Search Results
You are looking at 1 - 10 of 19 items for
- Author or Editor: Rodrigo Caballero x
- Refine by Access: All Content x
Abstract
Recent work using both simplified and comprehensive GCMs has shown that poleward moisture transport across midlatitudes follows Clausius–Clapeyron scaling at temperatures close to modern, but that it reaches a maximum at sufficiently elevated temperatures and then decreases with further warming. This study explores the reasons for this nonmonotonic behavior using a sequence of NCAR Community Atmosphere Model, version 3 (CAM3) simulations in an aquaplanet configuration spanning a broad range of climates. No significant change is found in the scale, structure, or organization of midlatitude eddies across these simulations. Instead, the high-temperature decrease in poleward moisture transport is attributed to the combined effect of decreasing eddy velocities and contracting mixing lengths. The contraction in mixing length is, in turn, a consequence of the decreasing eddy velocities in combination with constant eddy decorrelation time scales.
Abstract
Recent work using both simplified and comprehensive GCMs has shown that poleward moisture transport across midlatitudes follows Clausius–Clapeyron scaling at temperatures close to modern, but that it reaches a maximum at sufficiently elevated temperatures and then decreases with further warming. This study explores the reasons for this nonmonotonic behavior using a sequence of NCAR Community Atmosphere Model, version 3 (CAM3) simulations in an aquaplanet configuration spanning a broad range of climates. No significant change is found in the scale, structure, or organization of midlatitude eddies across these simulations. Instead, the high-temperature decrease in poleward moisture transport is attributed to the combined effect of decreasing eddy velocities and contracting mixing lengths. The contraction in mixing length is, in turn, a consequence of the decreasing eddy velocities in combination with constant eddy decorrelation time scales.
Abstract
The statistical equilibration of baroclinic waves in a two-level quasigeostrophic model subject to forcing and dissipation is studied. The model employed may be formulated in either spherical or Cartesian geometry and is restricted to a midlatitude channel. Parameters are chosen so that only up to three waves can become supercritical (one planetary- and two synoptic-scale waves). It is found that both geometries exhibit essentially two equilibration regimes as the forcing temperature gradient varies. At low forcing, the planetary-scale wave is not excited while the two synoptic-scale waves equilibrate with steady finite amplitude. In this regime, the equilibrated temperature gradient is sensitive to forcing; the authors argue that this is due to the barotropic governor effect. At higher forcing, the planetary wave becomes active and the solution is aperiodic. In this regime, the planetary wave acts to reduce the barotropic shear spun up by the synoptic waves, thereby limiting the role of the barotropic governor; the equilibrated temperature gradient then becomes much less sensitive to forcing. The Cartesian and spherical cases differ both in the structure of the equilibrated state and in the strength of the barotropic governor (which is greater on the sphere). These differences are related to the geometric curvature terms and not to the meridional variation of β.
Abstract
The statistical equilibration of baroclinic waves in a two-level quasigeostrophic model subject to forcing and dissipation is studied. The model employed may be formulated in either spherical or Cartesian geometry and is restricted to a midlatitude channel. Parameters are chosen so that only up to three waves can become supercritical (one planetary- and two synoptic-scale waves). It is found that both geometries exhibit essentially two equilibration regimes as the forcing temperature gradient varies. At low forcing, the planetary-scale wave is not excited while the two synoptic-scale waves equilibrate with steady finite amplitude. In this regime, the equilibrated temperature gradient is sensitive to forcing; the authors argue that this is due to the barotropic governor effect. At higher forcing, the planetary wave becomes active and the solution is aperiodic. In this regime, the planetary wave acts to reduce the barotropic shear spun up by the synoptic waves, thereby limiting the role of the barotropic governor; the equilibrated temperature gradient then becomes much less sensitive to forcing. The Cartesian and spherical cases differ both in the structure of the equilibrated state and in the strength of the barotropic governor (which is greater on the sphere). These differences are related to the geometric curvature terms and not to the meridional variation of β.
Abstract
This paper examines the trajectories followed by intense intrusions of moist air into the Arctic polar region during autumn and winter and their impact on local temperature and sea ice concentration. It is found that the vertical structure of the warming associated with moist intrusions is bottom amplified, corresponding to a transition of local conditions from a “cold clear” state with a strong inversion to a “warm opaque” state with a weaker inversion. In the marginal sea ice zone of the Barents Sea, the passage of an intrusion also causes a retreat of the ice margin, which persists for many days after the intrusion has passed. The authors find that there is a positive trend in the number of intrusion events crossing 70°N during December and January that can explain roughly 45% of the surface air temperature and 30% of the sea ice concentration trends observed in the Barents Sea during the past two decades.
