LONG-PERIOD VARIATIONS IN SEASONAL SEA-LEVEL PRESSURE OVER THE NORTHERN HEMISPHERE1

Abstract

The standardized departures of sea-level pressure from the normal over the Northern Hemisphere for each of the foiir seasons were examined by means of spectrum analysis. Emphasis was placed on interpreting the geographical distribution of relative spectral power at different periods, rather than the power distribution within the individual spectra. The predominant characteristic of the data was trend, found to represent real meteorological effects and not spurious results of changes in station locations or times of observation. Particularly in subtropical latitudes where the trend was greatest, the data were characterized by large 1- and 2-year-lag autocorrelations and pronounced “red noise” spectra. Exceptionally large 2-year-lag autocorrelation coefficients over several extensive regions of the hemisphere were related to marked departures of the data series from a linear first-order Markov process in those areas. Although not so strong in an absolute sense as the trend, several moderately strong quasi-periodicities were found that appeared to be related to specific geographical or climatological features. The seasonal sea-level pressure data from arid and semiarid regions in widely separated parts of the hemisphere displayed a rather strong peak of spectral power in the band centered near 21 yr. Data from an extensive area of the northern Pacific Ocean showed a broad spectral peak in the range of 5 to 6 yr, particularly in winter and summer. A pronounced spectral peak in the vicinity of 2 >$ yr was observed in the data series for winter over the Gulf of Alaska, the Mediterranean, and to a lesser extent off the east coast of the United States. All these areas are characterized by frequent and often intense cyclogenesis during the colder months. Although the large amount of trend in the data made if difficult to determine the significance of individual spectral peaks, the quasi-beinnial oscillation also appeared to be a real characteristic of the data. A moderate peak was observed in the spectrum of the data series for some subtropical and continental high-pressure areas, particularly in winter. A preponderance of the spectra at the individual Northern Hemisphere grid points displayed relative peaks in the range of 26 to 28 mo, most markedly for the winter data.