B. Geerts and E. Linacre
The subtropical ridge across the southern hemisphere sometimes builds into a poleward spreading high for a long period of time (typically 5 to 30 days), an event known as ‘blocking’ (Section 12.5). ‘Blocking highs’ were first studied by D.F. Rex in 1950 and later by Austin (1). To be called ‘blocking’, the high must split the mid-latitude jet stream meridionally into winds at least 45 degrees of latitude apart, and persist for 10 days or more. In other words, a blocking region is identified by a sustained split in the jet stream, with a polar jet at an unusually high latitude, and a well-defined subtropical jet at an anomalously low latitude. An unusually broad, quasi-stationary upper-level ridge extending to high latitudes is also referred to as a blocking event. The term 'blocking' is used because the large ridge in the polar jet blocks the normal progression of upper-level shortwaves and frontal disturbances in midlatitudes.
The definition of a blocking event is rather subjective, and various indices have been derived to quantify the blocking intensity. A blocking index essentially is the reverse of the zonal index, defined in Section 12.5. A low zonal index implies weak upper-level westerly winds, on average, in the mid-latitude belt, and this corresponds with a high blocking index. One blocking index is defined for any longitude as the difference in latitudes between the polar jet and the subtropical jet, for 5-day averaged data. The wider the spread between the two jet streams, the stronger the blocking event. Why blocking occurs, and whether it is a distinct, quasi-balanced atmospheric state, is still not clear. In general, a blocking event tends to occur a) more often in some regions than others (due to land/ocean contrast of temperatures, and mountains) and b) follow a rapid intensification of a low nearby.
Blocking is most frequent in winter and just west of New Zealand (the Tasman Sea)(2). Other regions of frequent blocking are the southeastern Pacific and the South Atlantic. Blocking is rare in the Indian ocean. Blocking is most frequent in summer, except near Australia, where it is most common in early winter, and it is least common in the southern hemisphere in late winter. The number of blocking events in the Pacific is doubled in La Niña years, compared with El Niño years.
Blocking also occurs in the northern hemisphere, mainly in late winter and spring (unlike the south), and mainly over Great Britain (see North Atlantic Oscillation) and the west coast of North America, in both cases centered around 55ºN.