Calculating the water and heat balances of the Baltic Sea using ocean modelling and available meteorological, hydrological and ocean data

In this paper we aim to analyse Baltic Sea water and heat balances for the BALTEX/BRIDGE study period and to put these into a climatic perspective. The study period—the three years starting October 1999—was a time of enhanced observational and modelling activities in the Baltic Sea region and of the major field activity of BALTEX Phase I programme. The present study follows the example of earlier work, where Baltic Sea modelling was used as a tool for synthesizing available data and closing the water and heat balances. The modelling approach was validated with independent data sets of observations from salinity and temperature. The model simulation was also compared with the coupled atmosphere’Baltic Sea model system, HIRLAM–BALTEX, whichwas run in a delayed data assimilation mode.

The results indicate that accurate long-term net water and net heat balances (mean errors over decadal time scales are about 600 m³ s^–1 and 2 W m^–2 respectively) can be calculated using current Baltic Sea modelling and meteorological and hydrological data available from the BALTEX data centres. The accuracy of the individual terms in the water and heat balances is, however, still unknown. The study illustrated that negative net precipitation rates are possible, with the year 2002 standing out from the rest of the 30-yr study period. The calculated inter-annual variability of the net heat loss between atmosphere and Baltic Sea during the BALTEX/BRIDGE period indicated large variations (±10 W m^–2). It has also been shown that the Baltic Sea annual mean temperature has not increased during the studied period despite an atmospheric warming of 1 °C. The reason has been explained by the heat balance that indicated no trend in the Baltic Sea net heat loss.