Mechanisms of Future Predicted Changes in the Zonal Mean MidLatitude Circulation
Abstract
Stateoftheart climate models predict the zonal mean midlatitude circulation will undergo a poleward shift and seasonally and hemispherically dependent intensity changes in the future. Here I review the mechanisms put forward to explain the zonal mean midlatitude circulation response to increased carbon dioxide (CO_{2}) concentration. The mechanisms are grouped according to their thermodynamic starting point, which are thought to arise from processes independent of the zonal mean midlatitude circulation response. There are 24 mechanisms and 8 thermodynamic starting points: (i) increased latent heat release aloft in the tropics, (ii) increased dry static stability and tropopause height outside the tropics, (iii) radiative cooling of the stratosphere, (iv) Hadley cell expansion, (v) increased specific humidity following the ClausiusClapeyron relation, (vi) cloud radiative effect changes, (vii) turbulent surface heat flux changes, and (viii) decreased surface meridional temperature gradient. I argue progress can be made by testing the thermodynamic starting points. I review recent tests of the increased latent heat release aloft in the tropics starting point, i.e., prescribing diabatic perturbations, quantifying the transient response to an abrupt CO_{2} increase and imposing latitudinally dependent CO_{2} concentration. Finally, I provide a future outlook for improving our understanding of predicted changes in the zonal mean midlatitude circulation.
Keywords
Circulation Climate changeIntroduction
The zonal mean midlatitude circulation on Earth encompasses surface westerlies, upper level jet streams, storm tracks, eddies or Rossby waves, and the Ferrel circulation. Decades of research has established that the midlatitudes is an eddydominated regime. More specifically, eddies dominate (1) the surface westerlies via eddy momentum flux convergence [50], (2) the thermally indirect Ferrel circulation via eddy heat fluxes [28, 48], and (3) the transport of moist static energy (MSE, i.e., the sum of dry static and latent energy) between the equator and the pole [62].
For over four decades stateoftheart climate models have predicted zonal mean midlatitude circulation changes in response to increased greenhouse gas (GHG) concentrations, primarily increased carbon dioxide (CO_{2}) concentration. The earliest prediction comes from Manabe and Wetherald [37] who showed zonal mean eddy kinetic energy (EKE) in the upper troposphere shifts poleward in response to a doubling of CO_{2} (see their Fig. 11). Hall et al. [18] showed a poleward shift also occurs for other storm track metrics, including lowlevel eddy temperature and moisture fluxes. The poleward shift of the zonal mean storm tracks has been reproduced in more recent climate model intercomparisons and is largest in the Southern Hemisphere (SH) [1, 2, 8]. In addition, storm track intensity increases in response to increased CO_{2} in the SH [47]. However, in the Northern Hemisphere (NH), the intensity changes are seasonally dependent: intensity increases during winter but decreases during summer [47].
The predicted zonal mean midlatitude circulation response is important because it is connected to changes in the hydrological cycle, individual storms, and extreme events [51]. To date, there is some observational evidence for the poleward shift of the zonal mean midlatitude circulation [3, 15] and the weakening of the NH storm track during summer [12]. However, in the SH, the observed poleward shift during SH summer most likely reflects the circulation response to ozone depletion [30].
Simulation does not equal understanding [21]. Confidence in the emergent zonal mean midlatitude circulation response to increased CO_{2} predicted by stateoftheart climate models requires a physically based explanation of the underlying mechanisms. Physically based explanations can be achieved using analytical models (equations), scaling arguments, or a hierarchy of numerical models of varying physical complexity. A success of climate theory has been the physically based explanation of several emergent zonal mean thermodynamic responses to increased CO_{2} predicted by stateoftheart climate models (Fig. 1, top), e.g., amplified warming aloft in the tropics, amplified warming at the surface in the Arctic, rising of the tropopause [13, 20, 64], and the wetgetwetter and drygetdrier response of the hydrological cycle [22]. Unfortunately, similar success does not exist for the response of the zonal mean midlatitude circulation to increased CO_{2} (Fig. 1, bottom). Instead, multiple mechanisms have been proposed, which have not been sufficiently tested.
Here I review the mechanisms that have been proposed to explain the predicted zonal mean midlatitude circulation response to increased CO_{2}. There are many possible ways to define and organize the mechanisms. I choose to define a mechanism broadly, i.e., I include all idealized modeling results that demonstrate a midlatitude circulation response by changing some factor even if the detailed chain of causality is not clear. I choose to organize the mechanisms according to their thermodynamic starting point, which are thought to arise from processes independent of the zonal mean midlatitude circulation response, e.g., tropical convective adjustment or stratospheric radiative adjustment. Before reviewing the mechanisms, I briefly review some relevant frameworks. I then review the mechanisms proposed to explain the (1) poleward shift, (2) intensity increase, and (3) intensity decrease response of the zonal mean midlatitude circulation to increased CO_{2}. I argue progress can be made by testing the thermodynamic starting points. I review recent tests of the thermodynamic starting point tied to tropical convective adjustment. Finally, I provide a future outlook for understanding the zonal mean midlatitude circulation response to increased CO_{2}.
