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Rodrigo Caballero - One of the best experts on this subject based on the ideXlab platform.

  • atmospheric bistability and abrupt transitions to superrotation wave jet resonance and Hadley Cell feedbacks
    Journal of the Atmospheric Sciences, 2020
    Co-Authors: Corentin Herbert, Rodrigo Caballero, Freddy Bouchet
    Abstract:

    AbstractStrong 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 co...

  • atmospheric bistability and abrupt transitions to superrotation wave jet resonance and Hadley Cell feedbacks
    arXiv: Atmospheric and Oceanic Physics, 2019
    Co-Authors: Corentin Herbert, Rodrigo Caballero, Freddy Bouchet
    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 feedbacks which 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.

  • the impact of methane thermodynamics on seasonal convection and circulation in a model titan atmosphere
    Icarus, 2009
    Co-Authors: Jonathan L Mitchell, Raymond T Pierrehumbert, Dargan M W Frierson, Rodrigo Caballero
    Abstract:

    We identify mechanisms controlling the distribution of methane convection and large-scale circulation in a simplified, axisymmetric model atmosphere of Titan forced by gray radiation and moist (methane) convection. The large-scale overturning circulation, or Hadley Cell, is global in latitudinal extent and provides fundamental control of precipitation and tropospheric winds. The precipitating, large-scale updraft regularly oscillates in latitude with seasons. The distance of greatest poleward excursion of the Hadley Cell updraft is set by the mass of the convective layer of the atmosphere; convection efficiently communicates seasonal warming of the surface through the cold and dense lower atmosphere, increasing the heat capacity of the system. The presence of deep, precipitating convection introduces three effects relative to the case with no methane latent heating: (1) convection is narrowed and enhanced in the large-scale updraft of the Hadley Cell; (2) the latitudinal amplitude of Hadley Cell updraft oscillations is decreased; and (3) a time lag is introduced. These effects are observable in the location and timing of convective methane clouds in Titan’s atmosphere as a function of season. A comparison of simulations over a range of convective regimes with available observations suggest methane thermodynamic–dynamic feedback is important in the Titan climate.

  • Hadley Cell bias in climate models linked to extratropical eddy stress
    Geophysical Research Letters, 2008
    Co-Authors: Rodrigo Caballero
    Abstract:

    [1] Recent theoretical and observational work shows that the Reynolds stresses due to large-scale extratropical eddies play a key role in determining Hadley Cell strength. This implies that errors in the representation of extratropical eddies in climate models could force a bias in the tropical circulation. Here, this hypothesis is assessed using output from 17 coupled climate models, focusing on Northern Hemisphere winter. Inter-model variability in Hadley Cell strength, tropical temperature and tropical humidity is found to be significantly correlated with inter-model differences in stationary eddy stress. Thus, a significant fraction of the tropical climate bias found in current climate models may be forced from the extratropics.

  • role of eddies in the interannual variability of Hadley Cell strength
    Geophysical Research Letters, 2007
    Co-Authors: Rodrigo Caballero
    Abstract:

    [1] There is evidence from simplified atmosphere models that extratropical eddies strongly affect the Hadley Cell mass flux. Here, we use reanalysis data to assess the role of eddy momentum transport in the interannual variability of northern hemisphere winter Hadley Cell strength. We define a Hadley Cell strength index and decompose it into an ENSO-related term and an uncorrelated remainder. Regressing these two time series onto the momentum budget shows that ENSO-related variability involves changes in momentum transport by tropical stationary waves, while non-ENSO variability is related to changes in the extratropical wave flux impinging on the tropics, with stationary waves dominant in the northern (winter) Cell and transients dominant in the southern (summer) Cell. Non-ENSO variability accounts for a large fraction of the interannual variance in Hadley Cell strength, supporting the hypothesis that extratropical eddy stresses drive much of the zonally symmetric variability in the tropics.

