The Structure and Driving Mechanisms of the Baltic Intrusions

Abstract

Data from closely spaced CTD profiling performed in the eastern Gotland Basin after the 1993 inflow event are used to study thermohaline intrusions in the Baltic Sea. Two CTD cross sections display abundant intrusive layers in the permanent halocline. Despite the overwhelming dominance of the salinity stratification, diffusive convection is shown to work in the Baltic halocline enhancing diapycnical mixing. To understand the driving mechanisms of observed intrusions, these are divided into different types depending on their structural features. Only two types of observed intrusions are suggested to be strongly influenced by diffusive convection: 1) relatively thin (3?5 m) and long (up to 8 km) intrusions inherent to high-baroclinicity regions and 2) relatively thick (?10 m) and short (2?5 km) intrusions inherent to low-baroclinicity regions. To verify this hypothesis the linear stability models of 3D and 2D double-diffusive interleaving in approximation of a finite-width front were used. It is shown that the horizontal and vertical scales of thick and short intrusions correspond well to the 3D rotational mode for a pure thermohaline front. Since mesoscale thermohaline fronts in the Baltic halocline are shown to be essentially baroclinic, the influence of baroclinicity on the rotational mode was studied, which resulted in more adequate estimates of the growth rate of the unstable modes. The thin and long intrusions are shown to be likely driven by 2D baroclinic instability triggered by diffusive convection. The model results demonstrated that diffusion convection can be considered as a possible driver for some intrusions observed in the Baltic halocline, while most of the intrusions have a non-double-diffusive origin. Nevertheless, diffusive convection can affect all types of observed intrusions, for example, by tilting them relative to isopycnals and thereby promoting diapycnal mixing and ventilation in the Baltic halocline.