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- Author or Editor: Lorenzo M. Polvani x
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Abstract
Analysis of model output from phase 5 of the Coupled Model Intercomparison Project (CMIP5) reveals that, in the zonal mean, the near-term projections of summertime changes of precipitation in the Southern Hemisphere (SH) subtropics are very widely scattered among the models. As a consequence, over the next 50 years, the CMIP5 multimodel mean projects no statistically significant trends in the SH subtropics in summer. This appears to be at odds with the widely reported, and robust, poleward expansion of the subtropical dry zones by the end of the twenty-first century.
This discrepancy between the shorter- and longer-term projections in SH summer, as shown here, rests in the recovery of the ozone hole in the coming decades, as a consequence of the Montreal Protocol. This is explicitly demonstrated by analyzing model experiments with the Whole Atmosphere Community Climate Model, version 4 (WACCM4), a high-top model with interactive stratospheric chemistry, and coupled to land, ocean, and sea ice components. Contrasting WACCM4 integrations of the representative concentration pathway 4.5 with and without trends in surface concentrations of ozone-depleting substances allows for demonstrating that stratospheric ozone recovery will largely offset the induced “wet gets wetter and dry gets drier” projections and the accompanying poleward expansion of the subtropical dry zone in the SH. The lack of near-term statistically significant zonal-mean changes in the SH hydrological cycle during summer is of obvious practical importance for many parts of the world, and it might also have implications for the Southern Ocean and the Antarctic continent.
Abstract
Analysis of model output from phase 5 of the Coupled Model Intercomparison Project (CMIP5) reveals that, in the zonal mean, the near-term projections of summertime changes of precipitation in the Southern Hemisphere (SH) subtropics are very widely scattered among the models. As a consequence, over the next 50 years, the CMIP5 multimodel mean projects no statistically significant trends in the SH subtropics in summer. This appears to be at odds with the widely reported, and robust, poleward expansion of the subtropical dry zones by the end of the twenty-first century.
This discrepancy between the shorter- and longer-term projections in SH summer, as shown here, rests in the recovery of the ozone hole in the coming decades, as a consequence of the Montreal Protocol. This is explicitly demonstrated by analyzing model experiments with the Whole Atmosphere Community Climate Model, version 4 (WACCM4), a high-top model with interactive stratospheric chemistry, and coupled to land, ocean, and sea ice components. Contrasting WACCM4 integrations of the representative concentration pathway 4.5 with and without trends in surface concentrations of ozone-depleting substances allows for demonstrating that stratospheric ozone recovery will largely offset the induced “wet gets wetter and dry gets drier” projections and the accompanying poleward expansion of the subtropical dry zone in the SH. The lack of near-term statistically significant zonal-mean changes in the SH hydrological cycle during summer is of obvious practical importance for many parts of the world, and it might also have implications for the Southern Ocean and the Antarctic continent.
Abstract
Observations show an increase in maximum precipitation extremes and a decrease in maximum temperature extremes over southeastern South America (SESA) in the second half of the twentieth century. The Community Earth System Model (CESM) Large Ensemble (LE) experiments are able to successfully reproduce the observed trends of extreme precipitation and temperature over SESA. Careful analysis of a smaller ensemble of CESM-LE single forcing experiments reveals that the trends of extreme precipitation and temperature over SESA are mostly caused by stratospheric ozone depletion. The underlying dynamical mechanism is investigated and it is found that, as a consequence of stratospheric ozone depletion and the resulting southward shift of tropospheric jet streams, anomalous easterly flow and more intense cyclones have occurred over SESA, which are favorable for heavier rainfall extremes and milder heat extremes.
Abstract
Observations show an increase in maximum precipitation extremes and a decrease in maximum temperature extremes over southeastern South America (SESA) in the second half of the twentieth century. The Community Earth System Model (CESM) Large Ensemble (LE) experiments are able to successfully reproduce the observed trends of extreme precipitation and temperature over SESA. Careful analysis of a smaller ensemble of CESM-LE single forcing experiments reveals that the trends of extreme precipitation and temperature over SESA are mostly caused by stratospheric ozone depletion. The underlying dynamical mechanism is investigated and it is found that, as a consequence of stratospheric ozone depletion and the resulting southward shift of tropospheric jet streams, anomalous easterly flow and more intense cyclones have occurred over SESA, which are favorable for heavier rainfall extremes and milder heat extremes.
