Thermal Overturning Circulation in an Arctic Pond

In a 1.5-m-deep arctic pond (68.626N,149.597W), bottom-water renewal observed during early summer was dominated by thermal overturning circulation, rather than by wind-driven overturning or vertical turbulent mixing. Water velocities and rates of viscous dissipation of turbulent energy were estimated using three pulse-coherent Nortek Aquadopp ADCPs, while temperature stratification was measured along three vertical profiles using 41 RBR Solo-T temperature loggers. Mixing displayed strong daily cycles, with a warm, <0.5-m-deep surface mixed layer during daytime, which cooled and thickened to ~0.75 m during night-time free convection. At greater depths, daily cycles of cooling and warming were also observed (+/-2°C), but strong stratification separated upper and lower layers. During both day and night, this stratification prevented significant vertical turbulent mixing between the surface mixed layer and underlying waters. Each night, in a layer extending 10 cm above the gently sloping bed near the pond’s inlet, temperatures were colder than those in underlying sediments, or those near the pond’s deepest point. We infer that these relatively cold waters were created by surface cooling in very shallow regions of the pond or the adjacent wetland, and cascaded down the bed to renew bottom waters. A bottom-water heat balance suggests about 40 m³ of bottom water renewal each night, sufficient to replace most bottom water despite the absence of deep-pond turbulent mixing. The observed thermal overturning circulation likely influences heat transport and biogeochemical cycling.

Primary Presenter: Stephen Henderson, Washington State University (steve_henderson@wsu.edu)

Authors:

Stephen Henderson, Washington State University  (steve_henderson@wsu.edu)

Sally MacIntyre, University of California, Santa Barabara  (sallymacintyre@ucsb.edu)