Abstract
This paper examines the trajectories followed by intense intrusions of moist air into the Arctic polar region during autumn and winter and their impact on local temperature and sea ice concentration. It is found that the vertical structure of the warming associated with moist intrusions is bottom amplified, corresponding to a transition of local conditions from a “cold clear” state with a strong inversion to a “warm opaque” state with a weaker inversion. In the marginal sea ice zone of the Barents Sea, the passage of an intrusion also causes a retreat of the ice margin, which persists for many days after the intrusion has passed. The authors find that there is a positive trend in the number of intrusion events crossing 70°N during December and January that can explain roughly 45% of the surface air temperature and 30% of the sea ice concentration trends observed in the Barents Sea during the past two decades.
Abstract
Equatorial superrotation is commonly observed in simulations of Earth and planetary climates, but it is almost without exception found to occur only at upper levels, with zero or easterly winds at the surface. Surface superrotation—a state with climatological zonal-mean westerlies at the equatorial surface—would lead to a major reorganization of the tropical ocean circulation with important consequences for global climate. Here, we examine the mechanisms that give rise to surface superrotation. We identify four theoretical scenarios under which surface superrotation may be achieved. Using an axisymmetric model forced by prescribed zonal-mean torques, we provide concrete examples of surface superrotation under all four scenarios. We also find that we can induce surface superrotation in a full-complexity atmospheric general circulation model, albeit in an extreme parameter range (in particular, convective momentum transport is artificially increased by almost an order of magnitude). We conclude that a transition to surface superrotation is unlikely in Earthlike climates, including ancient or future warm climates, though this conclusion is subject to the currently large uncertainties in the parameterization of convective momentum transport.
Abstract
Equatorial superrotation is commonly observed in simulations of Earth and planetary climates, but it is almost without exception found to occur only at upper levels, with zero or easterly winds at the surface. Surface superrotation—a state with climatological zonal-mean westerlies at the equatorial surface—would lead to a major reorganization of the tropical ocean circulation with important consequences for global climate. Here, we examine the mechanisms that give rise to surface superrotation. We identify four theoretical scenarios under which surface superrotation may be achieved. Using an axisymmetric model forced by prescribed zonal-mean torques, we provide concrete examples of surface superrotation under all four scenarios. We also find that we can induce surface superrotation in a full-complexity atmospheric general circulation model, albeit in an extreme parameter range (in particular, convective momentum transport is artificially increased by almost an order of magnitude). We conclude that a transition to surface superrotation is unlikely in Earthlike climates, including ancient or future warm climates, though this conclusion is subject to the currently large uncertainties in the parameterization of convective momentum transport.
Abstract
The salient features and drivers of wintertime warm and cold spells in the high Arctic are investigated. The analysis is based on the European Centre for Medium-Range Weather Forecasts interim reanalysis dataset. It is found that the warm spells are systematically associated with an intense sea level pressure and geopotential height anomaly dipole, displaying a low over the Arctic basin and a high over northern Eurasia. This configuration creates a natural pathway for extreme moisture influx episodes from the Atlantic sector into the Arctic (herein termed moisture intrusions). Anomalous cyclone frequency at the pole (largely attributable to local cyclogenesis) then favors a deep penetration of these intrusions across the Arctic basin. The large-scale circulation pattern associated with the warm spells further favors the advection of cold air across Siberia, leading to the so-called warm Arctic–cold Eurasia pattern previously discussed in the literature. On the contrary, cold Arctic extremes are associated with a severely reduced frequency of moisture intrusions and a persistent low pressure system over the pole. This effectively isolates the high latitudes from midlatitude air masses, favoring an intense radiative cooling of the polar region.
Abstract
The salient features and drivers of wintertime warm and cold spells in the high Arctic are investigated. The analysis is based on the European Centre for Medium-Range Weather Forecasts interim reanalysis dataset. It is found that the warm spells are systematically associated with an intense sea level pressure and geopotential height anomaly dipole, displaying a low over the Arctic basin and a high over northern Eurasia. This configuration creates a natural pathway for extreme moisture influx episodes from the Atlantic sector into the Arctic (herein termed moisture intrusions). Anomalous cyclone frequency at the pole (largely attributable to local cyclogenesis) then favors a deep penetration of these intrusions across the Arctic basin. The large-scale circulation pattern associated with the warm spells further favors the advection of cold air across Siberia, leading to the so-called warm Arctic–cold Eurasia pattern previously discussed in the literature. On the contrary, cold Arctic extremes are associated with a severely reduced frequency of moisture intrusions and a persistent low pressure system over the pole. This effectively isolates the high latitudes from midlatitude air masses, favoring an intense radiative cooling of the polar region.
