A model for the forecasting of oil drift has been developed at the SMHI. The model describes the pollution on the surface by treating it as a large nurnberof particles. These particles are transported by:-

• the surface current driven directly by the wind
• the background circulation driven by the inclination of the sealevel
• the turbulence of the sea

Usually the wind driven current is calculated by reducing the wind speed by a constant factor, the wind factor. Measurements show, however, that the windfactor varies with the state of the oil and also with the wind speed. An equation is derived where the velocity profile decreases logarithmically from the surface downwards and continuously transforms into an Ekman balance at a greater depth.

The background circulation; the currents driven by the inclination of the sea level, will be calculated by a separate vertically integrated model. The model gives the currents in the Baltic.

Every oil particle will also be exposed to a turbulent movement, which is calculated by random technics.

The velocity of each oil-particle can be written as

\V = \V_T + \V_B + \V' where

\V_T, is the oil velocity depending on the wind, \V_B is the backgroundcirculation and \V' is the turbulent oil movement.

It is important that this model be fast and easy to handle. At the same time, the results must be reliable and of good quality. The aim is to constantlyhave an effective "emergency forecasting program" ready to operate as soon as an accident occurs .

A physical lake model was employed to obtain a basis of discussing the impact of climate variability and climate change on the ecology of Lake Erken, Sweden. The validity of this approach was tested by running the PROBE-lake model for a 30-year period (STD) with observed meteorological data. The lake is adequately modelled, as seen in the comparison with actual lake observations. The validated lake model was then forced with meteorological data obtained from a regional climate model (RCM) with a horizontal resolution of 44 km for present (CLTR) and 2 x CO(2) (SCEN) climate conditions. The CUR lake simulation compares reasonably with the STD. Applying the SCEN simulation leads to a climate change scenario for the lake. The physical changes include elevated temperatures, shorter periods of ice cover combined with two of ten years being totally ice-free, and changes in the mixing regime. The ecological consequences of the physical simulation results are derived from the historical dataset of Lake Erken. Consequences of a warmer climate could imply increased nutrient cycling and lake productivity. The results suggest that an application of RCMs with a suitable resolution for lakes in combination with physical lake models allows projection of the responses of lakes to a future climate.

A mathematical model of the Himmerfjard estuary. divided into four basins. has been formulated and validated against measured data for 1986. The structure of each sub-basin is assumed to be horizontally homogeneous with vertical mean velocities based upon in- and outflows from adjacent basins and freshwater supply to each basin. The horizontal water exchange is formulated as a quasi-stationary Bernioulli flow, driven by horizontal pressure gradients over the sounds and instantaneously interleaved at a neutral buoyancy level. Thc salinity and temperature profiles measured outside the mouth of the estuary serve as forcing, as do the water level changes, the freshwater run-off and the local wind. Inherent in the model assumptions of the horizontal exchange over the sounds is that only a fraction, alpha, of the pressure gradient is used to accelerate each stratum. Variation of the alpha-value shows that the best statistical fit is found for alpha = 0.15 when compared with water exchange estimates based on measurements in one of the internal sounds tor almost an entire month. Using this alpha-value, in combination with standard mixing parameters and hypsographical data, the model satisfactorily captures the major features of the salinity and temperature-profiles development for the year 1986. This is substantiated by statistical analysis of the salinity profiles in the sub-basins for which different measures of similarity between simulated and measured data give the best fit for the same alpha-value as above.

The report presents sea ice data taken during the BEPERS-88 experiment in the Bothnian Bay. Several physical properties (crystal structure, salinity, temperature, density, porosity, and strength) are presented and analysed. In general the data iIlustrate that sea ice is a most komplex medium with  horizontal and vertical variations. Any analysis. of remotely sensed data must therefore carefully consider the ground truth data.

The coupling between a high-resolution weather forecasting model and an ocean model is investigated. It is demonstrated by several case studies that improvements of short-range weather forecasting in the area of the Baltic Sea require an accurate description of the lower boundary condition over sea. The examples are taken from summer situations without sea ice as well as from winter situations with extreme sea ice conditions. It is shown that the sea state variables used in the model influence the weather forecast both directly on the local scale due to the local impact of surface fluxes of latent and sensible heat and on regional and larger scales. The convective snowbands during winters with cold airmass outbreaks over the open water surfaces of the Baltic Sea are extreme examples of the influence of sea state variables on a regional scale, It is furthermore demonstrated that the sea state conditions may change considerably within forecasting periods up to 48 h. This implies the necessary application of ocean models, two-way interactively coupled to the weather forecasting model. The coupling of an advanced 2.5-dimensional ice-ocean model to the operational Swedish Meteorological and Hydrological Institute (SMHI) weather forecasting model HIRLAM is described. The ice-ocean model includes two-dimensional, horizontally resolved ice and storm surge models and a one-dimensional, vertically resolved ocean model applied to 31 Baltic Sea regions. The coupled model system is applied operationally in a data assimilation system at the SMHI. No data assimilation is applied in the operational ocean component: manual modifications to the sea state variables are introduced a few times every winter season. The application of this operational coupled model data assimilation system to the mesoscale reanalysis for the Baltic Sea Experiment (BALTEX) shows that it is necessary to apply data assimilation fur the sea state variables in order to avoid drift of the coupled model system toward less realistic model states. A successful application of a simple assimilation of SST observations is presented. The observed SSTs are first subject to a horizontal filter in order to minimize the effects of observational errors and to restrict the influence to a larger horizontal scale. Then the differences between these filtered temperature observations and the model SSTs are used to construct a modified sensible heat Aux that is applied as a form of a "nudging" term to the ocean model. It turns out that this "nudging" is successful in avoiding the drift away from realistic sea state conditions. The described atmosphere and ocean data assimilation scheme has been applied in a rerun of the BALTEX mesoscale reanalysis for the cold winter 1986/87. The quality of this reanalysis was assessed through the successful simulation of the convective snowbands in January 1987.

