0 1988, by the American Society of Limnology and Oceanography, Inc. Convective circulation in littoral water due to surface cooling

Cooling of littoral waters of a lake under calm conditions creates a horizontal temperature gradient because surface heat loss is distributed over progressively larger depth by plunging sheets (thermals). Resulting convective circulation establishes an exchange flow between littoral and open waters. Numerical simulations illustrate the development ofthe flow from the formation ofthermals to the establishment of a horizontal circulation. Circulation flow rate was found to be proportional toW’/fl,2<n<3(Wisth R l’h e ay etg number) for W values in the range 104-1O8. A laboratory visualization experiment at %?- lo9 confirmed the main features of the simulation. To consider applications to vegetated littoral slopes, we interpreted the results with a formula accounting for the added resistance offered to the flow by the aquatic plants. An example showed that convective Icirculation can be a significant factor in renewing littoral waters. In coastal waters heat transfer through the water surface may, under favorable condi-tions, generate horizontal temperature gra-dients. An example is the spring thermo-cline which occurs when solar radiation heats nearshore shallow waters faster than open waters and creates an inclined density in-terface (Csanady 1982). On a much shorter time and length scale a phenomenon of sim-ilar nature may occur during nighttime cooling of littoral waters, or on a seasonal scale in midlatitude lakes during early fall. The phenomenon does not seem to have been documented in the literature, but un-published data confirm its occurrence (J. Barko unpubl. data). A nearshore temper-ature gradient develops during cooling be-cause shallow water cools faster than deeper water when both regions are subjected to the same surface rate of cooling. The result is that the shallow nearshore water becomes negatively buoyant, and a convective cir-culation (underflow) is set in motion (Fig. 1). Since this circulation is driven by the surface rate of cooling (heat loss), it is sim-ilar to the convective circulation that arise