Jonathan L Mitchell - One of the best experts on this subject based on the ideXlab platform.

  • solsticial Hadley Cell ascending edge theory from supercriticality
    Journal of the Atmospheric Sciences, 2021
    Co-Authors: Spencer A Hill, Simona Bordoni, Jonathan L Mitchell
    Abstract:

    How far the Hadley circulation's ascending branch extends into the summer hemisphere is a fundamental but incompletely understood characteristic of Earth's climate. Here, we present a predictive, analytical theory for this ascending edge latitude based on the extent of supercritical forcing. Supercriticality sets the minimum extent of a large-scale circulation based on the angular momentum and absolute vorticity distributions of the hypothetical state were the circulation absent. We explicitly simulate this latitude-by-latitude radiative-convective equilibrium (RCE) state. Its depth-averaged temperature profile is suitably captured by a simple analytical approximation that increases linearly with $\sin\varphi$, where $\varphi$ is latitude, from the winter to the summer pole. This, in turn, yields a one-third power-law scaling of the supercritical forcing extent with the thermal Rossby number. In moist and dry idealized GCM simulations under solsticial forcing performed with a wide range of planetary rotation rates, the ascending edge latitudes largely behave according to this scaling.

  • constraints from invariant subtropical vertical velocities on the scalings of Hadley Cell strength and downdraft width with rotation rate
    Journal of the Atmospheric Sciences, 2021
    Co-Authors: Jonathan L Mitchell, Spencer A Hill
    Abstract:

    Weak-temperature-gradient influences from the tropics and quasigeostrophic influences from the extratropics plausibly constrain the subtropical-mean static stability in terrestrial atmospheres. Because mean descent acting on this static stability is a leading-order term in the thermodynamic balance, a state-invariant static stability would impose constraints on the Hadley Cells, which this paper explores in simulations of varying planetary rotation rate. If downdraft-averaged effective heating (the sum of diabatic heating and eddy heat flux convergence) too is invariant, so must be vertical velocity -- an "omega governor." In that case, the Hadley circulation overturning strength and downdraft width must scale identically -- the Cell can strengthen only by widening or weaken only by narrowing. Simulations in two idealized, dry GCMs with a wide range of planetary rotation rates exhibit nearly unchanging downdraft-averaged static stability, effective heating, and vertical velocity, as well as nearly identical scalings of the Hadley Cell downdraft width and strength. In one, eddy stresses set this scaling directly (the Rossby number remains small); in the other, eddy stress and bulk Rossby number changes compensate to yield the same, ({\sim}\Omega^{-1/3}) scaling. The consistency of this power law for Cell width and strength variations may indicate a common driver, and we speculate that Ekman pumping could be the mechanism responsible for this behavior. Extending to moist atmospheres, in an idealized aquaplanet GCM the subtropical static stability is also insensitive to rotation rate but the effective heating and vertical velocity are not.

  • dynamical constraints on the solsticial Hadley Cell ascending edge in earth s macroturbulent atmosphere
    arXiv e-prints, 2020
    Co-Authors: Spencer A Hill, Simona Bordoni, Jonathan L Mitchell
    Abstract:

    How far the ascending branch of the Hadley circulation extends into the summer hemisphere is a fundamental but incompletely understood characteristic of Earth's solsticial general circulation. Here, we present a predictive, analytical theory for this ascending edge latitude based on the extent of so-called supercritical forcing. Supercriticality sets the minimal extent of a large-scale circulation based on the angular momentum and absolute vorticity distributions of the hypothetical state were the circulation absent. Despite its origins in axisymmetric theory, supercriticality constitutes a conceptually valid predictor for the ascending edge even in zonally varying, macroturbulent atmospheres like Earth's, provided certain empirical conditions are met. Numerical simulations of latitude-by-latitude radiative-convective equilibrium (RCE) under Earth's solsticial forcing show that the supercritical forcing extent aligns well with the observed climatological boreal summer ascending edge. A simple analytical approximation to the solsticial RCE temperature profile, from Lindzen and Hou (1988) but with the temperature maximum located far off-equator, accurately approximates the RCE state and thereby the supercritical forcing extent. The accuracy of the resulting analytical predictor for the solsticial ascending edge is confirmed in moist and dry idealized GCMs under solsticial forcing with varying planetary rotation rate, $\Omega$. In particular, in the small-angle limit appropriate for Earth, the simulated ascending edges exhibit the theory's predicted $\Omega^{-2/3}$ scaling.

