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- Author or Editor: Chaim I. Garfinkel x
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Abstract
As the surface drag is increased in a comprehensive general circulation model (GCM), the upper-level zonal winds decrease and eddy momentum flux convergence into the jet core increases. Globally averaged eddy kinetic energy decreases, a response that is inconsistent with the conventional barotropic governor mechanism whereby decreased barotropic shears encourage baroclinic wave growth. As the conventional barotropic governor appears insufficient to explain the entire response in the comprehensive GCM, the nondivergent barotropic model on the sphere is used to demonstrate an additional mechanism for the effect of surface drag on eddy momentum fluxes and eddy kinetic energy. Analysis of the pseudomomentum budget shows that increased drag modifies the background meridional vorticity gradient, which allows for enhanced eddy momentum flux convergence and decreased eddy kinetic energy in the presence of a constant eddy source. This additional feedback may explain the changes in eddy momentum fluxes observed in the comprehensive GCM and was likely present in previous work on the barotropic governor.
Abstract
As the surface drag is increased in a comprehensive general circulation model (GCM), the upper-level zonal winds decrease and eddy momentum flux convergence into the jet core increases. Globally averaged eddy kinetic energy decreases, a response that is inconsistent with the conventional barotropic governor mechanism whereby decreased barotropic shears encourage baroclinic wave growth. As the conventional barotropic governor appears insufficient to explain the entire response in the comprehensive GCM, the nondivergent barotropic model on the sphere is used to demonstrate an additional mechanism for the effect of surface drag on eddy momentum fluxes and eddy kinetic energy. Analysis of the pseudomomentum budget shows that increased drag modifies the background meridional vorticity gradient, which allows for enhanced eddy momentum flux convergence and decreased eddy kinetic energy in the presence of a constant eddy source. This additional feedback may explain the changes in eddy momentum fluxes observed in the comprehensive GCM and was likely present in previous work on the barotropic governor.
Abstract
Experiments with the Whole Atmosphere Community Climate Model (WACCM) are used to understand the influence of the stratospheric tropical quasi-biennial oscillation (QBO) in the troposphere. The zonally symmetric circulation in thermal wind balance with the QBO affects high-frequency eddies throughout the extratropical troposphere. The influence of the QBO is strongest and most robust in the North Pacific near the jet exit region, in agreement with observations. Variability of the stratospheric polar vortex does not appear to explain the effect of the QBO in the troposphere in the model, although it does contribute to the response in the North Atlantic. Anomalies in tropical deep convection associated with the QBO appear to damp, rather than drive, the effect of the QBO in the extratropical troposphere. Rather, the crucial mechanism whereby the QBO modulates the extratropical troposphere appears to be the interaction of tropospheric transient waves with the axisymmetric circulation in thermal wind balance with the QBO. The response to QBO winds of realistic amplitude is stronger for perpetual February radiative conditions and sea surface temperatures than perpetual January conditions, consistent with the observed response in reanalysis data, in a coupled seasonal WACCM integration, and in dry model experiments described in Part I.
Abstract
Experiments with the Whole Atmosphere Community Climate Model (WACCM) are used to understand the influence of the stratospheric tropical quasi-biennial oscillation (QBO) in the troposphere. The zonally symmetric circulation in thermal wind balance with the QBO affects high-frequency eddies throughout the extratropical troposphere. The influence of the QBO is strongest and most robust in the North Pacific near the jet exit region, in agreement with observations. Variability of the stratospheric polar vortex does not appear to explain the effect of the QBO in the troposphere in the model, although it does contribute to the response in the North Atlantic. Anomalies in tropical deep convection associated with the QBO appear to damp, rather than drive, the effect of the QBO in the extratropical troposphere. Rather, the crucial mechanism whereby the QBO modulates the extratropical troposphere appears to be the interaction of tropospheric transient waves with the axisymmetric circulation in thermal wind balance with the QBO. The response to QBO winds of realistic amplitude is stronger for perpetual February radiative conditions and sea surface temperatures than perpetual January conditions, consistent with the observed response in reanalysis data, in a coupled seasonal WACCM integration, and in dry model experiments described in Part I.
