What happens when we remove GRACE or Ocean Bottom pressure from a GRACE+GPS+OBP joint inversion? Roelof Rietbroek, Sandra-Esther Brunnabend, Madlen Gebler, Mathias Fritsche, Jürgen Kusche, Christoph Dahle, Frank Flechtner, Schröter Jens, and Dietrich Reinhard

The movement of large masses, originating from hydrological and oceanographic variations, causes detectable variations in gravity and surface deformation. These may be detected by satellite gravimetry and a network of permanent GPS stations respectively. Alternatively, additional information on ocean bottom pressure(OBP) variations may be retrieved from simulations. Joint inversions offer a way to combine different data sources in order to obtain improved estimates of surface loading. This technique can be used to compensate for weaknesses in one dataset, by the strengths of the others. But what happens when one datasets is taken out of the equation? Here, we compute a joint inversion using a GPS+GRACE+OBP combination. Additionally, we purposely deteriorate the solution by removing either data from GRACE or OBP. The accuracy and resolution of the solutions is discussed. Furthermore, regions are identified where the restricted inversion is consistent with the full inversion, and where the results show strong hydrological signals

Assessment of eutrophication abatement scenarios for the Baltic Sea by multi-model ensemble simulations

Peer reviewe

Assessment of Uncertainties in Scenario Simulations of Biogeochemical Cycles in the Baltic Sea

Following earlier regional assessment studies, such as the Assessment of Climate Change for the Baltic Sea Basin and the North Sea Region Climate Change Assessment, knowledge acquired from available literature about future scenario simulations of biogeochemical cycles in the Baltic Sea and their uncertainties is assessed. The identification and reduction of uncertainties of scenario simulations are issues for marine management. For instance, it is important to know whether nutrient load abatement will meet its objectives of restored water quality status in future climate or whether additional measures are required. However, uncertainties are large and their sources need to be understood to draw conclusions about the effectiveness of measures. The assessment of sources of uncertainties in projections of biogeochemical cycles based on authors' own expert judgment suggests that the biggest uncertainties are caused by (1) unknown current and future bioavailable nutrient loads from land and atmosphere, (2) the experimental setup (including the spin up strategy), (3) differences between the projections of global and regional climate models, in particular, with respect to the global mean sea level rise and regional water cycle, (4) differing model-specific responses of the simulated biogeochemical cycles to long-term changes in external nutrient loads and climate of the Baltic Sea region, and (5) unknown future greenhouse gas emissions. Regular assessments of the models' skill (or quality compared to observations) for the Baltic Sea region and the spread in scenario simulations (differences among projected changes) as well as improvement of dynamical downscaling methods are recommended.Peer reviewe

Variationen des Meeresspiegels abgeleitet vom massenerhaltenden Finite Element Sea-Ice Ocean Model

During the last century sea level rise strongly increased compared to sea level change in the last 2000 years. The present study investigates global and regional sea level change, simulated with the finite element sea-ice ocean model (FESOM). The major goal is to separate sea level change into steric and eustatic contributions and to estimate the influence of Greenland and Antarctic ice sheet melt on global and regional sea level. Modeled steric height variations show realistic regional geophysical patterns compared with steric height variations derived from altimetry measurements and GRACE. Compared to the time before the 1990 s, an increased global trend in steric sea level rise is found in estimates derived from the model and from satellite measurements. Modeled ocean mass exhibits reasonable spatial structures. However, the trend in the global model mean cannot be trusted in FESOM as it strongly depends on the mass budget of the model, which is determined by uncertain mass fluxes. To account for this, global mean ocean mass variations need to be optimized to realistic values. To this end results from GRACE in combination with GPS data is used. Greenland and Antarctic ice sheet melting influence the global sea level mainly through the additional mass. The eustatic sea level rises by about 0.3 mm/yr for 100 Gt/yr of melt water. Additionally, the fresh water causes local steric variations in sea level that are transported farther by ocean currents. The ice sheet mass loss yields a decrease in gravitational attraction causing a sea level fall near the source of mass loss but also to a slight increase at long distance. This effect is computed for the Greenland ice sheet mass loss using Green s functions. It leads to a decreased sea level near the Greenland coast and to a slightly increased sea level in the Southern Ocean. The effect of different melting scenarios is investigated

Titel: Sea Level Variations derived from Mass Conserving Finite Element Sea-Ice Ocean Model; Untertitel: Study of Major Contributions to Sea Level Change in the Recent Past

During the last century sea level rise strongly increased compared to sea level change in the last 2000 years. The present study investigates global and regional sea level change, simulated with the finite element sea-ice ocean model (FESOM). The major goal is to separate sea level change into steric and eustatic contributions and to estimate the influence of Greenland and Antarctic ice sheet melt on global and regional sea level. Modeled steric height variations show realistic regional geophysical patterns compared with steric height variations derived from altimetry measurements and GRACE. Compared to the time before the 1990 s, an increased global trend in steric sea level rise is found in estimates derived from the model and from satellite measurements. Modeled ocean mass exhibits reasonable spatial structures. However, the trend in the global model mean cannot be trusted in FESOM as it strongly depends on the mass budget of the model, which is determined by uncertain mass fluxes. To account for this, global mean ocean mass variations need to be optimized to realistic values. To this end results from GRACE in combination with GPS data is used. Greenland and Antarctic ice sheet melting influence the global sea level mainly through the additional mass. The eustatic sea level rises by about 0.3 mm/yr for 100 Gt/yr of melt water. Additionally, the fresh water causes local steric variations in sea level that are transported farther by ocean currents. The ice sheet mass loss yields a decrease in gravitational attraction causing a sea level fall near the source of mass loss but also to a slight increase at long distance. This effect is computed for the Greenland ice sheet mass loss using Green s functions. It leads to a decreased sea level near the Greenland coast and to a slightly increased sea level in the Southern Ocean. The effect of different melting scenarios is investigated

Asymmetric response of the Atlantic Meridional Ocean Circulation to freshwater anomalies in a strongly-eddying global ocean model

The Atlantic Meridional Overturning Circulation (AMOC) responds sensitively to density changes in regions of deepwater formation. In this paper, we investigate the nonlinear response of the AMOC to large amplitude freshwater changes around Greenland using a strongly-eddying global ocean model. Due to a 0.5 Sv freshwater input, the maximum AMOC at 35° N decreases by about 50% over a 45 year period. The AMOC does not recover over a period of 50 years when the freshwater input is ceased at year 45. However, when reversing the sign of the freshwater input at year 45, the AMOC needs only about 10 years to fully recover. The mechanism that causes this asymmetric response in theAMOC is clarified using water mass transformation theory

Asymmetric response of the Atlantic Meridional Ocean Circulation to freshwater anomalies in a strongly-eddying global ocean model

The Atlantic Meridional Overturning Circulation (AMOC) responds sensitively to density changes in regions of deepwater formation. In this paper, we investigate the nonlinear response of the AMOC to large amplitude freshwater changes around Greenland using a strongly-eddying global ocean model. Due to a 0.5 Sv freshwater input, the maximum AMOC at 35° N decreases by about 50% over a 45 year period. The AMOC does not recover over a period of 50 years when the freshwater input is ceased at year 45. However, when reversing the sign of the freshwater input at year 45, the AMOC needs only about 10 years to fully recover. The mechanism that causes this asymmetric response in theAMOC is clarified using water mass transformation theory