The Baltic Sea climate is analysed based upon long-term oceanographic measurements. The objective of the work is to study the natural variability of present day climate with focus on the freshwater budget. The results are designed to be used for validation of climate models and for discrimination of the significance of modelled climate change scenarios. Almost 100 yr of observations are used in the study, including data for river runoff, water exchange through the Danish Straits (as calculated from river runoff and from sea level data from the Kattegat), salinity data from the Baltic Sea and the Kattegat, and oxygen content in the deep Baltic Sea. The analyses illustrate that freshwater supply to the Baltic shows large variations on time scales up to several decades. The long-term variations in freshwater storage are closely correlated to accumulated changes in river runoff. This indicates strong positive feedback between the amount of outflowing surface water from the Baltic Sea and the salinity of the inflowing Kattegat water. One implication of the study is that climate control simulations must cover several decades, probably up to 100 yr in order to capture the natural variability of present day climate. Also, models designed to study climate change for the Baltic Sea probably need to start integrating from the present day.KEY WORDS: Baltic Sea · Climate · Salinity · Freshwater · River runoff · OxygenResale or republication not permitted without written consent of the publisher

Relevant observed time series for the Baltic Sea region from the last 2 centuries were used to investigate climate variations and trends. These time series were: Stockholm air temperature and magnitude of seasonal temperature cycle, Stockholm sea level data, Baltic Sea maximum ice cover, and circulation types based on regional air pressure data. The definition of climate was analysed by considering how each parameter varies with the time scale. We found that 90% of the variance was for time scales shorter than 15 yr, the period then used as the climate-averaging time for all studied parameters. The results indicate positive trends for air temperature, sea level, and frequencies of anti-cyclonic circulation and westerly wind types over the last 200 yr. Negative trends were found for the magnitude of seasonal temperature cycle, sea-ice cover, and frequency of southwesterly wind. The major climate changes of the late 19th century were probably associated with the end of the 'Little Ice Age' and characterized by an unusual high frequency of cyclonic circulation. In the 20th century, pronounced positive trends were observed in sea level variation and anti-cyclonic circulation. In the most recent studied climate period (1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000), air temperature and sea level climate anomalies were positive and lay outside the range of last 200 yr normal variations. The study indicates that increased frequencies of anti-cyclonic circulation and westerly winds have resulted in a slightly warmer climate with reduced seasonal amplitude and reduced ice cover. Thereby, we support the hypothesis that the long-term climate change in the Baltic Sea region is at least partly related to changes in the atmospheric circulation.KEY WORDS: Baltic Sea · Climate · Atmospheric circulation · Temperature · Ice and sea levels Resale or republication not permitted without written consent of the publisherClim Res 25: [205][206][207][208][209][210][211][212][213][214][215][216] 2004 current climate (Heino et al. 1999, Crowley 2000, IPCC 2001, and some authors claim that warming since 1975 is likely to be partly due to increased amounts of greenhouse gases (Karlén et al. 1999, IPCC 2001. Similar warming may also have happened during the Medieval Warm Period (Crowley & Lowery 1999). Recent research using tree-ring-based chronologies indicates that the variability of recent decades may lies within the natural range (Esper at al. 2002). This illustrates the difficulty of climate research. Also, probability views based on studies using general circulation models (GCMs) may be questionable, as they often only resolve atmospheric circulation coarsely and exclude realistic treatment of some forcing mechanisms (Räisänen & Alexandersson 2003).It is often difficult to attribute a given change to a particular mechanism, and equally difficult to distinguish between trends and oscillations. It is, for example, an oversimplification to explain observed hemisphere mean surf...

Instantaneous velocity observations with high-space resolution at two sections in the Skagerrak has been repeated during the years 1975 to 1977. The velocity observations are presented together with hydrographic data. The mean currents show a cyclonic circulation. Apart from a narrow baroclinic coastal current, the mean velocities generally increase towards the bottom. The mean velocities at the bottom are highest close to and below the sill depth. In the upper layers, the direction of the circulation occasionally changes, whereas in the deeper layers, the cyclonic circulation is present in all the observations. The advective residence time for the water down to 400-500 m in the Skagerrak is some 100 days. The barotropic transport in the circulation has the same magnitude around the Skagerrak and at the eastern part of the sill west of Norway.

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