Response of the Upper Ocean to Northeast Pacific Atmospheric Rivers under Climate Change
Atmospheric rivers (ARs) are long, and narrow moisture transports essential to Earth’s water cycle because they are the primary mechanism for moving water from lower to higher latitudes. Many ARs occur over ocean basins because they tend to co-locate along the climatological storm tracks. However, to date, much of the climate change impacts literature is focused over land and population centers due to the potential for ARs to cause major flooding or relieve drought. Here, we focus on the upper ocean and analyze how climate change influences variables such as sea surface temperature and height, mixed layer depth, and surface energy fluxes. Understanding upper ocean impacts requires high fidelity to properly resolve the processes at play. For climate change scenarios, coupling an eddy-resolving ocean model to a 25km atmosphere allows us to resolve both ARs and ocean processes with greater accuracy.
ARs push water in the direction of the flow, increasing sea surface heights towards coastal communities as ARs approach land. Changes to sea surface height will be amplified under climate change and for the PNW by as much as 200%. ARs tend to produce deeper mixed layers upstream (and shallower downstream), which will be subdued under climate change. For example, mixed layers downstream of the AR will deepen. Heat flux responses are regionally dependent, but generally promote cooling upstream and warming downstream of ARs. Under climate change, for Southern Californian ARs, this process is amplified and is primarily driven by evaporation. For California and the Pacific Northwest, however, sensible heat is the dominant process driving the climate change response and is counter to the historical pattern.
Atmospheric rivers are important transport vehicles for Earth’s water cycle. Using a high-resolution, eddy-resolving Earth System Model, atmospheric river impacts on the upper ocean are investigated by analyzing historical and climate change simulations. For atmospheric rivers along the North American coastline, strong winds cause significant dynamic and thermodynamic upper ocean responses. They push ocean water towards the coast, measured by sea surface height, a process that is amplified under climate change. Mixed layers are deeper upstream of atmospheric rivers, and shallower downstream, however for climate change, shoaling downstream is subdued. Air-sea heat fluxes tend to promote ocean cooling upstream and warming downstream, although different regions have different climate change heat flux signals. Southern California heat flux changes due to warming are driven by evaporative processes and strengthen the ocean responses seen in historical simulations. The regions north are primarily dominated by sensible heat flux changes that counter historical patterns.