H. E. Markus Meier

Reconstructing the Development of Baltic Sea Eutrophication 1850–2006

A comprehensive reconstruction of the Baltic Sea state from 1850 to 2006 is presented: driving forces are reconstructed and the evolution of the hydrography and biogeochemical cycles is simulated using the model BALTSEM. Driven by high resolution atmospheric forcing fields (HiResAFF), BALTSEM reproduces dynamics of salinity, temperature, and maximum ice extent. Nutrient loads have been increasing with a noteworthy acceleration from the 1950s until peak values around 1980 followed by a decrease continuing up to present. BALTSEM shows a delayed response to the massive load increase with most eutrophic conditions occurring only at the end of the simulation. This is accompanied by an intensification of the pelagic cycling driven by a shift from spring to summer primary production. The simulation indicates that no improvement in water quality of the Baltic Sea compared to its present state can be expected from the decrease in nutrient loads in recent decades.

Simulated distributions of Baltic Sea-ice in warming climate and consequences for the winter habitat of the Baltic ringed seal

Sea-ice in the Baltic Sea in present and future climates is investigated. The Rossby Centre Regional Atmosphere-Ocean model was used to perform a set of 30-year-long time slice experiments. For each of the two driving global models HadAM3H and ECHAM4/OPYC3, one control run (1961-1990) and two scenario runs (2071-2100) based upon the SRES A2 and B2 emission scenarios were conducted. The future sea-ice volume in the Baltic Sea is reduced by 83% on average. The Bothnian Sea, large areas of the Gulf of Finland and Gulf of Riga, and the outer parts of the southwestern archipelago of Finland will become ice-free in the mean. The presented scenarios are used to study the impact of climate change on the Baltic ringed seal (Phoca hispida botnica). Climate change seems to be a major threat to all southern populations. The only fairly good winter sea-ice habitat is found to be confined to the Bay of Bothnia.

On the parameterization of mixing in three‐dimensional Baltic Sea models

As mixing plays a dominant role for the physics of an estuary like the Baltic Sea (seasonal heat storage, mixing in channels, deepwater mixing), different mixing parameterizations for use in three-dimensional (3-D) Baltic Sea models are discussed. Within the Swedish regional climate modeling program, SWECLIM, a 3-D coupled ice-ocean model for the Baltic Sea has been coupled with an improved version of the two-equation k-e turbulence model using a corrected dissipation term, flux boundary conditions to include the effect of a turbulence enhanced layer due to breaking surface gravity waves, and a parameterization for breaking internal waves. Results of multiyear simulations are compared with observations. The seasonal thermocline (the main focus of this paper) is simulated satisfactory. During the stagnation period between 1983 and 1993, simulated salinity in the lower layer of the Baltic Sea decreases as observed. Unsolved problems of the k-e model are discussed. To replace the controversial equation for dissipation, the performance of a hierarchy of k models has been tested and compared with the k-e model. In addition, it is shown that the results of the 1-D turbulence submodel depend very much on the dimensionality of the hydrodynamic model. Using the same turbulence parameterization, vertical velocity shear and density gradients are simulated differently in 1-D column models compared to 3-D ocean circulation models. Finally, the impact of two mixing parameterizations on Baltic Sea climate is discussed.

Comparing reconstructed past variations and future projections of the Baltic sea ecosystem first results from multi model ensemble simulations

Multi-model ensemble simulations for the marine biogeochemistry and food web of the Baltic Sea were performed for the period 1850‐2098, and projected changes in the future climate were compared with the past climate environment. For the past period 1850‐2006, atmospheric, hydrological and nutrient forcings were reconstructed, based on historical measurements. For the future period 1961‐2098, scenario simulations were driven by

Impact of Climate Change on Ecological Quality Indicators and Biogeochemical Fluxes in the Baltic Sea: A Multi-Model Ensemble Study

Multi-model ensemble simulations using three coupled physical–biogeochemical models were performed to calculate the combined impact of projected future climate change and plausible nutrient load changes on biogeochemical cycles in the Baltic Sea. Climate projections for 1961–2099 were combined with four nutrient load scenarios ranging from a pessimistic business-as-usual to a more optimistic case following the Helsinki Commission′s (HELCOM) Baltic Sea Action Plan (BSAP). The model results suggest that in a future climate, water quality, characterized by ecological quality indicators like winter nutrient, summer bottom oxygen, and annual mean phytoplankton concentrations as well as annual mean Secchi depth (water transparency), will be deteriorated compared to present conditions. In case of nutrient load reductions required by the BSAP, water quality is only slightly improved. Based on the analysis of biogeochemical fluxes, we find that in warmer and more anoxic waters, internal feedbacks could be reinforced. Increased phosphorus fluxes out of the sediments, reduced denitrification efficiency and increased nitrogen fixation may partly counteract nutrient load abatement strategies.

The Baltic Haline Conveyor Belt or The Overturning Circulation and Mixing in the Baltic

A study of the water-mass circulation of the Baltic has been undertaken by making use of a three dimensional Baltic Sea model simulation. The saline water from the North Atlantic is traced through the Danish Sounds into the Baltic where it upwells and mixes with the fresh water inflow from the rivers forming a Baltic haline conveyor belt. The mixing of the saline water from the Great Belt and Oresund with the fresh water is investigated making use of overturning stream functions and Lagrangian trajectories. The overturning stream function was calculated as a function of four different vertical coordinates (depth, salinity, temperature and density) in order to understand the path of the water and where it upwells and mixes. Evidence of a fictive depth overturning cell similar to the Deacon Cell in the Southern Ocean was found in the Baltic proper corresponding to the gyre circulation around Gotland, which vanishes when the overturning stream function is projected on density layers. A Lagrangian trajectory study was performed to obtain a better view of the circulation and mixing of the saline and fresh waters. The residence time of the water masses in the Baltic is calculated to be 26-29 years and the Lagrangian dispersion reaches basin saturation after 5 years.

