OBSERVATIONS OF SUB-ICE SURFACE SEICHE IN LAKE HOARE, ANTARCTICA, USING AN ACOUSTIC DOPPLER CURRENT PROFILER (ADCP)

Lake Hoare is a 34-m-deep basin in the McMurdo Dry Valleys that is covered with 3 to 5 m of ice throughout the year and has a maximum water temperature of 1°C. Summer inputs include a diurnal meltwater stream at the eastern end of the lake plus overflow from an adjacent pond at the western end, whereas output is limited to lake ice sublimation. Stratification is weak and vertical mixing of portions of the water column may have occurred recently. The goal of this project is to better understand the nature of shallow, sub-ice currents and mixing within the lake.

In January 2010, a SonTek Argonaut-XR 1.5 MHz ADCP was fixed at the bottom of an ice hole over the deepest portion of the lake to measure 3-D currents every 10 seconds for 32.5 hours. The ADCP has a water velocity resolution of 0.1 cm/sec and an accuracy of ± 0.5 cm/sec. Velocities and directions were investigated in ten 30-cm-thick cells spanning 3 m of the water column beginning 0.5 m below the ice-water boundary. In the cell closest to the boundary, horizontal currents had a mean velocity of 23.8 cm/sec and oscillated between10.1 and 37.5 cm/sec (ranges based on the mean ±1 standard deviation). These higher-than-expected velocities were independently validated using a handheld acoustic Doppler current meter lowered down the ice hole. Vertical currents had a mean velocity of -0.7 cm/sec and oscillated between +5.6 and -7.0 cm/sec. Horizontal currents had a mean direction of N 177°E and oscillated between N 71°E and N 282°E which roughly corresponds to the east and west ends of the lake. Such back-and-forth motion along the long-axis of a lake is characteristic of a seiche. Initial Fast Fourier Transforms identified dominant periods in horizontal velocity of 11, 3.7, and 2.5 minutes in several cells. These periods agree with the theoretical first-mode longitudinal and transverse surface seiches of 11 and 2.8 minutes, respectively, calculated from lake dimensions. Conductivity-temperature-depth profile data also show a thin layer of low density water with negative stability values immediately below the ice.

We hypothesize that low-density, diurnal, stream input generates a horizontal pressure gradient below the ice-water boundary which initiates longitudinal and transverse surface seiches. Alternatively, lake-ice meltwater or glacier-ice meltwater might also cause this behavior.