Characteristics of the water vapour transport over the Mackenzie River basin during the 1994/95 water year

The National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data were used to calculate the moisture flux convergence over the Mackenzie River basin (MRB) for a 10-year period from 1987 to 1996. The analysis of these results indicates that the 1994/95 water year was associated with the least amount of water vapour transport into the MRB over this selected period. This arose due to some unique features in the vertical structure of water vapour transport. In particular, the average annual level of maximum water vapour transport occurred at 700 hPa during 1994/95; this is higher than the 850-hPa level found for other years. Because of the lower humidity at higher levels in the atmosphere, the magnitude of the most significant water vapour transport was therefore less. Detailed analyses of the monthly data for the 1994/95 water year illustrated that the decrease in water vapour transport arose, in part, because of the year's deep mixed boundary layer attributable to its warm temperatures and dry surface conditions. Of course, the magnitude of the monthly moisture flux convergence changed substantially during this water year. The largest contribution occurred from July to October 1995, with the maximum in July. Two other peaks were found in October 1994 and February 1995. The moisture flux convergence at the transition period between seasons (May and June) was very weak and sometimes even negative. To understand better the components of the water balance for this water year, observational precipitation data were used to estimate evaporation. The water balance for this specific water year also showed that the precipitation in the 1995 summer was due mainly to the moisture flux convergence into the MRB, instead of the evaporation found in other studies. Correlation coefficients between the moisture flux convergence and precipitation were much higher for this water year than in other studies. The horizontal distribution of the moisture flux convergence across the basin during this water year displayed a band of high values between the Rocky Mountains and Great Bear Lake and another band of high values at the southern edge of the basin, which is closely linked with the topography in the MRB.