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Abstract
Given the rapidly changing Arctic climate, there is an urgent need for improved seasonal predictions of Arctic sea ice. Yet, Arctic sea ice prediction is inherently complex. Among other factors, wintertime atmospheric circulation has been shown to be predictive of summertime Arctic sea ice extent. Specifically, many studies have shown that the interannual variability of summertime Arctic sea ice extent (SIE) is anticorrelated with the leading mode of extratropical atmospheric variability, the Arctic Oscillation (AO), in the preceding winter. Given this relationship, the potential predictive role of stratospheric circulation extremes and stratosphere–troposphere coupling in linking the AO and Arctic SIE variability is examined. It is shown that extremes in the stratospheric circulation during the winter season, namely, stratospheric sudden warming (SSW) and strong polar vortex (SPV) events, are associated with significant anomalies in sea ice concentration in the Barents Sea in spring and along the Eurasian coastline in summer in both observations and a fully coupled, stratosphere-resolving general circulation model. Consistent with previous work on the AO, it is shown that SSWs, which are followed by the negative phase of the AO at the surface, result in sea ice growth, whereas SPVs, which are followed by the positive phase of the AO at the surface, result in sea ice loss, although the mechanisms in the Barents Sea and along the Eurasian coastline are different. The analysis suggests that the presence or absence of stratospheric circulation extremes in winter may play a nontrivial role in determining total September Arctic SIE when combined with other factors.
Abstract
Given the rapidly changing Arctic climate, there is an urgent need for improved seasonal predictions of Arctic sea ice. Yet, Arctic sea ice prediction is inherently complex. Among other factors, wintertime atmospheric circulation has been shown to be predictive of summertime Arctic sea ice extent. Specifically, many studies have shown that the interannual variability of summertime Arctic sea ice extent (SIE) is anticorrelated with the leading mode of extratropical atmospheric variability, the Arctic Oscillation (AO), in the preceding winter. Given this relationship, the potential predictive role of stratospheric circulation extremes and stratosphere–troposphere coupling in linking the AO and Arctic SIE variability is examined. It is shown that extremes in the stratospheric circulation during the winter season, namely, stratospheric sudden warming (SSW) and strong polar vortex (SPV) events, are associated with significant anomalies in sea ice concentration in the Barents Sea in spring and along the Eurasian coastline in summer in both observations and a fully coupled, stratosphere-resolving general circulation model. Consistent with previous work on the AO, it is shown that SSWs, which are followed by the negative phase of the AO at the surface, result in sea ice growth, whereas SPVs, which are followed by the positive phase of the AO at the surface, result in sea ice loss, although the mechanisms in the Barents Sea and along the Eurasian coastline are different. The analysis suggests that the presence or absence of stratospheric circulation extremes in winter may play a nontrivial role in determining total September Arctic SIE when combined with other factors.
Abstract
The impact of the Montreal Protocol on the potential intensity of tropical cyclones over the next 50 years is investigated with the Whole Atmosphere Community Climate Model (WACCM), a state-of-the-art, stratosphere-resolving atmospheric model, coupled to land, ocean, and sea ice components, with interactive stratospheric chemistry. An ensemble of WACCM runs from 2006 to 2065 forced with a standard future scenario is compared to a second ensemble in which ozone-depleting substances (ODS) are not regulated (the so-called World Avoided). It is found that by the year 2065, changes in the potential intensity of tropical cyclones in the World Avoided are nearly 3 times as large as for the standard scenario. The Montreal Protocol thus provides a strong mitigation of the adverse effects of intensifying tropical cyclones.
The relative importance of warmer sea surface temperatures (ozone-depleting substances are important greenhouse gases) and cooler lower-stratospheric temperatures (accompanying the massive destruction of the ozone layer) is carefully examined. It is found that the former are largely responsible for the increase in potential intensity in the World Avoided, whereas temperatures above the 70-hPa level—which plunge by nearly 15 K in 2065 in the World Avoided—have no discernible effect on potential intensity. This finding suggests that the modest (compared to the World Avoided) tropical ozone depletion of recent decades has not been a major player in determining the intensity of tropical cyclones, and neither will ozone recovery be in the coming half century.
