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Atmospheric blocking in the northern hemisphere Knox, John Lewis


Blocking is generally understood as the obstruction on a large scale of the normal west - to - east motion of mid-latitude pressure systems. It is a persistent phenomenon lasting from one to several weeks and the resulting prolonged weather regimes may have serious economic and social consequences. The recent Northern Hemisphere winters, starting with 1976-77, featured unusually large circulation anomalies, many of which can be directly related to prolonged episodes of large scale blocking. The intent of this study is to investigate the statistics and certain diagnostics of blocking in the Northern Hemisphere. The first of the three primary objectives is to present and interpret the spatial and temporal distribution of blocking during the past 33 years. We develop objective identification criteria, adaptable to machine processing methods, by relating the blocking anticyclone to its associated positive anomaly of 5-day mean 500MB height. Anomalies meeting the criteria are called 'blocking signatures.’ We present the seasonal frequency of occurrence of these signatures by longitude and by area. The results are in good agreement with published studies for the oceans, but they also reveal a high frequency of blocking signatures over the Northeastern Canadian Archipelago. This result, dubbed the 'Baffin Island Paradox' is further investigated and rationalized. A catalogue has been prepared which identifies the date, centre location and magnitude of every blocking signature which occurred from January 1, 1946 to December 31, 1978. A supplementary Catalogue identifies sequences of these signatures corresponding to actual blocking episodes The second objective is to investigate whether regions with high incidence of blocking, in either the developing or the mature stage, feature non-Gaussian distributions of 5-day mean geopotential. During winter, fields of significantly low kurtosis are found in certain mid-latitude regions where the genesis and amplification of blocking ridges are-frequently observed. Fields of significantly positive skewness are found in higher latitude regions where mature blocking episodes often interrupt the smaller fluctuations about the normal geopotential height. The final objective is to examine the association between the first six harmonics of the long wave pattern and the temporal and spatial characteristics of concurrent blocking episodes. Harmonics are calculated from profiles of daily 500MB height around latitude zones centred at 40°N and 60°N. Results for the northern zone are emphasized. It is found that there are spectral signatures distinctive to the regions where blocking anticyclones occur. Our results for the oceans are in general agreement with those of Austin (1980). During the strongly amplified meridional flow patterns associated with major blocking, we found that, at 60°N, more than 90% of the spatial variance of 500MB height is accounted for by wave components one to four. When the meridional regime gives way to predominantly zonal flow there is a marked reduction of spatial variance of 500MB height. During such regimes the higher harmonics (waves five and six) often make significant contributions (15 to 25%) to the total variance. The 'Baffin Island Paradox' is also studied using harmonics. It is found that in the majority of cases Baffin blocks originate from retrograding North Atlantic blocks. Finally, full latitude zonal harmonic analyses (15°N to pole, waves 1 to 4) are presented for three case studies of major blocking - (a) Greenland-North Atlantic, (b) Pacific Ocean-Alaska, and (c) Double Blocking. The harmonics often reveal two wave structures, one in the higher and other in the lower latitudes. The motion and growth characteristics of the two structures can be interpreted in terms of well-known features of total blocking systems.

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