The vernal bloom in heterogeneous convection: a numerical study of Baltic restratification

Abstract

The onset of the vernal phytoplankton bloom in heterogeneous convection at a temperature below the temperature of maximum density ( θ ρ ) is studied by means of an idealized numerical model. The modeled setting resembles the well-documented conditions in the Baltic Sea. For the low sea surface temperature found in early spring, the convection in this inland sea is driven by surface heating, in marked contrast to oceanic conditions, where cooling serves the purpose. The heterogeneity of the physical environment is enhanced by a lateral freshwater flux, which stratifies parts of the domain and allows the surface to warm above θ ρ in the stratified areas. Consequently, the same surface heat flux may cause convection in some parts and stabilisation in other parts of the domain. The development of a layer of anomalously cold water with θ< θ ρ is demonstrated numerically. The physical heterogeneity is strongly reflected in the development of the vernal bloom, with significant phytoplankton growth possible only in the stratified areas. The non-local organised structures in the mixing layer play a dominant role in the development of the bloom. An analysis of the model results shows the presence of biological intrusions during the bloom, highly reminiscent of their atmospheric counterparts.