Abstract
The impact of the Montreal Protocol on the potential intensity of tropical cyclones over the next 50 years is investigated with the Whole Atmosphere Community Climate Model (WACCM), a state-of-the-art, stratosphere-resolving atmospheric model, coupled to land, ocean, and sea ice components, with interactive stratospheric chemistry. An ensemble of WACCM runs from 2006 to 2065 forced with a standard future scenario is compared to a second ensemble in which ozone-depleting substances (ODS) are not regulated (the so-called World Avoided). It is found that by the year 2065, changes in the potential intensity of tropical cyclones in the World Avoided are nearly 3 times as large as for the standard scenario. The Montreal Protocol thus provides a strong mitigation of the adverse effects of intensifying tropical cyclones.
The relative importance of warmer sea surface temperatures (ozone-depleting substances are important greenhouse gases) and cooler lower-stratospheric temperatures (accompanying the massive destruction of the ozone layer) is carefully examined. It is found that the former are largely responsible for the increase in potential intensity in the World Avoided, whereas temperatures above the 70-hPa level—which plunge by nearly 15 K in 2065 in the World Avoided—have no discernible effect on potential intensity. This finding suggests that the modest (compared to the World Avoided) tropical ozone depletion of recent decades has not been a major player in determining the intensity of tropical cyclones, and neither will ozone recovery be in the coming half century.
Abstract
In this study the authors continue their investigation of the atmospheric energy budget of the Antarctic polar cap (the region poleward of 70°S) using integrations of the Whole Atmosphere Community Climate Model from the years 1960 to 2065. In agreement with observational data, it is found that the climatological mean net top-of-atmosphere (TOA) radiative flux is primarily balanced by the horizontal energy flux convergence over the polar cap. On interannual time scales, changes in the net TOA radiative flux are also primarily balanced by changes in the energy flux convergence, with the variability in both terms significantly correlated (positively and negatively, respectively) with the southern annular mode (SAM). On multidecadal time scales, twentieth-century stratospheric ozone depletion produces a negative trend in the net TOA radiative flux due to a decrease in the absorbed solar radiation within the atmosphere–surface column. The negative trend in the net TOA radiative flux is balanced by a positive trend in energy flux convergence, primarily in austral summer. This negative (positive) trend in the net TOA radiation (energy flux convergence) occurs despite a positive trend in the SAM, suggesting that the effects of the SAM on the energy budget are overwhelmed by the direct radiative effects of ozone depletion. In the twenty-first century, ozone recovery is expected to reverse the negative trend in the net TOA radiative flux, which would then, again, be balanced by a decrease in the energy flux convergence. Therefore, over the next several decades, ozone recovery will, in all likelihood, mask the effect of greenhouse gas warming on the Antarctic energy budget.
Abstract
In this study the authors continue their investigation of the atmospheric energy budget of the Antarctic polar cap (the region poleward of 70°S) using integrations of the Whole Atmosphere Community Climate Model from the years 1960 to 2065. In agreement with observational data, it is found that the climatological mean net top-of-atmosphere (TOA) radiative flux is primarily balanced by the horizontal energy flux convergence over the polar cap. On interannual time scales, changes in the net TOA radiative flux are also primarily balanced by changes in the energy flux convergence, with the variability in both terms significantly correlated (positively and negatively, respectively) with the southern annular mode (SAM). On multidecadal time scales, twentieth-century stratospheric ozone depletion produces a negative trend in the net TOA radiative flux due to a decrease in the absorbed solar radiation within the atmosphere–surface column. The negative trend in the net TOA radiative flux is balanced by a positive trend in energy flux convergence, primarily in austral summer. This negative (positive) trend in the net TOA radiation (energy flux convergence) occurs despite a positive trend in the SAM, suggesting that the effects of the SAM on the energy budget are overwhelmed by the direct radiative effects of ozone depletion. In the twenty-first century, ozone recovery is expected to reverse the negative trend in the net TOA radiative flux, which would then, again, be balanced by a decrease in the energy flux convergence. Therefore, over the next several decades, ozone recovery will, in all likelihood, mask the effect of greenhouse gas warming on the Antarctic energy budget.
