1. Introduction
Substantial changes to the zonal-mean atmospheric circulation are expected as a response to global warming. In some cases, there is good agreement on the expected sign of the response. For example, in the subtropics and extratropics, poleward expansion of the Hadley cell edge (Fu et al. 2006; Lu et al. 2007; Seidel and Randel 2007; Seidel et al. 2008), jet stream position (Kushner et al. 2001), and storm track latitude (Yin 2005) are all expected with climate change. These changes are projected to occur based on comprehensive climate models (Lu et al. 2007; Barnes and Polvani 2013; Simpson et al. 2014), and poleward Hadley cell edge shifts have been observed to occur in recent observations (Fu et al. 2006; Seidel and Randel 2007; Seidel et al. 2008; Davis and Birner 2017). These expansions are generally thought to be due to the amplified warming in the tropical upper troposphere, which increases the static stability in the subtropics (Lu et al. 2007; Frierson et al. 2007) and increases the equator-to-pole temperature gradient in the upper troposphere (Chen and Held 2007; Butler et al. 2010).
Expected changes in the deep tropical circulation as a result of global warming are more uncertain. While the multimodel mean of the zonal-mean precipitation and vertical ascent response to global warming in the tropics suggests a narrowing of the intertropical convergence zone (ITCZ; Lau and Kim 2015; Byrne and Schneider 2016a) due to increased moist static energy gradients, not all models agree on the sign of this response (Byrne and Schneider 2016a). Meridional shifts of the ITCZ position are also possible with climate change due to factors such as aerosol cleanup, cloud feedbacks, and changes in ocean circulation (Allen et al. 2015; Rotstayn et al. 2015; McFarlane and Frierson 2017) but these shifts are small in the multimodel mean projection of twenty-first century climate change (Byrne et al. 2018). In the recent observational record, a narrowing of the annual-mean ITCZ in both the Pacific (Wodzicki and Rapp 2016) and Atlantic (Byrne et al. 2018) has been reported.
Changes in the Hadley cell edge and eddy-driven jet position are not independent of changes in the deep tropical circulation. El Niño events provide a natural case study for tropically driven influence on the zonally averaged extratropical circulation. During El Niño events, tropical rainfall concentrates more closely on the equator and the Hadley cell accelerates (e.g., Fig. 5 of Adames and Wallace 2017; Seager et al. 2003). This shift and contraction of precipitation is thought to occur because of increased energy input to the atmosphere near the equator (Bischoff and Schneider 2014; Adam et al. 2016). Two factors lead to an increase in zonal winds within the upper branch of the Hadley cell: an equatorially confined updraft means that the angular momentum of the rising air is larger (since the distance to the rotation axis 
For phase 5 of the Coupled Model Intercomparison Project (CMIP5), aquaplanet simulations—that is, atmospheric general circulation models with no land surface or continents and forced by an idealized zonally symmetric sea surface temperature (SST) pattern—were performed with a variety of models (Taylor et al. 2012). These experiments are ideal for investigating the zonally symmetric circulations of interest in this study. The impact of global warming was simulated by imposing a globally uniform +4 K SST perturbation. The resulting circulations and warming responses for eight different models are shown in Fig. 1. It is evident that across these aquaplanet simulations, the midlatitude circulation responses are consistent in sign: The mass streamfunction responses consistently show a poleward shift in the edge of the Hadley cell and Ferrel cell (Figs. 1b,e) and the lower-tropospheric zonal wind response is always a dipole that represents a poleward shift in the maximum wind (Figs. 1c,f). However, the climatological circulations and +4 K responses in the deep tropics—equatorward of 15°—show remarkable variation across the models. The precipitation climatologies (Fig. 1a) vary from a wide double ITCZ with strongest precipitation at 10°S/N (CNRM-CM5) to a single narrow maximum at the equator (MRI-CGCM3). Furthermore, the +4 K changes in tropical precipitation range from widening to strong contraction of the region of maximum precipitation near the equator (Fig. 1d). Correspondingly, the deep tropical 500-hPa mass streamfunction response varies from about 
The time- and zonal-mean (left) tropical precipitation, (middle) 500-hPa mass streamfunction, and (right) 850-hPa zonal wind from eight aquaplanet models. (top) Simulations performed with the QOBS SST profile (Neale and Hoskins 2000a); (bottom) the response to a globally uniform +4 K SST perturbation. All data are averaged over the Southern and Northern Hemispheres, including for variables shown for both hemispheres (i.e., the precipitation).
