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.