Abstract
A dry primitive equation model is used to explain how the quasi-biennial oscillation (QBO) of the tropical stratosphere can influence the troposphere, even in the absence of tropical convection anomalies and a variable stratospheric polar vortex. QBO momentum anomalies induce a meridional circulation to maintain thermal wind balance. This circulation includes zonal wind anomalies that extend from the equatorial stratosphere into the subtropical troposphere. In the presence of extratropical eddies, the zonal wind anomalies are intensified and extend downward to the surface. The tropospheric response differs qualitatively between integrations in which the subtropical jet is strong and integrations in which the subtropical jet is weak. While fluctuation–dissipation theory provides a guide to predicting the response in some cases, significant nonlinearity in others, particularly those designed to model the midwinter subtropical jet of the North Pacific, prevents its universal application. When the extratropical circulation is made zonally asymmetric, the response to the QBO is greatest in the exit region of the subtropical jet. The dry model is able to simulate much of the Northern Hemisphere wintertime tropospheric response to the QBO observed in reanalysis datasets and in long time integrations of the Whole Atmosphere Community Climate Model (WACCM).
Abstract
A dry primitive equation model is used to explain how the quasi-biennial oscillation (QBO) of the tropical stratosphere can influence the troposphere, even in the absence of tropical convection anomalies and a variable stratospheric polar vortex. QBO momentum anomalies induce a meridional circulation to maintain thermal wind balance. This circulation includes zonal wind anomalies that extend from the equatorial stratosphere into the subtropical troposphere. In the presence of extratropical eddies, the zonal wind anomalies are intensified and extend downward to the surface. The tropospheric response differs qualitatively between integrations in which the subtropical jet is strong and integrations in which the subtropical jet is weak. While fluctuation–dissipation theory provides a guide to predicting the response in some cases, significant nonlinearity in others, particularly those designed to model the midwinter subtropical jet of the North Pacific, prevents its universal application. When the extratropical circulation is made zonally asymmetric, the response to the QBO is greatest in the exit region of the subtropical jet. The dry model is able to simulate much of the Northern Hemisphere wintertime tropospheric response to the QBO observed in reanalysis datasets and in long time integrations of the Whole Atmosphere Community Climate Model (WACCM).
Abstract
The distribution of near-surface and tropospheric temperature variability in midlatitudes is distinguishable from a Gaussian in meteorological reanalysis data; consistent with this, warm extremes occur preferentially poleward of the location of cold extremes. To understand the factors that drive this non-Gaussianity, a dry general circulation model and a simple model of Lagrangian temperature advection are used to investigate the connections between dynamical processes and the occurrence of extreme temperature events near the surface. The non-Gaussianity evident in reanalysis data is evident in the dry model experiments, and the location of extremes is influenced by the location of the jet stream and storm track. The cause of this in the model can be traced back to the synoptic evolution within the storm track leading up to cold and warm extreme events: negative temperature extremes occur when an equatorward propagating high–low couplet (high to the west) strongly advects isotherms equatorward over a large meridional fetch over more than two days. Positive temperature anomalies occur when a poleward propagating low–high couplet (low to the west) advects isotherms poleward over a large meridional fetch over more than two days. The magnitude of the extremes is enhanced by the meridional movement of the systems. Overall, horizontal temperature advection by storm track systems can account for the warm/cold asymmetry in the latitudinal distribution of the temperature extremes.
Abstract
The distribution of near-surface and tropospheric temperature variability in midlatitudes is distinguishable from a Gaussian in meteorological reanalysis data; consistent with this, warm extremes occur preferentially poleward of the location of cold extremes. To understand the factors that drive this non-Gaussianity, a dry general circulation model and a simple model of Lagrangian temperature advection are used to investigate the connections between dynamical processes and the occurrence of extreme temperature events near the surface. The non-Gaussianity evident in reanalysis data is evident in the dry model experiments, and the location of extremes is influenced by the location of the jet stream and storm track. The cause of this in the model can be traced back to the synoptic evolution within the storm track leading up to cold and warm extreme events: negative temperature extremes occur when an equatorward propagating high–low couplet (high to the west) strongly advects isotherms equatorward over a large meridional fetch over more than two days. Positive temperature anomalies occur when a poleward propagating low–high couplet (low to the west) advects isotherms poleward over a large meridional fetch over more than two days. The magnitude of the extremes is enhanced by the meridional movement of the systems. Overall, horizontal temperature advection by storm track systems can account for the warm/cold asymmetry in the latitudinal distribution of the temperature extremes.
