Climate dynamics

Abstract:

We demonstrate that open solar flux (Fs, derivable from geomagnetic data) exhibits stronger correlations with atmospheric circulation variations than conventionally-used measures of solar activity. The circulation anomalies are particularly enhanced over the North Atlantic/Eurasian sector, where there are large changes in the occurrence of blocking and the winter mean surface temperature differs by several degrees between high- and low-solar terciles. The relationship is stronger and simpler for Fs, being more linear between high- and low-solar winters. While the circulation anomalies strongly resemble the North Atlantic Oscillation they also extend deeper into Eurasia, especially in high-solar conditions. This distinct signature may be useful for the detection and attribution of observed changes and also the identification of dynamical mechanisms. © 2010 by the American Geophysical Union.

Abstract:

Climate science is coming under increasing pressure to deliver projections of future climate change at spatial scales as small as a few kilometres for use in impacts studies. But is our understanding and modelling of the climate system advanced enough to offer such predictions? Here we focus on the Atlantic-European sector, and on the effects of greenhouse gas forcing on the atmospheric and, to a lesser extent, oceanic circulations. We review the dynamical processes which shape European climate and then consider how each of these leads to uncertainty in the future climate. European climate is unique in many regards, and as such it poses a unique challenge for climate prediction. Future European climate must be considered particularly uncertain because (i) the spread between the predictions of current climate models is still considerable and (ii) Europe is particularly strongly affected by several processes which are known to be poorly represented in current models.

Abstract:

The robustness of stratospheric circulation changes under increased concentrations of carbon dioxide are investigated using the Met Office HadSM3-L64 model. Equilibrium climate change simulations employing forcing of two and four times pre-industrial CO2 are presented, with particular focus on the temperature response of the Arctic lower stratosphere during Northern Hemisphere winter. High CO2 loading provides the ability to attain the statistical significance of any response, typically a problem given the large component of interannual variability common to the region. In response to CO2, the expected global stratospheric cooling is modified by an anomalous dynamical warming of the Arctic winter lower stratosphere. This warming is shown to be associated with an increase in frequency of stratospheric sudden warming (SSW) events. At four times pre-industrial CO2, the frequency of SSW events per year is doubled with respect to the control simulation. Further, by comparing winters with and without SSW events, it is shown that the warming of the lower stratosphere cannot be achieved without the presence of a frequency modulation of SSW events. © 2010 Royal Meteorological Society and Crown.

Abstract:

Inspection of near-infrared images from Cassini's Imaging Science Subsystem and Visual and Infrared Mapping Spectrometer have revealed a new feature in Titan's haze structure: a narrow band of increased scattering by haze south of the equator. The band seems to indicate a region of very limited mixing in the lower stratosphere, which causes haze particles to be trapped there. This could explain the sharp separation between the two hemispheres, known as the north-south asymmetry, seen in images. The separation of the two hemispheres can also be seen in the stratosphere above 150 km using infrared spectra measured by Cassini's Composite Infrared Spectrometer. Titan's behaviour in the lower tropical stratosphere is remarkably similar to that of the Earth's tropical stratosphere, which hints at possible common dynamical processes. © 2009 Elsevier Inc. All rights reserved.

Abstract:

The climatology of a stratosphere-resolving version of the Met Office's climate model is studied and validated against ECMWF reanalysis data. Ensemble integrations are carried out at two different horizontal resolutions. Along with a realistic climatology and annual cycle in zonal mean zonal wind and temperature, several physical effects are noted in the model. The time of final warming of the winter polar vortex is found to descend monotonically in the Southern Hemisphere, as would be expected for purely radiative forcing. In the Northern Hemisphere, however, the time of final warming is driven largely by dynamical effects in the lower stratosphere and radiative effects in the upper stratosphere, leading to the earliest transition to westward winds being seen in the midstratosphere. A realistic annual cycle in stratospheric water vapor concentrations-the tropical "tape recorder"-is captured. Tropical variability in the zonal mean zonal wind is found to be in better agreement with the reanalysis for the model run at higher horizontal resolution because the simulated quasibiennial oscillation has a more realistic amplitude. Unexpectedly, variability in the extratropics becomes less realistic under increased resolution because of reduced resolved wave drag and increased orographic gravity wave drag. Overall, the differences in climatology between the simulations at high and moderate horizontal resolution are found to be small. © 2010 American Meteorological Society.