Selected recent research topics of the working group “Dynamics of Regional Climate Systems”
1. Long-term statistics of deep water ventilation
Despite the decrease of nutrient supply after the 1980s, recently observed oxygen consumption rates are higher than ever observed, limiting the impact of natural ventilation by oxygen-enriched saltwater intrusions. Oxygen consumption rates after saltwater inflows during subsequent stagnation periods were estimated from monitoring observations and model results for 1850-2015. In recent years, ventilating water that originates mainly from the surface layer has contained higher concentrations of organic matter, zooplankton and higher trophic levels. As a result, oxygen consumption in the water column has increased relatively more than oxygen consumption in the sediment, primarily due to respiration of zooplankton and higher trophic levels. Subsequently, natural ventilation has become less effective. However, the oxygenation effect of an inflow is not only determined by the strength of an MBI (Major Baltic Inflow) but also by preceding or subsequent smaller inflows. This can be demonstrated by the different effect of the 2003 and 2014 MBIs.
Meier, H. E. M., G. Väli, M. Naumann, K. Eilola, and C. Frauen, 2018: Recently accelerated oxygen consumption rates amplify deoxygenation in the Baltic Sea. J. Geophys. Res., 123, 3227-3240, https://doi.org/10.1029/2017JC013686
Neumann, T., H. Radtke and T. Seifert (2017). On the importance of Major Baltic Inflows for oxygenation of the central Baltic Sea. J. Geophys. Res. Oceans 122: 1090-1101, doi: 10.1002/2016jc012525
2. Climate variability and Baltic Sea ecosystem response
Climate variability of the past ~ 1000 years and trends during the instrumental period since about 1850 were studied with the help of regionalized climate model data and historical observations from the Baltic Sea region. It was shown that the Atlantic Multidecadal Oscillation (AMO) influences the low-frequency variability of temperature and salinity in the Baltic Sea and consequently the marine ecosystem. Hence, in addition to the North Atlantic Oscillation (NAO) also multi-decadal variations modify recent trends, e.g. in sea surface temperature (SST), which might otherwise mainly be attributed to anthropogenic induced warming. The annual mean SST trends have increased 10-fold during recent decades compared to long-term trends since 1850 and are mainly driven by changes in surface air temperature, coastal upwelling and offshore cloudiness. Climate change enhances the hydrological cycle in northern latitudes. A statistically significant trend in the North-South gradient of sea surface salinity for 1900-2004 was found, which is mainly attributed to increased river runoff from the northernmost catchment. The considerable increase in hypoxia since 1898 documented by historical oxygen measurements were mainly caused by changing riverborne nutrient loads and atmospheric deposition. The impact of other drivers like warming and eustatic sea level rise were comparatively smaller but still important depending on the variable.
Börgel, F., C. Frauen, T. Neumann, S. Schimanke, and H. E. M. Meier, 2018: Impact of the Atlantic Multidecadal Oscillation on Baltic Sea variability. Geophysical Research Letter, 45(18), 9880-9888, https://doi.org/10.1029/2018GL078943
Kniebusch, M., H. E. M. Meier, T. Neumann and Börgel, F., 2019: Temperature variability of the Baltic Sea since 1850 in model simulations and observations and attribution to atmospheric forcing. Journal of Geophysical Research – Oceans, under review.
Kniebusch, M., H. E. M. Meier, and H. Radtke, 2019: Changing salinity gradients in the Baltic Sea as a consequence of altered precipitation patterns in Northern Europe. Geophysical Research Letters, submitted.
Meier, H. E. M., K. Eilola, E. Almroth-Rosell, S. Schimanke, M. Kniebusch, A. Höglund, P. Pemberton, Y. Liu, G. Väli, and S. Saraiva, 2018: Disentangling the impact of nutrient load and climate changes on Baltic Sea hypoxia and eutrophication since 1850. Climate Dynamics, 1-22, https://doi.org/10.1007/s00382-018-4296-y
Meier, H. E. M., K. Eilola, E. Almroth-Rosell, S. Schimanke, M. Kniebusch, A. Höglund, P. Pemberton, Y. Liu, G. Väli, and S. Saraiva, 2018: Correction to: Disentangling the impact of nutrient load and climate changes on Baltic Sea hypoxia and eutrophication since 1850. Climate Dynamics, 1-3, https://doi.org/10.1007/s00382-018-4483-x
3. Assessment of uncertainties in Baltic Sea ecosystem modelling and projections
To improve the predictive capacity of regional climate models, an international Baltic Sea model intercomparison project (BMIP) was initiated by IOW. First results suggested that the mean circulation differs considerably between hindcast simulations although simulated temperatures and salinities agree relatively well with observations. Differences in barotropic velocities were explained by the usuage of different atmospheric forcing datasets. Hence, new coordinated experiments with high-resolution reanalysis data will be performed.