Precipitation and evaporation over the Baltic Sea are calculated for a one-year period from September 1998 to August 1999 by four different tools, the two atmospheric regional models HIRLAM and REMO, the oceanographic model PROBE-Baltic in combination with the SMHI (1 x 1)degrees database and Interpolated Fields, based essentially on ship measurements. The investigated period is slightly warmer and wetter than the climatological mean. Correlation coefficients of the differently calculated latent heat fluxes vary between 0.81 (HIRLAM and REMO) and 0.56 (SMHI/PROBE-Baltic and Interpolated Fields), while the correlation coefficients between model fluxes and measured fluxes range from 0.61 and 0.78. Deviations of simulated and interpolated monthly precipitation over the Baltic Sea are less than 5 mm in the southern Baltic and up to 20 mm near the Finnish coast for the one-year period. The methods simulate the annual cycle of precipitation and evaporation of the Baltic Proper in a similar manner with a broad maximum of net precipitation in spring and early summer and a minimum in late summer. The annual averages of net precipitation of the Baltic Proper range from 57 mm (REMO) to 262 turn (HIRLAM) and for the Baltic Sea from 96 turn (SMHI/PROBE-Baltic) to 209 rum (HIRLAM). This range is considered to give the uncertainty of present-day determination of the net precipitation over the Baltic Sea.

A model for the parameterization of lake temperatures and lake ice thicknesses in atmospheric models is presented. The model is verified independently, and it is also tested within the framework of the High Resolution Limited Area Model(HIRLAM), applied operationally for short range weather forecasting at the Swedish Meteorological and Hydrological Institute (SMHI). The lake model is a slab model based upon energy conservation and treats the lakes as well mixed boxes with depths represented by the mean depths. The model is forced by near surface fluxes calculated from total cloudiness, air temperature, air humidity and low-level winds. A data base, describing 92000 Swedish lakes. provides the model with lake mean depths, areal sizes and locations. When the model is used for parameterization of lake effects in the atmospheric model, all the smaller lakes and the fractions of larger lakes within each horizontal grid square of the atmospheric model are parameterized by four model lakes, representing the lake size distribution. The verification of the lake model is done by comparing it with a more advanced, vertically resolved model, including parameterization of turbulent mixing processes, as well as by comparison with observations. A sensitivity test shows great interannual variations of the ice-covered season, which implies that lake models should be used instead of climate data. The results from an experiment with two-way coupling of the lake model to the atmospheric model are verified by comparing forecasted weather parameters with routine meteorological observations. These results show that the impact of lake effects can reach several degrees C in air temperatures close to the surface.

The ice conditions in southern Baltic, Belt Sea, Öresund, Kattegat, and Skagerrak are discussed on the basis of a review of the last 50 years sea ice conditions. Also basic processes and equations are considered in the report.

During severe ice winters, ice formation starts in Öresund and the Belt Sea. In the middle of January, the first ice from the Kattegat is reported. Later, ice forms in, the southern Baltic Sea and the Skagerrak. The maximum ice extent is generally reached in the middle of February. The sea ice is easily moved by winds and currents, causing rafted and ridged ice. The break-up of the ice cover normally starts with ice drifting out to the sea, where it rnelts. By the beginning of April, most of the ice has rnelted.

The ice conditions are, besides the rneteorological forcing, also influenced by the hydrographic conditions. In one extrerne, relatively warm water of high salinity may fill up the basins and inhibit ice formation or rnelt the ice. In another extrerne, brackish Baltic Sea water may formathin surface layer, which rapidly can freeze.

Observations of salinity and oxygen isotope composition (delta(18)O) were made for the Beaufort shelf-Mackenzie estuary waters in September 1990, just prior to ice formation, and for both the water column and ice in April-May 1991, at the end of winter. These measurements are used to determine the apportioning of fresh water in the estuary between its two main sources, runoff and sea ice melt. Changes in disposition of water between seasons and amounts frozen into the growing ice sheet are also derived. Two domains are considered in order to construct a freshwater budget for the Mackenzie shelf, the nearshore within which landfast ice grows in winter and the outer shelf. Most of the winter inflow from the Mackenzie River appears to remain impounded as liquid under the ice within the landfast zone at the end of winter, and about 15% of it is incorporated into the landfast ice. Oxygen isotopes (delta(18)O) in ice cores collected from across the shelf record the progress beneath the ice of new Mackenzie inflow as it invades the nearshore throughout winter. Rates of spreading are about 0.2 cm s(-1) away from the coast and 1.3 cm s(-1) along the coast. As this inflow spreads across the shelf, it progressively shuts off convection driven by brine production at locations within the landfast ice. Salinity and delta(18)O in the offshore water column suggest that about 3 m of sea ice was for:med in the outer shelf domain. Since both brine and newly formed sea ice can be advected off the shelf, a complete budget for brine or sea ice production cannot be established without first measuring the advection of one of these two components.