Background on Relevant Frameworks
Before reviewing the mechanisms that have been proposed to explain the zonal mean midlatitude circulation response to increased CO_{2}, it is useful to review some relevant dynamical frameworks.
Barotropic Rossby Wave Dynamics
Baroclinic Rossby Wave Dynamics
Potential Vorticity Dynamics
Mean Available Potential Energy
Energy Budget
Recently, Barpanda and Shaw [2] and Shaw et al. [53] derived a MSE framework that connects changes in storm track position and intensity to (1) changes in energy input to the atmosphere (shortwave absorption, surface heat fluxes and outgoing longwave radiation or OLR) and (2) changes in the MSE flux by the stationary circulation (mean meridional circulation and stationary eddies). More specifically, a change in storm track intensity can be written as follows:
Mechanisms Explaining the Poleward Shift of the Zonal Mean MidLatitude Circulation
Here I review the mechanisms that have been proposed to explain the poleward shift of the zonal mean midlatitude circulation in response to increased CO_{2}. The mechanisms are grouped according to their thermodynamic starting point.
Increased Latent Heat Release Aloft in the Tropics
Butler et al. [4] showed a prescribed diabatic heating in the tropical upper troposphere shifts the storm track and jet poleward in dry dynamical core simulations. Butler et al. [5] argued the prescribed diabatic heating results in a poleward shift of the mean PV gradient, which enhances the eddy PV flux, i.e., \( {\varDelta } \overline {v^{\prime }P^{\prime }} \approx  K_{\text {eff}} \partial {\varDelta }\overline {P}/\partial y\) assuming ΔK_{eff} ≈ 0 (see Eq. 12), on the poleward side of the jet shifting the eddies and circulation poleward. Figure 8 in [5] is a schematic of their mechanism.
Riviere [49] showed that when the meridional temperature gradient in the upper troposphere is increased in a threelevel quasigeostrophic model, long waves become more unstable, and the increased eddy length scale favors anticyclonic wave breaking and a poleward shift of the jet.
Kidston and Vallis [27] and Lorenz [33] used barotropic models to show that the acceleration of the zonal wind aloft reduces the meridional gradient of the absolute vorticity on the flanks of the jet (\({\varDelta } \beta ^{*}= \partial ^{2} {\varDelta } \overline {u}/\partial y^{2}<0\), see Eq. 8), which affects the turning latitude (where ℓ = 0) on the poleward side of the jet. There is increased wave reflection from the poleward side of the jet, equatorward wave propagation (\({\varDelta } \ell \approx {\varDelta } \beta ^{*}/ 2\ell (\overline {u}c) > 0\) in the SH, see Eq. 7), a poleward momentum flux response (\({\varDelta } \overline {u^{\prime }v^{\prime }} \approx A^{2}k {\varDelta } \ell /2 < 0\) in the SH, see Eq. 6), and a poleward shift of the jet. This is analogous to scenario 1 in Fig. 2.
Lu et al. [36] showed that the eddy PV flux response to prescribed diabatic heating in the tropical upper troposphere in a dry dynamical core model leads to a poleward shift of the jet due to a change in effective diffusivity (\({\varDelta } \overline {v^{\prime }P^{\prime }} \approx  {\varDelta } K_{\text {eff}} \partial \overline {P}/\partial y\), see Eq. 12) rather than due to a change in the mean PV gradient, which was argued to be the important factor by [5] (see above). Their results were based on the finite amplitude wave activity budget. Figure 10 in [36] is a schematic of their mechanism.
Increased Dry Static Stability and Tropopause Height Outside the Tropics
Lorenz and DeWeaver [32] showed that artificially raising the tropopause poleward of the climatological jet shifts the jet poleward in dry dynamical core simulations. Interestingly, raising the tropopause equatorward of the climatological jet shifts the jet equatorward.
Frierson [14] argued increased subtropical dry static stability reduces eddy generation on the equatorward side of the jet (ΔA < 0 where ℓ > 0 in the SH), leading to an equatorward momentum flux response (\({\varDelta } \overline {u^{\prime }v^{\prime }} \approx Ak\ell {\varDelta } A > 0\) in the SH, see Eq. 6) and a poleward jet shift. This is analogous to scenario 2 in Fig. 2.
Chen et al. [10] and Lu et al. [35] argued that the increased extratropical tropopause height increases the meridional temperature gradient and zonal wind, and leads to faster eddy phase speeds (\({\varDelta } (\overline {u}  c) < 0\)), a poleward shift of the critical latitude (where \(\overline {u}=c\)) on the equatorward side of the jet, equatorward wave propagation (\({\varDelta } \ell \approx \beta ^{*}{\varDelta } (\overline {u}c)/(2\ell (\overline {u}c)^{2}>0\) in the SH, see Eq. 7), a poleward shift of the latitude of maximum absolute value of the eddy momentum flux, and a poleward shift of the jet. This is analogous to scenario 3 in Fig. 2.
