Showing papers by "Dag Klaveness published in 2016"

Thermobaric stratification and circulation in very deep freshwater lakes

Abstract: International Symposium on Stratified Flows , 2016 Thermobaric stratification and circulation in very deep freshwater lakes Bertram Boehrer 1* , Ryuji Fukuyama 2 , Lars Golmen 3,4 , Jarl Eivind Lovik 5 , Karsten Rahn 1 , Dag Klaveness 6 , Kazuhisa Chikita 7 Helmholtz-Centre for Environmental Research - UFZ, Magdeburg, Germany Hokkaido Institute of the environmental Sciences, Sapporo, Japan Norwegian Institute for Water Research (NIVA), Bergen, Norway Runde Environmental Centre, Runde, Norway Norwegian Institute for Water Research (NIVA). Ottestad, Norway University of Oslo, Department of Biosciences, Oslo, Norway Hokkaido University, Sapporo Japan *Corresponding author, e-mail Bertram.Boehrer@ufz.de KEYWORDS Deep lakes; thermobaricity, compressibility, water density, physical limnology ABSTRACT Themobaricity, i.e. temperature dependence of compressibility (of pure water) ∂  ∂ ρ in − situ  0 ≠ in − situ = ∂ T ∂ p ∂ T   ∂ p   leads to a shift of the temperature of maximum density by 0.02 °C/bar. This results not simply in a shift of temperatures in the deep waters, but also controls the deep recirculation in sufficiently deep lakes. This effect has been studied in Lake Baikal (e.g. Weiss 1991), but investigations in other lakes are rare (e.g. Crawford and Collier, 1997), although about half of the deep lakes are located in a climate where the thermobaric stratification is relevant. We distinguish two classes, i.e. horizontally homogeneous lakes and lakes with horizontal gradients. We demonstrate, how deep water recirculation is inhibited or accomplished. The temperature of maximum density for pure water can be drawn as a profile against depth T md (z). It starts at 4°C at the surface and reaches about 3.0°C at the base of a 500m deep lake. We present a simplistic model of a very deep pure water lake: surface temperatures are controlled by a user, while the model checks stability of the water column and does the required mixing, whenever instabilities occur. Forcing low surface temperatures through a model winter, the model reveals an asymmetry between autumn and spring recirculation. A period of homogeneous water temperature does not exist as in lakes not affected by thermobaricity. Over the winter period, the water column becomes subdivided into two layers that never completely mix with each other. No water property is separating both layers; this is accomplished by thermobaricity, as will be shown: the upper layer is directly affected by atmospheric heat transfer and presents itself for a large portion of the winter “inversely” temperature stratified. The intersection with the profile of T md marks the separation to the