ATMOSPHERIC BLOCKING
Mid-latitude atmospheric blocking is a meteorological phenomenon in which the climatologically prevailing Westerlies are temporarily reversed.
Atmospheric blocking variability is large both spatially and temporally. Individual blocking episodes are found to have a life-time anywhere between 5-25 days and covering a spatial scale between 40-100 degrees longitude.
Atmospheric blocking may exert a strong influence on the local climate and especially because of its persistence is known to play a role in extreme weather. Cold weather in winter, and warm/dry weather conditions in summer are often (though not always) associated with nearby blocking conditions. Two examples are shown below.
Further reading
Figure: diagnosing blocking. Explanation under “Further reading”.
Winter example: Example of midlatitude winter blocking situation. Shown are geopotential height in contours and temperature anomalies in shading. The geopotential height contours clearly resemble an Omega structure. Temperature advection associated with the blocking leads to significantly higher than usual temperatures to the West of the high pressure (over Greenland), and colder than usual conditions over much of the continent.
Summer example Example of summer blocking situation that lasted for many weeks throughout july 1976 and led to the driest july on record. The temperature response appears to be less extreme than in winter, with the (anomalously) high temperatures occurring right under the high pressure. The tripole-like structure of the geopotential height is well known in idealized fluid dynamical (numerical) experiments to be meta-stable. Also the theory of "modons" may be very relevant here.