The northern extension of the Baltic Sea, the Gulf of Bothnia, is a weakly stratified sea. One would therefore expect that the deep water of the Gulf is easily renewed through deep thermal convection, or even through deep haline convection, as the Gulf is more or less covered with ice every winter. However, the present study shows, through analysis of historical temperature, salinity and density data, that the deep water in the Gulf of Bothnia is mainly renewed by major inflows of Baltic Proper surface water. The penetrating water forms a dense bottom current in the Gulf. In the southern part of the Gulf, the Bothnian Sea, the volume flow of the bottom current is found to increase by 10%. It is therefore not likely that the bottom current properties are changed to any appreciable extent. The bottom current properties in the Bothnian Bay, on the other hand, are highly affected, as the volume flow is estimated to increase by 150% in this basin.

A new forecasting model for water cooling in the Gulf of Bothnia and Lake Vänern is presented. The model elements consist of a transient Ekman model, where the turbulent exchange coefficients are calculated with a two-equation model of turbulence, a heat flux package for calculating the net heat loss at the air/sea interface, and geometries based upon area/depth distributions for the Bothnian Bay, the North Bothnian Sea, Lake Vänern/Dalbosjön, and Lake Vänern/Värmlandssjön.

The model system can handle up to five different forecasts of maximum 30 days each simultaneously. It is constructed for easy and rapid interactive handling by letting the forecaster answer questions on a terminal screen.

During the winter navigation period of 1983/84 the model was tested in routine. The results from that test period were most satisfactory, and it is believed that the new forecasting model can serve as a reliable adviser for effective winter navigation planning during coming seasons.

The equation for steady, free floating ice is derived and analysed for a shallow sea. The analysis treats how accurate the free ice drift can be computed when variations in the ingoing parameters are introduced. Besides errors due to  unperfect winds, areas with large currents cause bad accuracy. If further more the bottom depth is neglected in these areas the accuracy become worse. Variable ice roughness and variable friction velocity introduce errors which are less important but still noticable in the computed ice drift.

This report presents sea ice core measurements taken from the Bothnian Bay, March, 1984. The measurements were taken at different sites in the skerries and the basin area. The ice cores were analysed with respect to crystallographic structure using polarized light. Some ice cores were also used for the determination of ice density and ice salinity. The data give a good insight into the ice structure in a sea ice cover, illustrating a complex structure with ice types of different origins, mixed and stratified within the ice cores.

The ice cores were classified with a structural scheme. In that scheme, the sea ice is classified into three main groups: granular ice, columnar ice and mixed ice. In the examined ice cores all three groups were well represented.

The Baltic Sea ice is strongly influenced by the atmospheric circulation and  shows large interannual variability. At the same time the Baltic Sea is one of the most investigated regions on earth with long ice time series. To detect trends in climate change and to relate these to natural or anthropogenic causes are of central importance in the present Baltic Sea research. This was also the main topic during the Fourth Workshop on Baltic Sea Ice Climate held in Norrköping, 22-24 May, 2002. The workshop was organised by SMHI, the SWECLIM program, the Department of Oceanography at the Earth Sciences Centre of Göteborg University, and the Swedish Maritime Administration.

Salinity and temperature variations in the Baltic proper and the Kattegat have been analyzed with a numerical ocean model and a large amount of observational data. In the model, the Baltic Sea is divided into 13 sub-basins with high vertical resolution, horizontally coupled by barotropic and baroclinic flows and vertically coupled to a sea-ice model which includes dynamics as well as thermodynamics. The model was integrated for a 15-year period (1980-1995) by using observed meteorological forcing data, river-runoff data and sea-level data from the Kattegat. The calculated 15-year median profiles of salinity and temperature in the different sub-basins are in good agreement with observations. However, the calculated mid-depth salinities in the Arkona Basin and Bornholm Basin were somewhat overestimated, and the calculated deep-water temperatures in the Arkona Basin and the Bornholm Basin are somewhat lower than the observed values. Frontal mixing and movements in the Kattegat and the entrance area of the Arkona Basin were important to consider in the model. Water masses were simulated well, and prescribing constant deep-water properties in the Kattegat proved to be a reasonable lateral boundary condition. Further, comparisons were made between observed and calculated seasonal and interannual variations of the hydrographic properties in the Eastern Gotland Basin, as well as the interannual variations of the annual maximum ice extent. We conclude that the model can simulate these variations realistically. The major Baltic inflow of 1993 was also simulated by the model, but the inflowing water was 1-2 degrees degrees too cold. Finally, the response times to changes in forcing of the Baltic proper and the Kattegat were investigated by performing the so-called lock-exchange experiment. Typical stratification spin-up times were of the order of 10 years for the Kattegat, and 100 years for the Baltic proper.