  • axisymmetric Hadley Cell theory with a fixed tropopause temperature rather than height
    Journal of the Atmospheric Sciences, 2020
    Co-Authors: Spencer A Hill, Simona Bordoni, Jonathan L Mitchell
    Abstract:

    AbstractAxisymmetric Hadley Cell theory has traditionally assumed that the tropopause height (Ht) is uniform and unchanged from its radiative–convective equilibrium (RCE) value by the Cells’ emerge...

  • combining thermodynamic and dynamic perspectives of tropical circulation to constrain the downdraft width of the Hadley Cell
    arXiv: Atmospheric and Oceanic Physics, 2019
    Co-Authors: Jonathan L Mitchell, Spencer A Hill
    Abstract:

    In the Hadley circulation downdraft, to leading order vertical potential temperature advection balances the "effective" heating, comprising the sum of diabatic heating and eddy heat flux divergence, placing a thermodynamic constraint on vertical velocity. Insofar as downdraft-averaged effective heating and static stability do not vary with planetary parameters, neither can vertical velocity --- an "omega governor." Separately, in the eddy-driven (i.e. small-Rossby-number) limit, extratropical eddy stresses also constrain the Cell strength dynamically. We combine these thermodynamic and dynamic mechanisms to derive new and identical scalings for the downdraft width and overturning strength with rotation rate. With the omega governor maintaining fixed vertical velocity, the downdraft must narrow or widen in order to attain the overturning strength dictated by the eddy stresses. We evaluate the validity of this new scaling using model simulations over a broad range of rotation rates and model forcing schemes.

Spencer A Hill - One of the best experts on this subject based on the ideXlab platform.

  • solsticial Hadley Cell ascending edge theory from supercriticality
    Journal of the Atmospheric Sciences, 2021
    Co-Authors: Spencer A Hill, Simona Bordoni, Jonathan L Mitchell
    Abstract:

    How far the Hadley circulation's ascending branch extends into the summer hemisphere is a fundamental but incompletely understood characteristic of Earth's climate. Here, we present a predictive, analytical theory for this ascending edge latitude based on the extent of supercritical forcing. Supercriticality sets the minimum extent of a large-scale circulation based on the angular momentum and absolute vorticity distributions of the hypothetical state were the circulation absent. We explicitly simulate this latitude-by-latitude radiative-convective equilibrium (RCE) state. Its depth-averaged temperature profile is suitably captured by a simple analytical approximation that increases linearly with $\sin\varphi$, where $\varphi$ is latitude, from the winter to the summer pole. This, in turn, yields a one-third power-law scaling of the supercritical forcing extent with the thermal Rossby number. In moist and dry idealized GCM simulations under solsticial forcing performed with a wide range of planetary rotation rates, the ascending edge latitudes largely behave according to this scaling.