Kidston et al. [25, 26] argued that increased dry static stability causes an increase in the eddy length scale, i.e., Rossby radius of deformation L_{R} = NH/f, thus Δk < 0, a decrease in the eddy phase speed relative to the background mean wind, a poleward shift of the critical line (where \(\overline {u}=c\)) on the poleward side of the jet, a poleward momentum flux response (\({\varDelta } \overline {u^{\prime }v^{\prime }} \approx A\ell {\varDelta } k < 0\) in the SH, see Eq. 6), leading eddies to dissipate further poleward, and shifting the jet poleward. This is analogous to scenario 1 in Fig. 2.
Radiative Cooling of the Stratosphere
Sigmond et al. [55] showed that doubling CO_{2} only in the middle atmosphere (above the climatological tropopause) shifts the jet poleward in a prescribed sea surface temperature (SST) atmospheric general circulation model (AGCM).
Butler et al. [4] showed that a prescribed diabatic cooling in the polar lower stratosphere shifts the storm track and jet poleward in dry dynamical core simulations.
Wu et al. [68] showed that the transient evolution of the poleward shift of the jet in response to CO_{2} doubling in an AGCM was dominated by changes in the stratosphere. More specifically, the subpolar lower stratospheric zonal wind response increased the index of refraction via the mean PV gradient (\({\varDelta } n^{2} \approx (\partial {\varDelta } \overline {P}/\partial y)/(\overline {u}c)/a>0\), see Eq. 9) on the poleward side of the jet such that eddies propagate further poleward and shift the jet poleward.
Hadley Cell Expansion
Increased CO_{2} leads to a poleward shift of the Hadley cell edge [34], and several theories have been proposed to explain its poleward shift [57]. Mbengue and Schneider [39, 40] showed the Hadley cell and storm track shifts were related in dry dynamical core simulations via MAPE changes that were dominated by the meridional temperature gradient response (\({\varDelta } \text {MAPE} \approx \frac {c_{p} p_{0}}{24g} (\sigma _{s}  \sigma _{t}^{\max \limits }) {L_{Z}^{2}} [ {\varGamma }]_{v} {\Delta }[ \{\partial _{y} \overline {T}\} ]^{2}_{v} \), see Eq. 14). Mbengue and Schenider [41] showed the expansion of the Hadley cell shifts the nearsurface meridional temperature gradient poleward, which shifts the latitude of maximum MAPE, and thus the storm track poleward in an EBM. An equation linking the storm track position to the Hadley cell edge and nearsurface meridional temperature gradient can be derived using the EBM (see equation 10 in [41]).
Increased Specific Humidity Following the ClausiusClapeyron Relation
Held [23] argued that if the total MSE flux is constant in response to increased CO_{2} (\({\varDelta } \langle \overline {v^{\prime }m^{\prime }} \rangle = {\varDelta } \langle \overline {Lv^{\prime }q^{\prime }} \rangle + {\varDelta } \langle \overline {v's^{\prime }} \rangle \approx 0\)), then increased latent energy flux following the ClausiusClapeyron relation (\({\varDelta } \langle \overline {Lv^{\prime }q^{\prime }} \rangle \approx \alpha {\varDelta } T \langle \overline {Lv^{\prime }q^{\prime }} \rangle < 0\) in the SH) should result in decreased DSE flux (\({\varDelta } \langle \overline {v's^{\prime }} \rangle >0 \) in the SH). Since the increased latent energy flux occurs on the equatorward side of the latitude of the maximum absolute value of the DSE flux, the corresponding DSE flux decrease leads to a poleward shift of the latitude of maximum absolute value of the DSE flux, i.e., a poleward shift of the dry storm track.
Shaw and Voigt [52] showed that if the total MSE flux is constant in response to SST warming (\({\varDelta } \langle \overline {v^{\prime }m^{\prime }} \rangle \approx 0\)) and the nearsurface meridional MSE gradient response to increased CO_{2} follows the ClausiusClapeyron relation (\(\partial {\varDelta } \overline {m}/\partial y \approx \alpha ^{2} {\varDelta } T L q\partial \overline {T}/ \partial \phi /a \)), then the eddy diffusivity change [\({\varDelta } D_{m} \approx  D_{m} (\alpha ^{2} {\varDelta } T L q\partial \overline {T}/ \partial \phi /a) / (\partial \overline {m}/\partial y)\), see Eq. 17 with \({\varDelta } \langle \overline {v^{\prime }m^{\prime }} \rangle \approx 0\) and \(\partial {\varDelta } \partial \overline {m}/\partial y \approx \alpha ^{2} {\varDelta } T L q\partial \overline {T}/ \partial \phi /a \)] shifts the maximum diffusivity, i.e., the storm track, poleward in aquaplanet models.
Cloud Radiative Effect Changes
Voigt and Shaw [65] showed that prescribing the cloud radiative response to warming in climatological prescribed SST aquaplanet simulations leads to a poleward jet shift. Voigt an Shaw [66] showed the poleward shift is dominated by the response of midlatitude (20–50^{∘}) high clouds.
