TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 51 (No. 2), pp. 171-185, 1995

Deep Convection in a Lake Triggered by Wind: Two-Dimensional Numerical Experiments with a Nonhydrostatic Model

Kazunori Akimoto, Kiyoshi Tanaka, Toshiyuki Awaji and Norihisa Imasato

Department of Geophysics, Faculty of Science, Kyoto University, Kyoto 606, Japan

(Received 18 May 1994; in revised form 5 September 1994; accepted 16 September 1994)

Abstract: We have investigated the fundamental processes of deep convection in a lake at high latitudes triggered by wind during spring or autumn and the associated deep water formation, executing vertically two-dimensional numerical experiments with a nonhydrostatic model. The water column in which a relatively cold mixed layer overlies a relatively warm layer becomes unstable, when the Ekman convergence on the shore due to along-shore wind deepens the mixed layer below the compensation depth, where water densities in both layers becomes equal to each other because of the thermobaric effect. At the onset of deep convection, the critical Rayleigh number agrees with that predicted by the linear theory. The onset time of deep convection is inversely proportional to the magnitude of wind stress. On the other hand, the onset time is minimal when water temperature in the mixed layer qmis 3.1°C because a change of qmhas two effects oppositely acting on the stability of the water column. After the first onset, deep convection occurs intermittently for a few days. The sinking of the mixed layer water occurs in a thermal-like shape, and its amount is 41~84% of the time-integrated Ekman transport when qm~ 3°C while it decreases to less than 10% for qmlower than 1.5°C. The present process can explain 30% of the amount of deep water renewal which is expected from the observation in Lake Baikal.

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