Abstract
It is widely appreciated that ozone-depleting substances (ODS), which have led to the formation of the Antarctic ozone hole, are also powerful greenhouse gases. In this study, we explore the consequence of the surface warming caused by ODS in the second half of the twentieth century over the Indo-Pacific Ocean, using the Whole Atmosphere Chemistry Climate Model (version 4). By contrasting two ensembles of chemistry–climate model integrations (with and without ODS forcing) over the period 1955–2005, we show that the additional greenhouse effect of ODS is crucial to producing a statistically significant weakening of the Walker circulation in our model over that period. When ODS concentrations are held fixed at 1955 levels, the forcing of the other well-mixed greenhouse gases alone leads to a strengthening—rather than weakening—of the Walker circulation because their warming effect is not sufficiently strong. Without increasing ODS, a surface warming delay in the eastern tropical Pacific Ocean leads to an increase in the sea surface temperature gradient between the eastern and western Pacific, with an associated strengthening of the Walker circulation. When increasing ODS are added, the considerably larger total radiative forcing produces a much faster warming in the eastern Pacific, causing the sign of the trend to reverse and the Walker circulation to weaken. Our modeling result suggests that ODS may have been key players in the observed weakening of the Walker circulation over the second half of the twentieth century.
Abstract
It is widely appreciated that ozone-depleting substances (ODS), which have led to the formation of the Antarctic ozone hole, are also powerful greenhouse gases. In this study, we explore the consequence of the surface warming caused by ODS in the second half of the twentieth century over the Indo-Pacific Ocean, using the Whole Atmosphere Chemistry Climate Model (version 4). By contrasting two ensembles of chemistry–climate model integrations (with and without ODS forcing) over the period 1955–2005, we show that the additional greenhouse effect of ODS is crucial to producing a statistically significant weakening of the Walker circulation in our model over that period. When ODS concentrations are held fixed at 1955 levels, the forcing of the other well-mixed greenhouse gases alone leads to a strengthening—rather than weakening—of the Walker circulation because their warming effect is not sufficiently strong. Without increasing ODS, a surface warming delay in the eastern tropical Pacific Ocean leads to an increase in the sea surface temperature gradient between the eastern and western Pacific, with an associated strengthening of the Walker circulation. When increasing ODS are added, the considerably larger total radiative forcing produces a much faster warming in the eastern Pacific, causing the sign of the trend to reverse and the Walker circulation to weaken. Our modeling result suggests that ODS may have been key players in the observed weakening of the Walker circulation over the second half of the twentieth century.
Abstract
Future emissions of greenhouse gases into the atmosphere are projected to result in significant circulation changes. One of the most important changes is the widening of the tropical belt, which has great societal impacts. Several mechanisms (changes in surface temperature, eddy phase speed, tropopause height, and static stability) have been proposed to explain this widening. However, the coupling between these mechanisms has precluded elucidating their relative importance. Here, the abrupt quadrupled-CO2 simulations of phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to examine the proposed mechanisms. The different time responses of the different mechanisms allow us to disentangle and evaluate them. As suggested by earlier studies, the Hadley cell edge is found to be linked to changes in subtropical baroclinicity. In particular, its poleward shift is accompanied by an increase in subtropical static stability (i.e., a decrease in temperature lapse rate) with increased CO2 concentrations. These subtropical changes also affect the eddy momentum flux, which shifts poleward together with the Hadley cell edge. Transient changes in tropopause height, eddy phase speed, and surface temperature, however, were found not to accompany the poleward shift of the Hadley cell edge. The widening of the Hadley cell, together with the increase in moisture content, accounts for most of the expansion of the dry zone. Eddy moisture fluxes, on the other hand, are found to play a minor role in the expansion of the dry zone.
Abstract
Future emissions of greenhouse gases into the atmosphere are projected to result in significant circulation changes. One of the most important changes is the widening of the tropical belt, which has great societal impacts. Several mechanisms (changes in surface temperature, eddy phase speed, tropopause height, and static stability) have been proposed to explain this widening. However, the coupling between these mechanisms has precluded elucidating their relative importance. Here, the abrupt quadrupled-CO2 simulations of phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to examine the proposed mechanisms. The different time responses of the different mechanisms allow us to disentangle and evaluate them. As suggested by earlier studies, the Hadley cell edge is found to be linked to changes in subtropical baroclinicity. In particular, its poleward shift is accompanied by an increase in subtropical static stability (i.e., a decrease in temperature lapse rate) with increased CO2 concentrations. These subtropical changes also affect the eddy momentum flux, which shifts poleward together with the Hadley cell edge. Transient changes in tropopause height, eddy phase speed, and surface temperature, however, were found not to accompany the poleward shift of the Hadley cell edge. The widening of the Hadley cell, together with the increase in moisture content, accounts for most of the expansion of the dry zone. Eddy moisture fluxes, on the other hand, are found to play a minor role in the expansion of the dry zone.
