Sensitivity of Atmospheric Rivers to Damping of Mid-latitude Sub-seasonal Variability

We examine how atmospheric rivers (ARs) over the North Pacific basin respond to relaxation in mid-latitudes in sets of re-forecasts made with the Community Atmosphere Model using super-parameterized convection. Initial conditions and boundary conditions are chosen in order to evenly sample boreal mid-winter of 2014/15. In two suites of experimental simulations, zonal and meridional wind, temperature, and specific humidity are relaxed within the mid-latitude band 40^o-70^o N, and compared with a control set of re-forecasts made without relaxation. In one experimental suite meteorological parameters are relaxed to a smooth seasonally-varying climatological cycle, and in the other they are relaxed to the smooth seasonal evolution of the control simulations. Much of the resultant changes are sensitive to the relaxation target, and relaxation to a climatological cycle results in warming and moistening in the eastern Pacific, which reinforces ARs. Independent of this, there is a consistent impact on the Pacific circulation and on different AR types that emerges after about a week. Damping sub-seasonal variability acts to anchor the West Pacific trough and deepen height anomalies at 40^o N, as well as systematically modify the duration of AR events. AR events with moisture originating from mid-latitudes tend to have shorter lifetimes, while AR events with moisture from the tropics last longer. The latter AR events also tend to become more zonally-oriented and can even extend across most of the Pacific basin by weeks 3-4. These changes in ARs translate to changes in landfall frequency, duration and location.