Low-frequency oscillations in the Northern Hemisphere

Multichannel singular spectrum analysis (MSSA) has been applied to 5-day mean grid point data of 500 hPa geopotential heights over the Northern Hemisphere, to explore low frequency oscillatory variabilities over the region. Two major low-frequency oscillations with periods of 60-70 and 30-40 days were found. These two quasi-periodicities are significantly (at the 99% confidence level) different from those found in the corresponding pure red noise process. Also, they were reproducible when using different window lengths for MSSA and when applying MSSA to two equal-length subsets of the basic data series. In addition, when one subset (first subset) is projected onto the patterns of the other subset, the resulting (projection) spatial-temporal principal components corresponding to the two oscillations are highly correlated with those ST-PCs directly derived from the first subset by MSSA, indicating that the oscillatory components obtained from one subset explain a significant amount of variance in the other subset. Some reliable information about the amplitudes of the oscillations has been also obtained by using composite analysis.