• Ambaum, M. H. P., and W. T. M. Verkley, 1995: Orography in a countour dynamics model of large-scale atmospheric flow. J. Atmos. Sci., 52 , 26432662.

    • Search Google Scholar
    • Export Citation
  • Appenzeller, C., and H. C. Davies, 1992: Structure of stratospheric intrusions into the troposphere. Nature, 358 , 570572.

  • Berggren, R., B. Bolin, and C-G. Rossby, 1949: An aerological study of zonal motion, its perturbations and break-down. Tellus, 1 (2) 1437.

    • Search Google Scholar
    • Export Citation
  • Charney, J. G., J. Shukla, and K. C. Mo, 1981: Comparison of a barotropic blocking theory with observation. J. Atmos. Sci., 38 , 762779.

    • Search Google Scholar
    • Export Citation
  • D'Andrea, F., and Coauthors. 1998: Northern Hemisphere atmospheric blocking as simulated by 15 atmospheric general circulation models in the period 1979–1988. Climate Dyn., 14 , 385407.

    • Search Google Scholar
    • Export Citation
  • Dole, R. M., 1978: The objective representation of blocking patterns. The General Circulation: Theory, Modeling and Observations, NCAR/CQ-6+1978-ASP, 404–426.

    • Search Google Scholar
    • Export Citation
  • Dole, R. M., and N. D. Gordon, 1983: Persistent anomalies of the extratropical Northern Hemisphere wintertime circulation: Geographical distribution and regional persistence characteristics. Mon. Wea. Rev., 111 , 15671586.

    • Search Google Scholar
    • Export Citation
  • Elliott, R. D., and T. B. Smith, 1949: A study of the effects of large blocking highs on the general circulation in the Northern-Hemisphere westerlies. J. Meteor, 6 , 6785.

    • Search Google Scholar
    • Export Citation
  • Green, J. S. A., 1977: The weather during July 1976: Some dynamical considerations of the drought. Weather, 32 , 120126.

  • Hoskins, B. J., 1997: A potential vorticity view of synoptic development. Meteor. Appl., 4 , 325334.

  • Hoskins, B. J., and F. P. Bretherton, 1972: Atmospheric frotogenesis models: Mathematical formulation and solutions. J. Atmos. Sci., 29 , 1137.

    • Search Google Scholar
    • Export Citation
  • Hoskins, B. J., and P. D. Sardeshmukh, 1987: A diagnostic study of the dynamics of the Northern Hemisphere winter of 1985–86. Quart. J. Roy. Meteor. Soc., 113 , 759778.

    • Search Google Scholar
    • Export Citation
  • Hoskins, B. J., and P. Berrisford, 1988: A potential vorticity perspective of the storm of 15–16 October 1987. Weather, 43 , 122129.

    • Search Google Scholar
    • Export Citation
  • Hoskins, B. J., M. E. McIntyre, and A. Robertson, 1985: On the use and significance of isentropic potential vorticity maps. Quart. J. Roy. Meteor. Soc., 111 , 877946.

    • Search Google Scholar
    • Export Citation
  • Lejenäs, H., and H. Økland, 1983: Characteristics of Northern Hemisphere blocking as determined from a long series of observational data. Tellus, 35A , 350362.

    • Search Google Scholar
    • Export Citation
  • Namias, J., 1947: Characteristics of the general circulation over the Northern Hemisphere during the abnormal winter 1946–1947. Mon. Wea. Rev., 75 , 145152.

    • Search Google Scholar
    • Export Citation
  • Pelly, J. L., and B. J. Hoskins, 2003: How well does the ECMWF Ensemble Prediction System predict blocking? Quart. J. Roy. Meteor. Soc., in press.

    • Search Google Scholar
    • Export Citation
  • Rex, D. F., 1950a: Blocking action in the middle troposphere and its effect upon regional climate. I. An aerological study of blocking action. Tellus, 2 , 196211.

    • Search Google Scholar
    • Export Citation
  • Rex, D. F., 1950b: Blocking action in the middle troposphere and its effect upon regional climate. II. The climatology of blocking action. Tellus, 2 , 275301.

    • Search Google Scholar
    • Export Citation
  • Rossby, C. G., 1951: On the dynamics of certain types of blocking. J. Chin. Geophys. Soc., 2 , 113.

  • Shutts, G. J., 1983: The propagation of eddies in diffluent jetstreams: Eddy vorticity forcing of “blocking” flow fields. Quart. J. Roy. Meteor. Soc., 109 , 737761.

    • Search Google Scholar
    • Export Citation
  • Shutts, G. J., 1986: A case study of eddy forcing during an Atlantic blocking episode. Advances in Geophysics, Vol. 29, Academic Press, 135–164.

    • Search Google Scholar
    • Export Citation
  • Swanson, K. L., 2000: Stationary wave accumulation and the generation of low-frequency variability on zonally varying flows. J. Atmos. Sci., 57 , 22622280.

