Over-lake meteorology and estimated bulk heat exchange of Great Slave Lake in 1998 and 1999

Abstract

Meteorological and thermistor moorings were deployed in Great Slave Lake during the Canadian Global Energy and Water Cycle Experiment (GEWEX) Enhanced Study (CAGES) in 1998 and 1999. Large-scale meteorology included influence from a record ENSO extending from 1997 to mid-1998. Meteorological variables varied across the lake especially during the lake-heating phase after ice breakup. Generally higher over-lake air temperature and surface water temperatures occurred in 1998, but larger vapor pressure gradients over water and ∼8% higher solar radiation was observed in 1999. Although wind speed averages were similar in both years, nearly 30% more over-lake storms with winds >10 m s-1 occurred in 1998. High sensitivity of the lake temperatures to surface wind forcing was observed in 1998 in the spring warming phase. Passive microwave imagery [from the Special Sensor Microwave Imager (SSM/I)) at 85 GHz showed a record 213 ice-free days in 1998 compared to 186 days in 1999. The extended ice-free period in 1998 significantly influenced lake temperature and heat content. Maximum heat content occurred on 5 August in 1998 (2.61 × 1019 J) compared to 23 August in 1999 (2.22 × 1019 J). The daily heat content formed the basis for deriving a bulk heat exchange. Five-day-averaged bulk heat exchange generally fluctuated between means of 282 (spring) to -225 (fall) W m-2 in 1998, compared to a smaller range of 171 (spring) to -202 (fall) W m-2 in 1999. Good correspondence between the bulk heat exchange based on lakewide heat content and computed heat flux at Inner Whaleback Island gives confidence in the seasonal heat flux and the heat content methodology. The seasonal trend and magnitudes of heat contents and bulk exchanges derived in this analysis form the basis for future research on the interannual variability in lakewide heat budgets and for developments leading to coupled climate-lake models.