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Papers by isabella bordi
Journal of the Atmospheric Sciences, 2009
The atmospheric general circulation is characterized by both single- and double-jet patterns. The... more The atmospheric general circulation is characterized by both single- and double-jet patterns. The double-jet structure of the zonal mean zonal wind is analyzed in Southern Hemisphere observations for the two calendar months of November and April. The observed features are studied further in an idealized quasigeostrophic and a simplified general circulation model (GCM). Results suggest that capturing the bimodality of the zonal mean flow requires the parameterization of momentum and heat fluxes associated with baroclinic instability of the three-dimensional fields. The role of eddy heat fluxes in generating the observed double-jet pattern is ascertained by using an analytical Eady model with stratospheric easterlies, in which a single wave disturbance interacts with the mean flow. In this model, the dual jets are generated by the zonal mean flow correction. Sensitivity of the results to the tropospheric vertical wind shear (or, equivalently, the meridional temperature gradient in the...
Nuovo Cimento C Geophysics Space Physics C, Sep 1, 2006
The effect of the stratosphere on the baroclinic adjustment of a nonlinear Eady model is presente... more The effect of the stratosphere on the baroclinic adjustment of a nonlinear Eady model is presented. The classical linear Eady model has been modified by including an additional layer (the stratosphere), Ekman dissipation at the bottom boundary and a Newtonian cooling at the surface and the tropopause, respectively; non-linearity is introduced by wave-mean flow interaction for a single eddy mode. Results for the rigid-lid case and for small troposphere/stratosphere stratification ratio are compared with those for the linear Eady model with Ekman dissipation at the surface. For these cases model solutions consist of a steady zonal correction and an eddy field with a travelling constant amplitude wave. The equilibrated field, as a function of small stratification ratio, shows that the minimum amplitude of the eddy component raises to a height close to the tropopause (its steering level), denoting that the wave solution becomes vertical evanescent. When realistic values for the static stability in the stratosphere are considered, the zonal correction is no more time independent and reveals a degree of chaotic behaviour, while the eddy field is fully chaotic. Effects of changes in the zonal wind vertical shear and a further decreasing static stability in the stratosphere are also analysed. Results suggest that the minimum amplitude is, in average, higher than the one computed for the classical rigid lid with Ekman dissipation at the surface. Thus, as in the linear Eady model, the stratosphere induces a stabilising effect on the baroclinic dynamics. Finally, the model solutions are compared with the time behaviour of a simplified General Circulation Model.
Tellus A: Dynamic Meteorology and Oceanography, 2002
Recently, a theory relating baroclinic neutrality and midlatitudes tropopause height has been pro... more Recently, a theory relating baroclinic neutrality and midlatitudes tropopause height has been proposed. However, GCM results have shown that the dependence of the theory on external parameters is not consistent with that displayed by these numerical experiments. In the present paper we suggest an analytic formula for baroclinic adjustment to the neutrality of Eady waves through tropopause modification. This formula extends considerably the abovementioned theory by taking into account both a simple representation of the stratosphere and the topography. These modifications alter the tropopause condition for a baroclinically neutral state and its sensitivity to the external parameters. In particular, the topography introduces a dependence on the tropospheric vertical wind shear of the neutrality condition. This feature is not present in other models that assume a background state with a zero potential vorticity gradient in the troposphere. We show, furthermore, that the modified neutrality condition has sensitivities that may resemble those displayed by GCM simulations, with respect to the parameters defining the background flow.
Annales Geophysicae, 2015
The long-term variability in stratospheric ozone mass mixing ratio (O 3) and temperature (T) from... more The long-term variability in stratospheric ozone mass mixing ratio (O 3) and temperature (T) from 1979 to 2013 is investigated using the latest reanalysis product delivered by the European Centre for Medium-Range Weather Forecasts (ECMWF), i.e., ERA-Interim. Moreover, using the Mg II index time series for the same time period, the response of the stratosphere to the 11-year Schwabe solar cycle is investigated. Results reveal the following features: (i) upward (downward) trends characterize zonally averaged O 3 anomalies in the upper (middle to lower stratosphere) stratosphere, while prevailing downward trends affect the T field. Mg II index data exhibit a weaker 24th solar cycle (though not complete) when compared with the previous two; (ii) correlations between O 3 and Mg II, T and Mg II, and O 3 and T are consistent with photochemical reactions occurring in the stratosphere and large-scale transport; and (iii) wavelet cross-spectra between O 3 and Mg II index show common power for the 11-year period, particularly in tropical regions around 30-50 hPa, and different relative phase in the upper and lower stratosphere. A comprehensive insight into the actual processes accounting for the observed correlation between ozone and solar UV variability would be gained from an improved bias correction of ozone measurements provided by different satellite instruments, and from the observations of the time behavior of the solar spectral irradiance.