  • constraints from invariant subtropical vertical velocities on the scalings of Hadley Cell strength and downdraft width with rotation rate
    Journal of the Atmospheric Sciences, 2021
    Co-Authors: Jonathan L Mitchell, Spencer A Hill
    Abstract:

    Weak-temperature-gradient influences from the tropics and quasigeostrophic influences from the extratropics plausibly constrain the subtropical-mean static stability in terrestrial atmospheres. Because mean descent acting on this static stability is a leading-order term in the thermodynamic balance, a state-invariant static stability would impose constraints on the Hadley Cells, which this paper explores in simulations of varying planetary rotation rate. If downdraft-averaged effective heating (the sum of diabatic heating and eddy heat flux convergence) too is invariant, so must be vertical velocity -- an "omega governor." In that case, the Hadley circulation overturning strength and downdraft width must scale identically -- the Cell can strengthen only by widening or weaken only by narrowing. Simulations in two idealized, dry GCMs with a wide range of planetary rotation rates exhibit nearly unchanging downdraft-averaged static stability, effective heating, and vertical velocity, as well as nearly identical scalings of the Hadley Cell downdraft width and strength. In one, eddy stresses set this scaling directly (the Rossby number remains small); in the other, eddy stress and bulk Rossby number changes compensate to yield the same, ({\sim}\Omega^{-1/3}) scaling. The consistency of this power law for Cell width and strength variations may indicate a common driver, and we speculate that Ekman pumping could be the mechanism responsible for this behavior. Extending to moist atmospheres, in an idealized aquaplanet GCM the subtropical static stability is also insensitive to rotation rate but the effective heating and vertical velocity are not.

  • dynamical constraints on the solsticial Hadley Cell ascending edge in earth s macroturbulent atmosphere
    arXiv e-prints, 2020
    Co-Authors: Spencer A Hill, Simona Bordoni, Jonathan L Mitchell
    Abstract:

    How far the ascending branch of the Hadley circulation extends into the summer hemisphere is a fundamental but incompletely understood characteristic of Earth's solsticial general circulation. Here, we present a predictive, analytical theory for this ascending edge latitude based on the extent of so-called supercritical forcing. Supercriticality sets the minimal extent of a large-scale circulation based on the angular momentum and absolute vorticity distributions of the hypothetical state were the circulation absent. Despite its origins in axisymmetric theory, supercriticality constitutes a conceptually valid predictor for the ascending edge even in zonally varying, macroturbulent atmospheres like Earth's, provided certain empirical conditions are met. Numerical simulations of latitude-by-latitude radiative-convective equilibrium (RCE) under Earth's solsticial forcing show that the supercritical forcing extent aligns well with the observed climatological boreal summer ascending edge. A simple analytical approximation to the solsticial RCE temperature profile, from Lindzen and Hou (1988) but with the temperature maximum located far off-equator, accurately approximates the RCE state and thereby the supercritical forcing extent. The accuracy of the resulting analytical predictor for the solsticial ascending edge is confirmed in moist and dry idealized GCMs under solsticial forcing with varying planetary rotation rate, $\Omega$. In particular, in the small-angle limit appropriate for Earth, the simulated ascending edges exhibit the theory's predicted $\Omega^{-2/3}$ scaling.

  • axisymmetric Hadley Cell theory with a fixed tropopause temperature rather than height
    Journal of the Atmospheric Sciences, 2020
    Co-Authors: Spencer A Hill, Simona Bordoni, Jonathan L Mitchell
    Abstract:

    AbstractAxisymmetric Hadley Cell theory has traditionally assumed that the tropopause height (Ht) is uniform and unchanged from its radiative–convective equilibrium (RCE) value by the Cells’ emerge...

  • combining thermodynamic and dynamic perspectives of tropical circulation to constrain the downdraft width of the Hadley Cell
    arXiv: Atmospheric and Oceanic Physics, 2019
    Co-Authors: Jonathan L Mitchell, Spencer A Hill
    Abstract:

    In the Hadley circulation downdraft, to leading order vertical potential temperature advection balances the "effective" heating, comprising the sum of diabatic heating and eddy heat flux divergence, placing a thermodynamic constraint on vertical velocity. Insofar as downdraft-averaged effective heating and static stability do not vary with planetary parameters, neither can vertical velocity --- an "omega governor." Separately, in the eddy-driven (i.e. small-Rossby-number) limit, extratropical eddy stresses also constrain the Cell strength dynamically. We combine these thermodynamic and dynamic mechanisms to derive new and identical scalings for the downdraft width and overturning strength with rotation rate. With the omega governor maintaining fixed vertical velocity, the downdraft must narrow or widen in order to attain the overturning strength dictated by the eddy stresses. We evaluate the validity of this new scaling using model simulations over a broad range of rotation rates and model forcing schemes.