Abstract
Recent studies have hypothesized that Arctic amplification, the enhanced warming of the Arctic region compared to the rest of the globe, will cause changes in midlatitude weather over the twenty-first century. This study exploits the recently completed phase 5 of the Coupled Model Intercomparison Project (CMIP5) and examines 27 state-of-the-art climate models to determine if their projected changes in the midlatitude circulation are consistent with the hypothesized impact of Arctic amplification over North America and the North Atlantic.
Under the largest future greenhouse forcing (RCP8.5), it is found that every model, in every season, exhibits Arctic amplification by 2100. At the same time, the projected circulation responses are either opposite in sign to those hypothesized or too widely spread among the models to discern any robust change. However, in a few seasons and for some of the circulation metrics examined, correlations are found between the model spread in Arctic amplification and the model spread in the projected circulation changes. Therefore, while the CMIP5 models offer some evidence that future Arctic warming may be able to modulate some aspects of the midlatitude circulation response in some seasons, the analysis herein leads to the conclusion that the net circulation response in the future is unlikely to be determined solely—or even primarily—by Arctic warming according to the sequence of events recently hypothesized.
Abstract
Recent studies have hypothesized that Arctic amplification, the enhanced warming of the Arctic region compared to the rest of the globe, will cause changes in midlatitude weather over the twenty-first century. This study exploits the recently completed phase 5 of the Coupled Model Intercomparison Project (CMIP5) and examines 27 state-of-the-art climate models to determine if their projected changes in the midlatitude circulation are consistent with the hypothesized impact of Arctic amplification over North America and the North Atlantic.
Under the largest future greenhouse forcing (RCP8.5), it is found that every model, in every season, exhibits Arctic amplification by 2100. At the same time, the projected circulation responses are either opposite in sign to those hypothesized or too widely spread among the models to discern any robust change. However, in a few seasons and for some of the circulation metrics examined, correlations are found between the model spread in Arctic amplification and the model spread in the projected circulation changes. Therefore, while the CMIP5 models offer some evidence that future Arctic warming may be able to modulate some aspects of the midlatitude circulation response in some seasons, the analysis herein leads to the conclusion that the net circulation response in the future is unlikely to be determined solely—or even primarily—by Arctic warming according to the sequence of events recently hypothesized.
Abstract
It has recently been shown that extreme stratospheric events (ESEs) are followed by surface weather anomalies (for up to 60 days), suggesting that stratospheric variability might be used to extend weather prediction beyond current time scales. In this paper, attention is drawn away from the stratosphere to demonstrate that the originating point of ESEs is located in the troposphere. First, it is shown that anomalously strong eddy heat fluxes at 100 hPa nearly always precede weak vortex events, and conversely, anomalously weak eddy heat fluxes precede strong vortex events, consistent with wave–mean flow interaction theory. This finding clarifies the dynamical nature of ESEs and suggests that a major source of stratospheric variability (and thus predictability) is located in the troposphere below and not in the stratosphere itself. Second, it is shown that the daily time series of eddy heat flux found at 100 hPa and integrated over the prior 40 days, exhibit a remarkably high anticorrelation (−0.8) with the Arctic Oscillation (AO) index at 10 hPa. Following Baldwin and Dunkerton, it is then demonstrated that events with anomalously strong (weak) integrated eddy heat fluxes at 100 hPa are followed by anomalously large (small) surface values of the AO index up to 60 days following each event. This suggests that the stratosphere is unlikely to be the dominant source of the anomalous surface weather regimes discussed in Thompson et al.
