Understanding Physical Drivers of the 2016 Marine Heatwave in the Northwest Atlantic

Abstract:

Marine heatwaves (MHWs), defined as discrete events of anomalously warm ocean temperatures, disrupt marine ecosystems and hence human industries like fisheries, tourism, and aquaculture. Under anthropogenic climate change more intense and frequent MHWs are expected. Here we investigate MHWs in the northwest Atlantic to understand their physical drivers with a focus on 2012 and 2016, the two warmest years on record. Using NOAA’s OISST v2.1, we analyze spatiotemporal patterns of sea surface temperature (SST) for 2012 and 2016 as well as a ‘cold’ year, 2004. Temperature profiles from expendable bathythermographs taken by the volunteer observing ship CMV Oleander are used to explore subsurface signals in the Mid-Atlantic Bight, continental slope, and Sargasso Sea. In addition, experiments with an eddy-active ocean general circulation model are used, namely, Nucleus for European Modeling of the Ocean v3.6. Sensitivity experiments that isolate effects of buoyancy and wind forcing are used to explore physical drivers of the 2016 MHW in contrast with the 2012 MHW, which was largely forced by an anomalous Jet Stream position. Differences in spatiotemporal patterns and seasonal progressions of the 2012 and 2016 SST anomalies suggest distinct drivers for each MHW. SST anomalies were initially greater in 2016 than in 2012, but by March 2012 was much warmer. 2012 warming extended along the shelf from Cape Hatteras to Nova Scotia, with cool anomalies in the Sargasso Sea. In contrast, the 2016 warm signal was centered on the slope and in the Sargasso Sea, with cool anomalies further north near the Grand Banks. The presence of large amplitude Gulf Stream meanders near 60W likely contributed to the 2016 MHW, suggesting the Gulf Stream played a greater role in development of this MHW compared to 2012. These warming signals are not well-captured by the model, likely due to deficits of the model in representation of Gulf Stream variability. In 2004, the spatial pattern was reversed from 2016, with cool anomalies in the Slope Sea and warming extending from the Sargasso Sea to the Grand Banks. MHWs in the Northeast U.S. are complex and may have distinct drivers in different years. Future work should include examining impacts of the Gulf Stream motions on the ecosystem of the continental shelf and slope.