Dargan M W Frierson - One of the best experts on this subject based on the ideXlab platform.

  • itcz width controls on Hadley Cell extent and eddy driven jet position and their response to warming
    Journal of Climate, 2019
    Co-Authors: Oliver Wattmeyer, Dargan M W Frierson
    Abstract:

    AbstractThe impact of global warming–induced intertropical convergence zone (ITCZ) narrowing onto the higher-latitude circulation is examined in the GFDL Atmospheric Model, version 2.1 (AM2.1), run...

  • the impact of methane thermodynamics on seasonal convection and circulation in a model titan atmosphere
    Icarus, 2009
    Co-Authors: Jonathan L Mitchell, Raymond T Pierrehumbert, Dargan M W Frierson, Rodrigo Caballero
    Abstract:

    We identify mechanisms controlling the distribution of methane convection and large-scale circulation in a simplified, axisymmetric model atmosphere of Titan forced by gray radiation and moist (methane) convection. The large-scale overturning circulation, or Hadley Cell, is global in latitudinal extent and provides fundamental control of precipitation and tropospheric winds. The precipitating, large-scale updraft regularly oscillates in latitude with seasons. The distance of greatest poleward excursion of the Hadley Cell updraft is set by the mass of the convective layer of the atmosphere; convection efficiently communicates seasonal warming of the surface through the cold and dense lower atmosphere, increasing the heat capacity of the system. The presence of deep, precipitating convection introduces three effects relative to the case with no methane latent heating: (1) convection is narrowed and enhanced in the large-scale updraft of the Hadley Cell; (2) the latitudinal amplitude of Hadley Cell updraft oscillations is decreased; and (3) a time lag is introduced. These effects are observable in the location and timing of convective methane clouds in Titan’s atmosphere as a function of season. A comparison of simulations over a range of convective regimes with available observations suggest methane thermodynamic–dynamic feedback is important in the Titan climate.

  • response of the zonal mean atmospheric circulation to el nino versus global warming
    Journal of Climate, 2008
    Co-Authors: Gang Chen, Dargan M W Frierson
    Abstract:

    Abstract The change in the zonal mean atmospheric circulation under global warming is studied in comparison with the response to El Nino forcing, by examining the model simulations conducted for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. In contrast to the strengthening and contraction of the Hadley Cell and the equatorward shift of the tropospheric zonal jets in response to El Nino, the Hadley Cell weakens and expands poleward, and the jets move poleward in a warmed climate, despite the projected “El Nino–like” enhanced warming over the equatorial central and eastern Pacific. The hydrological impacts of global warming also exhibit distinct patterns over the subtropics and midlatitudes in comparison to the El Nino. Two feasible mechanisms are proposed for the zonal mean circulation response to global warming: 1) The increase in static stability of the subtropical and midlatitude troposphere, a robust result of the quasi-moist adiabatic adjustment to the surface warming,...

  • response of the zonal mean atmospheric circulation to el nino versus global warming
    Journal of Climate, 2008
    Co-Authors: Jian Lu, Gang Chen, Dargan M W Frierson
    Abstract:

