Abstract
Abstract. In this study, the fine-scale responses of a stratified oligotrophic karstic lake (Kozjak Lake, Plitvice Lakes, Croatia; the lake fetch is 2.3 km, and the maximum depth is 46 m) to atmospheric forcing on the lake surface are investigated. Lake temperatures measured at a resolution of 2 min at 15 depths ranging from 0.2 to 43 m, which were observed during the 6 July–5 November 2018 period, were analyzed. The results show thermocline deepening from 10 m at the beginning of the observation period to 16 m at the end of the observation period, where the latter depth corresponds to approximately one-third of the lake depth. The pycnocline followed the same pattern, except that the deepening occurred throughout the entire period approximately 1 m above the thermocline. On average, thermocline deepening was 3–4 cm d−1, while the maximum deepening (12.5 cm d−1) coincided with the occurrence of internal seiches. Furthermore, the results indicate three different types of forcings on the lake surface; two of these forcings have diurnal periodicity – (1) continuous heat fluxes and (2) occasional periodic stronger winds – whereas forcing (3) corresponds to occasional nonperiodic stronger winds with steady along-basin directions. Continuous heat fluxes (1) produced forced diurnal oscillations in the lake temperature within the first 5 m of the lake throughout the entire observation period. Noncontinuous periodic stronger winds (2) resulted in occasional forced diurnal oscillations in the lake temperatures at depths from approximately 7 to 20 m. Occasional strong and steady along-basin winds (3) triggered both baroclinic internal seiches with a principal period of 8.0 h and barotropic surface seiches with a principal period of 9 min. Lake currents produced by the surface seiches under realistic-topography conditions generated baroclinic oscillations of the thermocline region (at depths from 9 to 17 m) with periods corresponding to the period of surface seiches (≈ 9 min), which, to the best of our knowledge, has not been reported in previous lake studies.
Highlights
Two-way interactions between lakes and the atmosphere occur through fluxes in heat, moisture, momentum, and mass at lake surfaces
The aim of the present study is to investigate the fine-scale evolution and dynamics of the vertical temperature profile driven by forcings on an oligotrophic lake surface
We analyzed vertical profiles of lake temperatures observed at a resolution of 2 min at 15 depths ranging from near the surface to near the bottom (i.e., 0.2–43 m) from 6 July to 5 November 2018
Summary
Two-way interactions between lakes and the atmosphere occur through fluxes in heat, moisture, momentum, and mass (such as carbon dioxide and methane emissions from lakes and the atmospheric deposition of nutrients) at lake surfaces. Authors have addressed various topics associated with the physical processes in lakes, such as lake stratification modeling (e.g., Stepanenko et al, 2014, 2016; Heiskanen et al, 2015); the improvement of numerical weather and climate models through coupling with lake models (e.g., Ljungemyr et al, 1996; Balsamo et al, 2012; Xue et al, 2017; Ma et al, 2019); the parameterization of lake-surface fluxes via transfer coefficients (e.g., Xiao et al, 2013); the estimation of the groundwater inflow rates based on observed vertical profiles of water temperature (e.g., Tecklenburg and Blume, 2017); and the influences of lakes on local and/or regional weather, the in-. The role of lakes in the regional climate has been investigated (e.g., Bryan et al, 2015) as well as lake responses to climate and land use change (e.g., Huziy and Shushama, 2017; Roberts et al, 2017; Hipsey et al, 2019)
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