ON THE CAUSES OF MAJOR BALTIC INFLOWS - AN ANALYSIS OF LONG-TIME SERIES

Conditions for life in the deep water of the Baltic Sea are strongly i nfluenced by inflows of highly saline and oxygenated water from the No rth Sea. These events - termed major Baltic inflows (MBI) - have episo dic character, and are the only mechanisms by which the central Baltic deep water is renewed. Although the cycle of water renewal is well do cumented, certain meteorological and oceanographic processes determini ng it are either not very well understood or even partly unknown. Base d on the data set of major inflows during the present century, long ti me series of relevant variables from the Baltic Sea itself (salinity, sea level), its drainage area (river runoff, precipitation), the whole Baltic region (air temperature) and from the North Atlantic and Europ e (sea level pressure) are analyzed using statistical methods. Charact eristic variations in the relevant meteorological, hydrological and oc eanographic variables before and during major events are calculated in order to identify conditions favouring or preventing: such events. Ma jor Baltic inflows are characterized by two phases: (1) high pressure over the Baltic region with easterly winds followed by (2) several wee ks of strong zonal wind and pressure fields over the North Atlantic an d Europe. Major events may occur when only one of these is well develo ped, the probability of strong events is high if both phases are well developed and closely spaced in time. Variations in river runoff to th e Baltic obviously have a greater impact on the occurrence of major ev ents then hitherto supposed. The decreasing frequency and intensity of major inflows since the mid-1970s and the complete absence of such ev ents from February 1983 to the beginning of 1993 is explained by incre ased zonal circulation linked with intensified precipitation in the Ba ltic region and increased river runoff to the Baltic. Possible anthrop ogenic impacts on changes in occurrence of major inflows due to river runoff regulations are indicated. The results should suggest to invest igate more intensively the causes of major Baltic inflows by using cou pled atmosphere/ocean/land surface models. (C) 1998 Elsevier Science L td. All rights reserved.