What do the latest reanalyses and models tell us about solar influences on climate?
EGU General Assembly Conference Abstracts 16 (2014)
A practical method to identify displaced and split stratospheric polar vortex events
Geophysical Research Letters 40:19 (2013) 5268-5273
Abstract:
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.Sensitivity of stratospheric dynamics and chemistry to QBO nudging width in the chemistry-climate model WACCM
Journal of Geophysical Research Atmospheres 118:18 (2013) 10-474
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.Revisiting the controversial issue of tropical tropospheric temperature trends
Geophysical Research Letters 40:11 (2013) 2801-2806
Abstract:
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.Multi-model analysis of Northern Hemisphere winter blocking: Model biases and the role of resolution
Journal of Geophysical Research Atmospheres 118:10 (2013) 3956-3971