Precursors to atmospheric blocking events

Abstract

Atmospheric blocking events disturb synoptic-scale features from their normal eastward progression, causing anomalous weather conditions for the duration of the blocking event. The essence of blocking can be captured by variation of potential temperature ([theta]trop) on the dynamically-defined tropopause, considered as the 2 potential vorticity unit (PVU) surface. A climatology is constructed on winter ERA-40 data (DJFM 1957/58-2001/02) of [theta]trop and wind. The climatology reveals a maximum in blocking activity over the eastern Atlantic. Long-lasting (> 10 days) Atlantic sector (0-45°W) blocks are then identified using a blocking index developed by Pelly and Hoskins (2003), which detects reversals of the usual meridional [theta]trop gradient. A composite of the 32 cases identified reveals that a blocking precursor signal, serving as an indicator of meridional flow preceding the onset of persistent Atlantic blocking episodes, can first be identified over the Pacific two weeks prior to genesis. The composite shows the precursor signal to be consistent with a transitioning positive-to-negative phase PNA, while the composite mature phase of Atlantic blocking resembles the negative phases of the NAO and AO. The results suggest that non-ephemeral blocking is a global, rather than localized, phenomenon.