Combined effects of global climate change and regional ecosystem drivers on an exploited marine food web

Changes in climate, in combination with intensive exploitation of marine resources, have caused large-scale reorganizations in many of the worlds marine ecosystems during the past decades. The Baltic Sea in Northern Europe is one of the systems most affected. In addition to being exposed to persistent eutrophication, intensive fishing, and one of the worlds fastest rates of warming in the last two decades of the 20th century, accelerated climate change including atmospheric warming and changes in precipitation is projected for this region during the 21st century. Here, we used a new multimodel approach to project how the interaction of climate, nutrient loads, and cod fishing may affect the future of the open Central Baltic Sea food web. Regionally downscaled global climate scenarios were, in combination with three nutrient load scenarios, used to drive an ensemble of three regional biogeochemical models (BGMs). An Ecopath with Ecosim food web model was then forced with the BGM results from different nutrient-climate scenarios in combination with two different cod fishing scenarios. The results showed that regional management is likely to play a major role in determining the future of the Baltic Sea ecosystem. By the end of the 21st century, for example, the combination of intensive cod fishing and high nutrient loads projected a strongly eutrophicated and sprat-dominated ecosystem, whereas low cod fishing in combination with low nutrient loads resulted in a cod-dominated ecosystem with eutrophication levels close to present. Also, nonlinearities were observed in the sensitivity of different trophic groups to nutrient loads or fishing depending on the combination of the two. Finally, many climate variables and species biomasses were projected to levels unseen in the past. Hence, the risk for ecological surprises needs to be addressed, particularly when the results are discussed in the ecosystem-based management context.

Extremes of Temperature, Oxygen and Blooms in the Baltic Sea in a Changing Climate

In the future, the Baltic Sea ecosystem will be impacted both by climate change and by riverine and atmospheric nutrient inputs. Multi-model ensemble simulations comprising one IPCC scenario (A1B), two global climate models, two regional climate models, and three Baltic Sea ecosystem models were performed to elucidate the combined effect of climate change and changes in nutrient inputs. This study focuses on the occurrence of extreme events in the projected future climate. Results suggest that the number of days favoring cyanobacteria blooms could increase, anoxic events may become more frequent and last longer, and salinity may tend to decrease. Nutrient load reductions following the Baltic Sea Action Plan can reduce the deterioration of oxygen conditions.

Sensitivity of the Baltic Sea salinity to the freshwater supply

The sensitivity of the Baltic Sea salinity to the freshwater supply is investigated using a 3-dimensional (3D) coupled sea-ice-ocean model. Todays climate is characterized by an average salinity of about 7.4‰ and a freshwater supply, including river runoff and net precipitation, of about 16 000 m 3 s -1 . As recent results of some regional climate models have suggested a significant increase in precipitation in the Baltic catchment area due to anthropogenic climate change, in this study the response of salinity in the Baltic Sea to changing freshwater inflow is investigated. Of special interest is the possibility of the Baltic Sea becoming a freshwater sea with 0 ‰ salinity in the future. There- fore, model simulations with modified river runoff and precipitation for 1902-1998 were performed. The model is forced with daily sea-level observations in the Kattegat, monthly basin-wide discharge data, and reconstructed atmospheric surface data. The reconstruction utilizes a statistical model to calculate daily sea-level pressure, and monthly surface-air temperature, dew-point temperature, pre- cipitation, and cloud-cover fields. It is assumed that the Kattegat deepwater salinity of about 33 ‰ will not change regardless of the changed freshwater supply. In most of the experiments the final stratifi- cation is almost in a steady state after 100 yr. We found that even for a freshwater supply increased by 100% compared to 1902-1998 the Baltic Sea cannot be classified as a freshwater sea. A pro- nounced halocline still separates the upper and lower layers in the Baltic Proper, limiting the impact of direct wind mixing to the surface layer. A calculated phase diagram suggests that the relationship between freshwater supply and average salinity of the final steady state is non-linear. The results of the 3D model are in agreement with an analytical steady-state model assumed to work for freshwa- ter changes smaller than 30%. The latter model was applied in scenarios for the average salinity of the Baltic Sea.

Simulated sea surface temperature and heat fluxes in different climates of the Baltic Sea.

The physical state of the Baltic Sea in possible future climates is approached by numerical model experiments with a regional coupled ocean-atmosphere model driven by different global simulations. Scenarios and recent climate simulations are compared to estimate changes. The sea surface is clearly warmer by 2.9 degrees C in the ensemble mean. The horizontal pattern of average annual mean warming can largely be explained in terms of ice-cover reduction. The transfer of heat from the atmosphere to the Baltic Sea shows a changed seasonal cycle: a reduced heat loss in fall, increased heat uptake in spring, and reduced heat uptake in summer. The interannual variability of surface temperature is generally increased. This is associated with a smoothed frequency distribution in northern basins. The overall heat budget shows increased solar radiation to the sea surface, which is balanced by changes of the other heat flux components.