Seasonality in the Secular warming of the Northern Continents

Surface air temperature (SAT) over the Northern Continents has increased dramatically in recent decades.  Observational data in multiple independent analyses reveal the linear trend in SAT to exhibit striking seasonal variations, with trends in winter and spring greater than those in summer and fall.  This exhibits strong regional variations as well – the seasonality is most pronounced over Alaska and western-central Canada and Mongolia and south-central Russia.  The seasonality is robust in both the recent record (1970-2014) and the century-long one (1902-2014). The SAT trends in the historical 20^th century simulations produced by climate system models used in the Fifth Assessment Report of the Intergovernmental Panel for Climate Change are also examined.  These vary in magnitude and distribution, but largely show a much weaker seasonality in SAT trends compared to observations, except for two models that perform well in this respect over North America. Thus, the seasonal variation in SAT trends offers an interesting new tool for evaluating climate models. Furthermore, the striking seasonality in SAT observations provides an excellent opportunity for better understanding the dynamical and thermodynamical processes generating the secular warming signal over the Northern Continents, and thus warrants an explanation. We discuss and investigate two viable theories in this work.