Shaw and Voigt [52] showed the atmospheric CRE (ACRE) response (top of atmosphere minus surface CRE) to warming in prescribed SST aquaplanet simulations implies a change in atmospheric energy transport (\({\varDelta } \langle \overline {vm} \rangle =2\pi a^{2} {\int \limits }^{\phi }_{\pi /2} {\cos \limits } \phi {\varDelta } F_{\text {ACRE}} ~ d \phi \)) that shifts the latitude of the maximum absolute value of the MSE transport poleward.
Ceppi and Hartmann [7] showed that prescribing the shortwave cloud radiative response to increased CO_{2} in a climatological slabocean aquaplanet simulation dominates the poleward shift of the midlatitude circulation.
Li et al. [31] showed that prescribing the highcloud radiative response in a dry dynamical core shifts the jet poleward.
Mechanisms Explaining the Intensification of the Zonal Mean MidLatitude Circulation
Here I review the mechanisms that have been proposed to explain the intensification of the zonal mean midlatitude circulation in response to increased CO_{2}. Once again, the mechanisms are grouped according to their thermodynamic starting point.
Increased Latent Heat Release Aloft in the Tropics

Held [20] argued that an increase in the meridional temperature gradient aloft should increase eddy energy (ΔA > 0), leading to an equatorward momentum flux response poleward of the source (\({\varDelta } \overline {u^{\prime }v^{\prime }} \approx Ak\ell {\varDelta } A > 0\) where ℓ < 0 in the SH, see Eq. 6), a poleward momentum flux response equatorward of the source (\({\varDelta } \overline {u^{\prime }v^{\prime }} \approx Ak\ell {\varDelta } A < 0\) where ℓ > 0 in the SH, see Eq. 6), convergence of eddy momentum flux into the source region and a strengthening of the jet.

O’Gorman [47] showed the EKE increase in response to increased CO_{2} in the SH and during NH winter follows an increase in MAPE. The MAPE increase in the SH was connected to the robust increase of the meridional temperature gradient in the upper troposphere (\({\varDelta } \text {MAPE} \approx \frac {c_{p} p_{0}}{24g} (\sigma _{s}  \sigma _{t}^{\max \limits }) {L_{Z}^{2}} [ {\varGamma }]_{v} {\Delta }[ \{ \partial _{y} \overline {T}\} ]^{2}_{v} > 0 \), see Eq. 14).
Turbulent Surface Heat Flux Changes
Shaw et al. [53] used the MSE framework for storm tracks to show that the SH storm track intensifies in response to SST warming in prescribed SST AGCM simulations following the increase in the equatortopole energy gradient induced by changes in surface heat fluxes, i.e., more evaporation in the tropics than at the South Pole (\({\varDelta } \langle \overline {v^{\prime }m^{\prime }} \rangle = 2\pi a^{2} {\int \limits }^{\pi /2}_{\phi _{s}} \cos \limits \phi {\varDelta } F_{\text {SHF}}~ d \phi > 0\), see Eq. 19).
Mechanisms Explaining the Weakening of the Zonal Mean MidLatitude Circulation
Here I review the mechanisms that have been proposed to explain the weakening of the zonal mean midlatitude circulation in response to increased CO_{2}. Recall that a weakening of the zonal mean midlatitude circulation occurs in response to increased CO_{2} during NH summer. Most of the mechanisms below were not proposed to explain the seasonality of the NH response to increased CO_{2}. Thus, it is not clear in all cases why the mechanisms below should apply only during NH summer. It is possible that different mechanisms operate in NH winter and summer and that more than one mechanism operates in a given season. Once again, the mechanisms are grouped according to their thermodynamic starting point.
Increased Specific Humidity Following the ClausiusClapeyron Relation
Held [23] argued that if the total MSE flux is constant in response to increased CO_{2} (\({\varDelta } \langle \overline {v^{\prime }m^{\prime }} \rangle \approx 0\)), then increased latent energy flux following the ClausiusClapeyron relation (\({\varDelta } \langle \overline {Lv^{\prime }q^{\prime }} \rangle \approx \alpha {\varDelta } T \langle \overline {Lv^{\prime }q^{\prime }} \rangle <0 \) in the SH) should result in decreased DSE flux (\({\varDelta } \langle \overline {v's^{\prime }} \rangle >0 \) in the SH), i.e., a weakening of the dry storm track.
Shaw and Voigt [52] showed that if one assumes the total MSE flux is constant in response to SST warming (\({\varDelta } \langle \overline {v^{\prime }m^{\prime }} \rangle \approx 0\)) and the nearsurface MSE gradient response to increased CO_{2} follows the ClausiusClapeyron relation (\(\partial {\varDelta } \overline {m}/\partial y \approx \alpha ^{2} {\varDelta } T L q\partial \overline {T} / \partial \phi /a < 0\)), then the eddy diffusivity decreases [ΔD_{m} ≈−D_{m}\((\alpha ^{2} {\varDelta } T L q\partial \overline {T} / \partial \phi /a) / (\partial \overline {m}/\partial y)<0\), see Eq. 17 with \({\varDelta } \langle \overline {v^{\prime }m^{\prime }} \rangle \approx 0\) and \(\partial {\varDelta } \overline {m}/\partial y \approx \alpha ^{2} {\varDelta } T L q\partial \overline {T} / \partial \phi /a < 0\)], i.e., the storm track weakens (see their Eq. 11) in aquaplanet models.
