Climate Risk Areas Initiative News

The First Mixed Layer Depth Climatology Over the Northeast U.S. Continental Shelf

  • 30 November 2021
The First Mixed Layer Depth Climatology Over the Northeast U.S. Continental Shelf

A new study, funded in part by CPO’s Climate Variability & Predictability (CVP) and Modeling, Analysis, Predictions, and Projections (MAPP) programs, is the first to construct a detailed description of the ocean mixed layer depth and its seasonal cycle for the Northeast U.S. continental shelf. 

The Northeast U.S. continental shelf is home to some of the worlds’ most productive and commercially valuable fisheries. Characterized by the contrast of the warm, northward-flowing Gulf Stream and the cold, southward-flowing Labrador Current converging together, the region has been significantly impacted by the changing climate, with a consistent rate of warming and notable extreme warming events known as marine heat waves. New research, published in Continental Shelf Research, from scientists at Northwestern University, Woods Hole Oceanographic Institution, and NOAA Fisheries, is focused on a key factor that determines the amount of warming experienced in the upper ocean -- mixed layer depth (MLD). 

Authors Cassia Cai, Young-Oh Kwon, Zhuomin Chen, and Paula Fratantoni computed MLD using an observational dataset from NOAA’s Northeast Fisheries Science Center (NEFSC). There were three primary goals of this study: (1) to describe the MLD seasonal cycle across the entire shelf, (2) to investigate the long-term change in MLD (specifically, 1993 to 2018), and (3) to assess the  GLobal Ocean ReanalYsis and Simulation project (GLORYS12V1) ocean reanalysis dataset against the observational estimates.

The team found clear MLD seasonal cycles, with the largest seasonality in the Gulf of Maine and smallest in the southern Mid-Atlantic Bight. While the reanalysis dataset showed similar MLD means across the region, the two datasets did not agree on the MLD’s interannual variability. The authors call for a more systematic assessment of multiple reanalysis products against observations from the Northeast U.S. coastal environment. They also hope their results can be a starting point for future studies on the drivers of MLD variability on the Northeast U.S continental shelf. These studies, in turn, could lead to better understanding of how changes in MLD variability influence changes in the upper ocean heat content.

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