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On the physical processes associated with the water budget and discharge of the Mackenzie basin during the 1994/95 water year

Abstract The Mackenzie River basin water budget and its discharge are investigated using the Canadian Meteorological Centre's Regional Finite Element/Global Environmental Multiscale (RFE/GEM) model‐based analyses and observations. A comprehensive water budget analysis for the Mackenzie basin illustrates that the annual convergence of moisture flux over the region is positive, and that the moisture available for precipitation originates mainly from moisture transport across the south‐west and north‐west boundaries of the basin. In the summer, however, the basin's moisture supply comes mainly from local evaporation. It is found that major atmospheric forcings for discharge are predominantly determined by 1) the nature of the circulation systems and the availability of moisture, and 2) the alteration of moisture gradients through horizontal twisting by the wind fields. The second process is dominant over most of the 1994/95 water year. The basin scale discharge in the autumn (spring) is, to a large extent, related to cyclonic (anticyclonic) circulation systems through moisture redistribution. This occurs due to the horizontal twisting of moisture gradients by the wind fields and the corresponding moisture advection with horizontally‐twisted moisture gradients. Over the southern Beaufort Sea, the overall storm frequency in the 1994/95 water year was very low compared with its climatology (Hudak and Young, this issue). However, this region was subjected to the highest percentage of Pacific‐origin storms on record during the 1994/95 water year. Synoptic scale weather systems associated with major snowfall and critical spring snowmelt events in the 1994/1995 water year are also examined. Results show that one particular storm in the autumn of 1994 produced much of the initial snowfall for the basin and it was associated with a deep low pressure system providing a great deal of moisture flux into the basin through its western boundaries. It is also shown that a rapid spring snowmelt event in April 1995 was associated with a shallow high pressure system. The descending motion associated with this high pressure system and strong horizontal advection induced by a south‐easterly low‐level jet were two major elements that contributed to the rapid heating of the basin surface and the subsequent rapid melting of the snow. This unusual snowmelt event was a contributing factor to the annual discharge peak being earlier than normal.