Variations in Reconstructed Ice Winter Severity in the Western Baltic from 1501 to 1995, and their implications for the North Atlantic Oscillation

Abstract

Variations in ice winter severity in the Western Baltic between 1501 and 1995 were investigated using an index time series derived from classified values of accumulated areal ice volume along the German Baltic coast, the time series back to 1701, having been extended to the beginning of the 16th century. When compared with the 1501–1995 mean, the Gaussian lowpass-filtered time series of the ice winter index numerals with a 40-year cutoff period shows increased severity (strong phases) in 1554–1576, 1593–1630, 1655–1710, and 1763–1860, while periods of decreased severity occurred in 1501–1553, 1577–1592, 1631–1654, 1711–1762, and from 1861 to the present. During the latter part of the ‘Little Ice Age’, especially during the 1655–1710 and 1763–1860 phases, the lowpass-filtered time series lay more than half a standard deviation above the arithmetic mean of the reference period 1901–1960, representing the present regime, for more than three decades. Between 1501 and 1860, the ice winter severity in the Western Baltic fluctuated around a level 55% higher than that during the present period. Using the contingency table published by Koslowski and Loewe, the frequency of events of weak westerly flow above the northeastern North Atlantic during the ‘Little Ice Age’ was estimated. The calculated values of weak westerly flow expected per decade suggest that strong phases of increased ice winter severity were characterized by frequent blocking situations (weak westerly flow), and that, contrarily, the weak phases of reduced ice winter severity between about 1575 and 1860 may be regarded as phases of increased zonal circulation.