Fluxes of water and energy between the surface and the atmosphere differ drastically between land and ocean, making midlatitude continental air typically drier than oceanic air at the same latitude. In this study, we define the “land wake” as the signature of continental air over the ocean downwind of large landmasses. We focus on the midlatitude North Atlantic, where oceanic air has anomalously low relative humidity downwind of North America for several hundred km, driven predominantly by high continental temperatures in summer and low continental moisture in winter.
We show that the response of the terrestrial carbon cycle to increased CO2 intensifies the land wake downwind of midlatitude North America using simulations from the Coupled Climate Carbon Cycle Model Intercomparison Project (C4MIP) where only the biogeochemical components of the model (and not atmospheric radiation) respond to increased CO2 concentrations. In summer, reduced terrestrial evapotranspiration results in hotter, drier air flowing from the continent to the ocean, intensifying the land wake diagnosed from relative humidity. The warmer continent also generates an anomalous low-level cyclone which brings warm, moist air from lower latitudes up the east coast of North America, dampening the reduction in oceanic relative humidity. In winter, increased leaf area due to CO2 fertilization leads to warmer continental temperatures due to reduced land albedo. With no substantial change in winter evapotranspiration, these increased temperatures reduce continental relative humidity. Offshore (eastward) winds are strong in winter, carrying the anomalously warm air over the ocean and generating an intensification of the land wake. These changes in temperature, specific humidity, and relative humidity of air downwind of the continent in turn lead to a reduction in low cloud cover over the coastal ocean, and increases of roughly 5 W/m2 in downwelling shortwave radiation reaching the ocean surface.