Modulation of Pacific-North America Teleconnections by Seasonality of the Basic State

Unlike the reminder of the year, summer temperatures over North America appear only weakly related to SST anomalies in the tropical Pacific. Wave activity fluxes and a linear barotropic model are used to explore the influence of the basic state of the Pacific sector on Rossby wave propagation, showing that zonal asymmetries of its summer configuration insulate North America from tropical influence. In contrast, we show that subtropical Pacific SST anomalies are predictive of large-scale heat waves in North America during summer. The very fact that the basic state limits the response of the extratropical Pacific to ENSO variability during summer appears to allow the weaker subtropical forcing to play a more central role. Atmosphere-ocean interactions lead to the growth of a coupled mode in response to an enhanced meridional SST gradient. Ducting of wave activity fluxes in the mid-latitude Pacific provides an atmospheric bridge to North America, where flux convergence is associated with the development of large-scale blocking and heat waves.