Variability of atmospheric circulation patterns associated with large volume changes of the Baltic Sea

Abstract

Abstract. Salinity and stratification in the deep basins of the Baltic Sea are linked to the occurrence of Major Baltic Inflows (MBIs) of higher saline water of North Sea origin, which occur sporadically and transport higher saline and oxygenated water to deeper layers. Since the mid-1970s, the frequency and intensity of MBIs have decreased. They were completely absent between February 1983 and January 1993. However, in spite of the decreasing frequency of MBIs, there was no obvious decrease of larger Baltic Sea volume changes (LVCs). A LVC is defined by a total volume change of at least 100 km3. LVCs can be identified from the sea level changes at Landsort which is known to represent the mean sea level of the Baltic Sea very well. Strong inflows leading to LVCs are associated to a special sequence of atmospheric circulation patterns. Our analysis based on Jenkinson-Collison circulation (JCC) types confirms that most effective inflows occur if about a month before the main inflow period, eastern air flow with anticyclonic vorticity over the western Baltic prevails. These conditions reduce the mean sea level of the Baltic Sea and lead to an increased saline stratification in the Belt Sea area. An immediate period of strong to very strong westerly winds trigger the inflow and force LVCs/MBIs. The lack of MBIs coincide with a negative trend of eastern types and a parallel increase of western type JCCs.

Spatial variability in the trends in extreme storm surges and weekly-scale high water levels in the eastern Baltic Sea

“…The stormy-season maxima of storm surges reveal almost no changes (below 1 cm/decade) on the south-western coast of the Western Estonian archipelago. The maxima of the total water level in this region are thus almost exclusively governed by the water volume of the entire Baltic Sea, equivalently, by the impact of specific sequences of wind events that are mirrored in the average water level (Lehmann and Post, 2015). A natural conjecture is that the wind speed in strong storms (that are responsible for such maxima) has not increased significantly in the Baltic Proper.…”

“…The largest impact to the Baltic Sea water volume arises from sequences of storm cyclones (Post and Kõuts, 2014) that force large amounts of the North Sea water to flow into the Baltic Sea over a few weeks (Stigebrandt and Gustafsson, 2003;Lehmann and Post, 2015). The associated water level increase in the entire sea may reach 1 m (Johansson et al, 2001) similarly to Chesapeake Bay (Bosley and Hess, 2001).…”

Major Baltic Inflow Statistics – Revised

“…3. There exists a mismatch between the MBI frequency in FM96 and the large volume change events (LVC) derived from mean sea level changes of the Baltic (Lehmann and Post, 2015;Lehmann et al, 2017). 4.…”

“…The weak point of these definitions is the lack of a direct link to water transports, since current measurements were only occasionally available. Thus, Lehmann and Post (2015) invented the concept of "Large Volume Changes" to derive the intensity of inflow events from the related change in total water volume of the Baltic. They found a significant difference between MBI and LVC frequency, especially since the late 1980s, and attributed the decreasing MBI frequency to a negative trend in eastern types of atmospheric circulation with a concurrent increase in western circulation types (according Jenkinson and Collison, 1977).…”

Low-Frequency Baltic Sea Level Spectrum

“…It must be caused by the influence of variations water exchange through DS and changes in the month of annual maximum river discharge (flood). Water exchange through straits causes large volume changes of the BS (Lehmann and Post, 2015) with the duration is about 40 days (Lehmann et al, 2012). These events cause the sea level rise by several dozens of centimeters (Soomere and Pindsoo, 2016) and may cause the variations of near-annual period.…”