Precursors to atmospheric blocking events

Abstract

Atmospheric blocking disturbs synoptic-scale features from their normal eastward progression, causing extreme weather conditions for the duration of the blocking event. Blocking precursors, as indicators of meridional flow preceding the onset of strong Atlantic blocking episodes, have been identified by using a modified version of the Tibaldi and Molteni blocking index (1990). Hovmoller diagrams of the modified index were found to be particularly useful in finding the potential cases, clearly showing a precursor signal propagating from the western United States down to the Atlantic to initiate the blocking pattern. A composite of the 14 cases identified showed the precursor pattern to be consistent with the positive phase of the PNA, while the composite mature phase of Atlantic blocking resembles the negative NAO and AO phases. The results suggest that blocking is a global, rather than localized, phenomenon.