Mechanisms of variability in decadal sea-level trends in the Baltic Sea over the 20th century


Coastal sea-level trends in the Baltic Sea display decadal-scale variations around a centennial trend. These long-term centennial trends are likely determined by climate change and centennial vertical land movements. In this study, we analyse the spatial and temporal characteristics of the decadal trend variations and investigate the links between coastal sea-level trends and atmospheric forcing on decadal time scale. This investigation mainly focuses on the identification of the possible impact of an underlying factor, apart from the effect of atmospheric circulation, on decadal sea-level trend anomalies. For this analysis, we use monthly means of long tide gauge records and gridded sea-surface-height (SSH) reconstructions. The SSH time series are constructed over the past 64 years and based on tide-gauge records and satellite altimetry. Climatic data sets are composed of the North Atlantic Oscillation (NAO) index, the Atlantic Multidecadal Oscillation (AMO) index, gridded sea-level-pressure (SLP), gridded near-surface air temperature and gridded precipitation fields. The analysis indicates that atmospheric forcing is a driving factor of decadal sea-level trends. However, its effect is geographically heterogeneous. The Baltic Sea can be classified into two parts according to atmospheric impacts on decadal sea-level trends: one part consists of the northern and eastern regions of the Baltic Sea, where this impact is large. The other one covers the southern Baltic Sea area, with a smaller impact of the atmospheric circulation. To identify the influence of the large-scale factors other than the simultaneous effect of atmospheric circulation on the Baltic Sea level trends, we filter out the direct signature of atmospheric circulation on the Baltic Sea level by a multivariate linear regression model and analysed the residuals of this regression model. These residuals hint at a common underlying factor that coherently drives the decadal sea-level trends into the similar direction in the whole Baltic Sea region. We found that this underlying effect is partly a consequence of precipitation contribution to the Baltic Sea basin in the previous season. The investigation on the relation between the AMO-index and sea-level trends implies that this detected underlying factor is not connected to oceanic forcing driven from the North Atlantic region.
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