Z. Long1,2, W. Perrie1,2, J. Gyakum3, R. Laprise5, D. Caya4,5

1Fisheries & Oceans Canada, Bedford Institute of Oceanography, Dartmouth NS, Canada 2Department of Engineering Math, Dalhousie University, Halifax NS, Canada 3McGill University, Montreal, Canada 4Ouranos Consortium, Montréal QC, Canada 5Université du Québec à Montréal, Montréal QC, Canada

It is well known that large lakes can perturb local weather and climate through mesoscale circulations, for example, lake-effects on storms and lake breezes, and the impacts on fluxes of heat, moisture, and momentum. However, for both large and small lakes, the importance of atmosphere-lake interactions in Northern Canada is largely unknown. Here, the Canadian Regional Climate Model (CRCM) is used to simulate seasonal timescales for the Mackenzie River Basin and northwest region of Canada, coupled to simulations of Great Bear Lake and Great Slave Lake using the Princeton Ocean Model (POM), to examine the interactions between large northern lakes and the atmosphere. We consider the lake impacts on the local water and energy cycles, and on regional seasonal climate. Verification of model results is achieved with atmospheric sounding and surface flux data collected during the Canadian GEWEX programme. The coupled atmosphere-lake model is shown to be able to successfully simulate the variation of surface heat fluxes and surface water temperatures, and to give a good representation of the vertical profiles of water temperatures, the warming and cooling processes, and the lake responses to the seasonal and interannual variation of surface heat fluxes. We show that these northern lakes can significantly influence the local water and energy cycles. Comparisons between our simulations and the observations of Schertzer et al. (2003) and Rouse et al. (2003) suggest the coupled lake-atmospheric model can provide good representations of the over-lake heat fluxes and water temperatures. In summer, the surface latent and sensible heat fluxes are small, and net downward radiation fluxes are dominant, and thus the lakes act as energy sinks. In the autumn, the net downward radiation fluxes are small, and the surface sensible heat and latent heat fluxes dominate the energy exchanges between the lakes and the atmosphere. During the (ice-free) warming phase, the northern lakes tend to be warmest near the shore areas and coldest in the deepest lake areas. During the cooling phase, the cooling process in Great Bear Lake is different from that of Great Slave Lake. In the fall, Great Slave Lake is coldest near the shore areas and warmest in deep water. In Great Bear Lake, the eastern areas of the lake are slightly colder than the western areas of the lake, but the water temperatures are more uniform than in Great Slave Lake. In addition, the coupled model was successful in simulating the vertical temperature profiles in both lakes, as well as the lakes' responses to the differing surface heat fluxes in the El Nino year 1998, compared to 1999. This study further suggests that, due to their large heat capacities, the northern lakes have significant impacts on surface heat and moisture fluxes. In summer, the surface temperatures over the lakes in coupled simulations are colder than uncoupled simulations; the lakes tend to reduce the fluxes of surface latent and sensible heat. However, in the fall, surface temperatures over the lakes are warmer than the uncoupled simulations; the lakes tend to increase the latent and sensible heat fluxes. During July-August, the net radiation used to warm these lakes is an average of 126 Wm-2 in Great Slave Lake and 155 Wm-2 in Great Bear Lake. This energy is released during the fall and winter. In October, the released energy through sensible and latent heat fluxes is about 135 Wm-2 in Great Slave Lake and 125 Wm-2 in Great Bear Lake. However, the simulated interannual variation of surface temperatures over the lake regions is slightly weak, comparing model results to observed 1998 and 1999 data. On the monthly time-scale, the impacts of both lakes on SLP are weak, although both lakes can notably impact the surface moisture processes over lake regions. In the autumn, the release of latent and sensible heat fluxes acts as a heat source, reducing the snow accumulation in the surface areas around the lakes. This is particularly evident in simulations around Great Bear Lake.