An Objectively Determined Blocking Index and its Northern Hemisphere Climatology

Abstract

A modified blocking index is defined based on vertically integrated potential vorticity. The application of this index identifies blocking activity over the Northern Hemisphere during all seasons. The index is developed by systematically identifying the magnitude and spatial scale that best characterizes persistent anticyclonic circulation anomalies in different seasons. By applying a systematic approach to the detection of blocking, the interannual, seasonal, and intraseasonal patterns of blocking frequency across the Northern Hemisphere are able to be characterized. The results are consistent with previous studies in finding that blocking is more frequent in the cold season months than in the warm season, although the results suggest that blocking occurs much more frequently in the summer and fall than many studies have previously reported. By examining blocking frequency monthly, interesting patterns of intraseasonal variability are found, especially over the central Pacific in August and the eastern Pacific in September and October, where blocking is nearly as frequent as in the winter. Possible explanations for this intraseasonal variability are discussed.

Blocking and its Response to Climate Change

“…The employed datasets vary in their spatial and temporal resolutions, the meteorological variable (geopotential height or materially conserved PV-based dynamical fields) and the vertical level (500 hPa, the upper troposphere-lower stratosphere or the dynamical tropopause). Although upper tropospheric fields may better capture blocking in all seasons [ 84 ], these choices ultimately produce similar blocking climatologies when compared systematically under the same method [ 85 – 87 ].…”

On the dynamical coupling between atmospheric blocks and heavy precipitation events: A discussion of the southern Alpine flood in October 2000

“…They persist between several days and several weeks. They are characterized by upper‐level negative potential vorticity (PV) anomalies in the central blocked region (Pelly and Hoskins, ; Schwierz et al ., ; Small et al ., ). Due to their longevity and stationariness they can play an important role in the formation of surface weather extremes (e.g.…”

A Dynamic Analysis of a Record Breaking Winter Season Blocking Event

“…Some of the techniques used here are not as well known in the literature on blocking but are effective at illustrating the exceptional nature of this event as it survived a regime change. A potential weakness in all of the techniques employed here is that they tend to emphasize the 500 hPa surface dynamics, somewhat arbitrarily; see [6]. Also, the temperature advection and other thermodynamic variables are not taken into account either.…”

“…The classical picture of blocking anticyclones consists in a long lived (5∼10 day minimum threshold of persistence), large amplitude, geopotential height anomaly [1][2][3][4]. More recently, some research groups have invoked the PV (potential vorticity) perspective in defining blocking (see, e.g., [5,6]). Both methods define blocking as a significant persistent anomaly, positive height anomalies in the more classical framework, or negative PV anomalies, respectively.…”