Abstract
This paper examines the wintertime northward moisture flux at 70°N from 1981–2005 in 31 of the CMIP5 models compared with the ERA-Interim reanalysis product. The models’ total zonally integrated northward moisture flux is found to agree reasonably well with the reanalysis, but with large compensating regional biases. Specifically, the models systematically underpredict the moisture flux in the Atlantic sector and overpredict it in the Pacific sector. The biases are predominantly due to misrepresentation of extreme moisture flux events, which are known to exert a significant control on Arctic climate. Biases in these high-intensity fluxes are almost entirely contributed by biases in the meridional velocity, suggesting a link with biases in storm-track activity at lower latitudes. The extent to which the deficit of moisture intrusions in the Atlantic sector and excess in the Pacific sector may account for biases in the climate of the respective sectors is assessed. Biases in the frequency of moisture intrusions explain roughly 17% of surface temperature and 24% of surface downward longwave radiation biases in the Atlantic sector, and about 14% and 16% of the gradient in these respective biases between the two sectors. The predicted bias gradients, while small in amplitude, are very highly correlated with the true bias gradients in the models, suggesting that the temperature bias directly induced by misrepresented intrusion statistics may be strongly amplified by sea ice feedback.
Abstract
This paper examines the wintertime northward moisture flux at 70°N from 1981–2005 in 31 of the CMIP5 models compared with the ERA-Interim reanalysis product. The models’ total zonally integrated northward moisture flux is found to agree reasonably well with the reanalysis, but with large compensating regional biases. Specifically, the models systematically underpredict the moisture flux in the Atlantic sector and overpredict it in the Pacific sector. The biases are predominantly due to misrepresentation of extreme moisture flux events, which are known to exert a significant control on Arctic climate. Biases in these high-intensity fluxes are almost entirely contributed by biases in the meridional velocity, suggesting a link with biases in storm-track activity at lower latitudes. The extent to which the deficit of moisture intrusions in the Atlantic sector and excess in the Pacific sector may account for biases in the climate of the respective sectors is assessed. Biases in the frequency of moisture intrusions explain roughly 17% of surface temperature and 24% of surface downward longwave radiation biases in the Atlantic sector, and about 14% and 16% of the gradient in these respective biases between the two sectors. The predicted bias gradients, while small in amplitude, are very highly correlated with the true bias gradients in the models, suggesting that the temperature bias directly induced by misrepresented intrusion statistics may be strongly amplified by sea ice feedback.
Abstract
Strong eastward jets at the equator have been observed in many planetary atmospheres and simulated in numerical models of varying complexity. However, the nature of the transition from a conventional state of the general circulation, with easterlies or weak westerlies in the tropics, to such a superrotating state remains unclear. Is it abrupt or continuous? This question may have far-reaching consequences, as it may provide a mechanism for abrupt climate change in a planetary atmosphere, both through the loss of stability of the conventional circulation and through potential noise-induced transitions in the bistability range. We study two previously suggested feedbacks that may lead to bistability between a conventional and a superrotating state: the Hadley cell feedback and a wave–jet resonance feedback. We delineate the regime of applicability of these two mechanisms in a simple model of zonal acceleration budget at the equator. Then we show using numerical simulations of the axisymmetric primitive equations that the wave–jet resonance feedback indeed leads to robust bistability, while the bistability governed by the Hadley cell feedback, although observed in our numerical simulations, is much more fragile in a multilevel model.
Abstract
Strong eastward jets at the equator have been observed in many planetary atmospheres and simulated in numerical models of varying complexity. However, the nature of the transition from a conventional state of the general circulation, with easterlies or weak westerlies in the tropics, to such a superrotating state remains unclear. Is it abrupt or continuous? This question may have far-reaching consequences, as it may provide a mechanism for abrupt climate change in a planetary atmosphere, both through the loss of stability of the conventional circulation and through potential noise-induced transitions in the bistability range. We study two previously suggested feedbacks that may lead to bistability between a conventional and a superrotating state: the Hadley cell feedback and a wave–jet resonance feedback. We delineate the regime of applicability of these two mechanisms in a simple model of zonal acceleration budget at the equator. Then we show using numerical simulations of the axisymmetric primitive equations that the wave–jet resonance feedback indeed leads to robust bistability, while the bistability governed by the Hadley cell feedback, although observed in our numerical simulations, is much more fragile in a multilevel model.