Abstract
The volcanic eruption of Mount Pinatubo in June 1991 is the largest terrestrial eruption since the beginning of the satellite era. Here, the monthly evolution of atmospheric temperature, zonal winds, and precipitation following the eruption in 14 CMIP5 models is analyzed and strong and robust stratospheric and tropospheric circulation responses are demonstrated in both hemispheres, with tropospheric anomalies maximizing in November 1991. The simulated Southern Hemisphere circulation response projects strongly onto the positive phase of the southern annular mode (SAM), while the Northern Hemisphere exhibits robust North Atlantic and North Pacific responses that differ significantly from that of the typical northern annular mode (NAM) pattern. In contrast, observations show a negative SAM following the eruption, and internal variability must be considered along with forced responses. Indeed, evidence is presented that the observed El Niño climate state during and after this eruption may oppose the eruption-forced positive SAM response, based on the El Niño–Southern Oscillation (ENSO) state and SAM response across the models. The results demonstrate that Pinatubo-like eruptions should be expected to force circulation anomalies across the globe and highlight that great care must be taken in diagnosing the forced response as it may not fall into typical seasonal averages or be guaranteed to project onto typical climate modes.
Abstract
The volcanic eruption of Mount Pinatubo in June 1991 is the largest terrestrial eruption since the beginning of the satellite era. Here, the monthly evolution of atmospheric temperature, zonal winds, and precipitation following the eruption in 14 CMIP5 models is analyzed and strong and robust stratospheric and tropospheric circulation responses are demonstrated in both hemispheres, with tropospheric anomalies maximizing in November 1991. The simulated Southern Hemisphere circulation response projects strongly onto the positive phase of the southern annular mode (SAM), while the Northern Hemisphere exhibits robust North Atlantic and North Pacific responses that differ significantly from that of the typical northern annular mode (NAM) pattern. In contrast, observations show a negative SAM following the eruption, and internal variability must be considered along with forced responses. Indeed, evidence is presented that the observed El Niño climate state during and after this eruption may oppose the eruption-forced positive SAM response, based on the El Niño–Southern Oscillation (ENSO) state and SAM response across the models. The results demonstrate that Pinatubo-like eruptions should be expected to force circulation anomalies across the globe and highlight that great care must be taken in diagnosing the forced response as it may not fall into typical seasonal averages or be guaranteed to project onto typical climate modes.
Abstract
Observational evidence indicates that the southern edge of the Hadley cell (HC) has shifted southward during austral summer in recent decades. However, there is no consensus on the cause of this shift, with several studies reaching opposite conclusions as to the relative role of changes in sea surface temperatures (SSTs) and stratospheric ozone depletion in causing this shift. Here, the authors perform a meta-analysis of the extant literature on this subject and quantitatively compare the results of all published studies that have used single-forcing model integrations to isolate the role of different factors on the HC expansion during austral summer. It is shown that the weight of the evidence clearly points to stratospheric ozone depletion as the dominant driver of the tropical summertime expansion over the period in which an ozone hole was formed (1979 to late 1990s), although SST trends have contributed to trends since then. Studies that have claimed SSTs as the major driver of tropical expansion since 1979 have used prescribed ozone fields that underrepresent the observed Antarctic ozone depletion.
Abstract
Observational evidence indicates that the southern edge of the Hadley cell (HC) has shifted southward during austral summer in recent decades. However, there is no consensus on the cause of this shift, with several studies reaching opposite conclusions as to the relative role of changes in sea surface temperatures (SSTs) and stratospheric ozone depletion in causing this shift. Here, the authors perform a meta-analysis of the extant literature on this subject and quantitatively compare the results of all published studies that have used single-forcing model integrations to isolate the role of different factors on the HC expansion during austral summer. It is shown that the weight of the evidence clearly points to stratospheric ozone depletion as the dominant driver of the tropical summertime expansion over the period in which an ozone hole was formed (1979 to late 1990s), although SST trends have contributed to trends since then. Studies that have claimed SSTs as the major driver of tropical expansion since 1979 have used prescribed ozone fields that underrepresent the observed Antarctic ozone depletion.
