Moored observations of turbulent mixing events in deep Lake Garda (I)

TL;DR

This study provides detailed yearlong observations of turbulent mixing in Lake Garda, revealing episodic deep convection, internal waves, and higher turbulence levels than open ocean, crucial for understanding deep lake dynamics.

Contribution

It offers the first high-resolution, yearlong mooring data in Lake Garda, highlighting the variability and mechanisms of deep water mixing in a large, deep lake.

Findings

Deep convection episodes occur in late winter driven by northerly winds.

Turbulence dissipation rates are about 10 times higher than in open ocean.

Internal waves and seiches significantly influence turbulence levels.

Abstract

Deep water circulation and mixing processes in deep lakes are largely unknown, although they are responsible for the transport of matter, nutrients and pollutants. Such a lack of knowledge cannot be reliably provided by numerical hydrodynamic modelling studies because detailed observations are typically not available to validate them. To overcome some of these deficiencies, a dedicated yearlong mooring comprising 100 high-resolution temperature sensors and a single current meter were located in the deeper half of the 344 m deepest point of the subalpine Lake Garda (Italy). The observations show peaks and calms of turbulent exchange, besides ubiquitous internal wave activity. In late winter, northerly winds activate episodic deep convective overturning, the dense water being subsequently advected along the lake-floor. Besides deep convection, such winds also set-up seiches and inertial waves that are associated with about 100 times larger turbulence dissipation rates than that by semidiurnal internal wave breaking observed in summer. In the lower 60 m above the lake-floor however, the average turbulence dissipation rate is approximately constant in value year-around, being about 10 times larger than open-ocean values, except during deep convection episodes.