The modeling of salinity and temperature in Gulf of Bothnia, Gulf of Finland, and Gulf of Riga is investigated by using a coupled sea ice-ocean Baltic Sea model. 18 years, from late 1980 to the end of 1998, have been investigated. The forcing data extracted taken from a gridded meteorological data base, sea level data from the Kattegat, and river runoff data to the different subbasins of the Baltic Sea from a hydrological data base. To improve the gridded meteorological data base a statistical model for the reduction of geostrophic winds to surface winds was developed. In the analysis it was shown that the calculated long-term salinity and temperature structures were stable and in good agreement with observations. This was made possible by using three different strait-flow models connecting the subbasins of the Baltic Sea. The seasonal and interannual variations of temperature and salinity were also well simulated by the model, implying that the coupling between the atmosphere and the Baltic Sea as well as the diapycnal mixing are reasonably well understood. The water cycle and the surface heat balance were calculated using the 18-year simulation. In the water-balance calculations it was shown that the volume flows from the large gulfs of the Baltic Sea were mainly due to baroclinic transports and that net precipitation added freshwater during the Studied period, particularly to the large gulfs. From the heat-balance calculation it is concluded that the Baltic Sea is almost in local balance with the atmosphere. The Bothnian Bay, Gulf of Finland and Gulf of Riga loose heat, whereas the Bothnian Sea gains heat, calculated as long-term means. (C) 2003 Elsevier Science Ltd. All rights reserved.

The freshwater content at the Mackenzie shelf/estuary is analyzed using observed and numerically simulated data. Field measurements cover the period from September 1986 to September 1987; calculations are based upon a time-dependent, one-dimensional model that treats the shelf as a single reservoir coupled to the surroundings through parameterizations of the inflows and outflows. The system is assumed to be controlled by transient Ekman flow dynamics, and forced by winds when the sea is ice free, and by tides when ice covered. Buoyancy fluxes due to river inflow, freezing and melting of ice, and outflow are added to the salinity conservation equation from which the freshwater content is calculated. Sensitivity studies from simulation of the 1986-1987 period show that the freshwater content of the Mackenzie shelf/estuary is highly influenced by freezing, ice advection off the shelf, and the wind-driven transport, all of which work effectively to remove fresh water from the shelf. Ice keels in the inner shelf may also reduce the export of fresh water to the outer shelf. The sensitivity of the system to altered river inflow from either hydroelectric development or climate change is also examined. For example, upstream storage would increase winter inflows and thus decrease the shelf's capacity to ventilate the halocline, while inflow reduction would enhance shelf ventilation.

This work analyzes long-term changes in the annual maximum ice extent in the Baltic Sea and Skagerrak between 1720 and 1997. It focuses on the sensitivity of the ice extent to changes in air temperature and on the relationships between the ice extent and large-scale atmospheric circulation. A significant regime shift in 1877 explains the decreasing trend in the ice extent. The regime shift indicates a change from a relatively cold climate regime to a relatively warm one, which is likely a result of changed atmospheric circulation. In addition, the analysis shows that a colder climate is associated with higher variability in the ice extent and with higher sensitivity of the ice extent to changes in winter air temperature. Moreover, the ice extent is fairly well correlated with the North Atlantic Oscillation (NAO) index during winter, which supports the results of earlier studies. However, the moving correlation analysis shows that the relationship between the NAO index and the ice extent is not stationary over time. A statistical model was established that links the ice extent and a set of circulation indices. It not only confirms the importance of the zonal how but also implies the impact of meridional wind and vorticity. The usefulness of the statistical model is demonstrated by comparing its performance with that of a numerical model and with independent observations. The statistical model achieves a skill close to that of the numerical model. We conclude that this model can be a useful tool in estimating the mean conditions of the ice extent from monthly pressures, allowing for the use of the general circulation model output for predictions of mean ice extent.

We have examined the effects of historical atmospheric depositions of sulphate, nitrate, and ammonium from land and shipping on the acid-base balance in the Baltic Sea. The modelling considers the 1750-2014 period, when land and ship emissions changed greatly, with increasing carbon dioxide concentrations, SOx, NOx, and NHx emissions, and nutrient loads. The present results indicate that Baltic Sea acidification due to the atmospheric deposition of acids peaked around 1980, with a pH cumulative decrease of approximately 10(-2) in surface waters. This is one order of magnitude less than the cumulative acidification due to increased atmospheric CO2. The acidification contribution of shipping is one order of magnitude less than that of land emissions. However, the pH trend due to atmospheric acids has started to reverse due to reduced land emissions, though the effect of shipping is ongoing. The effect of strong atmospheric acids on Baltic Sea water depends on the region and period studied. The largest total alkalinity sink per surface area is in the south-western Baltic Sea where shipping is intense. Considering the entire Baltic Sea over the 2001-2010 period, the pH changes are approximately -3 x 10(-3) to -11 x 10(-3) and -4 x 10(-4) to -16 x 10(-4) pH units attributable to all emissions and ship emissions only, respectively. The corresponding changes in total alkalinity are approximately -10 to -30 mu mol kg(-1) and -1 to -4 mu mol kg(-1) attributable to all emissions and ship emissions only, respectively. (C) 2015 Elsevier Ltd. All rights reserved.

