Trend and oscillations in the ice-cover duration of Lake Mendota, Wisconsin, USA

The 150-year time series of ice duration in Lake Mendota, Wisconsin was analysed to investigate possible effects of climate variability, and a non-parametric test confirmed a significant, decreasing trend. Singular-spectrum analysis (SSA) and the multi-taper method (MTM) of spectral analysis revealed a nonlinear trend and quasi-periodic oscillations. Analysis of the linearly detrended series showed significant frequencies ranging from quasi biennial to >100 years. The interannual and interdecadal signals detected are within the range of periodicity of the El Nino-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) modes of variability. A smoothing procedure was used to separate low frequencies, or trend, and higher frequencies. Four reconstructed components of the low-pass-filtered time series were sufficient to explain 95% of the filtered series variance. The reconstruction showed signals with periods longer than 20 years that could be associated with the PDO. The series showed a relatively constant trend between approximately 1895 and 1965, and a steep declining trend in the last 40 years. Twenty four reconstructed components of the high-frequency residuals were required to explain 90% of the variance in the residual series. The reconstruction showed interdecadal signals that could be associated with the PDO and signals with periods between two and seven years that could be associated with the Southern Oscillation Index (SOI). Results of a coherence analysis showed that ice-cover duration is significantly correlated with the PDO for periods longer than about 20 years, and with the SOI for periods between two and seven years. The results suggest a relationship between the decreasing ice duration since about 1970 and the accompanying increase in Northern Hemisphere mean annual surface temperature. This interpretation agrees with previous descriptions of observed historic changes that appear to be related to anthropogenic activity.