Early-winter mixing event associated with baroclinic motions in weakly stratified Lake Biwa

The annual overturn of lake water (termed holomixis) during winter is essential in maintaining the environment of warm monomictic lakes by transporting heat and organic and inorganic constituents; however, direct observation of wintertime mixing processes is limited. To better understand the detailed physical processes responsible for holomixis, this study investigated a cold low-oxygen water intrusion event on the northeastern slope of weakly stratified Lake Biwa (Japan), observed using mooring systems. The intrusion occurred concurrently with superinertial oscillations of near-bottom currents. The effective Wedderburn number suggests that basin-scale Kelvin waves were excited by northeastward winds (~4 m s-1) prior to the intrusion event. The results of a modal analysis suggest that the intrusion was caused by a combination of Kelvin and Poincare waves that locally uplifted the cold deep water. The heat budget analyses revealed that a substantial part of the intruded cold water was mixed diapycnally, induced by wind stirring, nighttime convection, and bottom friction. The superinertial currents enhanced the dissipation due to bottom friction by ~30%. The cold water intrusion and subsequent mixing provided local heat flux of 98 W m-2 over 6 days, which was 1.4 times larger than the average surface heat flux over the same period. In addition to previously studied processes, such as surface cooling and gravity currents from the shore, this study indicates that wind-induced baroclinic motions and subsequent diapycnal mixing of a stratified water column contribute to holomixis.