On the Development of Blocking Ridge Activity Over the Central North Pacific

Abstract

Monthly. mean atmospheric data taken over the North Pacific during the period 1950-70 are used to investigate blocking ridge activity over the central ocean. The blocking ridge is observed to be a finiteamplitude, quasi-stationary long wave, most often centered over the North Pacific at 170W, superimposed upon the quasi-zonal mid-latitude westerlies. The dominant length scale is 7000 km, the same dimensions as the width of the mid-latitude ocean. The growth time scale is 1-2 weeks, with the duration of blocking activity rarely exceeding 2 months in any given year. The blocking activity is confined almost exclusively to the autumn/winter months, where block development is closely coupled with the sensible heat transfer from the underlying ocean (anomalously small heat transfer under the ridge and anomalously large heat transfer under the associated troughs). Year-to-year variability in blocking ridge activity is found to have a dominant time scale of approximately 5 years from 1950-70 and to be inversely correlated (-0.79) with the strength of the autumn/winter mean mid-latitude westerlies (the mean formed using months not containing blocking activity). Further analysis shows that both blocking ridge activity and the strength of the westerly winds fluctuate together with the Southern Oscillation over this time period. These space/time scale considerations suggest that this regional blocking activity owes its existence to the marine environment. To test this idea, appeal is made to some theoretical work by Haltiner, where the baroclinic instability process was modified by sensible heat transfer from the ocean to the atmosphere. Haltiner found that for normal winter values of the background flow, the otherwise stable stationary long wave became unstable when sensible heat transfer was allowed. The wavelength for the unstable stationary wave was 7000-8000 km with a growth time scale of approximately 2 weeks. The scales are similar to that of blocking ridge activity over the North Pacific. In addition to good scale agreement with observations, Haltiner's theory is able to explain both the seasonal and year-to-year variability in blocking activity in terms of corresponding fluctuations in sensible heat transfer and the strength of the mean westerly winds.

Aspects of a Northern Hemisphere Atmospheric Blocking Climatology

“…This analog was challenged by Egger (1978) on the grounds of the argument that the turbulent nature of the hydraulic jump cannot be associated with the stability of a blocking anticyclone. Namias (1950) and White and Clark (1975) proposed that a blocking formation is a manifestation of baroclinic instability through the discharge of cold air that had been trapped in the polar cap because of the presence of strong midlatitude westerlies. Green (1977) emphasized the role of transient fluxes in the maintenance of the persistent European blocking anticyclone of July 1976.…”

A Climatology of Northern Hemisphere Blocking

“…1). Since then, there have been modifications to the original Rex definition, demanding lower durations or extensions (Treidl et al 1981) as well as new restrictions in latitude location (White and Clark 1975) to exclude semipermanent subtropical anticyclones.…”

Variations of the atlantic and pacific blocking anticyclones and their correlation in the Northern Hemisphere

“…Blocking is a large-scale, mid-latitude atmospheric phenomenon that has a profound effect on local and regional climates in the immediate blocking domain (Rex, 1950a, b;Illari, 1984) as well as in the regions upstream and/or downstream of the blocking event (Quiroz, 1984;White and Clark, 1975). Therefore, it has been of primary interest to many synoptic and dynamical meteorologists for decades.…”

“…Previous studies, using subjective or objective techniques, have derived a comprehensive set of statistical characteristics of blocking anticyclone, including location, frequency, duration, intensity, size, seasonal and regional distribution (Elliott and Smith, 1949;Rex, 1950 b;White and Clark, 1975;Lupo, 1995;Lejen/is and Okland, 1983). It is widely accepted that two regions, one over the Atlantic and the other over the Pacific, are the preferred regions for the occurrence of blocking.…”

“…Their synoptic criteria for identifying the blocking events, including their criteria for how long a block must persist to be identified as such, were different, and this points to a basic problem in identifying and cataloging blocking events. White and Clark (1975) analyzed monthly mean synoptic charts of sea level pressure and 700 hPa geopotential height in the Northern Hemisphere for the period of 1950-1970. They concluded that the frequency of blocking events over the north Pacific is at a maximum during autumn and winter (October to March) and a minimum during spring and summer.…”