Blocking signatures in the northern hemisphere: Frequency distribution and interpretation

Abstract

In a companion paper (Knox and Hay, 1983) the authors developed objective identification criteria, adaptable to machine processing methods, by relating the blocking anticyclone to its associated positive anomaly of 5-day mean 500 mb height. Anomalies meeting the criteria were called 'blocking signatures'. Moreover, by using an empirically derived pentad displacement threshold, it was possible to identify 'blocking signature sequences' corresponding to the life history of blocking episodes.In this paper we present and interpret the spatial and temporal distribution of blocking signatures during the 33 year period 1946 to 1978 inclusive. The frequency of occurrence is presented by area, and by longitude for each season and for the year. The results are, in general, in agreement with published studies for the oceans, but they also reveal a high frequency of blocking signatures over the North-eastern Canadian Archipelago. This particular result, 'the Baffin Island paradox', is further investigated and rationalized.The interannual variation of blocking signature frequency over the Northern Hemisphere showed substantial fluctuations, particularly with regard to the more persistent episodes. However, because of the small size of the sample (33 years) relative to the time scale of the fluctuations (order of one decade) it was not possible to draw statistical conclusions.KEY WORDS Blocking Blocking signature Blocking signature sequences Positive anomaly (of 5-day mean 500 mb height) Secular variation (of blocking signatures)

The relationship between Canadian Prairie dry and wet months and circulation anomalies in the mid‐troposphere

“…Barnston and Livezey (1987), to be referred to as BL, identify and describe the major modes for each calendar month using RPCA on monthly mean 70-kPa height fields. For the purposes of this paper, the results of H81 for the Summer, and BL for the individual months of April, May, June and July, will be taken into consideration, along with the findings of Knox and Hay (1985) concerning the low-frequency variability over the northeastern Canadian Archipelago. A discussion of results from the above investigations considered germane to our framework will be found in Appendix B.…”

“…The areas marked BAF and NSU have been singled out because these are locations of strong low-frequency variability. Moreover, BAF is also an area of high-frequency blocking in Spring and Summer, which is surprising because it also happens to be located in the quasipermanent Canadian trough, a paradox that is rationalized in Knox and Hay (1985). The PNA pattern is usually regarded as a mode that is amplified during the colder months but that attenuates with a decreasing wavelength while the summer season approaches.…”

Long Term Variations of Atmospheric Blocking in the Northern Hemisphere

“…studies based on investigation of persistent positive 500-hPa geopotential height anomalies, and studies focussing on characteristic features of the 500-hPa flow field during a blocking. An example of a study of atmospheric blocking using persistent positive 500-hPa geopotential height anomalies to identify blockings is Knox and Hay (1985). The other method of studying characteristic features of the flow field in the middle troposphere was introduced by Rex (1950).…”

“…Lejenas and Okland (1983) modified Rex criteria somewhat and formulated them in such a way that it was possible to use them in a computer program. There are of course several other studies of atmospheric blocking which belong to the two categories mentioned here (see for instance references in Qkland, 1983 andKnox andHay, 1985), and there are also some studies which are not directly related to these two categories. In this connection it is of interest to mention the work by Elliot and Smith (1949), who used sea-level pressure data to study variations in the extent of blocking action for January and February.…”

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“…The numbers of storm generated over the North Atlantic are closely related to temperature variations (Lamb, 1991), whilst, the NAO index reflects the strength of surface westerlies over Europe especially in the winter (Wilby et al, 1997), and thus is related to the pathways of storms or low-pressure cells. Furthermore, a strong highpressure system over Scandinavia can block low-pressure cells from moving eastwards, and divert them towards the Norwegian Sea or south-eastwards into the Bay of Biscay and English Channel (Knox and Hay, 1985) where the storm surges may be amplified.…”