    The change in the zonal mean atmospheric circulation under global warming is studied in comparison with the response to El Nino forcing, by examining the model simulations conducted for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. In contrast to the strengthening and contraction of the Hadley Cell and the equatorward shift of the tropospheric zonal jets in response to El Nino, the Hadley Cell weakens and expands poleward, and the jets move poleward in a warmed climate, despite the projected “El Nino–like” enhanced warming over the equatorial central and eastern Pacific. The hydrological impacts of global warming also exhibit distinct patterns over the subtropics and midlatitudes in comparison to the El Nino. Two feasible mechanisms are proposed for the zonal mean circulation response to global warming: 1) The increase in static stability of the subtropical and midlatitude troposphere, a robust result of the quasi-moist adiabatic adjustment to the surface warming, may stabilize the baroclinic eddy growth on the equatorward side of the storm tracks and push the eddy activity and the associated eddy-driven wind and subsidence poleward, leading to the poleward expansion of the Hadley Cell and the shift of midlatitude jets; 2) the strengthening of the midlatitude wind at the upper troposphere and lower stratosphere, arguably a consequence of increases in the meridional temperature gradient near the tropopause level due to the tropospheric warming and tropopause slope, may increase the eastward propagation of the eddies emanating from the midlatitudes, and thus the subtropical region of wave breaking displaces poleward together with the eddy-driven circulation. Both mechanisms are somewhat, if not completely, distinct from those in response to the El Nino condition.

  • width of the Hadley Cell in simple and comprehensive general circulation models
    Geophysical Research Letters, 2007
    Co-Authors: Dargan M W Frierson, Jian Lu, Gang Chen
    Abstract:

    [1] The width of the Hadley Cell is studied over a wide range of climate regimes using both simple and comprehensive atmospheric general circulation models. Aquaplanet, fixed sea surface temperature lower boundary conditions are used in both models to study the response of the Hadley Cell width to changes in both global mean temperature and pole-to-equator temperature gradient. The primary sensitivity of both models is a large expansion of the Hadley Cell with increased mean temperature. The models also exhibit a smaller increase in width with temperature gradient. The Hadley Cell widths agree well with a scaling theory by Held which assumes that the width is determined by the latitude where baroclinic eddies begin to occur. As surface temperatures are warmed, the latitude of baroclinic instability onset is shifted poleward due to increases in the static stability of the subtropics, which is increased in an atmosphere with higher moisture content.

Gang Chen - One of the best experts on this subject based on the ideXlab platform.

  • dependence of atmospheric transport into the arctic on the meridional extent of the Hadley Cell
    Geophysical Research Letters, 2020
    Co-Authors: Huang Yang, Darryn W Waugh, Clara Orbe, Gang Chen
    Abstract:

    Recent studies have shown a large spread in the transport of atmospheric tracers into the Arctic among a suite of chemistry climate models, and have suggested that this is related to the spread in ...

  • delineating the eddy zonal flow interaction in the atmospheric circulation response to climate forcing uniform sst warming in an idealized aquaplanet model
    Journal of the Atmospheric Sciences, 2013
    Co-Authors: Gang Chen, Jian Lu
    Abstract:

    The mechanisms of the atmospheric response to climate forcing are analyzed using an example of uniform SSTwarminginanidealizedaquaplanet model.A 200-memberensembleofexperiments isconductedwithan instantaneousuniformSSTwarming.Thezonal meancirculationchangesdisplaya rapidpolewardshift in the midlatitude eddy-driven westerlies and the edge of the Hadley Cell circulation and a slow equatorward contraction of the circulation in the deep tropics. The shift of the poleward edge of the Hadley Cell is predominantlycontrolledbytheeddymomentumflux.Italsoshiftstheeddy-drivenwesterliesagainstthesurface friction, at a rate much faster than the expectation from the natural variability of the eddy-driven jet (i.e., the e-folding time scale of the annular mode), with much less feedback between the eddies and zonal flow. The transient eddy‐zonal flow interactions are delineated using a newly developed finite-amplitude wave activity diagnostic of Nakamura. Applying it to the transient ensemble response to uniform SST warming reveals that the eddy-driven westerlies are shifted poleward by permitting more upward wave propagation in the middle and upper troposphere rather than reducing the lower-tropospheric baroclinicity. The increased upward wave propagation is attributed to a reduction in eddy dissipation of wave activity as a result of a weaker meridional potential vorticity (PV) gradient. The reduction allows more waves to propagate away from the latitudes of baroclinic generation, which, in turn, leads to more poleward momentum flux and a poleward shift of eddy-driven winds and Hadley Cell edge.

