Moisture recycling over the Mackenzie basin

Abstract

Moisture recycling over the Mackenzie basin is investigated by estimating the precipitation recycling ratio (the ratio of precipitation derived from local evaporation to the total precipitation within the basin) for the region with the National Centers for Environmental Prediction (NCEP) reanalysis dataset and the Meteorological Service of Canada (MSC) precipitation climatology. The results suggest that recycling is very active over the region during the warm season (April – August) and extremely inactive during the cold season. The annual recycling ratio estimated for the basin is about 0.25, which is close to that estimated by others for the Mississippi and Amazon basins despite the lower annual evapotranspiration over the Mackenzie basin.

The high recycling ratios and the recycling patterns estimated for the basin during the warm season are found to be consequences of the unique topographical and climatic settings characterizing the region. Analysis of conditions during the years having anomalous spring and summer precipitation suggests that the large‐scale atmospheric setting could act in concert with the basin's unique topographic and surface characteristics to increase or to decrease precipitation and its recycling over the basin, depending on whether the basin is under the influence of a persistent large‐scale low or a high pressure system. In the former case, much of the recycled precipitation would fall over the north‐western parts of the basin where the runoff ratios are relatively high, and thus enhance the summer discharge from the basin. When the basin is under the influence of a persistent high pressure system, much of the recycled precipitation would fall over the southern part of the basin where the runoff ratios are relatively low, and thus reduce the discharge from the basin. It is suggested that this latter effect might have contributed to the record low summer discharge from the basin during 1995.