Results from the first simulations with the Rossby Centre regional climate atmosphere (RCA) model were used to force 2 versions of process-oriented models of the Baltic Sea-one time-dependent, the other considering the mean state. The purpose was primarily to obtain a first scenario of the future state of the Baltic Sea. In addition, we looked at this exercise as a method to evaluate the consistency of the water cycle and the heat balance produced by atmospheric climate models. The RCA model is a high-resolution atmospheric regional model which is forced with lateral conditions from a global model. A large-scale Baltic drainage basin hydrological model, forced by the RCA model, was used to simulate river runoff. Using RCA model data from the control run we found that that the temperature and ice conditions in the Baltic Sea were reasonably realistic while the salinity field was poorly reproduced. We conclude that the modelling of the water cycle needs considerable improvement. We also conclude that the time for the Baltic Sea to respond to the water cycle is much longer than the integration period so far used with the RCA model. Forcing the ocean models with RCA model data from a future scenario with an enhanced greenhouse effect gives an increased sea-surface temperature and a much reduced extent of ice in the Baltic Sea due to climate warming. Also the salinity is reduced, which implies possible serious effects on the future marine life in the Baltic Sea. The results demonstrate that accurate atmospheric modelling of not only the heat balance but also the water cycle is crucial for Baltic Sea climate simulations.

Precipitation and evaporation rates over the Baltic Sea during the period 1981-1994 have been analyzed. The precipitation rate was based upon available meteorological weather, which were interpolated to grid paints using a two-dimensional objective analysis scheme, The evaporation rate was calculated using an ocean model, in which the Baltic Sea was treated as 13 sub-basins with high vertical resolution. Sea-surface temperatures as well as sea ice were calculated and verified extensively against temperature and ice-chart information, In the model, the latent heat flux was calculated according to a bulk formula parameterization. The evaporation rate was then calculated from the latent heat calculations and reduced by sea ice concentration, assuming that evaporation from sea ice is negligible. The long-term difference between precipitation and evaporation rates (the atmospheric fresh water inflow) is positive, which implies that the atmosphere adds fresh water to the Baltic Sea. For the period 1981 - 1994, the total mean atmospheric freshwater inflow was calculated to be 1986 m(3) s(-1). This is less than the total river runoff, but almost as large as the contribution from the River Neva, and thus an important source in the freshwater balance of the Baltic Sea. For the long time mean, the inclusion of sea ice increased (by reducing evaporation with 8%) the atmospheric freshwater inflow by 26% for the studied period, compared to an artificial case without ice in the Baltic Sea. The precipitation and evaporation over the Baltic Sea show, however, large interannual, seasonal and regional differences.

Currents and vertical mixing characteristics were investigated on the basis of time series of current meter and temperature data from a summer-stratified period in Lake Ontario. The experimental set up consisted of seven current meters distributed in one vertical line from 12 m below the surface to 1 m above the lake bottom at a total depth of 143 m. The period considered for the analysis was from June to September, 1991. The currents showed pronounced oscillations with two significant kinetic energy peaks, one at about 17 hours due to inertial motions, and one at 10 days, probably due to meteorological forcing. The current shear in the hypolimnion was strong enough to overcome stability and generate turbulence (Richardson numbers below 0.25) and there was probably turbulence enough available to keep the matter (almost neutral buoyant particles) in the whole Nepheloid bottom layer in suspension. In the thermocline region the turbulence was mainly damped (Richardson numbers above 1), but some events with lower Richardson numbers were also calculated indicating increased mixing during these events. By analysing filtered and unfiltered current meter data it was found that the shear-generated turbulence in the hypolimnion was mainly due to the meteorologically forced currents. In the thermocline region, however, the vertical shear associated with the inertial oscillation had a greater impact on the mixing.

For the first time a coupled ice-ocean model system simulating both  dynamic and thermodynamic processes has been run operationally for the Baltic Sea. The dynamical part of the system is based upon a two-dimensional coupled ice-ocean model that resolves the horizontal space and time, and the thermodynamic part of the system is based upon a one-dimensional coupled ice-ocean model that resolves the vertical space and time for different regions in the Baltic Sea. The system was forced using meteorological fields from the HIRLAM system and was run in operational mode during the winter of 1993/1994. The forecasted parameters were: water cooling, ice formation, ice growth and decay, ice drift and ridging, vertical mean currents, sea levels and water warming. The general experience from the winter was most satisfactory and the model system forms a good base for forecasts and further research.

During the winter of 1993/1994 a doser coupling between the ice-ocean system and the HIRLAM system has been tested, illustrating the importance of using accurate ice fields in the HIRLAM system.

The objectives of the present paper are to formulate and explore a coupled sea ice-ocean model and to examine the sensitivity of ice in the Baltic Sea to climate change. The model treats the Baltic Sea as 13 sub-basins with vertical resolution, horizontally coupled by estuarine circulation and vertically coupled to a sea ice model which includes both dynamic and thermodynamic processes. The reducing effect on the barotropic exchange due to sea ice in the entrance area is also added. The model was first verified with data from 3 test periods representing one mild, one normal and one severe ice winter. The maximum seasonal ice extent was then examined on the basis of simulated and observed data for the period 1980-1993. After that, some climate scenarios (both warm and cold) were examined. The seasonal, regional and interannual variations of sea ice were well described by the model, and the thermal response in the Baltic Sea can be realistically simulated applying forcing data from rather few stations. The Baltic Sea system is highly sensitive to climate change, particularly during the winter season, Warming may drastically decrease the number of winters classified as severe, forcing the climate towards more oceanic conditions. On the other hand, cooling will increase the number of severe winters, forcing the climate towards more sub-arctic conditions.