Abstract
It has recently been shown that extreme stratospheric events (ESEs) are followed by surface weather anomalies (for up to 60 days), suggesting that stratospheric variability might be used to extend weather prediction beyond current time scales. In this paper, attention is drawn away from the stratosphere to demonstrate that the originating point of ESEs is located in the troposphere. First, it is shown that anomalously strong eddy heat fluxes at 100 hPa nearly always precede weak vortex events, and conversely, anomalously weak eddy heat fluxes precede strong vortex events, consistent with wave–mean flow interaction theory. This finding clarifies the dynamical nature of ESEs and suggests that a major source of stratospheric variability (and thus predictability) is located in the troposphere below and not in the stratosphere itself. Second, it is shown that the daily time series of eddy heat flux found at 100 hPa and integrated over the prior 40 days, exhibit a remarkably high anticorrelation (−0.8) with the Arctic Oscillation (AO) index at 10 hPa. Following Baldwin and Dunkerton, it is then demonstrated that events with anomalously strong (weak) integrated eddy heat fluxes at 100 hPa are followed by anomalously large (small) surface values of the AO index up to 60 days following each event. This suggests that the stratosphere is unlikely to be the dominant source of the anomalous surface weather regimes discussed in Thompson et al.
Abstract
The extratropical circulation response to cooling of the polar-winter stratosphere in a simple AGCM is investigated. The AGCM is a dry hydrostatic primitive equation model with zonally symmetric boundary conditions and analytically specified physics. It is found that, as the polar-winter stratosphere is cooled, the tropospheric jet shifts poleward. This response projects almost entirely and positively (by convention) onto the AGCM's annular mode. At the same time, the vertical flux of wave activity from the troposphere to the stratosphere is reduced and the meridional flux of wave activity from high to low latitudes is increased. Thus, as the stratosphere is cooled, the stratospheric wave drag is reduced.
In order to understand this response, the transient adjustment of the stratosphere–troposphere system is investigated using an ensemble of “switch on” stratospheric cooling runs of the AGCM. The response to the switch-on stratospheric cooling descends from the upper stratosphere into the troposphere on a time scale that matches simple downward-control theory estimates.
The downward-control analysis is pursued with a zonally symmetric model that uses as input the thermal and eddy-driving terms from the eddying AGCM. With this model, the contributions to the response from the thermal and eddy-driving perturbations can be investigated separately, in the absence of eddy feedbacks. It is found that the stratospheric thermal perturbation, in the absence of such feedbacks, induces a response that is confined to the stratosphere. The stratospheric eddy-driving perturbation, on the other hand, induces a response that penetrates into the midtroposphere but does not account, in the zonally symmetric model, for the tropospheric jet shift. Furthermore, the tropospheric eddy-driving perturbation, in the zonally symmetric model, induces a strong upward response in the stratospheric winds. From these experiments and from additional experiments with the eddying AGCM, it is concluded that the stratospheric eddy-driving response induces a tropospheric response, but that the full circulation response results from a two-way coupling between the stratosphere and the troposphere.
Abstract
The extratropical circulation response to cooling of the polar-winter stratosphere in a simple AGCM is investigated. The AGCM is a dry hydrostatic primitive equation model with zonally symmetric boundary conditions and analytically specified physics. It is found that, as the polar-winter stratosphere is cooled, the tropospheric jet shifts poleward. This response projects almost entirely and positively (by convention) onto the AGCM's annular mode. At the same time, the vertical flux of wave activity from the troposphere to the stratosphere is reduced and the meridional flux of wave activity from high to low latitudes is increased. Thus, as the stratosphere is cooled, the stratospheric wave drag is reduced.
In order to understand this response, the transient adjustment of the stratosphere–troposphere system is investigated using an ensemble of “switch on” stratospheric cooling runs of the AGCM. The response to the switch-on stratospheric cooling descends from the upper stratosphere into the troposphere on a time scale that matches simple downward-control theory estimates.
The downward-control analysis is pursued with a zonally symmetric model that uses as input the thermal and eddy-driving terms from the eddying AGCM. With this model, the contributions to the response from the thermal and eddy-driving perturbations can be investigated separately, in the absence of eddy feedbacks. It is found that the stratospheric thermal perturbation, in the absence of such feedbacks, induces a response that is confined to the stratosphere. The stratospheric eddy-driving perturbation, on the other hand, induces a response that penetrates into the midtroposphere but does not account, in the zonally symmetric model, for the tropospheric jet shift. Furthermore, the tropospheric eddy-driving perturbation, in the zonally symmetric model, induces a strong upward response in the stratospheric winds. From these experiments and from additional experiments with the eddying AGCM, it is concluded that the stratospheric eddy-driving response induces a tropospheric response, but that the full circulation response results from a two-way coupling between the stratosphere and the troposphere.
