Climate dynamics

Abstract:

Three years of gravity wave observations from the High Resolution Dynamics Limb Sounder instrument on NASA's Aura satellite are examined. We produce estimates of the global distribution of gravity wave momentum flux as a function of individual observed wave packets. The observed distribution at the 25 km altitude level is dominated by the small proportion of wave packets with momentum fluxes greater than ∼0.5 mPa. Depending on latitude and season, these wave packets only comprise ∼7-25% of observations, but are shown to be almost entirely responsible for the morphology of the observed global momentum flux distribution. Large-amplitude wave packets are found to be more important over orographic regions than over flat ocean regions, and to be especially high in regions poleward of 40°S during austral winter. The momentum flux carried by the largest packets relative to the distribution mean is observed to decrease with height over orographic wave generation regions, but to increase with height at tropical latitudes; the mesospheric intermittency resulting is broadly equivalent in both cases. Consistent with previous studies, waves in the top 10% of the extratropical distribution are observed to carry momentum fluxes more than twice the mean and waves in the top 1% more than 10× the mean, and the Gini coefficient is found to characterize the observed distributions well. These results have significant implications for gravity wave modeling. Key Points Observed GW distribution dominated by wave packets with MF>0.5 mPa Intermittency higher over orography Gini coefficient confirmed as a good metric for wave intermittency ©2013. American Geophysical Union. All Rights Reserved.

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.

Abstract:

The consequences of different quasi-biennial oscillation (QBO) nudging widths on stratospheric dynamics and chemistry are analyzed by comparing two model simulations with the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model (WACCM) where the width of the QBO is varied between 22° and 8.5° north and south. The sensitivity to the nudging width is strongest in Northern Hemisphere (NH) winter where the Holton-Tan effect in the polar stratosphere, i.e., stronger zonal mean winds during QBO west phases, is enhanced for the wider compared to the narrower nudging case. The differences between QBO west and east conditions for the two model experiments can be explained with differences in wave propagation, wave-mean flow interaction, and the residual circulation. In the wider nudging case, a divergence anomaly in the midlatitude upper stratosphere/lower mesosphere occurs together with an equatorward anomaly of the residual circulation. This seems to result in a strengthening of the meridional temperature gradient and hence a significant strengthening of the polar night jet (PNJ). In the narrower nudging case, these circulation changes are weaker and not statistically significant, consistent with a weaker and less significant impact on the PNJ. Chemical tracers like ozone, water vapor, and methane react accordingly. From a comparison of westerly minus easterly phase composite differences in the model to reanalysis and satellite data, we conclude that the standard WACCM configuration (QBO22) generates more realistic QBO effects in stratospheric dynamics and chemistry during NH winter. Our study also confirms the importance of the secondary mean meridional circulation associated with the QBO for the Holton-Tan effect. Key Points The sensitivity to QBO nudging width is strongest in NH winterHolton-Tan effect in the polar stratosphere is enhanced for the wider nudgingWave-mean flow interactions explain differences between QBO west and east ©2013. American Geophysical Union. All Rights Reserved.

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 wavebreaking methodology of Pelly and Hoskins. It differs from the Tibaldi and Molteni index in viewing equatorward cutofflows 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. © 2013 American Meteorological Society.