The ice-ocean response to variable winds is analysed based upon two types of models. An analytical ice-ocean model with linear stress laws and forced by periodic winds is first derived. Secondly a numerical, vertically resolved ice-ocean model is introduced. In the numerical model, the ice-water stress law is calculated from a turbulence model and the wind stress is calculated on the basis of a square law formation. By comparing the ice-ocean stress law formulations, it is illustrated that the numerical model predicts an ice-ocean stress law that has a power slightly less than 2 compared to 1 for the analytical model. The numerical prediction is in good accordance with field observations and the slight deviation from 2 is due to wall effects close to the ice-water interface. It is then demonstrated that the ice-ocean response to variable winds could be well simulated by both models, but the analytical model did not capture the wind dependency properly (because of the linear approach). The ice and current factors are amplified at wind frequencies close to inertial (omega = -f) and damped at high positive and negative frequencies. The maximum ice and current factors at a wind frequency equal to the inertial oscillation are shown to be dependent only on the friction coefficients. With the constants applied in the present study, the maximum ice drift and current speed are equal to 7.8% and 5.5% of the wind speed, respectively. These steady state values are however quite unrealistic as they would require a uniformly changing wind direction for many inertial periods.

A sea ice forecasting system for the Baltic Sea is presented together with some illustrations. The model is a dynamic coupled model, consisting of both a sea ice and a storm surge model. The model was forced using wind and pressure fields from the HIRLAM system and was introduced in preoperational tests during the winter of 1992/93. In general, the results were most promising, but further work is needed, particularly the inclusion of thermodynamics to the model, a closer coupling between the ice-ocean model and the HIRLAM model, and the development of an automatic method for the generation of initial data to the model.

The objective of the present paper is to analyze the water and heat cycles of the Baltic Sea. The closure equations fur the water and heat cycles are formulated and the appropriate fluxes are calculated using the ocean model PROBE-Baltic forced by meteorological fields, river runoff and sea level data from the Kattegat. The time period considered is from November 1980 to November 1995. In the closing of the water cycle it is clear that river runoff, net precipitation (precipitation minus evaporation), in- and outflows through the Baltic Sea entrance area are the dominating flows. From the ocean model it is illustrated that the long-term water balance is consistent with the salinity in the Baltic Sea and that the net precipitation is positive during the studied period. For the closing of the heat cycle, the net heat loss to the atmosphere from the open water surface, as an annual moan, is in close balance with the solar radiation. The dominating fluxes in the net heat loss to the atmosphere are the sensible heat flux, the latent heat Aux and the net long wave radiation. The heat flux from water to ice also needs to be included in the modeling efforts. Heat flows associated with precipitation in the form of rain and snow can, as annual means, be neglected as well as the heat fluxes associated with river runoff, solar radiation through the ice and ice advecting out through the Baltic Sea entrance area. The total annual mean heat loss from the water body is in close balance with the annual change of heat storage in the water and the net heat exchange through the Baltic Sea entrance area is small. This illustrates that the Baltic Sea thermodynamically responds as a closed ocean basin.

A joint Swedish-Finnish sea-ice experiment was performed during March 1977. Measurements in the atmosphere, ice and sea were made during six days onboard the Finnish Research vessel Aranda stationed in the ice field in the Bay of Bothnia. During two days measurements were also carried out from thetwo Swedish icebreakers Atle and Tor. This report presents the data and some results from the Swedish group.

Autumn cooling in the Bay of Bothnia provides an opportunity for studying wind mixing, convection and restratification below the temperature for maximum density. Vertical temperature profiles for a 52 day period are analysed for the case of cooling of brackish sea water around the temperature of maximum density. A mathematical model, which is based on the conservation equations for momentum, heat and salt in their one- dimensional form and with an equation of state which is linear with respect to salinity but quadratic with respect to temperature, is presented. Turbulent exchange coefficients are calculated with a kinetic energy- dissipation model of turbulence.

Due to the fact that both salinity and temperature effect stratification and that buoyancy flux changes signat the temperature for maximum density several processes influence the cooling rate. The mathematical model describes these and the general development of the temperature profiles in a most satisfactory way.

The melt rates of fresh and saline drifting ice, heated from below, are examined using a one-dimensional ice/ocean model with high vertical resolution. The model is based on the conservation equations for heat, salt, and momentum and uses turbulence models to achieve closure. The model includes a low-Reynolds number turbulence model for the viscous region, coupled to a high-Reynolds number turbulence model for the outer boundary, and a discrete element approach to the parameterization of roughness. It is shown that the melt rate of drifting ice is sensitive to ice roughness and molecular salt diffusion, and it is found that bulk heat transfer coefficients vary within a rather narrow range in the examined interval.

