Cold Intrusions in Lake Baikal: Direct Observational Evidence for Deep-Water Renewal

We studied cold, deep-water intrusions in the South Basin of Lake Baikal on the basis of 2 yr of data (December 1995-November 1997) from near-bottom and near-surface thermistor strings, monthly conductivity-temperature-depth (CTD) profiles, and a near-bottom current meter, all collected near the South Basin maximum depth of 1,461 m. The data show intrusions into the greatest depths with temperatures of 0.08-0.20°C below ambient (∼3.33 to ∼3.38°C at maximum depth). The intrusions were observed three times per year between January and June, when surface water is always cooler than deep water, with durations of a few (at least 1-3) days. The estimated water input ranged from 1 to 10 km3 per event, and the annually accumulated volume was estimated to be 10-30 km3, which is significantly less than the steady-state indirect estimates of 30-70 km3 yr-1 to the permanently stratified deep water (depth > 300 m). This indicates that not all of the cold intrusions reach the deepest area. Because the cooling of the near-bottom waters was not accompanied by a significant increase in ion or particle content and because deep sediment traps did not contain significant enrichments of minerogenic particles, we concluded that Selenga River inflow is not a possible source of the cold intrusions. Two CTD profiles in June 1996 and 1997 showed lower temperature throughout the deep water, as expected from thermobaric instabilities. The required downwelling is definitely not occurring in the pelagic interior but most probably by near-coast counterclockwise currents. The source of the regularly occurring deep intrusions is clearly cold surface water, but the actual mechanism is uncertain.