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Twenty years of development: atmospheric blocking in climate models.

Midlatitude atmospheric blocking is a long-lasting quasi-stationary high pressure anomaly, typically occurring in the exit region of the Atlantic and Pacific jet streams. Initially identified in the 1950s, this phenomenon has long elicited the attention of scientists. Its impact on regional weather has been widely documented, including its effects on both winter and summer extreme events. In order to explain its rapid onset and anomalous persistence, a number of theories involving nonlinear dynamics have been proposed since, without really achieving a unique conclusion. Also, numerical models have always struggled in representing it. Blocking has been at the origin of most of the systematic error of numerical weather predictions in the extratropics, and to this day an incorrect simulation of blocking still considerably limits the quality of climate simulations.

Our research addresses this latter point; it provides a survey of the climate models participating in successive intercomparison exercises, analyzing the long-term evolution of their skills in simulating Northern Hemisphere atmospheric blocking. Results from the AMIP-1 (1992), CMIP-3 (2007), and CMIP-5 (2012) model intercomparison projects are taken into consideration, including almost 100 global climate models, among which many are different--and successive--versions of the same model. The key result is that in the last 20 years, although large improvements are seen over the Pacific Ocean, only minor advancements have been achieved over the Euro-Atlantic sector. Note that while this result is valid on average, some individual models have nevertheless improved and do show good performances in both sectors.

Some of the most recent general circulation models (GCMs) still exhibit the same negative bias as in the 1990s, associated with large geopotential height systematic errors. Indeed, the bias associated with the zonal flow (i.e., when blocking is not occurring) is larger than the bias due to blocking, suggesting that blocking is not the only culprit. In general, each model shows different patterns of systematic error, pointing to the fact that reasons behind it can be model-dependent. However, increased horizontal resolution seems to be able to alleviate the Euro-Atlantic blocking bias. Interestingly, negligible differences emerge among ocean-coupled or atmosphere-only simulations, suggesting weak relevance of sea surface temperature biases. Finally, an analysis of the simulated blocking durations highlights a chronic underestimation of the persistence of the blocking events, especially evident over the Euro-Atlantic region.

Atmospheric blocking remains challenging, but the climate community seems to be moving forward on the right track, with slow but constant improvements: increasing computational resources and improved dynamics and parametrizations likely will be able to further mitigate the present-day bias. Results from the upcoming CMIP-6 campaign are expected to follow this trend. --Paolo Davini (Laboratoire de Meteorologie Dynamique) and F. D'Andrea, "Northern Hemisphere atmospheric blocking representation in Global Climate Models: Twenty years of improvements?," in the December Journal of Climate.

Caption: DJF Blocking Frequency 1979-1988

1979-88 December-January-February (DJF) blocking frequency. Multi-model ensemble mean for the different intercomparison projects are grouped into atmoshphere-only (AMIPs) and coupled (CMIPs) models, and are compared to the reference ECMWF interim reanalysis (ERA-Interim) model. Longitudes are plotted from 90[degrees]E to 2700[degrees]W in order to better frame the two areas of maximum blocking frequency; the peak on the left corresponds to the Euro-Atlantic blocking sector, while the one on the right corresponds to the Pacific blocking sector.
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Title Annotation:PAPERS OF NOTE
Publication:Bulletin of the American Meteorological Society
Date:Feb 1, 2017
Words:543
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