Although ozone appears in the Earth’s atmosphere in a small abundance, it plays a key role in the... more Although ozone appears in the Earth’s atmosphere in a small abundance, it plays a key role in the energy balance of the planet through its involvement in radiative processes. Its absorption of solar radiation leads to the temperature increase with height defining the tropopause and the stratosphere. Moreover, excluding water vapor, O3 is the third most important contributor (after CO2 and CH4) to the greenhouse radiative forcing. Thus, the total removal of O3 content in an Earth-like atmosphere may cause interesting response of the climate system that deserves further investigation. The present paper addresses this issue by means of a global climate model where the atmosphere is coupled with a passive ocean of a given depth. The model, after reaching the statistical equilibrium under present climate conditions, is perturbed by a sudden switch off of the O3 content. Results obtained for the new equilibrium suggest that the model gets in a colder state mainly because of the water vapo...
Monthly Weather …, 2007
The observed low-frequency variability of the zonally averaged atmospheric circulation in the win... more The observed low-frequency variability of the zonally averaged atmospheric circulation in the winter hemisphere is found to be amenable to an interpretation where the subtropical jet is flanked by a secondary midlatitude one. Observations also suggest that the link between the stratosphere and the troposphere modulates the variability of the tropospheric double-jet structure. Moreover, the summer hemisphere is characterized by a strong midlatitude jet sided by an intermittent subtropical one and easterly winds in the stratosphere. This work addresses the question about the role of eddies in generating and maintaining these key features of the general circulation by means of a simplified general circulation model. Model solutions for different parameter settings and external radiative forcings in the stratosphere are studied with and without eddies active on the system. The following main findings are noted. 1) Eddy dynamics alone, through the baroclinic instability processes in an atmosphere subjected to radiative forcing and dissipation, may account for the observed meridional variance of the tropospheric jets. 2) The Hadley cell can extend to the pole overlying the Ferrel cell, a feature supported by observations in the summer hemisphere. 3) The meridional temperature gradient reversal in the summer stratosphere contributes to the observed lowfrequency variability introducing an intermittent formation of a subtropical jet and the occurrence of easterlies in the tropical stratosphere. 4) Poleward propagation of the zonal wind anomaly is, when it occurs, related to the activity of synoptic eddies.
Journal of the Atmospheric Sciences, 2009
The atmospheric general circulation is characterized by both single- and double-jet patterns. The... more The atmospheric general circulation is characterized by both single- and double-jet patterns. The double-jet structure of the zonal mean zonal wind is analyzed in Southern Hemisphere observations for the two calendar months of November and April. The observed features are studied further in an idealized quasigeostrophic and a simplified general circulation model (GCM). Results suggest that capturing the bimodality of the zonal mean flow requires the parameterization of momentum and heat fluxes associated with baroclinic instability of the three-dimensional fields. The role of eddy heat fluxes in generating the observed double-jet pattern is ascertained by using an analytical Eady model with stratospheric easterlies, in which a single wave disturbance interacts with the mean flow. In this model, the dual jets are generated by the zonal mean flow correction. Sensitivity of the results to the tropospheric vertical wind shear (or, equivalently, the meridional temperature gradient in the...
Nuovo Cimento C Geophysics Space Physics C, Sep 1, 2006
The effect of the stratosphere on the baroclinic adjustment of a nonlinear Eady model is presente... more The effect of the stratosphere on the baroclinic adjustment of a nonlinear Eady model is presented. The classical linear Eady model has been modified by including an additional layer (the stratosphere), Ekman dissipation at the bottom boundary and a Newtonian cooling at the surface and the tropopause, respectively; non-linearity is introduced by wave-mean flow interaction for a single eddy mode. Results for the rigid-lid case and for small troposphere/stratosphere stratification ratio are compared with those for the linear Eady model with Ekman dissipation at the surface. For these cases model solutions consist of a steady zonal correction and an eddy field with a travelling constant amplitude wave. The equilibrated field, as a function of small stratification ratio, shows that the minimum amplitude of the eddy component raises to a height close to the tropopause (its steering level), denoting that the wave solution becomes vertical evanescent. When realistic values for the static stability in the stratosphere are considered, the zonal correction is no more time independent and reveals a degree of chaotic behaviour, while the eddy field is fully chaotic. Effects of changes in the zonal wind vertical shear and a further decreasing static stability in the stratosphere are also analysed. Results suggest that the minimum amplitude is, in average, higher than the one computed for the classical rigid lid with Ekman dissipation at the surface. Thus, as in the linear Eady model, the stratosphere induces a stabilising effect on the baroclinic dynamics. Finally, the model solutions are compared with the time behaviour of a simplified General Circulation Model.
