Some Singularities and Irregularities in the Seasonal Progression of the 700 mb Height Field

Abstract

A climatological investigation of singularities in the seasonal progression of 5-day means of 700 mb height was undertaken for the region 30?90°N, from 160°E eastward to 0°, for the period 1947?76. A harmonic analysis was performed at each of the 127 gridpoints (10° longitude by 10° latitude spacing) in order to determine the field of deviations (for each 5-day period) between the long-term mean value and the first harmonic value (which represents the smooth seasonal progression of heights). The Students's t-test was applied at each gridpoint to test whether the long-term (population) mean is significantly different from the first harmonic value (i.e., to test if the deviation is zero). The significance of the field of t-statistics for each 5-day period was estimated using a modification of the technique presented by Livezey and Chen (1981). The results of the analyses described above indicate that the 700 mb height field deviates significantly from the first harmonic during several times of the year. A pattern characterized by height rises centered over Alaska from late December through late January is terminated abruptly by the January thaw. In late February, a regional singularity (manifasted as rapid height rises), perhaps related to the termination of the February minimum in Hawaiian rainfall, is found in the eastern Pacific. The ?end of winter? in the eastern North Pacific is heralded by a very rapid northward shift of the westerlies from late February through early March. During the spring, the rapid rise of heights experienced in most regions is interrupted by a southward advance and strengthening of the westerlies in the eastern Pacific during late May and June. During summer, heights over most of the domain rise to ?excessively high? values relative to the first harmonic; the only exception is significantly negative deviations in eastern North America. This phenomenon results in the most deviant 5-day period (29 July?2 August) in terms of percentage of map area (50.1%) having significantly non-zero deviations. An Indian summer type pattern, characterized by weak positive deviations over most of North America and negative deviations in polar regions, persists from late September through October. This pattern exhibits the greatest persistence, between successive 5-day periods, of all the singularities found. Examination of the second harmonic of 5-day 700 mb heights suggests the existence of a semi-annual, cast-west oscillation of height in the North Pacific. This phenomenon seems to explain, to a large extent, most of the singularities and irregularities described above. At two of the four extrema are found the ?January thaw? and ?Indian Summer.? A six-month periodicity in (monthly) North Pacific and sea surface temperature (SST), in phase with the atmospheric oscillation was also found. It is speculated that the apparent association between these two phenomena is due to advection of long-term mean SST's by long-term mean-wind-induced surface currents as proposed by Weare et al. (1976).