    • Search Google Scholar
    • Export Citation
  • Swanson, K. L., 2001: Blocking as a local instability to zonally varying flows. Quart. J. Roy. Meteor. Soc., 127 , 13411355.

  • Thorpe, A. J., 1985: Diagnosis of balanced vortex structure using potential vorticity. J. Atmos. Sci., 42 , 397406.

  • Tibaldi, S., and F. Molteni, 1990: On the operational predictability of blocking. Tellus, 42A , 343365.

  • Vautard, R., and B. Legras, 1988: On the source of midlatitude low-frequency variability. Part II: Nonlinear equilibration of weather regimes. J. Atmos. Sci., 45 , 28452867.

    • Search Google Scholar
    • Export Citation
  • White, W. B., and N. E. Clarke, 1975: On the development of blocking ridge activity over the central North Pacific. J. Atmos. Sci., 32 , 489502.

    • Search Google Scholar
    • Export Citation
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A New Perspective on Blocking

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  • 1 Department of Meteorology, University of Reading, United Kingdom
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Abstract

It is argued that the essential aspect of atmospheric blocking may be seen in the wave breaking of potential temperature (θ) on a potential vorticity (PV) surface, which may be identified with the tropopause, and the consequent reversal of the usual meridional temperature gradient of θ. A new dynamical blocking index is constructed using a meridional θ difference on a PV surface. Unlike in previous studies, the central blocking latitude about which this difference is constructed is allowed to vary with longitude. At each longitude it is determined by the latitude at which the climatological high-pass transient eddy kinetic energy is a maximum. Based on the blocking index, at each longitude local instantaneous blocking, large-scale blocking, and blocking episodes are defined. For longitudinal sectors, sector blocking and sector blocking episodes are also defined. The 5-yr annual climatologies of the three longitudinally defined blocking event frequencies and the seasonal climatologies of blocking episode frequency are shown. The climatologies all pick out the eastern North Atlantic–Europe and eastern North Pacific–western North America regions. There is evidence that Pacific blocking shifts into the western central Pacific in the summer. Sector blocking episodes of 4 days or more are shown to exhibit different persistence characteristics to shorter events, showing that blocking is not just the long timescale tail end of a distribution. The PV–θ index results for the annual average location of Pacific blocking agree with synoptic studies but disagree with modern quantitative height field–based studies. It is considered that the index used here is to be preferred anyway because of its dynamical basis. However, the longitudinal discrepancy is found to be associated with the use in the height field index studies of a central blocking latitude that is independent of longitude. In particular, the use in the North Pacific of a latitude that is suitable for the eastern North Atlantic leads to spurious categorization of blocking there. Furthermore, the PV–θ index is better able to detect Ω blocking than conventional height field indices.

Corresponding author address: Prof. B. J. Hoskins, Department of Meteorology, University of Reading, Earley Gate, P.O. Box 243, Reading RG6 6BB, United Kingdom. Email: b.j.hoskins@rdg.ac.uk

Abstract

It is argued that the essential aspect of atmospheric blocking may be seen in the wave breaking of potential temperature (θ) on a potential vorticity (PV) surface, which may be identified with the tropopause, and the consequent reversal of the usual meridional temperature gradient of θ. A new dynamical blocking index is constructed using a meridional θ difference on a PV surface. Unlike in previous studies, the central blocking latitude about which this difference is constructed is allowed to vary with longitude. At each longitude it is determined by the latitude at which the climatological high-pass transient eddy kinetic energy is a maximum. Based on the blocking index, at each longitude local instantaneous blocking, large-scale blocking, and blocking episodes are defined. For longitudinal sectors, sector blocking and sector blocking episodes are also defined. The 5-yr annual climatologies of the three longitudinally defined blocking event frequencies and the seasonal climatologies of blocking episode frequency are shown. The climatologies all pick out the eastern North Atlantic–Europe and eastern North Pacific–western North America regions. There is evidence that Pacific blocking shifts into the western central Pacific in the summer. Sector blocking episodes of 4 days or more are shown to exhibit different persistence characteristics to shorter events, showing that blocking is not just the long timescale tail end of a distribution. The PV–θ index results for the annual average location of Pacific blocking agree with synoptic studies but disagree with modern quantitative height field–based studies. It is considered that the index used here is to be preferred anyway because of its dynamical basis. However, the longitudinal discrepancy is found to be associated with the use in the height field index studies of a central blocking latitude that is independent of longitude. In particular, the use in the North Pacific of a latitude that is suitable for the eastern North Atlantic leads to spurious categorization of blocking there. Furthermore, the PV–θ index is better able to detect Ω blocking than conventional height field indices.

Corresponding author address: Prof. B. J. Hoskins, Department of Meteorology, University of Reading, Earley Gate, P.O. Box 243, Reading RG6 6BB, United Kingdom. Email: b.j.hoskins@rdg.ac.uk

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