Dynamical downscaling of future hydrographic changes over the Northwest Atlantic Ocean

Projected climate changes along the U.S. East and Gulf Coasts were examined using the eddy-resolvingRegional Ocean Modeling System (ROMS). First, a control (CTRL) ROMS simulation was performed usingboundary conditions derived from observations. Then climate change signals, obtained as mean seasonalcycle differences between the recent past (1976–2005) and future (2070–99) periods in a coupled global cli-mate model under the RCP8.5 greenhouse gas trajectory, were added to the initial and boundary conditionsof the CTRL in a second (RCP85) ROMS simulation. The differences between the RCP85 and CTRLsimulations were used to investigate the regional effects of climate change. Relative to the coarse-resolutioncoupled climate model, the downscaled projection shows that SST changes become more pronounced nearthe U.S. East Coast, and the Gulf Stream is further reduced in speed and shifted southward. Moreover, thedownscaled projection shows enhanced warming of ocean bottom temperatures along the U.S. East and Gulf Coasts, particularly in the Gulf of Maine and the Gulf of Saint Lawrence. The enhanced warming was relatedto an improved representation of the ocean circulation, including topographically trapped coastal oceancurrents and slope water intrusion through the Northeast Channel into the Gulf of Maine. In response toincreased radiative forcing, much warmer than present-day Labrador Subarctic Slope Waters entered the Gulf of Maine through the Northeast Channel, warming the deeper portions of the gulf by more than 48ºC.