Long-term Change in the Mixed Layer Depth on the Northeast U.S. Continental Shelf

Abstract:

The Northwest Atlantic Ocean, in particular the coastal ocean on the Northeast U.S. Continental Shelf (NES), has experienced rapid warming over recent decades, leading to disruptions in coastal ecosystems. The mixed layer depth (MLD) can impact the amount of warming experienced by the surface water. Naturally, a question is whether the changes in the MLD act to hinder or exacerbate this warming. Although the NES has been extensively studied, a full description of the mixed layer and its long-term change in depth is lacking. Here, we compute the MLD using a 0.2°C temperature threshold and analyze the MLD in the 5 eco-regions of the NES using an observational dataset collected between 1977 and 2019 by NOAA’s Northeast Fisheries Science Center hydrography program. This dataset includes over 60,000 profiles of temperature and salinity. There is greater data coverage in the Spring and Fall (31.9% and 29.3% of all profiles, respectively) and fewer in Winter (13.94%). The data is filtered to remove bottle observations, incomplete profiles, and instances where the MLD is detected at the surface layer, resulting in 51,196 usable profiles. The overall deepest MLD (mean:78.16m) is observed in the western Gulf of Maine (GoM) in Winter while the shallowest MLD (mean: 8.91m) is observed in the southern Mid-Atlantic Bight (MAB) in Summer. Within a region, the MLD trend varies significantly across seasons. The long-term change in depth for each region as well as a comparison of our new observational MLD data with the GLORYS12v1 ocean reanalysis MLD will be further discussed. With this, we can identify potential drivers for the observed MLD changes.