All colored regions in the hovmoller plot below depict regions where the flow is blocked according to the blocking index. The color scheme denotes the strength of the blocked flow as determined from the 500 hPa geopotential height gradient measured from the blocking ridge equatorward (denoted GHGS in blocking index definition). The presence of an intense cutoff low equatorward of the blocking ridge can act to enhance the blocking strength.

• Observed Southern Hemisphere Blocking
  
  
• Blocking Climatology Northern Hemisphere and Southern Hemisphere
  
  

• GFS Hovmoeller (NH)
• GFS 500 mb Days 1-4 (NH)
• GFS 500 mb Days 5-9 (NH)
• GFS 500 mb Days 10-14 (NH)
• GFS 500 mb Days 1-4 (SH)
• GFS 500 mb Days 5-9 (SH)
• GFS 500 mb Days 10-14 (SH)
• GFS Ensemble Hovmoeller
• GFS Forecasts (Southern Hemisphere)
• GFS Ensemble Forecasts (Southern Hemisphere)

• Individual Neutral DJF Seasons [W-,C-,N]
• Individual Warm DJF Seasons [W+,W]
• Individual Cold DJF Seasons [C+,C]

Atmospheric blocking is commonly referred to as the situation when the normal zonal flow is interrupted by strong and persistent meridional flow. The normal eastward progression of synoptic disturbances is obstructed leading to episodes of prolonged extreme weather conditions. On intraseasonal time scales the persistent weather extremes can last from several days up to a few weeks, often accompanied by significant temperature and precipitation anomalies. Examples of the 500 hPa height and anomaly fields associated with mature blocking episodes over the northeastern Atlantic and the North Pacific. These two regions are preferred areas for atmospheric blocking during the northern hemisphere cold season.

A common finding among scientific studies is that these long-lived weather extremes are associated with recurrent atmospheric flow anomalies. Numerous studies have found that the poor forecast skill beyond a few days results principally from the inability of numerical weather prediction models to simulate the onset and evolution of blocking flows.

• Definition of Blocking Index Definition
  
  

Carrera, M. L., R. W. Higgins, and V. E. Kousky, 2004: Downstream weather impacts associated with atmospheric blocking over the Northeast Pacific. J. Climate, in press.

Chen, W. Y., and H. M. Van den Dool, 1995: Low-frequency anomalies in the NMC MRF model and reality. J. Climate, 8, 1369-1385.

Chen, W. Y., and H. M. Van den Dool, 1997: Asymmetric impact of tropical SST anomalies on atmospheric internal variability over the North Pacific. J. Atmos. Sci., 54, 725-740.

Chen, W. Y., and H. M. Van den Dool, 1999: Significant change of extratropical natural variability and potential predictability associated with the El Nino/Southern Oscillation Tellus, 51A, 790-802..

Higgins, R. W., and S. D. Schubert, 1994: Simulated life cycles of persistent anticyclonic anomalies over the North Pacific: Role of synoptic-scale eddies. J. Atmos. Sci., 51, 3238-3260.

Higgins, R. W., and S. D. Schubert, 1996: Simulations of persistent North Pacific circulation anomalies and interhemispheric teleconnections. J. Atmos. Sci., 53, 188-207.

Higgins, R. W., and K. C. Mo, 1997: Persistent North Pacific circulation anomalies and the tropical intraseasonal oscillation. J. Climate, 10, 223-244.