JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 119 (2014) 2811-2830
PHYSICAL REVIEW D 90 (2014) ARTN 012010
JOURNAL OF THE ATMOSPHERIC SCIENCES 71 (2014) 391-409
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 119 (2014) 2329-2355
Northern winter climate change: Assessment of uncertainty in CMIP5 projections related to stratosphere-troposphere coupling
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 119 (2014) ARTN 2013JD021403
Stratospheric variability in 20th Century CMIP5 simulations of the Met Office climate model: High-top versus low-top
J CLIM 26 (2013) 5
JOURNAL OF CLIMATE 26 (2013) 2668-2682
Global observations of gravity wave intermittency and its impact on the observed momentum flux morphology
Journal of Geophysical Research: Atmospheres Blackwell Publishing Ltd 118 (2013) 10980-10993
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.
Sensitivity of stratospheric dynamics and chemistry to QBO nudging width in the chemistry-climate model WACCM
Journal of Geophysical Research: Atmospheres Blackwell Publishing Ltd 118 (2013) 10464-10474
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.
ATMOSPHERIC CHEMISTRY AND PHYSICS 13 (2013) 3945-3977
Multi-model analysis of Northern Hemisphere winter blocking: Model biases and the role of resolution
Journal of Geophysical Research: Atmospheres Blackwell Publishing Ltd 118 (2013) 3956-3971
Blocking of the tropospheric jet stream during Northern Hemisphere winter (December-January-February) is examined in a multi-model ensemble of coupled atmosphere-ocean general circulation models (GCMs) obtained from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The CMIP5 models exhibit large biases in blocking frequency and related biases in tropospheric jet latitude, similar to earlier generations of GCMs. Underestimated blocking at high latitudes, especially over Europe, is common. In general, model biases decrease as model resolution increases. Increased blocking frequency at high latitudes in both the Atlantic and Pacific basins, as well as more realistic variability of Atlantic jet latitude, are associated with increased vertical resolution in the mid-troposphere to lowermost stratosphere. Finer horizontal resolution is associated with higher blocking frequency at all latitudes in the Atlantic basin but appears to have no systematic impact on blocking near Greenland or in the Pacific basin. Results from the CMIP5 analysis are corroborated by additional controlled experiments using selected GCMs. Key PointsCMIP5 models have large blocking biases and associated jet biasesIncreased spatial resolution is associated with reduced blocking and jet biasesVertical and horizontal resolution give blocking changes in different regions ©2013. American Geophysical Union. All Rights Reserved.
The impact of stratospheric resolution on the detectability of climate change signals in the free atmosphere
GEOPHYSICAL RESEARCH LETTERS 40 (2013) 937-942
Multi-model analysis of Northern Hemisphere winter blocking: Model biases and the role of resolution
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 118 (2013) 3956-3971
Geophysical Research Letters 40 (2013) 434-439
Variability in solar irradiance has been connected to changes in surface climate in the North Atlantic through both observational and climate modelling studies which suggest a response in the atmospheric circulation that resembles the North Atlantic Oscillation or its hemispheric equivalent the Arctic Oscillation. It has also been noted that this response appears to follow the changes in solar irradiance by a few years, depending on the exact indicator of solar variability. Here we propose and test a mechanism for this lag based on the known impact of atmospheric circulation on the Atlantic Ocean, the extended memory of ocean heat content anomalies, and their subsequent feedback onto the atmosphere. We use results from climate model experiments to develop a simple model for the relationship between solar variability and North Atlantic climate. © 2013. American Geophysical Union. All Rights Reserved.
Journal of Geophysical Research: Atmospheres Blackwell Publishing Ltd 118 (2013) 13405-13420
The surface response to 11 year solar cycle variations is investigated by analyzing the long-term mean sea level pressure and sea surface temperature observations for the period 1870-2010. The analysis reveals a statistically significant 11 year solar signal over Europe, and the North Atlantic provided that the data are lagged by a few years. The delayed signal resembles the positive phase of the North Atlantic Oscillation (NAO) following a solar maximum. The corresponding sea surface temperature response is consistent with this. A similar analysis is performed on long-term climate simulations from a coupled ocean-atmosphere version of the Hadley Centre model that has an extended upper lid so that influences of solar variability via the stratosphere are well resolved. The model reproduces the positive NAO signal over the Atlantic/European sector, but the lag of the surface response is not well reproduced. Possible mechanisms for the lagged nature of the observed response are discussed. Key Points 11-year solar signal detected over N. Atlantic/Europe Signal is evident if data are lagged by ~3 years HadGEM climate model simulates signal but not the lag ©2013. The Authors.
