Use of existing hydrographic infrastructure to forecast the environmental spawning conditions for Eastern Baltic cod

The semi-enclosed nature and estuarine characteristics, together with its strongly alternating bathymetry, make the Baltic Sea prone to much stronger interannual variations in the abiotic environment, than other spawning habitats of Atlantic cod (Gadus morhua). Processes determining salinity and oxygen conditions in the basins are influenced both by long term gradual climate change, e.g. global warming, but also by short-term meteorological variations and events. Specifically one main factor influencing cod spawning conditions, the advection of highly saline and well-oxygenated water masses from the North Sea, is observed in irregular frequencies and causes strong interannual variations in stock productivity. This study investigates the possibility to use the available hydrographic process knowledge to predict the annual spawning conditions for Eastern Baltic cod in its most important spawning ground, the Bornholm Basin, only by salinity measurements from a specific location in the western Baltic. Such a prediction could serve as an environmental early warning indicator to inform stock assessment and management. Here we used a hydrodynamic model to hindcast hydrographic property fields for the last 40+ years. High and significant correlations were found for months early in the year between the 33m salinity level in the Arkona Basin and the oxygen-dependent cod spawning environment in the Bornholm Basin. Direct prediction of the Eastern Baltic cod egg survival in the Bornholm Basin based on salinity values in the Arkona Basin at the 33 m depth level is shown to be possible for eggs spawned by mid-age and young females, which currently predominate the stock structure. We recommend to routinely perform short-term predictions of the Eastern Baltic cod spawning environment, in order to generate environmental information highly relevant for stock dynamics. Our statistical approach offers the opportunity to make best use of permanently existing infrastructure in the western Baltic to timely provide scientific knowledge on the spawning conditions of Eastern Baltic cod. Furthermore it could be a tool to assist ecosystem-based fisheries management with a cost-effective implementation by including the short term predictions as a simple indicator in the annual assessments

Towards an improved mechanistic understanding of major saltwater inflows into the Baltic Sea

Oral presentation at the 11th Baltic Sea Science Congress, Rostock, June 2017

A statistical approach to coastal upwelling in the Baltic Sea based on the analysis of satellite data for 1990-2009

A statistical analysis of Baltic Sea upwelling has been carried out to cover, for the first time, the entire sea area for the period 1990-2009. Weekly composite SST maps based on NOAA/AVHRR satellite data were used to evaluate the location and frequency of upwelling. The results obtained were analysed and compared with earlier studies with excellent agreement. Our study enables the most intense upwelling areas in the entire Baltic Sea to be evaluated. According to the analysis of 443 SST maps, the most common upwelling regions are found off the Swedish south and east coasts (frequency 10-25%), the Swedish coast of the Bothnian Bay (16%), the southern tip of Gotland (up to 15%), and the Finnish coast of the Gulf of Finland (up to 15%). Pronounced upwelling also occurs off the Estonian coast and the Baltic east coast (up to 15%), the Polish coast and the west coast of Rügen (10-15%); otherwise the upwelling frequency was between 5 and 10%. Additionally, simulated SST distributions derived from a Baltic Sea numerical model were analysed for the same period. Furthermore, at specific positions close to the coastline, surface winds based on the SMHI meteorological data base were analysed for the same 20-year period. Wind components parallel to the coast were discriminated into favourable and unfavourable winds forcing upwelling. The obtained frequencies of upwelling-favourable winds fit very well the observed upwelling frequencies derived from satellite SST maps. A positive trend of upwelling frequencies along the Swedish east coast and the Finnish coast of the Gulf of Finland was calculated for the period 1990-2009

Pathways of deep cyclones associated with large volume changes (LVCs) and major Baltic inflows (MBIs)