Citation: Journal of Climate 32, 4; 10.1175/JCLI-D-18-0434.1
Given the connections between the ITCZ position/width to both the Hadley cell edge and eddy-driven jet position discussed above and known to occur during El Niño events, we postulate that possible changes in the ITCZ width under global warming would have an impact on the higher-latitude circulation response. Specifically, a narrowing of the ITCZ (Lau and Kim 2015; Byrne and Schneider 2016a) would lead to a smaller-than-otherwise-expected poleward Hadley cell edge and eddy-driven jet shift. Furthermore, the ITCZ width changes associated with, for example, a globally uniform +4 K SST perturbation in aquaplanet models are highly uncertain, not agreeing even on sign. We suggest that the varying deep tropical circulation responses contribute to the spread in higher-latitude circulation responses, which agree in sign but not quantitatively.
The rest of this paper is structured as follows: Section 2 describes the atmospheric model and experimental setup, as well as the diagnostics used to describe the atmospheric circulation. Section 3 presents the main results of the study, including the relationship between the deep tropical and higher-latitude circulation responses to global warming in a specified SST model. Section 4 provides analogous results of simulations performed with a slab ocean model. Section 5 quantifies the relationship between the deep tropical and higher-latitude circulations across the CMIP5 aquaplanet experiments. Finally, a summary and discussion of results is provided in section 6.
2. Model and diagnostics
a. GFDL-AM2.1 model experiments
The time- and zonal-mean (a) SST, (b) tropical precipitation, (c) 500-hPa mass streamfunction, and (d) 850-hPa zonal wind for the six experiments with differing 
Citation: Journal of Climate 32, 4; 10.1175/JCLI-D-18-0434.1
Simulations are 20 years long for each experiment, after a 1-yr spinup. All quantities shown are for the time and zonal mean. Since the SST profiles used are symmetric across the equator, the output is averaged across the two hemispheres before the circulation diagnostics are computed. All results are similar for the individual Southern and Northern Hemisphere data. Experiments with the same atmospheric model coupled to a slab ocean will be described in section 4.
b. Circulation metrics
We will use the Hadley cell extent and latitude of the eddy-driven jet as metrics for the subtropical and midlatitude zonal-mean atmospheric circulation. Following previous work, we define the Hadley cell extent as the first zero crossing of the 500-hPa mass streamfunction poleward of its tropical extremum (e.g., Frierson et al. 2007) and label it 
c. CMIP5 aquaplanet experiments
Although not the primary focus of this work, results from a multimodel comparison of aquaplanet experiments will be briefly discussed. These data were shown in Fig. 1 and consists of six models from the CMIP5 archive that provided the necessary data (CNRM-CM5, HadGEM2-A, IPSL-CM5A-LR, MRI-CGCM3, MPI-ESM-LR, and MPI-ESM-MR) and additional data from equivalent experiments that were performed by the authors using the GFDL-AM2.1 (Anderson et al. 2004) and the NCAR CAM5.3 models (Neale et al. 2012). All eight of these experiments use the QOBS SST profile and otherwise follow specifications from the aquaplanet experiment (Neale and Hoskins 2000a; Taylor et al. 2012).
