publications

The Atlantic Multidecadal Oscillation controls the impact of the North Atlantic Oscillation on North European climate

Florian Börgel, Claudia Frauen, Thomas Neumann and H. E. Markus Meier., https://iopscience.iop.org/article/10.1088/1748-9326/aba925

European climate is heavily influenced by the North Atlantic Oscillation (NAO). However, the spatial structure of the NAO is varying with time, affecting its regional importance. By analyzing an 850-year global climate model simulation of the last millennium it is shown that the variations in the spatial structure of the NAO can be linked to the Atlantic Multidecadal Oscillation (AMO). The AMO changes the zonal position of the NAO centers of action, moving them closer to Europe or North America. During AMO+ states, the Icelandic Low moves further towards North America while the Azores High moves further towards Europe and vice versa for AMO- states. The results of a regional downscaling for the East Atlantic/European domain show that AMO-induced changes in the spatial structure of the NAO reduce or enhance its influence on regional climate variables of the Baltic Sea such as sea surface temperature, ice extent, or river runoff.

Commentary: Lake or Sea? The unknown future of central Baltic Sea herring

H. E. Markus Meier, Florian Börgel, Claudia Frauen and Hagen Radtke, 2020, Commentary: Lake or Sea? The unknown future of central Baltic Sea herring, Front. Ecol. Evol., https://www.frontiersin.org/articles/10.3389/fevo.2020.00055/full

Since the 1970s the weight of 3-year old central Baltic Sea (BS) herring (Clupea harengus) decreased causing a loss in catch which is equivalent to ∼100 million € (Dippner et al., 2019). Dippner et al. (2019) suspected that the decline in weight was related to a change in the prey community caused by a reduction in surface layer (< 30 m) salinity. They concluded that the changes in herring weight are “indirectly influenced by the Atlantic Multidecadal Oscillation (AMO) via a complex mediator chain”, i.e. increased sea surface temperature (SST) in the North Atlantic (NA), increased precipitation in the BS catchment area, increased river runoff, decreased surface layer salinity in the Baltic proper and decreased zooplankton (Pseudocalanus acuspes) biomass and herring weight. We argue that observational records are too short to verify a causal relationship between AMO and herring weight, because the presented herring weight data cover only 35 years (1974-2010). Further, Dippner et al. (2019) proposed that “this observed trend is also projected for the future in regional climate change scenarios”. We argue that past correlations between multidecadal climate variations and herring weight should not be extrapolated into the future because the underlying processes of past and expected future changes may differ.

Validator – a Web-Based Interactive Tool for Validation of Ocean Models at Oceanographic Stations

Radtke, H, Börgel, F, Brunnabend, S-E, Eggert, A, Kniebusch, M, Meier, H E M, Neumann, D, Neumann, T and Placke, M 2019 Validator – a Web-Based Interactive Tool for Validation of Ocean Models at Oceanographic Stations. Journal of Open Research Software, 7: 18. DOI: https://doi.org/10.5334/jors.259

Numerical ocean models, like other geoscientific models, are a strongly simplified representation of real oceans. They are used as tools to answer research questions about the real-world systems. Therefore, their thorough validation is essential to ensure that the conclusions drawn from the model experiment are valid in reality. We demonstrate a software which allows an interactive model validation through a web interface based on the R Shiny framework. At pre-defined stations, different kinds of plots can be rendered within a few seconds, according to the user’s choice, allowing a live validation of different model parameters even in model simulations which are still running. This makes it different from validation approaches which generate a pre-defined set of plots after the calculations have finished and make it particularly useful for model tuning purposes. Observation data can be read in from text files or can be extracted from a database.

Once set up, the validation tool requires no technical skills to use. It can be used for single- or multi-model validation and allows saving the generated plots as high-resolution images suitable for use in scientific publications.

A Linux operating system is required for the Validator app, but via a virtual machine, the software can run on Windows or MacOS hosts as well. A Dockerfile is supplied which allows to test the software with example data without installation.

Temperature variability of the Baltic Sea since 1850 and attribution to atmospheric forcing variables

Kniebusch, M., Meier, H. E. M., Neumann, T., & Börgel, F. (2019). Temperature variability of the Baltic Sea since 1850 and attribution to atmospheric forcing variables. Journal of Geophysical Research: Oceans, 124. https://doi.org/10.1029/2018JC013948

The Baltic Sea is highly impacted by global warming and other anthropogenic changes and is one of the fastest‐warming marginal seas in the world. To detect trends in water temperature and to attribute them to atmospheric parameters, the results of two different ocean circulation models driven by reconstructed atmospheric forcing fields for the period 1850‐2008 were analysed. The model simulations were analysed at temporal and spatial scales from seasonal to centennial and from intra‐basin to basin, respectively. The strongest 150‐year trends were found in the annual mean bottom temperature of the Bornholm Deep (0.15 K/decade) and in summer mean sea surface temperature (SST) in Bothnian Bay (0.09‐0.12 K/decade). A comparison of the time periods 1856‐2005 and 1978‐2007 revealed that the SST trends strengthened 10‐fold. An attribution analysis showed that most of the SST variability could be explained by the surface air temperature (SAT), i.e., sensible heat flux, and the latent heat flux. Wind parallel to the coast and cloudiness additionally explained SST variability in the coastal zone affected by the variations in upwelling and in offshore areas affected by the variations in solar radiation, respectively. In contrast, the high variability in stratification caused by fresh‐ and saltwater inflows does not impact the long‐term variability in the SST averaged over the Baltic Sea. The strongest SST trends since the 1980s can be explained by the superposition of global warming and a shift from the cold to the warm phase of the Atlantic Multidecadal Oscillation (AMO).

Impact of the Atlantic Multidecadal Oscillation on Baltic Sea Variability

Börgel, F., C. Frauen, T. Neumann, S. Schimanke, and H. E. M. Meier. (2018), Impact of the Atlantic Multidecadal Oscillation on Baltic Sea variability, Geophys. Res. Lett., 45. https://doi.org/10.1029/2018GL078943

Coastal seas are of great importance to society. A prominent example of such a coastal sea is the Baltic Sea, since it is strongly impacted by human activities. However, besides the human footprint there are also natural phenomena, that influence the Baltic Sea. Especially climate phenomena over the North Atlantic can have a strong impact on the Baltic Sea. One such phenomenon is the so‐called Atlantic Multidecadal Oscillation (AMO), a seesaw between warm and cold sea surface temperatures in the North Atlantic with a period of 60‐90 years. Reliable observations only exist for a period of about 150 years, which is too short to study multidecadal time scales. Therefore, we use an 850 years long model simulation. Our results show that changes in North Atlantic sea surface temperature associated with the AMO influence the atmospheric circulation, which impacts the rain and snowfall over the Baltic Sea region. This in turn enhances or decreases the river runoff into the Baltic Sea and thus impacts the Baltic Sea salinity. Thus, the AMO has a strong influence on the Baltic Sea.