Increased Dry Static Stability Outside the Tropics
O’Gorman [47] showed the weakening of the zonal mean midlatitude EKE during NH summer in response to increased CO_{2} follows the decrease in MAPE, which is partly related to increased dry static stability (\({\varDelta } \text {MAPE} \approx \frac {c_{p} p_{0}}{24g} (\sigma _{s}  \sigma _{t}^{\max \limits }) {L_{Z}^{2}} {\varDelta } [ {\varGamma }]_{v} [ \{ \partial _{y} \overline {T}\} ]^{2}_{v} < 0 \), see Eq. 14).
Decreased Surface Meridional Temperature Gradient
In addition to the stability changes discussed above, Gertler and O’Gorman [16] showed that the weakening of the surface meridional temperature gradient in midlatitudes around NH land also contributes to the decrease of nonconvective MAPE (\({\varDelta } \text {MAPE} \approx \frac {c_{p} p_{0}}{24g} (\sigma _{s}  \sigma _{t}^{\max \limits }) {L_{Z}^{2}} [ {\varGamma }]_{v} {\Delta }[ \{ \partial _{y} \overline {T}\} ]^{2}_{v} < 0 \), see Eq. 14).
Turbulent Surface Heat Flux Changes
Shaw et al. [53] used the MSE framework for storm tracks to show that the storm track weakens in response to increased CO_{2} over land during NH summer following the decreased equatortopole energy gradient induced by changes in surface heat fluxes over land, i.e., land warms more than the atmosphere and ocean (\({\varDelta } \langle \overline {v^{\prime }m^{\prime }} \rangle = {\int \limits }^{\pi /2}_{\phi _{s}} a \cos \limits \phi {\varDelta } F_{\text {SHF}}~ d \phi < 0\), see Eq. 19).
Testing Mechanisms
 1.
Increased latent heat release aloft in the tropics
 2.
Increased dry static stability and tropopause height outside the tropics
 3.
Radiative cooling of the stratosphere
 4.
Hadley cell expansion
 5.
Increased specific humidity following the ClausiusClapeyron relation
 6.
Cloud radiative effect changes
 7.
Turbulent surface heat flux changes
 8.
Decreased surface meridional temperature gradient
To date, there exist several different tests of thermodynamic starting points explaining the zonal mean midlatitude circulation response to increased CO_{2}: (1) prescribing diabatic perturbations, (2) quantifying the transient response to an abrupt CO_{2} increase, and (3) imposing latitudinally dependent CO_{2} concentration. In what follows, I review how these approaches have been used to test whether the increased latent heat release aloft in the tropics starting point (see section “Increased Latent Heat Release Aloft in the Tropics”), hereafter referred to as the tropical starting point, can explain the annual mean poleward shift of the zonal mean midlatitude circulation response to increased CO_{2} in the SH.
For the prescribed diabatic perturbation approach, the test of the tropical starting point is conducted by prescribing a diabatic pertubation in the tropical upper troposphere, which mimics the tropical temperature response in CMIP5 models (Fig. 1, top), in a dry dynamical core model. If there is no poleward shift then the tropical starting point is falsified. The results of these tests show (1) raising the tropopause equatorward of the climatological jet shifts the jet equatorward [32] and (2) prescribing diabatic heating in the tropical upper troposphere leads to a poleward shift of the midlatitude circulation only if the heating extends into the subtropics [4, 36, 59, 61].
It is difficult to interpret the results of tests that prescribe diabatic perturbations (e.g., prescribing diabatic heating in dry dynamical core simulations or prescribing the cloud radiative response to increased CO_{2} in aquaplanet simulations) for several reasons. Prescribed diabatic perturbations require knowledge of the equilibrium response to increased CO_{2}, which is shaped by the circulation. The perturbations, including their meridional and vertical extent, have not been shown to occur in the absence of the circulation response, i.e., in radiativeconvective equilibrium either without [60] or with the climatological circulation. Thus, it is possible the meridional extension of the prescribed diabatic heating into the subtropics is shaped by the circulation and encodes the circulation response. In order for tests involving prescribed diabatic perturbations to be useful the perturbations must be shown to occur in response to increased CO_{2} in radiativeconvective equilibrium either without [60] or with the climatological circulation.
Overall, the recent tests discussed above do not support the tropical starting point and its underlying mechanisms (see section “Increased Latent Heat Release Aloft in the Tropics”) as an explanation for the poleward shift of the SH midlatitude circulation in response to increased CO_{2}. More specifically, (1) the meridional temperature gradient and subtropical jet strength evolve on a different timescale than the midlatitude circulation shift in response to an abrupt CO_{2} increase and (2) increased CO_{2} in the tropics does not produce a significant poleward shift. Instead, the tests lend support to the increased dry static stability and tropopause height outside the tropics starting point (see section “??”). However, more tests are needed, including with climate models across the model hierarchy, before falsifying the tropical starting point. The results above demonstrate the importance of using climate models in novel ways to test mechanisms.
