Abstract
An investigation of singularities associated with the semiannual cycle of Northern Hemisphere (25–90°N) 700 mb heights was conducted using 33 years of data. Extension of the harmonic analyses of Lanzante (1983) to a larger domain revealed the considerable importance of the semiannual cycle to the seasonal progression of heights over subtropical Asia; this is undoubtedly a reflection of the Asiatic monsoon. From a global perspective, the semiannual cycle varies in such a fashion that this Asiatic region is out of phase with a region stretching from northeastern Siberia to the Gulf of Alaska, as well as the subtropical Atlantic and eastern Pacific. Furthermore, it is suggested that the semiannual cycle is reflected regionally in the persistence of height anomalies as reported by van den Dool and Livezey (1984).
Indices for nine regions of relatively large explained variance and uniform phase angle of the second harmonic of heights were derived in order to quantify interannual variations in the semiannual cycle. Frequency distribution of parameters, based on 25-day and seasonal aggregates of the daily indices were found to be largely Gaussian. It is concluded that (for the time averages considered) there is no indication that the indices come from two populations representing either the occurrence or nonoccurrence of an amplified semiannual cycle. Finally, interannual relationships among the nine indices were investigated through auto- and cross-correlation analyses; the results were largely negative.
