Recent satellite-measured SST trends and 21stcentury climate model SST projections in North American east coast Large Marine Ecosystems

Monthly OISST data over the 35 year period 1982 – 2016 are used to quantify and compare SST trends in the four east coast North American Large Marine Ecosystems (LMEs). Trends > 0.4^oC decade^-1over the Gulf of Maine and Scotian Shelf are the strongest warming in the study area, and deeper basins within the Gulf and mid Scotian Shelf have trends ~ 0.5^oC decade^-1. The inner shelf of the South Atlantic Bight is the only region cooling over the study period, with maximum rates of stronger than -0.7^oC decade^-1. Seasonally separated trends show that warming in summer months (June-September) is significantly stronger than that of winter – spring months (February – April), especially over the Gulf of Maine, but the cooling trends in the South Atlantic Bight are dominated by winter months. The 26 models in the CMIP5 ensemble show that these global climate models project increasing SST over all four LMEs over the 21^stcentury, strongest over the Mid Atlantic Bight-Gulf of Maine (LME 7) and Scotian Shelf-Gulf of St Lawrence (LME 8), with mean trends of 0.35 and 0.42^oC decade^-1 respectively. By 2050, the CMIP5 ensemble mean projects SST averaged over the MAB - Gulf of Maine region to be 2^oC (summer) and 1.5^oC (winter) warmer than the 30-year baseline period 1982-2011. By 2099, this region is projected to be ~ 4.5 and 3.3^oC (summer, winter, respectively) warmer than the baseline period. In the period of OISST-CMIP5 temporal overlap (1982-2016), the CMIP5 ensemble mean effectively captures the summer-winter seasonal differences in SST trends in all four LME regions and the OISST trends are within the range of the ensemble. However, the CMIP5 ensemble mean over-predicts LME SST trends south of Cape Hatteras, but under-predicts LME trends by 0.1-0.2^oC decade^-1 at latitudes higher than this.