In addition, new projections and sensitivity studies were performed. Using a coupled physical-biogeochemical model the impact of past and accelerated future global mean sea level rise (GSLR) upon water exchange and oxygen conditions in the Baltic Sea was investigated. In future high-end projections (> ~ 1 m) the impact of GSLR is projected to become important by reinforced saltwater inflows causing increased vertical stratification compared to present-day conditions. Contrary to intuition, reinforced ventilation of the deep water may not lead to overall improved oxygen conditions but causes instead expanded dead bottom areas accompanied with increased internal phosphorus loads from the sediments and increased risk for cyanobacteria blooms.
Within Baltic Earth, IOW coordinated an international working group that assessed the impact of the implementation of the Baltic Sea Action Plan (BSAP) on the future environmental status by analyzing multi-model ensemble simulations for the 21st century. It was found that the assumptions on bioavailable nutrient loads during present and future periods considerably differ between the models causing a substantial spread in projections. Nevertheless, the implementation of the BSAP will lead to a significant improvement of the environmental status of the Baltic Sea. The biggest uncertainties are related to (1) unknown current and future bioavailable nutrient loads from land and atmosphere, (2) the experimental setup, (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.
Placke, M., H. E. M. Meier, U. Gräwe, T. Neumann, C. Frauen and Y. Liu, 2018: Long-term mean circulation of the Baltic Sea as represented by various ocean circulation models, 2018. Frontiers in Marine Science, 5:287. https://doi.org/10.3389/fmars.2018.00287
Meier, H. E. M., A. Höglund, E. Almroth-Rosell, and K. Eilola, 2017: Impact of accelerated future global mean sea level rise on hypoxia in the Baltic Sea. Climate Dynamics, 49, 163-172, doi:10.1007/s00382-016-3333-y.
Meier, H. E. M., M. Edman, K. Eilola, M. Placke, T. Neumann, H. Andersson, S.-E. Brunnabend, C. Dieterich, C. Frauen, R. Friedland, M. Gröger, B. G. Gustafsson, E. Gustafsson, A. Isaev, M. Kniebusch, I. Kuznetsov, B. Müller-Karulis, A. Omstedt, V. Ryabchenko, S. Saraiva, and O. P. Savchuk, 2018: Assessment of eutrophication abatement scenarios for the Baltic Sea by multi-model ensemble simulations. Frontiers in Marine Science, 5:440, https://doi.org/10.3389/fmars.2018.00440
Meier, H. E. M., M. Edman, K. Eilola, M. Placke, T. Neumann, H. Andersson, S.-E. Brunnabend, C. Dieterich, C. Frauen, R. Friedland, M. Gröger, B. G. Gustafsson, E. Gustafsson, A. Isaev, M. Kniebusch, I. Kuznetsov, B. Müller-Karulis, M. Naumann, A. Omstedt, V. Ryabchenko, S. Saraiva, and O. P. Savchuk, 2019: Assessment of uncertainties in scenario simulations of biogeochemical cycles in the Baltic Sea. Frontiers in Marine Science, in press.
4. Development of a regional climate system model for climate studies in the Baltic Sea region
The semi-enclosed Baltic Sea is located in Northern Europe and accommodates a complex marine ecosystem. It is impacted by changing climate. However, corresponding information provided by sparse long-term observations are limited and coarse resolution global models do not resolve local air-sea interactions. Hence, a regional climate system model (IOW-RCSM) is currently under development to study regional past and future climate change in the Baltic Sea (Figure 1). In a first step, the atmospheric model COSMO-CLM (Rockel et al., 2008) is coupled to the ocean model MOM-5 (Griffies, 2012) with the coupler OASIS3-MCT (Valcke et al., 2015). The Model MOM-5 is already internally coupled to the sea-ice model SIS (Winton, 2000) and the ecosystem model ERGOM (Neumann, 2010). A first version of this coupled model system is currently in the test phase. In the production phase, simulation will be performed for the recent past and evaluated with observations available at the IOW and other databases. They will provide information about the current state of the Baltic Sea. In addition, the IOW-RCSM will be used to study the Baltic Sea from a paleo perspective.
Figure 1: Components of the regional climate system model for the Baltic Sea Region (IOW-RCSM) that is currently under development
Griffies, S.M., (2012), Elements of the Modular Ocean Model (MOM). GFDL Ocean, Group Technical Resport No. 7, NOAA/Geophysical Fluid Dynamics Laboratory, 618pp.
Rockel, B., A. Will and A. Hense, (2008) The Regional Climate Model COSMO-CLM (CCLM), Meteorologische Zeitschrift Vol. 17 No. 4, p. 347 – 348, doi: 10.1127/0941-2948/2008/0309.
Valcke, S., T. Craig, and L. Coquart, (2015), OASIS3-MCT User Guide, OASIS3-MCT 3.0, Technical Report, TR/CMGC/15/38, CERFACS/CNRS SUC URA No 1875, Toulouse, France.
Winton, M., (2000), A reformulated three-layer sea ice model, Journal of Atmospheric and Ocean Technology, 17, 525-531, doi: 10.1175/1520-0426(2000)017<0525:ARTLSI>2.0.CO;2.