Heat fluxes al the ice-ocean interface and ice thickness are investigated by comparing field data from the Coordinated Eastern Arctic Experiment (CEAREX) drift phase with model calculations. The calculations are based on two types of models. The first one is a one-dimensional ice-ocean model with high vertical resolution. This model is based on the conservation equations for heat, salt, and momentum and uses turbulence models to achieve closure. A discrete element approach is also introduced to explicitly parameterize the ice roughness. The second model is a simple one-dimensional bulk heat transfer model. In this version, the interfacial salinity is modelled on the basis of salt conservation at the ice-ocean interface. The bulk heat transfer model is then calibrated using the former model. The two models predict ocean heat fluxes that are quite variable in time owing to short-term variations in the ice drift. Both models calculate realistic ice thicknesses. It is demonstrated that the observed time variation in ice thickness from eight different experimental sites with varying initial thicknesses and bottom topographies can be reproduced by applying bulk heat transfer coefficients in the range (2.8 +/- 1) x 10(-4). Horizontal variation of the thermal state within a single pack ice floe results in simultaneous freezing and melting over relatively small spatial scales. When modeling or averaging ice data in space these aspects need to be considered.

The Baltic Sea Experiment (BALTEX) is one of the five continental-scale experiments of the Global Energy and Water Cycle Experiment (GEWEX). More than 50 research groups from 14 European countries are participating in this project to measure and model the energy and water cycle over the large drainage basin of the Baltic Sea in northern Europe. BALTEX aims to provide a better understanding of the processes of the climate system and to improve and to validate the water cycle in regional numerical models for weather forecasting and climate studies. A major effort is undertaken to couple interactively the atmosphere with the vegetated continental surfaces and the Baltic Sea including its sea ice. The intensive observational and modeling phase BRIDGE, which is a contribution to the Coordinated Enhanced Observing Period of GEWEX, will provide enhanced datasets for the period October 1999-February 2002 to validate numerical models and satellite products. Major achievements have been obtained in an improved understanding of related exchange processes. For the first time an interactive atmosphere-ocean-land surface model for the Baltic Sea was tested. This paper reports on major activities and some results.

BALTEX is an environmental research network dealing with the Earth system of the entire Baltic Sea drainage basin. Important elements include the water and energy cycle, climate variability and change, water management and extreme events, and related impacts on biogeochemical cycles. BALTEX was founded in 1993 as a GEWEX continental-scale experiment and is currently in its second 10 yr phase. Phase I (1993-2002) was primarily dedicated to hydrological, meteorological and oceanographic processes in the Baltic Sea drainage basin, hence mostly dealt with the physical aspects of the system. Scientific focus was on the hydrological cycle and the exchange of energy between the atmosphere, the Baltic Sea and the surface of its catchment. The BALTEX study area was hydrologically defined as the Baltic Sea drainage basin. The second 10 yr phase of BALTEX (Phase II: 2003-12) has strengthened regional climate research, water management issues, biogeochemical cycles and overarching efforts to reach out to stakeholders and decision makers, as well as to foster communication and education. Achievements of BALTEX Phase II have been the establishment of an assessment report of regional climate change and its impacts on the Baltic Sea basin (from hydrological to biological and socio-economic), the further development of regional physical climate models and the integration of biogeochemical and ecosystem models. BALTEX features a strong infrastructure, with an international secretariat and a publication series, and organizes various workshops and conferences. This article gives an overview of the BALTEX programme, with an emphasis on Phase II, with some examples from BALTEX-related research.

This work presents a regional climate model, the Rossby Centre regional Atmospheric model(RCA1), recently developed from the High Resolution Limited Area Model (HIRLAM). The changes in the HIRLAM parametrizations, necessary for climate-length integrations, are described. A regional Baltic Sea ocean model and a modeling system for the Nordic inland lake systems have been coupled with RCA1. The coupled system has been used to downscale 10-year time slices from two different general circulation model (GCM) simulations to provide high-resolution regional interpretation of large-scale modeling. A selection of the results from the control runs, i.e. the present-day climate simulations, are presented: large-scale free atmospheric fields, the surface temperature and precipitation results and results for the on-line simulated regional ocean and lake surface climates. The regional model modifies the surface climate description compared to the GCM simulations, but it is also substantially affected by the biases in the GCM simulations. The regional model also improves the representation of the regional ocean and the inland lakes, compared to the GCM results.

Precipitation is one of the main components in the water balance, and probably the component determined with the greatest uncertainties. In the present paper we focus on precipitation (mainly rain) over the Baltic Sea as a part of the BAL-TEX project to examine the present state of the art concerning different precipitation estimates over that area. Several methods are used, with the focus on 1) interpolation of available synoptic stations; 2) a mesoscale analysis system including synoptic, automatic, and climate stations, as well as weather radar and an atmospheric model; and 3) measurements performed on ships. The investigated time scales are monthly and yearly and also some long-term considerations are discussed. The comparison shows that the differences between most of the estimates, when averaged over an extended period and a larger area, are in the order of 10-20%, which is in the same range as the correction of the synoptic gauge measurements due to wind and evaporation losses. In all data sets using gauge data it is important to include corrections for high winds. To improve the structure of precipitation over sea more focus is to be put on the use of radar data and combinations of radar data and other data. Interpolation methods that do not consider orographic effects must treat areas with large horizontal precipitation gradients with care. Due to the large variability in precipitation in time and space, it is important to use long time periods for climate estimates of precipitation. Ship measurements are a valuable contribution to precipitation information over sea, especially for seasonal and annual time scales.

