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Abstract:

The frequency of persistent atmospheric blocking events in the 40-yr ECMWF Re-Analysis (ERA-40) is compared with the blocking frequency produced by a simple first-order Markov model designed to predict the time evolution of a blocking index [defined by the meridional contrast of potential temperature on the 2-PVU surface (1 PVU ≡ 1 × 10-6 K m2 kg-1 s-1)]. With the observed spatial coherence built into the model, it is able to reproduce the main regions of blocking occurrence and the frequencies of sector blocking very well. This underlines the importance of the climatological background flow in determining the locations of high blocking occurrence as being the regions where the mean midlatitude meridional potential vorticity (PV) gradient is weak. However, when only persistent blocking episodes are considered, the model is unable to simulate the observed frequencies. It is proposed that this persistence beyond that given by a red noise model is due to the self-sustaining nature of the blocking phenomenon. © 2009 American Meteorological Society.

Abstract:

The atmospheric circulation changes predicted by climate models are often described using sea level pressure, which generally shows a strengthening of the mid-latitude westerlies. Recent observed variability is dominated by the Northern Annular Mode (NAM) which is equivalent barotropic, so that wind variations of the same sign are seen at all levels. However, in model predictions of the response to anthropogenic forcing, there is a well-known enhanced warming at low levels over the northern polar cap in winter. This means that there is a strong baroclinic component to the response. The projection of the response onto a NAM-like zonal index varies with height. While at the surface most models project positively onto the zonal index, throughout most of the depth of the troposphere many of the models give negative projections. The response to anthropogenic forcing therefore has a distinctive baroclinic signature which is very different to the NAM. Copyright 2008 by the American Geophysical Union.

Abstract:

A synoptic situation termed 'high-latitude blocking' (HLB) is shown to occur frequently in both the Atlantic and Pacific sectors, and to result in flow anomalies very similar to those associated with the negative phase of the Northern Annular Mode (NAM) in the respective sector. There is a weak but significant link between the occurrence of HLB in the two sectors, with Atlantic HLB tending to lead Pacific HLB by 1-3 days. This link arises from rare events in which both sectors are almost simultaneously affected by a large-scale wave-breaking event which distorts the polar trough over Northern Canada. In several cases the tropospheric wave-breaking occurs in tandem with a large-scale disturbance of the stratospheric polar vortex. There is, therefore, a physical link between the Atlantic and Pacific sectors, but analysis suggests that this does not contribute to determining the pattern of the NAM, as conventionally defined from monthly mean data. However, an alternative version of the NAM, derived directly from daily data, does appear to reflect this physical link. These conflicting results highlight the sensitivity of the NAM to the period over which data are averaged. Copyright © 2008 Royal Meteorological Society.