A Simple Model of Stratospheric Dynamics Including Solar Variability (original) (raw)
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A simple model of solar variability influence on climate
Advances in Space Research, 2004
We present a simple dynamic model of solar variability influence on climate, which is truncated from the stratospheric wavezonal flow interaction dynamics over a b-plane. The model consists of three ordinary differential equations controlled by two parameters: the initial amplitude of planetary waves and the vertical gradient of the zonal wind. The changes associated with the solar UV variability, as well as with seasonal variations, are introduced as periodic modulations of the zonal wind gradient. Influence of the Quasi-Biennial Oscillation is included as a periodic change of the width of the latitudinal extent of the b-plane. The major climate response to these changes is seen through modulation of the number of cold and warm winters.
Journal of Atmospheric and Solar-Terrestrial Physics, 2012
The impact of the 11-year solar cycle on the stratosphere and, in particular, on the polar regions is investigated using simulations from the Chemistry Climate Model (CCM) LMDz-Reprobus. The annual solar signal clearly shows a stratospheric response largely driven by radiative and photochemical processes, especially in the upper stratosphere. A month-by-months analysis suggests that dynamical feedbacks play an important role in driving the stratospheric response on short timescales. CCM outputs on a 10 days frequency indicate how, in the northern hemisphere, changes in solar heating in the winter polar stratosphere may influence the upward propagation of planetary waves and thus their deposition of momentum, ultimately modifying the strength of the mean stratospheric overtuning circulation at middle and high latitudes. The model results emphasize that the main temperature and wind responses in the northern hemisphere can be explained by a different timing in the occurrence of Sudden Stratospheric Warmings (SSWs) that are caused by small changes in planetary wave propagation depending on solar conditions. The differences between simulations forced by different solar conditions indicate successive positive and negative responses during the course of the winter. The solar minimum simulation generally indicates a slightly stronger polar vortex early in the winter while the solar maximum simulation experiences more early SSWs with a stronger wave-mean flow interaction and reduced zonal wind at mid-latitudes in the upper stratosphere. The opposite response is observed during mid-winter, in February, with more SSWs simulated for solar minimum conditions while solar maximum conditions are associated with a damped planetary wave activity and a reinforced vortex after the initial stratospheric warming period. In late winter, the response is again reversed, as noticed in the temperature differences, with major SSW mostly observed in the solar maximum simulation and less intense final warmings simulated for solar minimum conditions. Due to the non-zonal nature of SSWs, the stratospheric response presents high regional variability during the northern hemisphere winter. As a result, successive positive and negative responses are observed during the course of the winter.
Journal of the Meteorological Society of Japan. Ser. II
The structure of stationary planetary waves in the winter stratosphere is computed by. means of a steady-state hemispheric quasi-geostrophic model with a zonal basic state and lower boundary forcing obtained from climatology. The nonlinear wave solution is found to resemble rather closely the linear one, despite the large wave amplitudes which distort considerably the westerly zonal flow. Zonal wavenumber one is the most affected by the wave-wave interactions, experiencing an increase in amplitude and a decrease in westward phase tilt in the northerly regions of the middle stratosphere. Comparison of the solutions to the corresponding climatological wave structure indicates that the inclusion of the nonlinear terms leads to an improvement of the structure of wavenumber one. An examination of the 5-year January climatological basic state reveals a distinct linear relationship between the zonal streamfunction and the tonal potential vorticity in middle and northerly latitudes. Consequently, the wave-wave interactions are to a first approximation a result of the presence of the model dissipation. Weak dissipation in this region implies only weak interactions, which explains the quasi-linear structure of the solutions.
Simulations of stratospheric flow regimes during northern hemisphere winter
Advances in Space Research, 2004
The interaction of the solar influence and the Quasi-Biennial Oscillation (QBO) on the northern hemisphere winter circulation and temperatures is investigated. If we can reproduce and understand this interaction in a relatively simple model of the atmosphere it should help to identify the important mechanisms that enable the solar influence to extend to the lower atmosphere and influence our climate. A stratosphere-mesosphere model of the atmosphere has been employed to carry out a wide range of regime studies to determine the sensitivity of stratospheric sudden warmings to various forcings, including the amplitude of tropospheric planetary wave forcing, the QBO and a simple representation of the solar-induced zonal wind anomaly. The results suggest that warmings are sensitive to the background flow and forcing in the subtropical northern upper stratosphere in early winter. This is the region in which the Aleutian High is initiated and thence grows in amplitude, extending polewards and downwards as the warming evolves. Both QBO and solar anomalies of similar amplitude are found in this region of the atmosphere and therefore have the potential to interact with each other. Published by Elsevier Ltd on behalf of COSPAR.
Impact of stratospheric variability on tropospheric climate change
Climate Dynamics, 2010
An improved stratospheric representation has been included in simulations with the Hadley Centre HadGEM1 coupled ocean atmosphere model with natural and anthropogenic forcings for the period 1979-2003. An improved stratospheric ozone dataset is employed that includes natural variations in ozone as well as the usual anthropogenic trends. In addition, in a second set of simulations the quasi biennial oscillation (QBO) of stratospheric equatorial zonal wind is also imposed using a relaxation towards ERA-40 zonal wind values. The resulting impact on tropospheric variability and trends is described. We show that the modelled cooling rate at the tropopause is enhanced by the improved ozone dataset and this improvement is even more marked when the QBO is also included. The same applies to warming trends in the upper tropical troposphere which are slightly reduced. Our stratospheric improvements produce a significant increase of internal variability but no change in the positive trend of annual mean global mean near-surface temperature. Warming rates are increased significantly over a large portion of the Arctic Ocean. The improved stratospheric representation, especially the QBO relaxation, causes a substantial reduction in near-surface temperature and precipitation response to the El Chichón eruption, especially in the tropical region. The winter increase in the phase of the northern annular mode observed in the aftermath of the two major recent volcanic eruptions is partly captured, especially after the El Chichón eruption. The positive trend in the southern annular mode (SAM) is increased and becomes statistically significant which demonstrates that the observed increase in the SAM is largely subject to internal variability in the stratosphere. The possible inclusion in simulations for future assessments of full ozone chemistry and a gravity wave scheme to internally generate a QBO is discussed.