  • response of the zonal mean atmospheric circulation to el nino versus global warming
    Journal of Climate, 2008
    Co-Authors: Gang Chen, Dargan M W Frierson
    Abstract:

    Abstract The change in the zonal mean atmospheric circulation under global warming is studied in comparison with the response to El Nino forcing, by examining the model simulations conducted for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. In contrast to the strengthening and contraction of the Hadley Cell and the equatorward shift of the tropospheric zonal jets in response to El Nino, the Hadley Cell weakens and expands poleward, and the jets move poleward in a warmed climate, despite the projected “El Nino–like” enhanced warming over the equatorial central and eastern Pacific. The hydrological impacts of global warming also exhibit distinct patterns over the subtropics and midlatitudes in comparison to the El Nino. Two feasible mechanisms are proposed for the zonal mean circulation response to global warming: 1) The increase in static stability of the subtropical and midlatitude troposphere, a robust result of the quasi-moist adiabatic adjustment to the surface warming,...

  • response of the zonal mean atmospheric circulation to el nino versus global warming
    Journal of Climate, 2008
    Co-Authors: Jian Lu, Gang Chen, Dargan M W Frierson
    Abstract:

    The change in the zonal mean atmospheric circulation under global warming is studied in comparison with the response to El Nino forcing, by examining the model simulations conducted for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. In contrast to the strengthening and contraction of the Hadley Cell and the equatorward shift of the tropospheric zonal jets in response to El Nino, the Hadley Cell weakens and expands poleward, and the jets move poleward in a warmed climate, despite the projected “El Nino–like” enhanced warming over the equatorial central and eastern Pacific. The hydrological impacts of global warming also exhibit distinct patterns over the subtropics and midlatitudes in comparison to the El Nino. Two feasible mechanisms are proposed for the zonal mean circulation response to global warming: 1) The increase in static stability of the subtropical and midlatitude troposphere, a robust result of the quasi-moist adiabatic adjustment to the surface warming, may stabilize the baroclinic eddy growth on the equatorward side of the storm tracks and push the eddy activity and the associated eddy-driven wind and subsidence poleward, leading to the poleward expansion of the Hadley Cell and the shift of midlatitude jets; 2) the strengthening of the midlatitude wind at the upper troposphere and lower stratosphere, arguably a consequence of increases in the meridional temperature gradient near the tropopause level due to the tropospheric warming and tropopause slope, may increase the eastward propagation of the eddies emanating from the midlatitudes, and thus the subtropical region of wave breaking displaces poleward together with the eddy-driven circulation. Both mechanisms are somewhat, if not completely, distinct from those in response to the El Nino condition.

  • width of the Hadley Cell in simple and comprehensive general circulation models
    Geophysical Research Letters, 2007
    Co-Authors: Dargan M W Frierson, Jian Lu, Gang Chen
    Abstract:

    [1] The width of the Hadley Cell is studied over a wide range of climate regimes using both simple and comprehensive atmospheric general circulation models. Aquaplanet, fixed sea surface temperature lower boundary conditions are used in both models to study the response of the Hadley Cell width to changes in both global mean temperature and pole-to-equator temperature gradient. The primary sensitivity of both models is a large expansion of the Hadley Cell with increased mean temperature. The models also exhibit a smaller increase in width with temperature gradient. The Hadley Cell widths agree well with a scaling theory by Held which assumes that the width is determined by the latitude where baroclinic eddies begin to occur. As surface temperatures are warmed, the latitude of baroclinic instability onset is shifted poleward due to increases in the static stability of the subtropics, which is increased in an atmosphere with higher moisture content.