Abstract
A dry general circulation model is used to investigate the connections between Rossby wave breaking and the latitude of the midlatitude tropospheric eddy-driven jet. An ensemble of experiments is constructed in which the jet latitude is influenced by a midlatitude tropospheric temperature anomaly that resembles observed climate change and by the imposition of a stratospheric polar vortex, and the distribution of Rossby wave breaking frequency is examined for each experiment. The shift in wave breaking per degree latitude of jet shift is then compared for three different sources of jet movement: the tropospheric baroclinic forcing imposed in midlatitudes, the imposition of a stratospheric polar vortex, and the internal variability of the midlatitude eddy-driven jet. It is demonstrated that all three sources of jet movement produce a similar change in Rossby wave breaking frequency per degree of jet shift. Hence, it is difficult (if not impossible) to isolate the ultimate cause behind the shift in Rossby wave breaking in response to the two external forcings.
Abstract
A dry general circulation model is used to investigate the connections between Rossby wave breaking and the latitude of the midlatitude tropospheric eddy-driven jet. An ensemble of experiments is constructed in which the jet latitude is influenced by a midlatitude tropospheric temperature anomaly that resembles observed climate change and by the imposition of a stratospheric polar vortex, and the distribution of Rossby wave breaking frequency is examined for each experiment. The shift in wave breaking per degree latitude of jet shift is then compared for three different sources of jet movement: the tropospheric baroclinic forcing imposed in midlatitudes, the imposition of a stratospheric polar vortex, and the internal variability of the midlatitude eddy-driven jet. It is demonstrated that all three sources of jet movement produce a similar change in Rossby wave breaking frequency per degree of jet shift. Hence, it is difficult (if not impossible) to isolate the ultimate cause behind the shift in Rossby wave breaking in response to the two external forcings.
Abstract
An intermediate complexity general circulation model is used to investigate the transient response of the NH winter stratosphere to modulated ultraviolet (UV) radiation by imposing a stepwise, deliberately exaggerated UV perturbation and analyzing the lagged response. Enhanced UV radiation is accompanied by an immediate warming of the tropical upper stratosphere. The warming then spreads into the winter subtropics due to an accelerated Brewer–Dobson circulation in the tropical upper stratosphere. The poleward meridional velocity in the subtropics leads to an increase in zonal wind in midlatitudes between 20° and 50°N due to Coriolis torque. The increase in midlatitude zonal wind is accompanied by a dipole in Eliassen–Palm flux convergence, with decreased convergence near the winter pole and increased convergence in midlatitudes (where winds are strengthening due to the Coriolis torque); this dipole subsequently extends the anomalous westerlies to subpolar latitudes within the first 10 days. The initial radiatively driven acceleration of the Brewer–Dobson circulation due to enhanced shortwave absorption is replaced in the subpolar winter stratosphere by a wave-driven deceleration of the Brewer–Dobson circulation, and after a month the wave-driven deceleration of the Brewer–Dobson circulation encompasses most of the winter stratosphere. Approximately a month after UV is first modified, a significant poleward jet shift is evident in the troposphere. The results of this study may have implications for the observed stratospheric and tropospheric responses to solar variability associated with the 27-day solar rotation period, and also to solar variability on longer time scales.
Abstract
An intermediate complexity general circulation model is used to investigate the transient response of the NH winter stratosphere to modulated ultraviolet (UV) radiation by imposing a stepwise, deliberately exaggerated UV perturbation and analyzing the lagged response. Enhanced UV radiation is accompanied by an immediate warming of the tropical upper stratosphere. The warming then spreads into the winter subtropics due to an accelerated Brewer–Dobson circulation in the tropical upper stratosphere. The poleward meridional velocity in the subtropics leads to an increase in zonal wind in midlatitudes between 20° and 50°N due to Coriolis torque. The increase in midlatitude zonal wind is accompanied by a dipole in Eliassen–Palm flux convergence, with decreased convergence near the winter pole and increased convergence in midlatitudes (where winds are strengthening due to the Coriolis torque); this dipole subsequently extends the anomalous westerlies to subpolar latitudes within the first 10 days. The initial radiatively driven acceleration of the Brewer–Dobson circulation due to enhanced shortwave absorption is replaced in the subpolar winter stratosphere by a wave-driven deceleration of the Brewer–Dobson circulation, and after a month the wave-driven deceleration of the Brewer–Dobson circulation encompasses most of the winter stratosphere. Approximately a month after UV is first modified, a significant poleward jet shift is evident in the troposphere. The results of this study may have implications for the observed stratospheric and tropospheric responses to solar variability associated with the 27-day solar rotation period, and also to solar variability on longer time scales.
