Evaluation of Northern Hemisphere Blocking Climatology in the Global Environment Multiscale Model

Abstract

The performance of the Global Environmental Multiscale (GEM) model, the Canadian operational numerical model, in reproducing atmospheric low-frequency variability is evaluated in the context of Northern Hemisphere blocking climatology. The validation is conducted by applying a comprehensive but relatively simple blocking detection algorithm to a 20-yr (1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) integration of the GEM model in climate mode. The comparison to reanalysis reveals that, although the model can reproduce Northern Hemisphere blocking climatology reasonably well, the maximum blocking frequency over the North Atlantic and western Europe is generally underestimated and its peak season is delayed from late winter to spring. This contrasts with the blocking frequency over the North Pacific, which is generally overestimated during all seasons. These misrepresentations of blocking climatology are found to be largely associated with the biases in climatological background flow. The modeled stationary waves show a seasonal delay in zonal wavenumber 1 and an eastward extension in zonal wavenumber-2 components consistent with blocking frequency biases. High-frequency eddies are, however, consistently underestimated both in the North Atlantic and Pacific, indicating that the biases in eddy fields might not be the main reason for the blocking biases in the North Pacific.

Evaluating Indices of Blocking Anticyclones in Terms of Their Linear Relations With Surface Hot Extremes

“…All blocking indices used here are two dimensional and defined on Z500 (Table ). They are Dole and Gordon (), Dunn‐Sigouin et al (), Scherrer et al (), Masato et al (), and Martineau et al (), hereafter DG83, D13, S06, M13, and M17, with some modifications (see supporting information Text S1). We also tested a variation of DG83 (denoted as DG83p), which replaces the duration requirement with temporal smoothing by a persistence factor (Text S1).…”

Major cause of unprecedented Arctic warming in January 2016: Critical role of an Atlantic windstorm

“…Blocking is detected using a recently developed algorithm39, which is a hybrid algorithm that combines two widely used indices: the Dole and Gordon type index40 and the Tibaldi and Molteni type index41. This method searches for persistent contiguous areas of geopotential height anomalies, as in the Dole and Gordon type index, which occurs in conjunction with the meridional reversals of the 500-hPa geopotential height as in the Tibaldi and Molteni type index.…”

The Relationship Between Extratropical Cyclone Steering and Blocking Along the North American East Coast

“…The algorithm produces a data set in which each latitude/longitude grid point is labeled as either a block or not a block in 6‐hourly increments. Since there are multiple block detection algorithms (see Barnes et al, ), we repeat our analysis using the blocking algorithm of Dunn‐Sigouin et al (). It identifies anomalies in the 500 hPa geopotential height ( Z 500 ) field and tests for a reversal in the meridional gradient of the geopotential height field.…”