Highlights: • Frequencies and pathways of deep cyclones forcing large volume changes (LVCs) and major Baltic inflows (MBIs) have been determined and analyzed. • Deep cyclones associated with LVCs and MBIs follow 4 main routes over the Baltic Sea area. • For the period 1950–2010, there is a positive trend of the number of deep cyclones associated with LVCs. Abstract: Large volume changes (LVCs) and major Baltic inflows (MBIs) are essential processes for the water exchange and renewal of the stagnant water in the Baltic Sea deep basins. These strong inflows are known to be forced by persistent westerly wind conditions. In this study, MBIs are considered as subset of LVCs transporting with the large water volume a big amount of highly saline and oxygenated water into the Baltic Sea. Since the early 1980s the frequency of MBIs has dropped drastically from 5 to 7 events to only one inflow per decade, and long lasting periods without MBIs became the usual state. Only in January 1993, 2003 and December 2014 MBIs occurred that were able to interrupt the stagnation periods in the deep basins of the Baltic Sea. However, in spite of the decreasing frequency of MBIs, there is no obvious decrease of LVCs. The Landsort sea level is known to reflect the mean sea level of the Baltic Sea very well, and hence LVCs have been calculated for the period 1887–2015 filtering daily time series of Landsort sea surface elevation anomalies. The cases with local minimum and maximum difference resulting in at least 60 km3 of water volume change excluding the volume change due to runoff have been chosen for a closer study (1948–2013) of characteristic pathways of deep cyclones. The average duration of LVCs is about 40 days. During this time, 5–6 deep cyclones move along characteristic storm tracks. Furthermore, MBIs are characterized by even higher cyclonic activity compared to average LVCs. We obtained four main routes of deep cyclones which were associated with LVCs, but also with the climatology. One is approaching from the west at about 56–60°N, passing the northern North Sea, northern Denmark, Sweden and the Island of Gotland. A second broad corridor of frequent cyclone pathways enters the study area north of Scotland between 60 and 66°N turning north-eastwards along the northern coast of Scandinavia. This branch bifurcates into smaller routes. One at about 62°N passing Oslo, southern Sweden and entering the central Baltic Sea, and another less frequent one at about 65°N, crossing Scandinavia south-eastwards passing the Sea of Bothnia and entering Finland. The conditions for LVCs to happen are temporal clustering of deep cyclones in certain trajectory corridors. We also found an increasing linear trend of the number of deep cyclones for the period 1950–2010

Modeled and Measured salinity, oxygen and derived cod egg survival data for the Arkona Basin and the Bornholm Basin in the Baltic Sea

This data set contains several tables with modeled and measured data and also correlations coefficient data used in the publication. The model data are aggregations of areas in the Arkona Basin (AB) and Bornholm Basin (BB) from the Kiel Baltic Sea Ice and Ocean model (BSIOM) for the period 1971 to 2015. See the puplication for details about horizontal aggregation. Raw Data of the Model is not included in this data deposition, but are given by the authors upon request and when credited in any use of the data. Monthly Means of the AB and BB are used for detailed correlation analysis with time lags (raw data is given in the files "ModelData_O2_BornholmBasin_MM_ModelDepths.." and "ModelData_S_ArkonaBasin_MM_modelDepths.."). Resulting data of all combinations for depth layers in either Basin, and Monthly lags up to a full year, is given in the "ModelData_correlation_ABMM_vs_BBMM.txt" file. Monthly profiles of the Bornholm Basin were also used to produce monthly means of S,T,O2 and cod egg survival chances on certain levels of water density (1009, 1010, 1011, 1012 and 1013 kg/m^3) given in the file "ModelData_O2_BornholmBasin_MM_onDensLayers.txt" Model data for the Bornholm Basin were also used to produce a time series of daily resolution of the mean oxygen content below the halocline. Data is given in the file "ModelData_O2_BornholmBasin_DM_rough_layers.txt" Measured salinity at the 33m depth level in a daily resolution is taken from the Arkona Basin platform (permanent measuring buoy) operated by the Federal Maritime and Hydrography Agency, Hamburg, Germany. Given in file "Measured_S_ArkonaBasin_DM_33m_corrected.txt