Description of stability and neutrally buoyant transport in freshwater lakes

Abstract

The concept of potential density, introduced by oceanographers to describe the vertical stability of a water column, may be inadequate if the water temperature is close to the temperature of maximum density, Tmd, where the thermal expansion coefficient changes its sign. Because Tmd decreases with increasing pressure, potential density especially fails to provide a reliable means for the analysis of the local stability of a water column in deep, cold freshwater lakes. A new quantity called quasi-density is introduced. Its vertical gradient correctly describes the local stability of a water column for every water body (warm, cold, fresh, or salty). Application of this concept to field data from Lake Baikal shows that quasi-density-in contrast to potential density - is an ideal tool to assess vertical stability. In a two- or three-dimensional field of potential temperature and salinity, the neutral surface is defined by the direction along which an infinitesimal isentropic displacement of a water parcel is buoyancy-free. The neutral surfaces do not define a potential; that is, there is no scalar property that is constant along the neutral surface. Isentropic transport over finite distances is not buoyancy-free along neutral surfaces or along surfaces of constant potential density (isopycnals. We define the neutral track as the path along which isentropic transport of a water parcel is buoyancy-free. In general, each water parcel defines a different neutral track. Neutral track and neutral surface are complementary concepts used to assess the potential movement of a water parcel over some distance. The latter describes the path of a water parcel that after each infinitesimal displacement completely exchanges its identity with the characteristics of its new environment, whereas the former describes a parcel that totally keeps its identity specified by potential temperature and salinity.