Abstract
Using the CMIP5 multimodel ensemble (MME) historical experiments, the modulation of the stratospheric El Niño–Southern Oscillation (ENSO) teleconnection by the Pacific decadal oscillation (PDO) is investigated in this study. El Niño (La Niña) significantly impacts the extratropical stratosphere mainly during the positive (negative) PDO in the MME. Although the composite tropical ENSO SST intensities are similar during the positive and negative PDO in models, the Pacific–North American (PNA) responses are only significant when the PDO and ENSO are in phase. The local SST anomalies in the North Pacific can constructively (destructively) interfere with the tropical ENSO forcing to influence the extratropical eddy height anomalies when the PDO and ENSO are in (out of) phase. The difference between the positive and negative PDO in El Niño or La Niña winters filters out the tropical SST forcing, permitting the deduction of the extratropical SST contribution to the atmospheric response. The composite shows that the cold (warm) SST anomalies in the central North Pacific associated with the positive (negative) PDO have a similar impact to that of the warm (cold) SST anomalies in the tropical Pacific, exhibiting a positive (negative) PNA-like response, enhancing (weakening) the upward propagation of waves over the western coast of North America. The composite difference between the positive and negative PDO in El Niño or La Niña winters, as well as in eastern Pacific ENSO or central Pacific ENSO winters, presents a highly consistent atmospheric response pattern, which may imply a linear interference of the PDO’s impact with ENSO’s.
Abstract
Using the CMIP5 multimodel ensemble (MME) historical experiments, the modulation of the stratospheric El Niño–Southern Oscillation (ENSO) teleconnection by the Pacific decadal oscillation (PDO) is investigated in this study. El Niño (La Niña) significantly impacts the extratropical stratosphere mainly during the positive (negative) PDO in the MME. Although the composite tropical ENSO SST intensities are similar during the positive and negative PDO in models, the Pacific–North American (PNA) responses are only significant when the PDO and ENSO are in phase. The local SST anomalies in the North Pacific can constructively (destructively) interfere with the tropical ENSO forcing to influence the extratropical eddy height anomalies when the PDO and ENSO are in (out of) phase. The difference between the positive and negative PDO in El Niño or La Niña winters filters out the tropical SST forcing, permitting the deduction of the extratropical SST contribution to the atmospheric response. The composite shows that the cold (warm) SST anomalies in the central North Pacific associated with the positive (negative) PDO have a similar impact to that of the warm (cold) SST anomalies in the tropical Pacific, exhibiting a positive (negative) PNA-like response, enhancing (weakening) the upward propagation of waves over the western coast of North America. The composite difference between the positive and negative PDO in El Niño or La Niña winters, as well as in eastern Pacific ENSO or central Pacific ENSO winters, presents a highly consistent atmospheric response pattern, which may imply a linear interference of the PDO’s impact with ENSO’s.
Abstract
Extreme stratospheric vortex states are often associated with extreme heat flux and upward wave propagation in the troposphere and lower stratosphere; however, the factors that dictate whether an upward-directed wave in the troposphere will reach the bottom of the vortex versus being reflected back to the troposphere are not fully understood. Following Charney and Drazin, an analytical quasigeostrophic planetary-scale model is used to examine the role of the tropopause inversion layer (TIL) in wave propagation and reflection. The model consists of three different layers: troposphere, TIL, and stratosphere. It is shown that a larger buoyancy frequency in the TIL leads to weaker upward transmission to the stratosphere and enhanced reflection back to the troposphere, and thus reflection of wave packets is sensitive not just to the zonal wind but also to the TIL’s buoyancy frequency. The vertical–zonal cross section of a wave packet for a more prominent TIL in the analytical model is similar to the corresponding wave packet for observational events in which the wave amplitude decays rapidly just above the tropopause. Similarly, a less prominent TIL both in the model and in reanalysis data is associated with enhanced wave transmission and a weak change in wave phase above the tropopause. These results imply that models with a poor representation of the TIL will suffer from a bias in both the strength and phase of waves that transit the tropopause region.
