Sensitivity of Atmospheric Rivers to Damping of Midlatitude Subseasonal Variability

Abstract

AbstractWe examine how atmospheric rivers (ARs) over the North Pacific basin respond to relaxation in midlatitudes in sets of reforecasts made with the Community Atmosphere Model using superparameterized convection. The study focuses on January–March 2015 with weekly staggered initial conditions and sea surface temperature prescribed. In one set of reforecasts, zonal and meridional winds, temperature, and specific humidity are relaxed within the midlatitude band 40–70°N and compared with a control set of reforecasts made without relaxation. Meteorological parameters are relaxed to the smooth seasonal evolution of the control simulations (the seasonal mean, trend, and parabolic fit), about which subseasonal deviations are damped. Relaxation has a systematic impact on the Pacific circulation and on different AR types. Damping subseasonal variability acts to anchor the West Pacific trough and deepen height anomalies at 40°N, as well as modify the total number and duration of AR events. Although largely offset by changes in the number of events, individual AR events with moisture originating from midlatitudes tend to have shorter lifetimes, while AR events with moisture from the tropics tend to last longer.