Abstract
Shock-capturing numerical methods are employed to integrate the fully nonlinear, rotating 1D shallow-water equations starting from steplike nongeostrophic initial conditions (a Rossby adjustment problem). Such numerical methods allow one to observe the formation of multiple bores during the transient adjustment process as well as their decay due to rotation. It is demonstrated that increasing the rotation and/or the nonlinearity increases the rate of decay. Additionally, the time required for adjustment to be completed and its dependence on nonlinearity is examined; this time is found to be highly measure dependent. Lastly, the final adjusted state of the system is observed through long time integrations. Although the bores that form provide a mechanism for dissipation, their decay results in a final state in very good agreement with the one computed by well-known (dissipationless) conservation methods.
Abstract
Shock-capturing numerical methods are employed to integrate the fully nonlinear, rotating 1D shallow-water equations starting from steplike nongeostrophic initial conditions (a Rossby adjustment problem). Such numerical methods allow one to observe the formation of multiple bores during the transient adjustment process as well as their decay due to rotation. It is demonstrated that increasing the rotation and/or the nonlinearity increases the rate of decay. Additionally, the time required for adjustment to be completed and its dependence on nonlinearity is examined; this time is found to be highly measure dependent. Lastly, the final adjusted state of the system is observed through long time integrations. Although the bores that form provide a mechanism for dissipation, their decay results in a final state in very good agreement with the one computed by well-known (dissipationless) conservation methods.
Abstract
The 1987 Montreal Protocol regulating emissions of chlorofluorocarbons (CFCs) and other ozone-depleting substances (ODSs) was motivated primarily by the harm to human health and ecosystems arising from increased exposure to ultraviolet-B (UV-B) radiation associated with depletion of the ozone layer. It is now known that the Montreal Protocol has helped reduce radiative forcing of the climate system since CFCs are greenhouse gases (GHGs), and that ozone depletion (which is now on the verge of reversing) has been the dominant driver of atmospheric circulation changes in the Southern Hemisphere in the last half century.
This paper demonstrates that the Montreal Protocol also significantly protects Earth’s hydroclimate. Using the Community Atmospheric Model, version 3 (CAM3), coupled to a simple mixed layer ocean, it is shown that in the “world avoided” (i.e., with CFC emissions not regulated), the subtropical dry zones would be substantially drier, and the middle- and high-latitude regions considerably wetter in the coming decade (2020–29) than in a world without ozone depletion. Surprisingly, these changes are very similar, in both pattern and magnitude, to those caused by projected increases in GHG concentrations over the same period. It is further shown that, by dynamical and thermodynamical mechanisms, both the stratospheric ozone depletion and increased CFCs contribute to these changes. The results herein imply that, as a consequence of the Montreal Protocol, changes in the hydrological cycle in the coming decade will be only half as strong as what they otherwise would be.
Abstract
The 1987 Montreal Protocol regulating emissions of chlorofluorocarbons (CFCs) and other ozone-depleting substances (ODSs) was motivated primarily by the harm to human health and ecosystems arising from increased exposure to ultraviolet-B (UV-B) radiation associated with depletion of the ozone layer. It is now known that the Montreal Protocol has helped reduce radiative forcing of the climate system since CFCs are greenhouse gases (GHGs), and that ozone depletion (which is now on the verge of reversing) has been the dominant driver of atmospheric circulation changes in the Southern Hemisphere in the last half century.
This paper demonstrates that the Montreal Protocol also significantly protects Earth’s hydroclimate. Using the Community Atmospheric Model, version 3 (CAM3), coupled to a simple mixed layer ocean, it is shown that in the “world avoided” (i.e., with CFC emissions not regulated), the subtropical dry zones would be substantially drier, and the middle- and high-latitude regions considerably wetter in the coming decade (2020–29) than in a world without ozone depletion. Surprisingly, these changes are very similar, in both pattern and magnitude, to those caused by projected increases in GHG concentrations over the same period. It is further shown that, by dynamical and thermodynamical mechanisms, both the stratospheric ozone depletion and increased CFCs contribute to these changes. The results herein imply that, as a consequence of the Montreal Protocol, changes in the hydrological cycle in the coming decade will be only half as strong as what they otherwise would be.
