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Abstract:

This paper aims to understand the physical processes causing the large spread in the storm track projections of the CMIP5 climate models. In particular, the relationship between the climate change responses of the storm tracks, as measured by the 2-6 day mean sea level pressure variance, and the equator-to-pole temperature differences at upper- and lower-tropospheric levels is investigated. In the southern hemisphere the responses of the upper- and lower-tropospheric temperature differences are correlated across the models and as a result they share similar associations with the storm track responses. There are large regions in which the storm track responses are correlated with the temperature difference responses, and a simple linear regression model based on the temperature differences at either level captures the spatial pattern of the mean storm track response as well explaining between 30 and 60 % of the inter-model variance of the storm track responses. In the northern hemisphere the responses of the two temperature differences are not significantly correlated and their associations with the storm track responses are more complicated. In summer, the responses of the lower-tropospheric temperature differences dominate the inter-model spread of the storm track responses. In winter, the responses of the upper- and lower-temperature differences both play a role. The results suggest that there is potential to reduce the spread in storm track responses by constraining the relative magnitudes of the warming in the tropical and polar regions. © 2013 Springer-Verlag Berlin Heidelberg.

Abstract:

The two-way relationship between Rossby wave-breaking (RWB) and intensification of extratropical cyclones is analysed over the Euro-Atlantic sector. In particular, the timing, intensity and location of cyclone development are related to RWB occurrences. For this purpose, two indices based on potential temperature are used to detect and classify anticyclonic and cyclonic RWB episodes from ERA-40 reanalysis data. Results show that explosive cyclogenesis over the North Atlantic (NA) is fostered by enhanced occurrence of RWB on days prior to the cyclone's maximum intensification. Under such conditions, the eddy-driven jet stream is accelerated over the NA, thus enhancing conditions for cyclogenesis. For explosive cyclogenesis over the eastern NA, enhanced cyclonic RWB over eastern Greenland and anticyclonic RWB over the subtropical NA are observed. Typically only one of these is present in any given case, with the RWB over eastern Greenland being more frequent than its southern counterpart. This leads to an intensification of the jet over the eastern NA and enhanced probability of windstorms reaching western Europe. Explosive cyclones evolving under simultaneous RWB on both sides of the jet feature a higher mean intensity and deepening rates than cyclones preceded by a single RWB event. Explosive developments over the western NA are typically linked to a single area of enhanced cyclonic RWB over western Greenland. Here, the eddy-driven jet is accelerated over the western NA. Enhanced occurrence of cyclonic RWB over southern Greenland and anticyclonic RWB over Europe is also observed after explosive cyclogenesis, potentially leading to the onset of Scandinavian blocking. However, only very intense developments have a considerable influence on the large-scale atmospheric flow. Non-explosive cyclones depict no sign of enhanced RWB over the whole NA area. We conclude that the links between RWB and cyclogenesis over the Euro-Atlantic sector are sensitive to the cyclone's maximum intensity, deepening rate and location. © 2013 Royal Meteorological Society.

Abstract:

Long records of the latitude and speed of the North Atlantic eddy-driven jet stream since 1871 are presented from the newly available Twentieth Century Reanalysis. These jet variations underlie the variability associated with patterns such as the North Atlantic Oscillation (NAO) and have considerable societal impact through variations in the prevailing westerly winds. While the NAO combines variations in the latitude and speed of the jet, these two characteristics are shown to have quite different seasonal cycles and interannual variability, suggesting that they may have different dynamical influences. In general, the features exhibited in shorter records are shown to be robust, for example the strong skewness of the NAO distribution. Related to this is a clear multimodality of the jet latitude distribution, which suggests the existence of preferred positions of the jet. Decadal variations in jet latitude are shown to correspond to changes in the occurrence of these preferred positions. However, it is the speed rather than the latitude of the jet that exhibits the strongest decadal variability, and in most seasons this is clearly distinct from a white-noise representation of the seasonal means. When viewed in this longer term context, the variations of recent decades do not appear unusual and recent values of jet latitude and speed are not unprecedented in the historical record. © 2013 Royal Meteorological Society.

Abstract:

The future changes of atmospheric blocking over the Euro-Atlantic sector, diagnosed from an ensemble of 17 global-climate simulations obtained with the ECHAM5/MPI-OM model, are shown to be largely explainable from the change of the 500 hPa mean zonal circulation and its variance. The reduction of the blocking frequency over the Atlantic and the increased frequency of easterly upper-level flow poleward of 60°N are well explained by the changes of mean zonal circulation. In winter and autumn an additional downstream shift of the frequency maximum is simulated. This is also seen in a subset of the CMIP5 models with RCP8.5. To explain this downstream shift requires the inclusion of the changing variance. It is suggested that the increased downstream variance is caused by the stronger, more eastward extending future jet, which promotes Rossby wave breaking and blocking to occur further downstream. The same relation between jet-strength and central-blocking longitude is found in the variability of the current climate. © 2013 Springer-Verlag Berlin Heidelberg.

Abstract:

The atmospheric circulation over the North Atlantic-European sector experienced exceptional but highly contrasting conditions in the recent 2010 and 2012 winters (November-March, with the year dated by the relevant January). Evidence is given for the remarkably different locations of the eddy-driven westerly jet over the North Atlantic. In the 2010 winter the maximum of the jet stream was systematically between 308 and 408N (south jet regime), whereas in the 2012 winter it was predominantly located around 558N (north jet regime). These jet features underline the occurrence of either weak flow (2010) or strong and persistent ridges throughout the troposphere (2012). This is confirmed by the very different occurrence of blocking systems over the North Atlantic, associated with episodes of strong cyclonic (anticyclonic) Rossby wave breaking in 2010 (2012) winter. These dynamical features underlie strong precipitation and temperature anomalies over parts of Europe, with detrimental impacts on many socioeconomic sectors. Despite the highly contrasting atmospheric states, mid- and high-latitude boundary conditions do not reveal strong differences in these two winters. The two winters were associated with opposite ENSO phases, but there is no causal evidence of a remote forcing from the Pacific sea surface temperatures. Finally, the exceptionality of the two winters is demonstrated in relation to the last 140 years. It is suggested that these winters may be seen as archetypes of North Atlantic jet variability under current climate conditions. © 2013 American Meteorological Society.