A global blocking climatology published by this group for events that occurred during the late 20th century examined the comprehensive list of characteristics that included block intensity. In addition to confirming the results of other published climatologies, they found that Northern Hemisphere blocking was stronger than Southern Hemisphere events and winter events are stronger than summer ones. This work also examined the interannual variability of blocking as related to El Nino. Since this time, there is evidence that the occurrence of blocking has increased globally. A comparison of blocking characteristics during the first part of the 21st century to those in the late 20th century shows that the number of blocking events and their duration have increased in the Northern and Southern Hemisphere. The intensity of blocking has decreased by about nine percent in the Northern Hemisphere, but there was little change in the intensity of Southern Hemisphere events. Additionally, there is little or no change in the genesis regions of blocking. An examination of variability related to El Nino and Southern Oscillation reveals that the variability found in the earlier work has reversed. This could either be the result of interdecadal variability or a change in the climate. Additionally, a more detailed investigation of seasonal and regional characteristics are examined.
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