Future Outlook
Future progress in understanding the zonal mean midlatitude circulation response to increased CO_{2} relies on (1) carefully designed climate model experiments that attempt to falsify the thermodynamic starting points and assumptions underlying the proposed mechanisms and (2) subsequently using the remaining mechanisms to create emergent constraints. Several approaches can be used to test mechanisms, for example, (1) imposing diabatic perturbations based on the response to increased CO_{2} in radiativeconvective equilibrium either in the absence [60] or presence of the climatological circulation, (2) quantifying the transient evolution of the circulation response to an abrupt CO_{2} increase [9, 11, 17, 36, 42, 67, 68], (3) imposing latitudinally dependent CO_{2} concentration [54], (4) quantifying the seasonality of the mechanisms, and (5) nudging parameters to their climatological value, e.g., turning off wind induced surface heat exchange [43]. Emergent constraints (see [19]) are needed in order to quantify and compare the modeled mechanisms to those in the real atmosphere. Finally, in addition to recommending more be done to falsify existing mechanisms, I would also recommend any new mechanism proposed in the future should (1) be simple, (2) be able to predict the circulation response without running a model, (3) involve an equation (to ensure transparency), and (4) be falsifiable.
Notes
Acknowledgments
TAS thanks the section editor Isla Simpson and two anonymous reviewers for their comments and Molly Menzel for providing Fig. 3.
Funding Information
TAS acknowledges support from the National Science Foundation (AGS1742944) and the David and Lucile Packard Foundation.
Compliance with Ethical Standards
Conflict of Interest
The authors declare that they have no conflict of interest.
References
 1.Barnes EA, Polvani LM. Response of the midlatitude jets, and of their variability, to increased greenhouse gases in the CMIP5 models. J Clim 2013;26:7117–7135.Google Scholar
 2.Barpanda P, Shaw T. Using the moist static energy budget to understand stormtrack shifts across a range of time scales. J Atmos Sci 2017;74:2427–2446.Google Scholar
 3.Bender FAM, Ramanathan V, Tselioudis G. Changes in extratropical storm track cloudiness 19832008: observational support for a poleward shift. Clim Dyn 2012;28:2037–2053.Google Scholar
 4.Butler AH, Thompson DWJ, Heikes R. The steadystate atmospheric circulation response to climate changelike thermal forcings in a simple general circulation Model. J Clim 2010;23:3474–3496.Google Scholar
 5.Butler AH, Thompson DWJ, Birner T. Isentropic slopes, downgradient eddy fluxes, and the extratropical atmospheric circulation response to tropical tropospheric heating. J Atmos Sci 2011;68:2292–2305.Google Scholar
 6.Ceppi P, Hartmann DL. Connections between clouds, radiation, and midlatitude dynamics: a review. Curr Clim Chang Rep 2015;1:94–102.Google Scholar
 7.Ceppi P, Hartmann DL. Clouds and the atmospheric circulation response to warming. J Clim 2016;29: 783–799.Google Scholar
 8.Chang EKM, Guo Y, Xia X. 2012. CMIP5 multimodel ensemble projection of storm track change under global warming. J Geophys Res. https://doi.org/10.1029/2012JD018578.Google Scholar
 9.Chemke R, Polvani LM. Exploiting the abrupt 4xCO2 scenario to elucidate tropical expansion mechanisms. J Clim 2019;32:859–875.Google Scholar
 10.Chen G, Lu J, Frierson DMW. Phase speed spectra and the latitude of surface westerlies: interannual variability and global warming trend. J Clim 2008;21:5942–5959.Google Scholar
 11.Chen G, Lu J, Sun L. Delineating the eddyzonal flow interaction in the atmospheric circulation response to climate forcing: uniform SST warming in an idealized aquaplanet model. J Atmos Sci 2013;70:2214–2233.Google Scholar
 12.Coumou D, Lehmann J, Beckmann J. The weakening summer circulation in the Northern Hemisphere midlatitudes. Science 2015;348:324–327.Google Scholar
 13.Cronin TW, Jansen MF. 2016. Analytic radiativeadvective equilibrium as a model for highlatitude climate. Geophys. Res. Lett. https://doi.org/10.1002/2015GL067172.Google Scholar
 14.Frierson DMW. Midlatitude static stability in simple and comprehensive general circulation models. J Atmos Sci 2008;65:1049–1062.Google Scholar
 15.Fu Q, Johanson CM, Wallace JM, Reichler T. Enhanced midlatitude tropospheric warming in satellite measurements. Science 2006;312:1179.Google Scholar
 16.Gertler CG, O’Gorman PA. 2019. Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer. Proc. Nat. Acad. Sciences. https://doi.org/10.1073/pnas.1812312116.Google Scholar
 17.Grise K, Polvani LM. Understanding the time scales of the tropospheric circulation response to abrupt CO_{2} forcing in the Southern Hemisphere: Seasonality and the role of the stratosphere. J Clim 2017;30:8497–8515.Google Scholar
 18.Hall NJ, Hoskins BJ, Valdes PJ, Senior CA. Storm tracks in a highresolution GCM with doubled carbon dioxide. Quart J Roy Met Soc 1994;120:1209–1230.Google Scholar
 19.Hall A, Cox P, Huntingford C, Klein S. Progressing emergent constraints on future climate change. Nat Clim Chang 2019;9:269–278.Google Scholar
 20.Held IM. Largescale dynamics and global warming. Bull Amer Met Soc 1993;74:228–241.Google Scholar
 21.Held IM. 2005. The gap between simulation and understanding in climate modeling. Bull. Amer. Met. Soc. https://doi.org/10.1175/BAMS86111609.Google Scholar
 22.Held IM, Soden BJ. Robust responses of the hydrological cycle to global warming. J Clim 2006;19:5686–5699.Google Scholar
 23.Held IM. 2015. Poleward atmospheric energy transport. https://www.gfdl.noaa.gov/blog/held/62polewardatmosphericenergytransport.
