THE WESTERN BALTIC SEA-ICE SEASON IN TERMS OF A MASS-RELATED SEVERITYINDEX 1879-1992 - (II) - SPECTRAL CHARACTERISTICS AND ASSOCIATIONS WITH THE NAO, QBO, AND SOLAR-CYCLE

In this sequel paper, the investigation of the ice winter severity in the Western Baltic is continued in the frequency domain. Spectral anal ysis of the time series of the mass-related ice index (V-A Sigma) reve als prominent quasicycles with periods of 2.3, 5.8 and 7.8 years. The same cycles stand out in the variance spectrum of the winter North Atl antic Oscillation (NAG) index. Variations in the quasibiennial Q(2)(3) and intermediate I-5(10) periodicity ranges (indices give the range l imits in years) account for 68.2% of the linear correlation (-0.46) be tween both time series. A maximum entropy subinterval analysis shows t hat the spectral variance composition of the V-A Sigma series undergoe s significant temporal changes. The relative contribution of the Q(2)( 3) range is down from 52.7% during 1899(+/-20) to almost half of this value during 1972(+/-20). The significance of the I-5(10) and of short periodic oscillations in the S-3(5) range has in turn increased from about 15% each to 36.8 and 30.7%, respectively. Predictions of ice win ter severity based on the spectral characteristics of the full record are thus bound to fail. Some predictive skill for severe ice winters, which occurred with an overall frequency of 24.6%, may be realized fro m an apparent association with the 11-year solar activity cycle. This association has been most pronounced since the 1950s, about the time t he 8-year rhythm started dominating variability. 9 out of 10 severe ic e winters eventuated in phase with either high (3) or low solar activi ty (6). Moreover, almost all of the most severe ice winters occurred w hen the phase of the quasibiennial stratospheric wind oscillation (QBO ) at the Equator was east (west) and, at the same time, solar activity was low (high). The peaks in the V-A Sigma spectrum after WWII reflec t the dominance of singular 8-year ice events of modulated strength. T his quasicyclic recurrence of strong ice winters in conjunction with t he apparent statistical Solar Cycle-QBO-Ice Winter Severity relationsh ip may be cautiously used in ice winter forecasting.