Abstract
The authors evaluate 23 coupled atmosphere–ocean general circulation models from phase 5 of CMIP (CMIP5) in terms of their ability to simulate the observed climatological mean energy budget of the Antarctic atmosphere. While the models are shown to capture the gross features of the energy budget well [e.g., the observed two-way balance between the top-of-atmosphere (TOA) net radiation and horizontal convergence of atmospheric energy transport], the simulated TOA absorbed shortwave (SW) radiation is too large during austral summer. In the multimodel mean, this excessive absorption reaches approximately 10 W m−2, with even larger biases (up to 25–30 W m−2) in individual models. Previous studies have identified similar climate model biases in the TOA net SW radiation at Southern Hemisphere midlatitudes and have attributed these biases to errors in the simulated cloud cover. Over the Antarctic, though, model cloud errors are of secondary importance, and biases in the simulated TOA net SW flux are instead driven mainly by biases in the clear-sky SW reflection. The latter are likely related in part to the models’ underestimation of the observed annual minimum in Antarctic sea ice extent, thus underscoring the importance of sea ice in the Antarctic energy budget. Finally, substantial differences in the climatological surface energy fluxes between existing observational datasets preclude any meaningful assessment of model skill in simulating these fluxes.
Abstract
The authors evaluate 23 coupled atmosphere–ocean general circulation models from phase 5 of CMIP (CMIP5) in terms of their ability to simulate the observed climatological mean energy budget of the Antarctic atmosphere. While the models are shown to capture the gross features of the energy budget well [e.g., the observed two-way balance between the top-of-atmosphere (TOA) net radiation and horizontal convergence of atmospheric energy transport], the simulated TOA absorbed shortwave (SW) radiation is too large during austral summer. In the multimodel mean, this excessive absorption reaches approximately 10 W m−2, with even larger biases (up to 25–30 W m−2) in individual models. Previous studies have identified similar climate model biases in the TOA net SW radiation at Southern Hemisphere midlatitudes and have attributed these biases to errors in the simulated cloud cover. Over the Antarctic, though, model cloud errors are of secondary importance, and biases in the simulated TOA net SW flux are instead driven mainly by biases in the clear-sky SW reflection. The latter are likely related in part to the models’ underestimation of the observed annual minimum in Antarctic sea ice extent, thus underscoring the importance of sea ice in the Antarctic energy budget. Finally, substantial differences in the climatological surface energy fluxes between existing observational datasets preclude any meaningful assessment of model skill in simulating these fluxes.
Abstract
Stratospheric ozone is expected to recover by the end of this century because of the regulation of ozone-depleting substances by the Montreal Protocol. Targeted modeling studies have suggested that the climate response to ozone recovery will greatly oppose the climate response to rising greenhouse gas (GHG) emissions. However, the extent of this cancellation remains unclear since only a few such studies are available. Here, a much larger set of simulations performed for phase 5 of the Coupled Model Intercomparison Project is analyzed, which includes ozone recovery. It is shown that the closing of the ozone hole will cause a delay in summertime [December–February (DJF)] Southern Hemisphere climate change between now and 2045. Specifically, it is found that the position of the jet stream, the width of the subtropical dry zones, the seasonality of surface temperatures, and sea ice concentrations all exhibit significantly reduced summertime trends over the first half of the twenty-first century as a consequence of ozone recovery. After 2045, forcing from GHG emissions begins to dominate the climate response. Finally, comparing the relative influences of future GHG emissions and historic ozone depletion, it is found that the simulated DJF tropospheric circulation changes between 1965 and 2005 (driven primarily by ozone depletion) are larger than the projected changes in any future scenario over the entire twenty-first century.
Abstract
Stratospheric ozone is expected to recover by the end of this century because of the regulation of ozone-depleting substances by the Montreal Protocol. Targeted modeling studies have suggested that the climate response to ozone recovery will greatly oppose the climate response to rising greenhouse gas (GHG) emissions. However, the extent of this cancellation remains unclear since only a few such studies are available. Here, a much larger set of simulations performed for phase 5 of the Coupled Model Intercomparison Project is analyzed, which includes ozone recovery. It is shown that the closing of the ozone hole will cause a delay in summertime [December–February (DJF)] Southern Hemisphere climate change between now and 2045. Specifically, it is found that the position of the jet stream, the width of the subtropical dry zones, the seasonality of surface temperatures, and sea ice concentrations all exhibit significantly reduced summertime trends over the first half of the twenty-first century as a consequence of ozone recovery. After 2045, forcing from GHG emissions begins to dominate the climate response. Finally, comparing the relative influences of future GHG emissions and historic ozone depletion, it is found that the simulated DJF tropospheric circulation changes between 1965 and 2005 (driven primarily by ozone depletion) are larger than the projected changes in any future scenario over the entire twenty-first century.
