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

Regional sea level change in response to ice mass loss in Greenland, the West Antarctic and Alaska

Besides the warming of the ocean, sea level is mainly rising due to land ice mass loss of the major ice sheets in Greenland, the West Antarctic, and the Alaskan Glaciers. However, it is not clear yet how these land ice mass losses influence regional sea level. Here, we use the global Finite Element Sea-ice Ocean Model (FESOM) to simulate sea surface height (SSH) changes caused by these ice mass losses and combine it with the passive ocean response to varying surface loading using the sea level equation. We prescribe rates of fresh water inflow, not only around Greenland, but also around the West Antarctic Ice Sheet and the mountain glaciers in Alaska with approximately present-day amplitudes of 200, 100, and 50 Gt/yr, respectively. Perturbations in sea level and in freshwater distribution with respect to a reference simulation are computed for each source separately and in their combination. The ocean mass change shows an almost globally uniform behavior. In the North Atlantic and Arctic Ocean, mass is redistributed toward coastal regions. Steric sea level change varies locally in the order of several centimeters on advective time- scales of decades. Steric effects to local sea level differ significantly in different coastal locations, e.g., at North American coastal regions the steric effects may have the same order of magnitude as the mass driven effect, whereas at the European coast, steric effects remain small during the simulation period

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

A cold and fresh ocean surface in the Nordic Seas during MIS 11: Significance for the future ocean

Paleoceanographical studies of Marine Isotope Stage (MIS) 11 have revealed higher-than-present sea surface temperatures (SSTs) in the North Atlantic and in parts of the Arctic but lower-than-present SSTs in the Nordic Seas, the main throughflow area of warm water into the Arctic Ocean. We resolve this contradiction by complementing SST data based on planktic foraminiferal abundances with surface salinity changes using hydrogen isotopic compositions of alkenones in a core from the central Nordic Seas. The data indicate the prevalence of a relatively cold, low-salinity, surface water layer in the Nordic Seas during most of MIS 11. In spite of the low-density surface layer, which was kept buoyant by continuous melting of surrounding glaciers, warmer Atlantic water was still propagating northward at the subsurface thus maintaining meridional overturning circulation. This study can help to better constrain the impact of continuous melting of Greenland and Arctic ice on high-latitude ocean circulation and climate

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