Report on the 3rd SPARC DynVar Workshop on Modelling the Dynamics and Variability of the Stratosphere-Troposphere System
Geophysical Research Letters 40 (2013) 5268-5273
Extreme variability of the stratospheric polar vortex during winter can manifest as a displaced vortex event or a split vortex event. The influence of this vortex disruption can extend downwards and affect surface weather patterns. In particular, vortex splitting events have been associated with a negative Arctic Oscillation pattern. An assessment of the impacts of climate change on the polar vortex is therefore important, and more climate models now include a wella-resolved stratosphere. To aid this analysis, we introduce a practical thresholda-based method to distinguish between displaced and split vortex events. It requires only geopotential height at 10 hPa to measure the geometry of the vortex using twoa-dimensional moment diagnostics. It captures extremes of vortex variability at least, as well as previous methods when applied to reanalysis data, and has the advantage of being easily employed to analyze climate model simulations. Key Points It is important to distinguish split and displaced vortex events Current methods to do so are not easily-applicable to climate models A new method is easily-applicable and can accurately identify these events ©2013. American Geophysical Union. All Rights Reserved.
Geophysical Research Letters 40 (2013) 2801-2806
Controversy remains over a discrepancy between modeled and observed tropical upper tropospheric temperature trends. This discrepancy is reassessed using simulations from the Coupled Climate Model Inter-comparison Project phase 5 (CMIP 5) together with radiosonde and surface observations that provide multiple realizations of possible "observed" temperatures given various methods of homogenizing the data. Over the 1979-2008 period, tropical temperature trends are not consistent with observations throughout the depth of the troposphere, and this primarily stems from a poor simulation of the surface temperature trends. This discrepancy is substantially reduced when (1) atmosphere-only simulations are examined or (2) the trends are considered as an amplification of the surface temperature trend with height. Using these approaches, it is shown that within observational uncertainty, the 5-95 percentile range of temperature trends from both coupled-ocean and atmosphere-only models are consistent with the analyzed observations at all but the upper most tropospheric level (150 hPa), and models with ultra-high horizontal resolution (≤ 0.5° × 0.5°) perform particularly well. Other than model resolution, it is hypothesized that this remaining discrepancy could be due to a poor representation of stratospheric ozone or remaining observational uncertainty. © 2013 American Geophysical Union. All Rights Reserved.
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES 118 (2013) 13405-13420
Journal of Geophysical Research: Atmospheres 117 (2012)
Trends in the position of the DJF Austral jet have been analyzed for multimodel ensemble simulations of a subset of high- and low-top models for the periods 1960-2000, 2000-2050, and 2050-2098 under the CMIP5 historical, RCP4.5, and RCP8.5 scenarios. Comparison with ERA-Interim, CFSR and the NCEP/NCAR reanalysis shows that the DJF and annual mean jet positions in CMIP5 models are equatorward of reanalyses for the 1979-2006 mean. Under the RCP8.5 scenario, the mean jet position in the high-top models moves 3 degrees poleward of its 1860-1900 position by 2098, compared to just over 2 degrees for the low-top models. Changes in jet position are linked to changes in the meridional temperature gradient. Compared to low-top models, the high-top models predict greater warming in the tropical upper troposphere due to increased greenhouse gases for all periods considered: up to 0.28K/decade more in the period 2050-2098 under the RCP8.5 scenario. Larger polar lower-stratospheric cooling is seen in high-top models: -1.64K/decade compared to -1.40K/decade in the period 1960-2000, mainly in response to ozone depletion, and -0.41K/decade compared to -0.12K/decade in the period 2050-2098, mainly in response to increases in greenhouse gases. Analysis suggests that there may be a linear relationship between the trend in jet position and meridional temperature gradient, even under strong forcing. There were no clear indications of an approach to a geometric limit on the absolute magnitude of the poleward shift by 2100. © 2012. American Geophysical Union. All Rights Reserved.