Maintenance of the Stratospheric Structure in an Idealized General Circulation Model
This work explores the maintenance of the stratospheric structure in a primitive equation model that is forced by a Newtonian cooling with a prescribed radiative equilibrium temperature field. Models such as this are well suited to analyze and address questions regarding the nature of wave propagation and tropospherestratosphere interactions. The focus lies on the lower to midstratosphere and the mean annual cycle, with its large interhemispheric variations in the radiative background state and forcing, is taken as a benchmark to be simulated with reasonable verisimilitude. A reasonably realistic basic stratospheric temperature structure is a necessary first step in understanding stratospheric dynamics.
Journal of The Atmospheric Sciences, 1998
A 48-yr integration was performed using the Geophysical Fluid Dynamics Laboratory SKYHI tropospherestratosphere-mesosphere GCM with an imposed zonal momentum forcing designed to produce a quasi-biennial oscillation (QBO) in the tropical stratosphere. In response to this forcing, the model generates a QBO in the tropical circulation that includes some very realistic features, notably the asymmetry between the strength of the descending easterly and westerly shear zones, and the tendency for the initial westerly accelerations to appear quite narrowly confined to the equator. The extratropical circulation in the Northern Hemisphere (NH) winter stratosphere is affected by the tropical QBO in a manner similar to that observed. In particular, the polar vortex is generally weaker in winters in which there are easterlies in the tropical middle stratosphere. Roughly twothirds of the largest midwinter polar warmings occur when the equatorial 30-mb winds are easterly, again in rough agreement with observations. Despite this effect, however, the total interannual variance in the zonalmean extratropical circulation in the model apparently is slightly decreased by the inclusion of the tropical QBO. The observed QBO dependence of the winter-mean stratospheric extratropical stationary wave patterns is also quite well reproduced in the model.
Longitudinal Variations of the Stratospheric Quasi-biennial Oscillation
Journal of the Atmospheric Sciences, 2004
The longitudinal dependence of interannual variations of tropical stratospheric wind is examined in a detailed general circulation model simulation and in the limited observations available. A version of the SKYHI model is run with an imposed zonally symmetric zonal momentum source that forces the zonal-mean zonal wind evolution in the tropical stratosphere to be close to an estimate of the observed zonal wind based on radiosonde observations at Singapore during the period 1978-99. This amounts to a kind of simple assimilation model in which only the zonal-mean wind field in the tropical stratosphere is assimilated, and other quantities are allowed to vary freely. A total of five experiments were run, one covering the full 1978-99 period and four for 1989-99.
Climatology and trends in the forcing of the stratospheric zonal-mean flow
The momentum budget of the Transformed Eulerian-Mean (TEM) equation is calculated using the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-40) and the National Centers for Environmental Prediction (NCEP) Reanalysis 2 (R-2). This study outlines the considerable contribution of unresolved waves, deduced to be gravity waves, to the forcing of the zonal-mean flow. A trend analysis, from 1980 to 2001, shows that the onset and break down of the Northern Hemisphere (NH) stratospheric polar night jet has a tendency to occur later in the season in the more recent years. This temporal shift follows long-term changes in planetary wave activity that are mainly due to synoptic waves, with a lag of one month. In the Southern Hemisphere (SH), the polar vortex shows a tendency to persist further into the SH summertime. This also follows a statistically significant decrease in the intensity of the stationary EP flux divergence over the 1980-2001 period. Ozone depletion is well known for strengthening the polar vortex through the thermal wind balance. However, the results of this work show that the SH polar vortex does not experience any significant long-term changes until the month of December, even though the intensification of the ozone hole occurs mainly between September and November. This study suggests that the decrease in planetary wave activity in November provides an important feedback to the zonal wind as it delays the breakdown of the polar vortex. In addition, the absence of strong eddy feedback before November explains the lack of significant trends in the polar vortex in the SH early spring. A long-term weakening in the Brewer-Dobson (B-D) circulation in the polar region is identified in the NH winter and early spring and during the Correspondence to: E. Monier (emonier@mit.edu) SH late spring and is likely driven by the decrease in planetary wave activity previously mentioned. During the rest of the year, there are large discrepancies in the representation of the B-D circulation and the unresolved waves between the two reanalyses, making trend analyses unreliable.
Journal of Atmospheric and Solar-Terrestrial Physics, 2005
The response to the 11-year solar cycle is investigated using a 3-D mechanistic stratosphere-mesosphere model, in particular the zonal asymmetry in the solar signal. Two model simulations are performed, one using solar forcing corresponding to solar minimum and the other corresponding to solar maximum. It is seen that the solar signal is strongly zonally asymmetric in northern hemisphere winter, with variations of up to 20 K at 25 km. The model results are compared to the solar signal seen from five rocket-sonde sites and one lidar site, all located at mid-latitude, but at different longitudes. Although there are some significant differences between the model results and the observations, the model results help to explain why there are significant variations in the solar signal in the observations made at different longitudinal positions. Such longitudinal dependence obviously does not appear in zonal mean results, hence this raises questions about the meaning of comparing zonal mean model results with results from local observations. r