Assessment of atmosphere-ocean general circulation model simulations of winter northern hemisphere atmospheric blocking

Abstract

An assessment of six coupled AtmosphereOcean General Circulation Models (AOGCMs) is undertaken in order to evaluate their ability in simulating winter atmospheric blocking highs in the northern hemisphere. The poor representation of atmospheric blocking in climate models is a long-standing problem (e.g. D'Andrea et al. in Clim Dyn 4:385-407, 1998), and despite considerable effort in model development, there is only a moderate improvement in blocking simulation. A modified version of the Tibaldi and Molteni (in Tellus A 42:343-365, 1990) blocking index is applied to daily averaged 500 hPa geopotential fields, from the ERA-40 reanalysis and as simulated by the climate models, during the winter periods from 1957 to 1999. The two preferred regions of blocking development, in the Euro-Atlantic and North Pacific, are relatively well captured by most of the models. However, the prominent error in blocking simulations consists of an underestimation of the total frequency of blocking episodes over both regions. A more detailed analysis revealed that this error was due to an insufficient number of medium spells and long-lasting episodes, and a shift in blocking lifetime distributions towards shorter blocks in the Euro-Atlantic sector. In the Pacific, results are more diverse; the models are equally likely to overestimate or underestimate the frequency at different spell lengths. Blocking spatial signatures are relatively well simulated in the Euro-Atlantic sector, while errors in the intensity and geographical location of the blocks emerge in the Pacific. The impact of models' systematic errors on blocking simulation has also been analysed. The time-mean atmospheric circulation biases affect the frequency of blocking episodes, and the maximum event duration in the Euro-Atlantic region, while they sometimes cause geographical mislocations in the Pacific sector. The analysis of the systematic error in timevariability has revealed a negative relationship between the high-frequency variability of the transient eddies in the areas affected by blocking and blocking frequency. The blocking responses to errors in the low-frequency variability are different according to the region considered; the amplitude of the low-frequency variability is positively related to the blocking frequency and persistence in the Euro-Atlantic sector, while no such consistency is observed in the Pacific.

A global perspective on atmospheric blocking using GPS radio occultation – one decade of observations

“…Hence, blocking research mainly relies on model output and reanalysis data rather than using direct observations. However, most models show only limited skill in blocking representation, as has been noted by many studies in the past (D'Andrea et al, 1998;Vial and Osborn, 2012;Barnes et al, 2012;Anstey et al, 2013;Christensen et al, 2013;Dunn-Sigouin and Son, 2013;Masato et al, 2013). Recently, Davini and D'Andrea (2016) showed that current climate models still underrepresent blocking occurrence by up to 50 %, particularly in the Euro-Atlantic blocking region.…”

The influence of atmospheric circulation on the mid-Holocene climate of Europe: a data–model comparison

“…For instance, much of the warming that occurred in the late 20th century in Europe has been attributed to the increased number of winters with a high-index AO/NAO (Hurrell, 1995;Visbeck et al, 2001). High-index AO/NAO conditions are also thought to have occurred during the Medieval Climate Anomaly, providing a dynamic explanation for the winter warmth experienced over Europe at this time .…”

“…One of the main sources of uncertainty in determining regional climate change in the mid-latitude regions such as Europe is the atmospheric circulation, a feature of the climate system that models have difficulty to simulate (Gillett, 2005;van Ulden and van Oldenborgh, 2006;Woollings, 2010;Vial and Osborn, 2012;Brands et al, 2013).…”

Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia–New Zealand Sector

“…Assessing Northern Hemisphere (NH) blocking characteristics across a range of models from phase 3 of the Coupled Model Intercomparison Project (CMIP3), Vial and Osborn (2012) found the frequency of blocking episodes to be underestimated owing to an excessive number of shortlived blocking events at the expense of prolonged blocking episodes. Such deficiencies have often been attributed to insufficient horizontal resolution, where small-scale features, such as atmospheric eddies important for maintaining larger-scale blocking patterns through positive feedbacks, are insufficiently resolved (Kug and Jin 2009;Scaife et al 2011, and references therein).…”