 24.Karoly DJ, Hoskins BJ. Threedimensional propagation of planetary waves. J Met Soc Jpn 1982;60:109–123.Google Scholar
 25.Kidston J, Dean SM, Renwick JA, Vallis GK. 2010. A robust increase in the eddy length scale in the simulation of future climates Geophys. Res. Lett. https://doi.org/10.1029/2009GL041615.Google Scholar
 26.Kidston J, Vallis GK, Dean SM, Renwick JA. 2011. Can the increase in the eddy length scale under global warming cause the poleward shift of the jet streams. J Clim. https://doi.org/10.1175/2010JCLI3738.1.Google Scholar
 27.Kidston J, Vallis GK. 2012. The relationship between the speed and the latitude of an eddydriven jet in a stirred barotropic model. J Atmos Sci. https://doi.org/10.1175/JASD110300.1.Google Scholar
 28.Kuo HL. Forced and free meridional circulations in the atmosphere. J Meteorol 1956;13:561–568.Google Scholar
 29.Kushner PJ, Held IM. A test, using atmospheric data of a method for estimating oceanic eddy diffusivity. Geophys Res Lett 1998;25:4213–4216.Google Scholar
 30.Lee S, Feldstein SB. 2013. Detecting ozone and greenhouse gas driven wind trends with observational data. Science. https://doi.org/10.1126/science.1225154.Google Scholar
 31.Li Y, Thompson DWJ, Bony S, Merlis TM. Thermodynamic control on the poleward shift of the extratropical jet in climate change simulations: the role of rising high clouds and their radiative effects. J Clim 2018; 32:917–934.Google Scholar
 32.Lorenz DJ, DeWeaver ET. 2007. Tropopause height and zonal wind response to global warming in the IPCC scenario integrations. J Geophys Res. https://doi.org/10.1029/2006JD008087.
 33.Lorenz DJ. Understanding midlatitude jet variability and change using rossby wave chromatography: polewardshifted jets in response to external forcing. J Atmos Sci 2014;71:2370–2389.Google Scholar
 34.Lu J, Vecchi GA, Reichler T. 2007. Expansion of the Hadley cell under global warming. Geophys. Res Lett. https://doi.org/10.1029/2006GL028443.
 35.Lu J, Chen G, Frierson DMW. Response of the zonal mean atmospheric circulation to El Nino versus global warming. J Clim 2008;21:5835–5851.Google Scholar
 36.Lu J, Sun L, Wu Y, Chen G. The role of subtropical irreversible PV mixing in the zonal mean circulation response to global warminglike thermal forcing. J Clim 2014;27:2297–2316.Google Scholar
 37.Manabe S, Wetherald RT. The effects of doubling CO_{2} concentration in a general circulation model. J Atmos Sci 1975;32:3–15.Google Scholar
 38.Matsuno T. Vertical propagation of stationary planetary waves in winter Northern Hemisphere. J Atmos Sci 1970;27:871–883.Google Scholar
 39.Mbengue C, Schneider T. Storm track shifts under climate change: what can be learned from largescale dry dynamics. J Clim 2013;26:9923–9930.Google Scholar
 40.Mbengue C, Schneider T. Stormtrack shifts under climate change: toward a mechanistic understanding using baroclinic mean available potential energy. J Atmos Sci 2017;74:93–110.Google Scholar
 41.Mbengue C, Schneider T. Linking Hadley circulation and storm tracks in a conceptual model of the atmospheric energy balance. J Atmos Sci 2018;75:841–856.Google Scholar
 42.Menzel ME, Waugh D, Grise K. 2019. Disconnect between Hadley cell and subtropical jet variability and response to increased CO_{2}. Geophys. Res Lett. https://doi.org/10.1029/2019GL083345.Google Scholar
 43.Muller CJ, Romps DM. Acceleration of tropical cyclogenesis by selfaggregation feedbacks. Proc Nat Acad Sci 2018;115:2930–2935.Google Scholar
 44.Nakamura N, Zhu D. Finiteamplitude wave activity and diffusive flux of potential vorticity in eddymean flow interaction. J Atmos Sci 2010;67:2701–2716.Google Scholar
 45.Nakamura N, Solomon A. Finiteamplitude wave activity and mean flow adjustments in the atmospheric general circulation. Part I: quasigeostrophic theory and analysis. J Atmos Sci 2010;67:3967–3983.Google Scholar
 46.O’Gorman PA, Schneider T. Energy of midlatitude transient eddies in idealized simulations of changed climates. J Clim 2008;21:5797–5806.Google Scholar
 47.O’Gorman PA. Understanding the varied response of the extratropical storm tracks to climate change. Proc Nat Acad Sci 2010;107:19176–19180.Google Scholar