Abstract
The authors present a new, observationally based estimate of the atmospheric energy budget for the Antarctic polar cap (the region poleward of 70°S). This energy budget is constructed using state-of-the-art reanalysis products from ECMWF [the ECMWF Interim Re-Analysis (ERA-Interim)] and Clouds and the Earth's Radiant Energy System (CERES) top-of-atmosphere (TOA) radiative fluxes for the period 2001–10. The climatological mean Antarctic energy budget is characterized by an approximate balance between the TOA net outgoing radiation and the horizontal convergence of atmospheric energy transport, with the net surface energy flux and atmospheric energy storage generally being small in comparison. Variability in the energy budget on intraseasonal-to-interannual time scales bears a strong signature of the southern annular mode (SAM), with El Niño–Southern Oscillation (ENSO) having a smaller impact. The energy budget framework is shown to be a useful alternative to the SAM for interpreting surface climate variability in the Antarctic region.
Abstract
The authors present a new, observationally based estimate of the atmospheric energy budget for the Antarctic polar cap (the region poleward of 70°S). This energy budget is constructed using state-of-the-art reanalysis products from ECMWF [the ECMWF Interim Re-Analysis (ERA-Interim)] and Clouds and the Earth's Radiant Energy System (CERES) top-of-atmosphere (TOA) radiative fluxes for the period 2001–10. The climatological mean Antarctic energy budget is characterized by an approximate balance between the TOA net outgoing radiation and the horizontal convergence of atmospheric energy transport, with the net surface energy flux and atmospheric energy storage generally being small in comparison. Variability in the energy budget on intraseasonal-to-interannual time scales bears a strong signature of the southern annular mode (SAM), with El Niño–Southern Oscillation (ENSO) having a smaller impact. The energy budget framework is shown to be a useful alternative to the SAM for interpreting surface climate variability in the Antarctic region.
Abstract
A new method for integrating shallow water equations, the contour-advective semi-Lagrangian (CASL) algorithm, is presented. This is the first implementation of a contour method to a system of equations for which exact potential vorticity invertibility does not exist. The new CASL method fuses the recent contour-advection technique with the traditional pseudospectral (PS) method. The potential vorticity field, which typically develops steep gradients and evolves into thin filaments, is discretized by level sets separated by contours that are advected in a fully Lagrangian way. The height and divergence fields, which are intrinsically broader in scale, are treated in an Eulerian way: they are discretized on an fixed grid and time stepped with a PS scheme.
In fact, the CASL method is similar to the widely used semi-Lagrangian (SL) method in that material conservation of potential vorticity along particle trajectories is used to determine the potential vorticity at each time step from the previous one. The crucial difference is that, whereas in the CASL method the potential vorticity is merely advected, in the SL method the potential vorticity needs to be interpolated at each time step. This interpolation results in numerical diffusion in the SL method.
By directly comparing the CASL, SL, and PS methods, it is demonstrated that the implicit diffusion associated with potential vorticity interpolation in the SL method and the explicit diffusion required for numerical stability in the PS method seriously degrade the solution accuracy compared with the CASL method. Moreover, it is shown that the CASL method is much more efficient than the SL and PS methods since, for a given solution accuracy, a much coarser grid can be used and hence much faster computations can be performed.
Abstract
A new method for integrating shallow water equations, the contour-advective semi-Lagrangian (CASL) algorithm, is presented. This is the first implementation of a contour method to a system of equations for which exact potential vorticity invertibility does not exist. The new CASL method fuses the recent contour-advection technique with the traditional pseudospectral (PS) method. The potential vorticity field, which typically develops steep gradients and evolves into thin filaments, is discretized by level sets separated by contours that are advected in a fully Lagrangian way. The height and divergence fields, which are intrinsically broader in scale, are treated in an Eulerian way: they are discretized on an fixed grid and time stepped with a PS scheme.
In fact, the CASL method is similar to the widely used semi-Lagrangian (SL) method in that material conservation of potential vorticity along particle trajectories is used to determine the potential vorticity at each time step from the previous one. The crucial difference is that, whereas in the CASL method the potential vorticity is merely advected, in the SL method the potential vorticity needs to be interpolated at each time step. This interpolation results in numerical diffusion in the SL method.