Tellus A: Dynamic Meteorology and Oceanography, 2002
Recently, a theory relating baroclinic neutrality and midlatitudes tropopause height has been pro... more Recently, a theory relating baroclinic neutrality and midlatitudes tropopause height has been proposed. However, GCM results have shown that the dependence of the theory on external parameters is not consistent with that displayed by these numerical experiments. In the present paper we suggest an analytic formula for baroclinic adjustment to the neutrality of Eady waves through tropopause modification. This formula extends considerably the abovementioned theory by taking into account both a simple representation of the stratosphere and the topography. These modifications alter the tropopause condition for a baroclinically neutral state and its sensitivity to the external parameters. In particular, the topography introduces a dependence on the tropospheric vertical wind shear of the neutrality condition. This feature is not present in other models that assume a background state with a zero potential vorticity gradient in the troposphere. We show, furthermore, that the modified neutrality condition has sensitivities that may resemble those displayed by GCM simulations, with respect to the parameters defining the background flow.
Annales Geophysicae, 2015
The long-term variability in stratospheric ozone mass mixing ratio (O 3) and temperature (T) from... more The long-term variability in stratospheric ozone mass mixing ratio (O 3) and temperature (T) from 1979 to 2013 is investigated using the latest reanalysis product delivered by the European Centre for Medium-Range Weather Forecasts (ECMWF), i.e., ERA-Interim. Moreover, using the Mg II index time series for the same time period, the response of the stratosphere to the 11-year Schwabe solar cycle is investigated. Results reveal the following features: (i) upward (downward) trends characterize zonally averaged O 3 anomalies in the upper (middle to lower stratosphere) stratosphere, while prevailing downward trends affect the T field. Mg II index data exhibit a weaker 24th solar cycle (though not complete) when compared with the previous two; (ii) correlations between O 3 and Mg II, T and Mg II, and O 3 and T are consistent with photochemical reactions occurring in the stratosphere and large-scale transport; and (iii) wavelet cross-spectra between O 3 and Mg II index show common power for the 11-year period, particularly in tropical regions around 30-50 hPa, and different relative phase in the upper and lower stratosphere. A comprehensive insight into the actual processes accounting for the observed correlation between ozone and solar UV variability would be gained from an improved bias correction of ozone measurements provided by different satellite instruments, and from the observations of the time behavior of the solar spectral irradiance.
Although ozone appears in the Earth’s atmosphere in a small abundance, it plays a key role in the... more Although ozone appears in the Earth’s atmosphere in a small abundance, it plays a key role in the energy balance of the planet through its involvement in radiative processes. Its absorption of solar radiation leads to the temperature increase with height defining the tropopause and the stratosphere. Moreover, excluding water vapor, O3 is the third most important contributor (after CO2 and CH4) to the greenhouse radiative forcing. Thus, the total removal of O3 content in an Earth-like atmosphere may cause interesting response of the climate system that deserves further investigation. The present paper addresses this issue by means of a global climate model where the atmosphere is coupled with a passive ocean of a given depth. The model, after reaching the statistical equilibrium under present climate conditions, is perturbed by a sudden switch off of the O3 content. Results obtained for the new equilibrium suggest that the model gets in a colder state mainly because of the water vapo...
Monthly Weather …, 2007
The observed low-frequency variability of the zonally averaged atmospheric circulation in the win... more The observed low-frequency variability of the zonally averaged atmospheric circulation in the winter hemisphere is found to be amenable to an interpretation where the subtropical jet is flanked by a secondary midlatitude one. Observations also suggest that the link between the stratosphere and the troposphere modulates the variability of the tropospheric double-jet structure. Moreover, the summer hemisphere is characterized by a strong midlatitude jet sided by an intermittent subtropical one and easterly winds in the stratosphere. This work addresses the question about the role of eddies in generating and maintaining these key features of the general circulation by means of a simplified general circulation model. Model solutions for different parameter settings and external radiative forcings in the stratosphere are studied with and without eddies active on the system. The following main findings are noted. 1) Eddy dynamics alone, through the baroclinic instability processes in an atmosphere subjected to radiative forcing and dissipation, may account for the observed meridional variance of the tropospheric jets. 2) The Hadley cell can extend to the pole overlying the Ferrel cell, a feature supported by observations in the summer hemisphere. 3) The meridional temperature gradient reversal in the summer stratosphere contributes to the observed lowfrequency variability introducing an intermittent formation of a subtropical jet and the occurrence of easterlies in the tropical stratosphere. 4) Poleward propagation of the zonal wind anomaly is, when it occurs, related to the activity of synoptic eddies.