Winter and Summer Northern Hemisphere Blocking in CMIP5 Models

Abstract

The frequencies of atmospheric blocking in both winter and summer and the changes in them from the twentieth to the twenty-first centuries as simulated in 12 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are analyzed. The representative concentration pathway 8.5 (RCP8.5) high emission scenario runs are used to represent the twenty-first century. The analysis is based on the wave-breaking methodology of Pelly and Hoskins. It differs from the Tibaldi and Molteni index in viewing equatorward cutoff lows and poleward blocking highs in equal manner as indicating a disruption to the westerlies. One-dimensional and two-dimensional diagnostics are applied to identify blocking of the midlatitude storm track and also at higher latitudes. Winter blocking frequency is found to be generally underestimated. The models give a decrease in the European blocking maximum in the twenty-first century, consistent with the results in other studies. There is a mean twenty-first-century winter poleward shift of high-latitude blocking but little agreement between the models on the details. In summer, Eurasian blocking is also underestimated in the models, whereas it is now too large over the high-latitude ocean basins. A decrease in European blocking frequency in the twenty-first-century model runs is again found. However, in summer there is a clear eastward shift of blocking over eastern Europe and western Russia, in a region close to the blocking that dominated the Russian summer of 2010. While summer blocking decreases in general, the poleward shift of the storm track into the region of frequent high-latitude blocking may mean that the incidence of storms being obstructed by blocks may actually increase.

Impact of observed North Atlantic multidecadal variations to European summer climate: a linear baroclinic response to surface heating

“…Moreover, the composite of 500 hPa geopotential height further shows a strong high pressure anomaly over the same region which altogether indicates a blocking high characteristic. This high pressure anomaly falls under the climatological location of the European blocking region (Masato et al 2013). A blocking high leads to warmer and drier conditions for an extended period of time.…”

North American extreme temperature events and related large scale meteorological patterns: a review of statistical methods, dynamics, modeling, and trends

“…Otherwise differences among model patterns consist of horizontal shifts or amplitude variations in the circulation anomaly pattern features. CMIP5 models generally underestimate the regional frequency of winter blocking events while summertime blocking events occurring over the high latitude oceanic basins are typically overestimated (Masato et al 2013). Conversely, Westby et al (2013) found serious deficiencies in the representation of the PDO by CMIP5 models with direct impacts upon the modulation of anomalous temperature regimes.…”

Climate SPHINX: evaluating the impact of resolution and stochastic physics parameterisations in the EC-Earth global climate model

“…One of the most important challenges for the current generation of climate models is the simulation of atmospheric blocking (Anstey et al, 2013;Masato et al, 2013;DunnSigouin and Son, 2013;Davini and D'Andrea, 2016). Blocking is a recurrent weather pattern typically occurring in the Northern Hemisphere at the exit of the Atlantic and Pacific jet stream, more frequently during the winter season but observed throughout the year (Rex, 1950;Tibaldi and Molteni, 1990).…”