Investigating the Transient and Stationary Eddy Response to Anomalous North American Snow Cover

Three modeling studies are presented, which investigate the influence of anomalous North American snow cover on atmospheric dynamics and surface climate. The overall goal is to identify physical mechanisms and pathways by which anomalous North American snow cover may influence regional to hemispheric scale atmospheric circulation and downstream climate. Much of the recent research into snow -- climate interactions has focused on the influence of Siberian/Eurasian snow cover while comparatively little attention has been paid to North America. While the dynamics of the atmospheric response to Siberian/Eurasian snow anomalies are well-documented the limited research studies focused on North American have largely been exploratory in nature. The effects of North American snow anomalies on large-scale atmospheric dynamics, in particular transient and stationary eddy behavior, has not been studied in any detail.;A second study expands on these findings and imposes more comprehensive, full-year, North American snow perturbations. A robust transient eddy response begins in fall and propagates around the mid-latitudes, reaching maximum extent and magnitude in spring. Robust responses in associated surface climate variables are observed downstream over Eurasia in late winter -- early spring. Steepened temperature gradients over North America initiate and maintain increased baroclinicity, which leads to enhanced and extended mid-latitude storm tracks. This physical pathway is posited to be the cause of the downstream surface climate response. Additionally, active, incoherent winter and active, coherent spring stationary wave responses are also observed.;A third modeling study investigates the stationary wave response during the winter and spring seasons through the use of linear and fully nonlinear stationary wave models. These models are forced by GCM output diabatic heating, transient activity and orography from high and low snow experiments. The linear stationary wave model indicates that all the individual forcing components exhibit coherent responses of varying amplitude to snow during both winter and spring. During winter the diagnostic stationary nonlinear component dominates the stationary wave response while diabatic heating dominates in the spring season. The substantial contribution of stationary nonlinearity in both winter and spring suggests that nonlinearity is important for the stationary wave response to snow. The fully nonlinear stationary wave model is used to further decompose the response into parts due to direct nonlinear effects for each forcing and nonlinear interaction effects between forcings. Comparison of the linear and nonlinear model responses also facilitates the identification of any stationary wave response that may be due to statistical sampling instead of the physical snow forcing. The part of the winter and spring stationary waves responses that is snow induced is largely due to the coherent direct nonlinear effects of heating and complex nonlinear interactions between heating, transients and orography. Hence order and structure can be found underlying an ambiguous overall winter stationary wave response to snow. The relatively more coherent overall spring season response can likewise be attributed to physically meaningful component forcings.;The findings in this thesis make a number of important scientific contributions. The identification of a physically-based pathway involving transient eddy activity represents a new contribution to our understanding of the snow → climate relationship. This finding expands our knowledge of land surface -- atmosphere interactions and demonstrates that for similar types of surface boundary forcing highly varied and complex responses may emerge. Further, establishing a hemispheric-scale response to large scale snow anomalies, while shown previously for Siberia, has not been previously demonstrated for NA. The nature of this response, i.e. an enhanced, robust, hemispheric-scale enhancement of transient eddy activity has not been previously reported. Finally, the detailed investigation into dynamical response of the atmosphere to a land surface perturbation, through the decomposition of the winter and spring stationary wave responses, yields new insights into the mechanisms that influence planetary stationary waves and how they reinforce, counter or otherwise interact with each other.;The first study completes a sequence of earlier GCM modeling experiments which initially focused on Eurasian snow cover, and now turns its attention towards North American snow cover. Anomalous fall (SON) snow cover is shown to elicit a downstream climate response over Eurasia. A positive surface temperature (negative sea level pressure) response is observed over eastern Eurasia in the winter season (DJF). It is shown that inclusion of orography is essential for these effects to manifest. However, a plausible physical pathway for the modeled snow climate teleconnection is not immediately clear. Modest, though active stationary and transient eddy responses are shown and are hypothesized to play a role in the modeled downstream climate response.