Abstract
The midlatitude poleward atmospheric energy transport increases in radiatively forced simulations of warmed climates across a range of models from comprehensive coupled general circulation models (GCMs) to idealized aquaplanet moist GCMs to diffusive moist energy balance models. These increases have been rationalized from two perspectives. The energetic (or radiative) perspective takes the atmospheric energy budget and decomposes energy flux changes (radiative forcing, feedbacks, or surface fluxes) to determine the energy transport changes required by the budget. The diffusive perspective takes the net effect of atmospheric macroturbulence to be a diffusive energy transport down-gradient, so transport changes can arise from changes in mean energy gradients or turbulent diffusivity. Here, we compare these perspectives in idealized moist, gray-radiation GCM simulations over a wide range of climates. The energetic perspective has a dominant role for radiative forcing in this GCM, with cancellation between the temperature feedback components that account for the GCM’s nonmonotonic energy transport changes in response to warming. Comprehensive CMIP5 simulations have similarities in the Northern Hemisphere to the idealized GCM, although a comprehensive GCM over several CO2 doublings has a distinctly different feedback evolution structure. The diffusive perspective requires a non-constant diffusivity to account for the idealized GCM-simulated changes, with important roles for the eddy velocity, dry static stability, and horizontal energy gradients. Beyond diagnostic analysis, GCM-independent a priori theories for components of the temperature feedback are presented that account for changes without knowledge of a perturbed climate state, suggesting that the energetic perspective is the more parsimonious one.
Abstract
The midlatitude poleward atmospheric energy transport increases in radiatively forced simulations of warmed climates across a range of models from comprehensive coupled general circulation models (GCMs) to idealized aquaplanet moist GCMs to diffusive moist energy balance models. These increases have been rationalized from two perspectives. The energetic (or radiative) perspective takes the atmospheric energy budget and decomposes energy flux changes (radiative forcing, feedbacks, or surface fluxes) to determine the energy transport changes required by the budget. The diffusive perspective takes the net effect of atmospheric macroturbulence to be a diffusive energy transport down-gradient, so transport changes can arise from changes in mean energy gradients or turbulent diffusivity. Here, we compare these perspectives in idealized moist, gray-radiation GCM simulations over a wide range of climates. The energetic perspective has a dominant role for radiative forcing in this GCM, with cancellation between the temperature feedback components that account for the GCM’s nonmonotonic energy transport changes in response to warming. Comprehensive CMIP5 simulations have similarities in the Northern Hemisphere to the idealized GCM, although a comprehensive GCM over several CO2 doublings has a distinctly different feedback evolution structure. The diffusive perspective requires a non-constant diffusivity to account for the idealized GCM-simulated changes, with important roles for the eddy velocity, dry static stability, and horizontal energy gradients. Beyond diagnostic analysis, GCM-independent a priori theories for components of the temperature feedback are presented that account for changes without knowledge of a perturbed climate state, suggesting that the energetic perspective is the more parsimonious one.
Abstract
Current estimates of the height of the Laurentide Ice Sheet (LIS) at the Last Glacial Maximum (LGM) range from around 3000 to 4500 m. Modeling studies of the LGM, using low-end estimates of the LIS height, show a relatively weak and northeastward-tilted winter jet in the North Atlantic, similar to the modern jet, while simulations with high-end LIS elevations show a much more intense and zonally oriented jet. Here, an explanation for this response of the Atlantic circulation is sought using a sequence of LGM simulations spanning a broad range of LIS elevations. It is found that increasing LIS height favors planetary wave breaking and nonlinear reflection in the subtropical North Atlantic. For high LIS elevations, planetary wave reflection becomes sufficiently prevalent that a poleward-directed flux of wave activity appears in the climatology over the midlatitude North Atlantic. This entails a zonalization of the stationary wave phase lines and thus of the midlatitude jet.
Abstract
Current estimates of the height of the Laurentide Ice Sheet (LIS) at the Last Glacial Maximum (LGM) range from around 3000 to 4500 m. Modeling studies of the LGM, using low-end estimates of the LIS height, show a relatively weak and northeastward-tilted winter jet in the North Atlantic, similar to the modern jet, while simulations with high-end LIS elevations show a much more intense and zonally oriented jet. Here, an explanation for this response of the Atlantic circulation is sought using a sequence of LGM simulations spanning a broad range of LIS elevations. It is found that increasing LIS height favors planetary wave breaking and nonlinear reflection in the subtropical North Atlantic. For high LIS elevations, planetary wave reflection becomes sufficiently prevalent that a poleward-directed flux of wave activity appears in the climatology over the midlatitude North Atlantic. This entails a zonalization of the stationary wave phase lines and thus of the midlatitude jet.