By using a process-oriented ocean model forced with data from a gridded synoptic database, net precipitation values (precipitation minus evaporation) over the Baltic Sea are obtained. For a range of realistic meteorological forcing the average annual value obtained from an 18 yr (1981-1998) simulation ranges between 1100 and 2500 m(3) s(-1). The monthly variations are significant with the highest values occurring in early summer and even negative values in late autumn. Ice is an important factor, and the net precipitation is close to zero in the southern basins with no ice. Calculated net precipitation for a 98 yr period (1901-1998) using river runoff and maximum ice extent indicates that the investigated 18 yr period was wetter than the almost 100 yr climate mean. A realistic climate estimate of net precipitation during the 20th century is estimated to be 1500 +/-1000 m(3) s(-1). The evaluation of 2 present day regional climate simulations indicated high precipitation, low evaporation, and thus excessive net precipitation compared to the climate estimate from this investigation. When simulating the effect of increased greenhouse gases, the change in net precipitation was positive but small due to the compensating effects of increased precipitation and increased evaporation associated with increased temperature and reduced ice.

In this study, turbulent heat flux data from two sites within the Baltic Sea are compared with estimates from two models. The main focus is on the latent heat flux. The measuring sites are located on small islands close to the islands of Bornholm and Gotland. Both sites have a wide wind direction sector with undisturbed over-water fetch. Mean parameters and direct fluxes were measured on masts during May to December 1998. The two models used in this study are the regional-scale atmospheric model HIRLAM and the ocean model PROBE-Baltic. It is shown that both models overestimate the sensible and latent heat fluxes. The overestimation can, to a large extent, be explained by errors in the air-water temperature and humidity differences. From comparing observed and modelled data, the estimated 8-month mean errors in temperature and humidity are up to 1 degreesC and 1 g kg(-1),respectively. The mean errors in the sensible and latent heat fluxes for the same period are approximately 15 and 30 W m(-2), respectively. Bulk transfer coefficients used for calculating heat and humidity fluxes at the surface were shown to agree rather well with the measurements, at least for the unstable data. For stable stratification, the scatter in data is generally large, and it appears that the bulk formulation chosen overestimates turbulent heat fluxes.

The frazil ice dynamics in a turbulent Ekman layer have been investigated using a mathematical model. The model is based on the conservation equations for mean momentum, energy and salinity, and employs a two-equation turbulence model for the determination of turbulent diffusion coefficients. A crystal number continuity equation is used for the prediction of the frazil ice dynamics. This equation considers several processes of importance, as for example turbulent diffusion, gravitational up-drift, flocculation/break-up and growth. The results focus on the frazil ice characteristics in the upper layers of the ocean, like suspended ice volume, ice crystals per m(3), vertical distributions, etc. From the idealized calculations, it is indicated that a large number of ice crystals can be mixed into the ocean during freezing. However, the amount of ice in suspension, measured as vertically integrated ice thickness, adds only a minor part to the total surface ice budget. Small crystals are mixed deep in the ocean while the large ones are found only in the top of the mixed layer. Knowledge about the vertical distribution of ice crystals of different sizes, which is calculated from the model, should be of importance when analysing processes as formation of ice covers in the ocean and ice-sediment or ice-algae interaction. (C) 1998 Elsevier Science B.V. All rights reserved.

The objective of the work presented is to formulate a mathematical description of frazil ice dynamics. The formulation is to be in balance with the current knowledge of the physical processes, for example secondary nucleation. As the knowledge of some of these processes is fragmentary, this means that a conceptually simple formulation is sought. A number of processes are known to influence the supercooling rate and the frazil ice production. The present formulation accounts for the following processes: initial seeding, secondary nucleation, gravitational removal, growth due to cooling of water volume and flocculation/break up. Equations are formulated for these present considering a resolution in time and radius of particles but not in space (well-mixed jar). The equations are solved using a simple explicit numerical scheme. Preliminary results indicate that the model can be calibrated to describe the experimental results reported in the literature. It is mainly the supercooling curves that are used for comparison but some information about the crystal size distribution is also considered. It is to be noted that the model is calibrated to fit the experiments, due to the lack of detailed mathematical description of some of the physical processes. Sensitivity analysis is also used in order to establish that the model behaves according to experimental findings and expectations. The main conclusion of the study is that a fairly simple mathematical model can be formulated and calibrated, which fits the experimental data reported in the literature hitherto. It is further concluded that a resolution in radial space gives additional insight into the dynamics of the process. The evolution of the size distribution and its sensitivity to seeding and dissipation rate has been predicted with results that look physically plausible.

One of the sub-projects within the sea ice research programme carried out at SMHI deals with the thermodynamics of the sea and the ice. In these studies the sea surface temperature plays a fundamental r ole. During the latest years considerable efforts have been made in order to obtain more temperature observations, in particular from the open sea. Various vessels have been equipped with new instruments, the collection of observations improved, the sea surface temperature distribution analyzed every second day and all information stored in digital form.

The instruments are discribed and their specifications given in the report. Various observational methods are compared and examples of sea surface temperature analyses for the period July 1973 - July 1974  illustrating yearly variations, tendency to circulation patterns, coastal effects, up-welling etc. are given.

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.