CLIMATE CHANGE AND LAKE VANDA, ANTARCTICA: PAST, PRESENT AND FUTURE SCENARIOS

Lake Vanda is a 77 m deep, perennially ice covered, terminal lake located in the McMurdo Dry Valleys of Antarctica. The primary source of inflowing water to Lake Vanda is the Onyx River, which flows 32 km inland from the Wright Lower Glacier. Ice ablation and evaporation are the only outputs. Over the past 50 years, lake level has risen 12 m; this is the largest documented environmental change in the region, and is likely the result of increased meltwater production on the coastal Wright Lower Glacier. In the 1960’s, Vanda had a 29 m-thick saline bottom layer, overlain by an isothermal, 20 m-thick thermohaline convection cell, and a 12 m-thick stepped thermal gradient below ice. Convection was driven by solar heating of the bottom layer owing to high light transmission though ice and water. Bottom temperatures exceeded 25 °C. Lake rise added a second convection cell, now 19 m thick, above the first. In Dec. 2013, the bottom temperature was 21.37 °C, the lower cell was 6.53 °C, 1610 µS/cm and 1000.736 kg/m³, and upper cell was 4.43 °C, 955 µS/cm and 1000.404 kg/m³. Comparisons of temperature profiles suggest that the pycnocline between these cells marks a previous lake surface, ~25 m below the current surface, that probably prevailed in the early 20^th Century. Convective mixing most likely depends on the heat flux and lake level. While the position and thickness of the lower convection cell remained constant over time, temperature declined since 1960 by 0.03 °C/y due to increased depth and/or heat loss to the upper cell.

In Dec. 2013, mean lake level dropped 3 cm from the 2^nd to 14^th and rose 5 cm from the 15^th to 23^rd in response to flow from the Onyx River, which began on the 8^th. Over the same period, in situ profile monitoring with a distributed temperature sensor (DTS), plus three temperature sondes moored at fixed depths, showed simultaneous temperature variations which may be indicative of convection.

We envision four future scenarios: (1) Continued melting, lake level rise, and heat loss from the bottom; convection stops. (2) Increased surface area yields a stable level where ablation balances inflow; solutes increase in the upper cell until both cells mix. (3) Reduced meltwater production and/or increased aridity drops level; results same as (2). (4) Unstable climate conditions with fluctuating level; each rise generates a new convective cell.