She et al.
Operational Oceanography and Earth System Science
Frontiers In Earth Science | www.frontlersln.org
8
February 2020 | Volume 8 | Article 7
has increased relatively more than oxygen consumption in the
sediment. Subsequently, natural ventilation has become less
effective representing a positive feedback for hypoxia (Meier
etal., 2018b).
Observations from field campaigns from the northern Baltic
Sea suggested that the flow regimes are intermittent and that
hydraulic control occurs in only about 55% of the cases, i.e., less
frequently than anticipated (Green et al., 2006). Further, in wider
gravitational flows, transverse Ekman circulation was identified
to be an important process for the generation of mixing (Umlauf
and Arneborg, 2009a,b).
Recently, there has been increased research into the Baltic
Sea coastal zone, particularly into upwelling, nutrient retention
and the coastal filter capacity of nutrients (e.g., Edman et al.,
2018). Estimates suggest that the coastal filter of the entire
Baltic Sea removes 16% of nitrogen and 53% of phosphorus
inputs from land (Asmala et al., 2017). Simulated long-term
nutrient retention was found to be associated with the physical
characteristics of a water body, such as the surface area, depth
and residence time of the water.
Progress was also made in understanding the large-scale
circulation, water mass transformations, and mixing processes
in the Baltic Sea using high-resolution ocean circulation models
that were running for many decades together with Eulerian
concentration and age tracers (e.g., Meier, 2005, 2007). The
model results illustrate possible pathways and ages of either
inflowing saline water from the North Sea or freshwater
originating from the various rivers. Freshwater is found to be
subject to an efficient recirculation in the Baltic (e.g., Rodhe
and Winsor, 2002). These simulations are complementary to an
interesting tracer release experiment in the deep water of the
central Gotland Basin showing a considerable increase in vertical
mixing rates after the tracer reached the lateral boundaries of the
basin (Holtermann and Umlauf, 2012; Holtermann et al., 2012).
Hence, boundary mixing is perhaps the key process of basin-scale
vertical mixing. For further details, the reader is referred to the
review article by Omstedt et al. (2014) and the original literature
cited therein.
Climate and Environmental Observations
and Reanalyses
Nowadays, meteorological databases (both station data and highresolution
gridded datasets) are freely available with high quality
to force ocean models on decadal and even centennial time
scales. For instance, the regional reanalysis project Uncertainties
in Ensembles of Regional ReAnalyses (UERRA, http://www.
uerra.eu) delivers homogenous atmospheric surface fields for
the period 1961 until today. In addition, oceanographic data
became more easily accessible and new important measurement
platforms, such as the MARNET stations (https://www.iowarnemuende.de/marnet-en.html),
long-term moorings, e.g.,
in the Gotland Deep region, FerryBoxes, and satellites, have
provided temporally and spatially better resolved observations.
River runoff data are crucial for the understanding of the Baltic
Sea dynamics and new catchment-wide high-resolution datasets
based on process-based hydrological modeling calibrated to
available station data are now available. However, a homogeneous
hydrological dataset that covers the entire period from the
1960s to the present day comparable to atmospheric reanalysis
data is still missing. Further, available nutrient load and other
environmental data are nowadays collected and stored in publicly
available databases.
A big step forward to understand climate variability in
the Baltic Sea region was the development of historical
reconstructions of atmospheric, hydrological and oceanic
datasets since around 1850. With the help of Baltic Sea models,
the impact of increasing nutrient loads and climate change on
the marine ecosystem was detected and attributed to the various
drivers of the system. We have now a better understanding of the
natural variability in the Baltic Sea region and how large-scale
atmospheric circulation affects the Baltic Sea climate variability
(e.g., Borgel et al., 2018). During recent decades, changes in largescale
atmospheric circulation have caused a north-eastward shift
in low-pressure tracks consistent with a more zonal circulation
over the Baltic Sea basin (e.g., Trenberth et al., 2007). The decadal
and multi-decadal regional variability of the past climate is partly
explained by the North Atlantic Oscillation (NAO, mainly during
winter) and the Atlantic Multi-decadal Oscillation (AMO).
Despite the pronounced internal variability, trends were detected
that could probably be attributed to anthropogenic climate
change on centennial time scale (e.g., Kniebusch et al., 2019a,b).
A highlight was the revision of the empirically derived barotropic
saltwater inflow statistics for 1887 until present that shows
no statistically significant trend but the same multi-decadal
variability as in precipitation data (Mohrholz, 2018). Further,
based upon model results it was concluded that stagnation
periods such as the one between 1983 and 1992 are part of the
natural variability of the system and occur once per 100 years on
average (Schimanke and Meier, 2016).
While atmospheric reanalysis data have long been used to
force ocean models, long-term reanalyses for the ocean on multidecadal
time scales became only recently available including both
physical and biogeochemical variables (e.g., Liu et al., 2017).
Ocean reanalysis data play an important role for the development
and evaluation of ocean models (Placke et al., 2018).
Climate and Environmental Modeling
Within BALTEX, the first coupled atmosphere-ice-ocean regional
models were developed about 20 years ago (Gustafsson et al.,
1998; Hagedorn et al., 2000; Doscher et al., 2002; Schrum et al.,
2003). Nowadays several coupled models for the Baltic Sea—
North Sea system are under development (e.g., Groger et al.,
2013; Tian et al., 2013, 2016; Van Pham et al., 2014; Wang
et al., 2015; Ho-Hagemann et al., 2017). Regional climate models
contributed to a better quantitative understanding of the energy
and water cycles of the Baltic Sea basin. However, especially
processes important for the regional water cycle are still not
well-understood causing, inter alia, precipitation and runoff
biases over the catchment area in long-term atmosphere climate
simulations with considerable impact on the quality of ocean
climate simulations (Meier et al., 2019).
These models will be the future tools to investigate the
dynamics of regional Earth systems. Within Baltic Earth a
