Cooperative Institute for Research in Environmental Sciences

Synoptic forcing of precipitation in the Mackenzie and Yukon River basins

E.N. Cassano and J.J. Cassano

2010, International Journal of Climatology , 30, 658-674.

The relationship between near surface atmospheric circulation, as characterized by sea level pressure patterns, and precipitation in northwestern North America and adjacent regions is presented. A synoptic climatology of sea level pressure patterns based on daily sea level pressure anomalies from the ERA40 reanalysis dataset was created using the method of self-organizing maps. This objective analysis identified all major near-surface atmospheric circulation patterns in the region and illustrated the progression of dominant circulation patterns throughout the seasons, with strong Aleutian Low patterns dominant in the winter and patterns characterized by low pressure over land areas and the Beaufort/Chukchi Seas in the summer. These synoptic patterns were then related to daily precipitation in this area with a particular focus on the Mackenzie and Yukon River basins. The highest daily precipitation values, for both the Mackenzie and Yukon basins, were associated with patterns that occur most frequently in the summer, likely associated with increased frequency of cyclones and convective events that occur over land in that season. During winter, the largest positive precipitation anomalies were along the coastal mountain range in southeastern Alaska associated with Aleutian Lows bringing warm, moist flow from the south resulting in upslope flow on the windward side of these mountains. These patterns were responsible for many of the large precipitation events in the winter in the Mackenzie Basin as some of the orographically forced precipitation was advected over the mountains and into the basin. The largest precipitation events in the winter in the Yukon basin occurred with patterns that have a low pressure center to the southwest of the basin. This synoptic pattern results in southerly flow advecting moisture into the basin to the west of the higher topography which bounds much of the southern boundary of the Yukon watershed.