Extreme winter warming events over the Mackenzie Basin: Dynamic and thermodynamic contributions

The Mackenzie River of Canada is one of the great rivers of the world. Its basin is characterized by a highly variable topography and its climate is subject to many important cold-region phenomena. Over the last few decades, the Mackenzie Basin has also been experiencing a pronounced winter warming. In this study, a number of extreme basin warming events and related processes during the winter are investigated using surface and rawinsonde data. By documenting these events, we have found that the basin warming is mainly associated with low pressure systems in and near the basin, and with extratropical and subpolar high pressure systems in the vicinity of the basin. The dynamic and thermodynamic contributions (due to atmospheric circulations, topography, low-level temperature inversion and their interactions) to the basin warming are also examined. It is shown that basin warming events occur through the horizontal advection of warm air from west and south of the basin, and through adiabatic descent induced by both the topography and the low and/or high pressure system, particularly when the low-level temperature inversion occurs.