 48.Pfeffer RL. Wavemean flow interactions in the atmosphere. J Atmos Sci 1981;38:1340–1359.Google Scholar
 49.Riviere G. A dynamical interpretation of the poleward shift of the jet streams in global warming scenarios. J Atmos Sci 2011;68:1253–1272.Google Scholar
 50.Schneider T. 2006. The general circulation of the atmosphere. Annu. Rev. Earth Planet. Sci. https://doi.org/10.1146/annurev.earth.34.031405.125144.Google Scholar
 51.Shaw T, Baldwin M, Barnes EA, Caballero R, Garfinkel CI, Hwang YT, Li C, O’Gorman PA, Riviere G, Simpson I, Voigt A. 2016. Storm track processes and the opposing influences of climate change. Nature Geoscience. https://doi.org/10.1038/NGEO2783.Google Scholar
 52.Shaw T, Voigt A. 2016. What can moist thermodynamics tell us about circulation shifts in response to uniform warming? Geophys. Res Lett. https://doi.org/10.1002/2016GL068712.Google Scholar
 53.Shaw T, Barpanda P, Donohoe A. A moist static energy framework for zonalmean stormtrack intensity. J Atmos Sci 2018;75:1979–1994.Google Scholar
 54.Shaw T, Tan Z. 2018. Testing latitudinally dependent explanations of the circulation response to increased CO_{2} using aquaplanet models. Geophys. Res Lett. https://doi.org/10.1029/2018GL078974.Google Scholar
 55.Sigmond M, Siegmund PC, Manzini E, Kelder H. A simulation of the separate climate effects of middleatmospheric and tropospheric CO_{2} doubling. J Clim 2004;17:2352–2367.Google Scholar
 56.Simpson I, Shaw T, Seager R. A diagnosis of the seasonally and longitudinally varying midlatitude circulation response to global warming. J Atmos Sci 2014;71:2489–2515.Google Scholar
 57.Staten PW, Lu J, Grise K, Davis SM, Birner T. Reexamining tropical expansion. Nat Clim Chang 2018;8:768–775.Google Scholar
 58.Stevens B, Giorgetta M, Esch M, Mauritsen T, Crueger T, Rast S, et al. 2013. Atmospheric component of the MPIM earth system model: ECHAM6. J. Adv. Mod. Earth Sys. https://doi.org/10.1002/jame.20015.Google Scholar
 59.Sun L, Chen G, Lu J. Sensitivities and mechanisms of the zonal mean atmospheric circulation response to tropical warming. J Atmos Sci 2013;70:2487–2504.Google Scholar
 60.Tan Z, Lachmy O, Shaw T. The sensitivity of the jet stream response to climate change to radiative assumptions. J Adv Model Earth Sys 2019;11:1–23.Google Scholar
 61.Tandon N, Gerber EP, Sobel AH, Polvani LM. Understanding hadley cell expansion versus contraction: Insights from simplified models and implications for recent observations. J Clim 2013;26:4304–4321.Google Scholar
 62.Trenberth KE, Stepaniak DP. Covariability of components of poleward atmospheric energy transports on seasonal and interannual timescales. J Clim 2003;16:3691–3705.Google Scholar
 63.Vallis GK. Atmospheric and oceanic fluid dynamics. Cambridge: Cambridge University Press; 2006.Google Scholar
 64.Vallis GK, ZuritaGotor P, Cairns C, Kidston J. Response of the largescale structure of the atmosphere to global warming. Quart J Roy Met Soc 2015;141:1479–1501.Google Scholar
 65.Voigt A, Shaw T. 2015. Circulation response to warming shaped by radiative changes of clouds and water vapour. Nature Geoscience. https://doi.org/10.1038/NGEO2345.Google Scholar
 66.Voigt A, Shaw T. Impact of regional atmospheric cloud radiative changes on shifts of the extratropical jet stream in response to global warming. J Clim 2016;29:8399–8421.Google Scholar
 67.Wu Y, Seager R, Ting M, Naik N, Shaw T. Circulation response to an instantaneous doubling of carbon dioxide. Part I: model experiments and transient thermal response in the troposphere. J Clim 2012;25: 2862–2879.Google Scholar
 68.Wu Y, Seager R, Shaw T, Ting M, Naik N. Atmospheric circulation response to an instantaneous doubling of carbon dioxide. part II: atmospheric transient adjustment and its dynamics. J Clim 2013;26:918–935.Google Scholar
 69.Yin JH. 2005. A consistent poleward shift of the storm tracks in simulations of 21st century climate. Geophys. Res Lett. https://doi.org/10.1029/2005GL023684.Google Scholar
Copyright information
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.