Modelling circulation dynamics in the northern Baltic Sea

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Title: Modelling circulation dynamics in the northern Baltic Sea
Author: Westerlund, Antti
Date: 2018-11
Language: en
Belongs to series: Finnish Meteorological Institute Contributions 145
ISBN: 978-952-336-055-6
ISSN: 0782-6117
Abstract: Circulation and surface layer dynamics are of significant importance, for example, when considering how hazardous substances or nutrients are transported in the sea. The earliest studies mapping circulation patterns in the northern Baltic Sea were done before the Second World War and were based on lightship observations. Although the number of available observation points was low, these studies showed that there is a cyclonic long-term surface circulation pattern in the northern subbasins. Even today, there are considerable research gaps and uncertainties in knowledge. For example, observational data still has insufficient coverage, descriptions of processes in numerical models need tuning to the conditions of the Baltic Sea and model forcing data can have large uncertainties. With modern analysis methods and new observational datasets, gaps in the current understanding of Baltic Sea circulation patterns can be identified and analyzed. In this thesis, circulation dynamics were investigated in the northern Baltic Sea with numerical hydrodynamic modelling. The complex dynamics of the brackish Baltic Sea put hydrodynamic models to the test. Several different model configurations were applied and developed further, including a high-resolution configuration of the NEMO (Nucleus for European Modelling of the Ocean) model for the Gulf of Finland (GoF). Methods such as machine learning algorithms, new data from automated observational platforms and ensemble forecasting were applied. Circulation patterns in the GoF were investigated with the self-organizing map (SOM) algorithm. The cyclonic circulation pattern visible in earlier studies was not seen in the GoF in the overall means calculated from the model results for the studied periods 2007–2013 and 2012–2014. SOM analysis of currents in the GoF revealed that they are highly variable and complex. There was significant inter-annual and intra-annual variability in the circulation patterns. A connection between wind forcing and the characteristic patterns from the SOM analysis was found. Analysis emphasized the estuary-like nature of the GoF. The results showed that circulation in the GoF changes rapidly between normal estuarine circulation and reverse estuarine circulation. The fact that the dominant wind direction is from the southwest supports this reversal. The cyclonic mean circulation pattern seems to appear only if the normal estuarine circulation is common enough for it to emerge during the averaging period. Small changes to wind direction distribution can have a significant effect on the longterm circulation patterns. Upwelling events on a timescale of days to weeks can also affect long-term circulation patterns. The NEMO model proved to be a suitable tool for the studies of circulation in the northern sub-basins of the Baltic Sea. It quality seems comparable to other models commonly used in the GoF and Bothnian Sea. The GoF is still a challenging environment for circulation modelling. Salinity gradients in the GoF are still not reproduced in a satisfactory manner by the models. More information is required on how well the models reproduce true circulation patterns and, for example, upwelling frequency and intensity. The need for accurate model inputs, especially wind forcing, was demonstrated. The value of observations (especially the better spatial coverage of current measurements) was once again emphasized. Furthermore, the results highlighted that care must be taken to make sure that models and observations represent the same thing when they are compared.
Subject: Baltic Sea
Gulf of Finland
Bothnian Sea
hydrodynamic modelling
circulation modelling

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