Spatial population synchrony has been the focus of theoretical and empirical studies for decades, in the hopes of understanding mechanisms and interactions driving ecological dynamics. In many systems, it is well-known that seasonality plays a critical role in the density-dependence structure of the populations, yet this has hardly received any attention in synchrony studies. Here, we propose a protocol that allows to elucidate deterministic and stochastic sources of spatial synchrony, while accounting for geographic-and season-specific density dependence. We apply our protocol to seasonally-sampled time series of sub-arctic gray-sided voles, known for marked spatial synchrony. Dissociating seasonal density-dependence contributions to the total observed synchrony reveals differential strength and shape of synchrony patterns by season. Mild winter weather reveals to be an important driver of vole spatial synchrony, with lagged effects in the fall. This has direct implications to the future population dynamics of such species when facing climate change.