Spatiotemporal variability of thermal front features in the Baltic Sea 2010–2011

Abstract

Regions of the formation of the thermal front in the Baltic Sea (a direct manifestation of the lacustrine thermal bar), and its specific features, were analyzed on the basis of subsurface temperature and salinity. Data were obtained from 25 horizontal tows along sections in the southern and central parts of the Baltic Sea during spring 2010 and autumn/winter 2010/2011. The width of the front was approximately 5–30 km, and the front lifetime was 1.5 months. Horizontal temperature ranged from 0.7 to 2.5°C; thus, the temperature gradient was one- to twofold larger than the long-term monthly mean equivalent. Analysis of hourly temperature and salinity data from the Arkona basin and at the Darss Sill, obtained at 2 m depth, indicated that the surface temperature increased during the transition through temperatures of maximum density at a rate of approximately 0.01–0.02°C/h between 3–5 days; which is 1.4- to 5-fold higher than values before and after this period. The thermal front simultaneously propagated along the main sea axis (due to the significant salinity and buoyancy flux variations from south to north), and from the shallow parts towards the deep parts of the Baltic Sea. Therefore, the horizontal advection of the cold/warm waters clearly contributes to the speed increase of the thermal front at the end of the respective season. The speed of the thermal front propagation from south to north was approximately 28 km/day at the end of the spring period of 2010 (based on field data). This was considerably higher in comparison with the typical values of the lacustrine thermal bar speeds; however, it accords with estimates for a basin with depth/salinity horizontal variation.