[21-Feb-2024] Ryan, S., Ummenhofer, C., Gawarkiewicz, G., Taenzer, L., and Parfitt, R.
Presented at the 2024 Ocean Sciences MeetingThe Northeast U.S. continental shelf is a highly productive and economically important region that has experienced robust changes in upper-ocean properties in recent decades. Warming rates exceed the global and North Atlantic rates and in particular several episodes of anomalously warm temperatures, so called marine heatwaves (MHWs), have had devastating impacts on regional fisheries over the past decade. MHWs can be driven by various atmospheric and oceanic processes, such as increased heat flux into the ocean, advection of warmer waters, or a general shift in the large-scale circulation. The upper ocean density structure can pre-condition the shelf for MHWs by regulating the impact of surface warming, i.e., a thin surface layer will heat faster. The role of salinity and its contribution to density over the shelf has not been investigated in the MHW context. Here, we use a suite of observational data, including in-situ and remote-sensing data to assess the spatio-temporal variability of salinity and related upper ocean density structure on seasonal to interannual time scales with focus on the implications for MHWs. The interplay of cool, fresh shelf water and warm, saline slope water depicts a challenge as both water masses have competing effects on density. During summer, seasonal freshwater input via river discharge creates a thin fresh surface layer that can extend all the way to the shelf break and can contribute significantly to the build-up of stratification in addition to the thermal stratification. In contrast, a positive salinity onshore flux from the slope can have compensating effects on density and drive destratification, while the offshore water is also generally much warmer and onshore intrusions can drive MHWs in themselves. Understanding these (at times) competing processes and their seasonal and interannual variability is an important step in characterizing different MHW events, including surface vs. subsurface events, and their drivers.