Abstract
Extreme stratospheric vortex states are often associated with extreme heat flux and upward wave propagation in the troposphere and lower stratosphere; however, the factors that dictate whether an upward-directed wave in the troposphere will reach the bottom of the vortex versus being reflected back to the troposphere are not fully understood. Following Charney and Drazin, an analytical quasigeostrophic planetary-scale model is used to examine the role of the tropopause inversion layer (TIL) in wave propagation and reflection. The model consists of three different layers: troposphere, TIL, and stratosphere. It is shown that a larger buoyancy frequency in the TIL leads to weaker upward transmission to the stratosphere and enhanced reflection back to the troposphere, and thus reflection of wave packets is sensitive not just to the zonal wind but also to the TIL’s buoyancy frequency. The vertical–zonal cross section of a wave packet for a more prominent TIL in the analytical model is similar to the corresponding wave packet for observational events in which the wave amplitude decays rapidly just above the tropopause. Similarly, a less prominent TIL both in the model and in reanalysis data is associated with enhanced wave transmission and a weak change in wave phase above the tropopause. These results imply that models with a poor representation of the TIL will suffer from a bias in both the strength and phase of waves that transit the tropopause region.
Abstract
A yet unexplained feature of the tropical wavenumber–frequency spectrum is its parity distribution, i.e., the distribution of power between the meridionally symmetric and antisymmetric components of the spectrum. Due to the linearity of the decomposition to symmetric and antisymmetric components and the Fourier analysis, the total spectral power equals the sum of the power contained in each of these two components. However, the spectral power need not be evenly distributed between the two components. Satellite observations and reanalysis data provide ample evidence that the parity distribution of the tropical wavenumber–frequency spectrum is biased toward its symmetric component. Using an intermediate-complexity model of an idealized moist atmosphere, we find that the parity distribution of the tropical spectrum is nearly insensitive to large-scale forcing, including topography, ocean heat fluxes, and land–sea contrast. On the other hand, we find that a small-scale (stochastic) forcing has the capacity to affect the parity distribution at large spatial scales via an upscale (inverse) turbulent energy cascade. These results are qualitatively explained by considering the effects of triad interactions on the parity distribution. According to the proposed mechanism, any bias in the small-scale forcing, symmetric or antisymmetric, leads to symmetric bias in the large-scale spectrum regardless of the source of variability responsible for the onset of the asymmetry. As this process is also associated with the generation of large-scale features in the tropics by small-scale convection, the present study demonstrates that the physical process associated with deep convection leads to a symmetric bias in the tropical spectrum.
Abstract
A yet unexplained feature of the tropical wavenumber–frequency spectrum is its parity distribution, i.e., the distribution of power between the meridionally symmetric and antisymmetric components of the spectrum. Due to the linearity of the decomposition to symmetric and antisymmetric components and the Fourier analysis, the total spectral power equals the sum of the power contained in each of these two components. However, the spectral power need not be evenly distributed between the two components. Satellite observations and reanalysis data provide ample evidence that the parity distribution of the tropical wavenumber–frequency spectrum is biased toward its symmetric component. Using an intermediate-complexity model of an idealized moist atmosphere, we find that the parity distribution of the tropical spectrum is nearly insensitive to large-scale forcing, including topography, ocean heat fluxes, and land–sea contrast. On the other hand, we find that a small-scale (stochastic) forcing has the capacity to affect the parity distribution at large spatial scales via an upscale (inverse) turbulent energy cascade. These results are qualitatively explained by considering the effects of triad interactions on the parity distribution. According to the proposed mechanism, any bias in the small-scale forcing, symmetric or antisymmetric, leads to symmetric bias in the large-scale spectrum regardless of the source of variability responsible for the onset of the asymmetry. As this process is also associated with the generation of large-scale features in the tropics by small-scale convection, the present study demonstrates that the physical process associated with deep convection leads to a symmetric bias in the tropical spectrum.
