, and the wider fishing industry communities informed of relevant results, and engage in a range of outreach and educational activities, including efforts to engage underrepresented minorities. The proposed work is aligned with US CLIVAR Science Plan Addendum "Research Challenge on Climate at the Coasts," and is expected to generate broad interest from both the scientific community and the general public. The project identifies seven hypotheses covering the three areas of interest identified above. For the first, concerning forcing mechanisms: H1A - changes in western boundary currents related to a weakening AMOC or changing wind-driven gyres in subtropical and subpolar basins contribute to the observed SSH anomaly; H1B - a change in surface wind-stress curl forces the observed SSH anomaly; H1C – an increase in the number of warm core rings contribute to both mass and steric heat fluxes associated with increased WCR activity contribute to the observed SSH changes. For the second, concerning the cross-shelf connection of SSH variability: H2A - the cross-shelf transport in the bottom Ekman layer plays an important role in the cross-shelf SSH connectivity; H2B - the surface wind-driven Ekman layer plays an important role as well; H2C - eddy PV flux increased and contributed to enhanced cross-shelf connectivity in SSH anomaly. For the third, concerning connections between Grand Banks and Cape Hatteras: H3A - the SSH anomaly on the Newfoundland and Scotian shelves have impacted the along-shelf flows considerably, likely enhancing the equatorward flow from Newfoundland to Cape Hatteras. Both modeling and data analysis will be used to address these hypotheses. The fairly wide range of data used will include: gridded SSH altimetry, global ocean and atmosphere reanalyses, observation-based heat fluxes, evaporation and wind stress, and local multi-year in-situ data from the OOI Pioneer Array and the Oleander line. Modeling will utilize a new Northwest Atlantic Regional Circulation Model, which encompasses the entire shelf and slope region of the Labrador Sea and the Middle Atlantic Bight with a submesoscale-permitting horizontal resolution of 2-km and 40 vertical layers. A two-layer model will be used in parallel with the realistic regional circulation models for process modelling, to help guide analyses and interpretations of data analyses and NWA model simulations, as to help isolate key processes and mechanisms. This model uses realistic bathymetry and forcing and resolves the barotropic and the 1st baroclinic modes. The study will lead to a better understanding of how dynamical processes interact in a rapidly changing climate system, how coastal oceans respond to climate-induced changes in the open ocean, and inform predictability of the Northeast US shelf circulation. 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.