By directly comparing the CASL, SL, and PS methods, it is demonstrated that the implicit diffusion associated with potential vorticity interpolation in the SL method and the explicit diffusion required for numerical stability in the PS method seriously degrade the solution accuracy compared with the CASL method. Moreover, it is shown that the CASL method is much more efficient than the SL and PS methods since, for a given solution accuracy, a much coarser grid can be used and hence much faster computations can be performed.
Abstract
Weather regimes defined through cluster analysis concisely categorize the anomalous regional circulation pattern on any given day. Owing to their persistence and low dimensionality, regimes are increasingly used in subseasonal-to-seasonal prediction and in analysis of climate variability and change. However, a limitation of existing regime classifications for North America is their seasonal dependence, with most existing studies defining regimes for winter only. Here, we normalize the seasonal cycle in daily geopotential height variance and use empirical orthogonal function analysis combined with k-means clustering to define a new set of year-round North American weather regimes: the Pacific Trough, Pacific Ridge, Alaskan Ridge, and Greenland High regimes. We additionally define a “No Regime” state to represent conditions close to climatology. To demonstrate the robustness of the classification, a thorough assessment of the sensitivity of the clustering solution to various methodological choices is provided. The median persistence of all four regimes, obtained without imposing a persistence criterion, is found to be one week, approximately 3 times longer than the median persistence of the No Regime state. Regime-associated temperature and precipitation anomalies are reported, together with the relationship between the regimes and modes of climate variability. We also quantify historical trends in the frequency of the regimes since 1979, finding a decrease in the annual frequency of the Pacific Trough regime and an increase in the summertime frequency of the Greenland High regime. This study serves as a foundation for the future use of these regimes in a variety of weather and climate applications.
Significance Statement
Weather regimes provide a simple way of classifying daily large-scale regional weather patterns into a few predefined types. Existing methods usually define regimes for a specific season (typically winter), which limits their use, or provides only a minimal assessment of their robustness. In this study, we objectively quantify four weather regimes for use year-round over North America, while we classify near-normal conditions as No Regime. The four regimes represent persistent large-scale weather types that last for about a week and occasionally much longer. Our new classification can be applied to subseasonal-to-seasonal forecasts and climate model output to diagnose recurrent weather types across the North American continent.
Abstract
Weather regimes defined through cluster analysis concisely categorize the anomalous regional circulation pattern on any given day. Owing to their persistence and low dimensionality, regimes are increasingly used in subseasonal-to-seasonal prediction and in analysis of climate variability and change. However, a limitation of existing regime classifications for North America is their seasonal dependence, with most existing studies defining regimes for winter only. Here, we normalize the seasonal cycle in daily geopotential height variance and use empirical orthogonal function analysis combined with k-means clustering to define a new set of year-round North American weather regimes: the Pacific Trough, Pacific Ridge, Alaskan Ridge, and Greenland High regimes. We additionally define a “No Regime” state to represent conditions close to climatology. To demonstrate the robustness of the classification, a thorough assessment of the sensitivity of the clustering solution to various methodological choices is provided. The median persistence of all four regimes, obtained without imposing a persistence criterion, is found to be one week, approximately 3 times longer than the median persistence of the No Regime state. Regime-associated temperature and precipitation anomalies are reported, together with the relationship between the regimes and modes of climate variability. We also quantify historical trends in the frequency of the regimes since 1979, finding a decrease in the annual frequency of the Pacific Trough regime and an increase in the summertime frequency of the Greenland High regime. This study serves as a foundation for the future use of these regimes in a variety of weather and climate applications.
Significance Statement
Weather regimes provide a simple way of classifying daily large-scale regional weather patterns into a few predefined types. Existing methods usually define regimes for a specific season (typically winter), which limits their use, or provides only a minimal assessment of their robustness. In this study, we objectively quantify four weather regimes for use year-round over North America, while we classify near-normal conditions as No Regime. The four regimes represent persistent large-scale weather types that last for about a week and occasionally much longer. Our new classification can be applied to subseasonal-to-seasonal forecasts and climate model output to diagnose recurrent weather types across the North American continent.
