Description: During 2002 and 2003 exceptional inflow events have been registered. In January 2003 a massive inflow of highly saline, cold and extremely oxygen‐rich water from the North Sea was recorded at Darss Sill. This event is considered to be the most important inflow since 1993. A coupled model system for the Baltic Sea region, called BALTIMOS, was developed in the frame of DEKLIM/BALTEX by linking existing model components for the atmosphere (model REMO), for the ocean including sea ice (model BSIOM), for the hydrology (model LARSIM) as well as for lakes. The model system consists of high resolution model components: 1/6° (∼18 km) with 20 vertical levels; ocean‐ice 5 km with 60 vertical levels, hydrology 1/6°. The model domain covers the whole drainage basin of the Baltic Sea as well as major parts of Europe. The exceptional inflow events have been simulated successfully with BALTIMOS. The simulation was initialized at 1st of February 2002 and the model has been run until October 2003. This period includes the exceptional warm water inflow in autumn 2002 and the major Baltic inflow in January 2003. Different inflow characteristics are presented and discussed. The simulated volume transport for the major inflow in January 2003 amounts to about 250 km3, half of which was of salinity 17 PSU which corresponds to a salt transport of 2.7 × 1012 kg.

Description: A three-dimensional hydrodynamic model has been used to analyse temporally and spatially resolved circulation patterns in the Baltic Sea with special emphasis on drifting particles representing larval fish. The main purpose of this study was (i) to investigate potential drift patterns of larval fish, (ii) to identify its intra- and inter-annual variability for time periods based on the timing of spawning and (iii) to analyse its seasonal and spatial variability in dependence of the atmospheric forcing conditions. For the time period 1979–1998 temporally and spatially resolved simulated flow fields were used to describe the potential drift from the centre of main reproductive effort of Baltic cod (Gadus morhua). The results of the model runs demonstrate a general change in circulation pattern from retention during a first decade (1979–1988) to dispersion in the following decade (1989–1998). This increase in dispersion was related to an increase in the variability of the local wind forcing conditions over the Baltic. The more frequent occurrence of dispersion in spring of the recent decade was accompanied by a strong decay in biomass of one of the main larval fish feeding component in the central basin, the calanoid copepod Pseudocalanus elongatus. Larger dispersion of this prey organism may have affected the spatial overlap and thus the contact rates between predator and prey. Hence, this may have resulted in a food limitation for early life stages of Baltic cod and potentially contributed to the pronounced shift in cod peak spawning time from spring to late summer. Early life stages of cod originating from late spawning fish, benefited from a stronger dispersion in late summer and autumn, into shallow coastal areas with higher calanoid abundance.

Description: In order to clarify mechanisms influencing the reproductive success of Baltic cod (Gadus morhua L.), a modelling exercise was performed to examine the effects of the wind-driven circulation on the transport of early life stages between the western and eastern Baltic. Because the different stocks spawn in different areas and environments at different times of the year, the occurrence of variable age/length distributions of juveniles within the different potential nursery areas can be explained by the circulation pattern. A three-dimensional circulation model of the Baltic was utilized to investigate the temporal evolution of egg and larval distributions of the western Baltic cod stock, which spawns preferentially in the Danish Straits, in Kiel Bay as well as in Mecklenburg Bay. For different scenarios (1988 and 1993), within- and between-year variability of egg and larval transport showed large differences, primarily due to variations in wind forcing. In 1988, relatively low and variable wind forcing prevailed, whereas, due to sustained strong, mainly westerly, winds, in January 1993, the recent major Baltic inflow to the Baltic Sea occurred. Differences in contributions of early life stages from the western to the eastern cod stocks, depending on the physical forcing conditions, suggest that this process can be controlled by variations of atmospheric forcing conditions. The potential for early life stages from the western Baltic cod stock to drift into the Arkona Basin and the Bornholm Basin, and to contribute there to the juvenile population, has been recognized as being mainly due to strong westerly winds. During cold winters, retention of eggs, larvae and juveniles within their original spawning grounds may predominate. Transport of cod early life stages from the Øresund, as well as from the Great Belt, can occur only during periods of strong westerly winds, but significant eastwards orientated drift from Kiel Bay and Mecklenburg Bay was also evident during periods of minor westerly wind influence.