3. Specified SST simulation results
a. Control experiment climatologies
Using the QOBS SST profile, the GFDL-AM2.1 model simulates a double ITCZ with precipitation maximized around 5°S/N (Fig. 1a, blue line). This is despite the maximum surface temperature being at the equator. This double-ITCZ structure also occurs in other aquaplanet models with the QOBS SST profile (e.g., Fig. 1a; Neale and Hoskins 2000b; Möbis and Stevens 2012; Oueslati and Bellon 2013; Medeiros et al. 2016). Its existence can be dependent on the convection scheme used and the vertical, horizontal, and temporal resolutions (Williamson 2008; Möbis and Stevens 2012; Retsch et al. 2017) and is a result of complex feedbacks between surface winds, evaporation, and convective heating (Möbis and Stevens 2012; Oueslati and Bellon 2013). For the GFDL-AM2.1 model, this double-ITCZ pattern is highly sensitive to the exact SSTs in the deep tropical region. In particular, the SST perturbations of Eq. (2) cause fundamental reorganizations of the tropical circulation (and its response to warming, although discussion of this is delayed until the next section). The QOBS and flatter distributions, 
As the deep tropical circulation varies with 
Climatological values (first number in each entry) of Hadley cell strength 
The mechanism for a higher-latitude response to the ITCZ narrowing and Hadley cell strengthening was discussed in the introduction and is briefly repeated here. Insofar as angular momentum is conserved, air parcels that begin their poleward flow in the upper branch of the Hadley circulation closer to the equator will gain a larger component of zonal wind (Held et al. 2000). Furthermore, a stronger Hadley cell will lead to winds that more closely follow angular momentum conservation (i.e., eddy stresses will play a relatively lesser role in the zonal-mean wind budget). Both of these factors will force a Hadley cell edge that is closer to the equator (Held et al. 2000; Kang and Lu 2012). Furthermore, a stronger subtropical jet will also in turn lead to an equatorward-shifted eddy-driven jet via changes in critical lines or the region of maximum baroclinicity (Lee and Kim 2003; Barnes and Hartmann 2011; Ceppi et al. 2013).
Indeed, as the ITCZ narrows across the control simulations, there is a significant strengthening of the subtropical jet (Figs. 3a,b) that results in an equatorward shift of the critical line separating the equatorial easterlies and higher-latitude westerlies. A simple calculation assuming the conservation of angular momentum 
(a) The time- and zonal-mean zonal wind averaged between 150 and 225 hPa for the six experiments with differing 
Citation: Journal of Climate 32, 4; 10.1175/JCLI-D-18-0434.1
b. Global warming response
To model the effects of global warming onto the atmospheric circulation, a globally uniform +4 K SST perturbation is added to each of the six experiments discussed in the previous section. The precipitation responses to this perturbation vary strongly across the six cases (Fig. 4). For the QOBS SST profile, there is a striking rearrangement of the deep tropical circulation, with the double-ITCZ pattern being replaced by a single precipitation maximum at the equator (Fig. 4c). Comparing to other atmospheric models run with the same configuration, GFDL-AM2.1 stands out by having the largest mass streamfunction and tropical precipitation response (Fig. 1). However, when using the SST profile that is flattened near the equator (
Time and zonal mean of precipitation for the six different SST profiles (black; reproduced from Fig. 2b) and the same SST profiles with a globally uniform +4 K perturbation (red). The x axis is plotted in terms of the sine of latitude.
Citation: Journal of Climate 32, 4; 10.1175/JCLI-D-18-0434.1
The Hadley circulation response to the +4 K SST perturbation also varies nonmonotonically as a function of the control SST profile used (Fig. 5). Corresponding to the contraction of the ITCZ seen for 
Time-mean mass streamfunction for the six different SST profiles (contours) and the response of the streamfunction (shading) to a globally uniform +4 K perturbation. The x axis is plotted in terms of the sine of latitude and data are averaged over the Southern and Northern Hemispheres. The interval for the line contours is 
Citation: Journal of Climate 32, 4; 10.1175/JCLI-D-18-0434.1
Figure 6 shows the zonal-mean zonal wind control climatology and response for all six experiments. Qualitatively, the response is similar in all cases, with a poleward shift of the eddy-driven jet and acceleration of the westerlies on the upper flank of the subtropical jet, as expected from a rising tropopause (Santer et al. 2003; Lorenz and DeWeaver 2007), and increasing tropical to extratropical temperature gradient in the upper troposphere (Chen and Held 2007; Butler et al. 2010). However, the magnitude of the dipole in winds representing the poleward shift varies strongly between cases (e.g., cf. Figs. 6b and 6f). Furthermore, there are subtle but important differences in the response of the upper-tropospheric subtropical winds between cases. The experiments that have a substantial contraction of the ITCZ and strengthening of the Hadley cell 
As in Fig. 5, but for the zonal-mean zonal wind. The interval for the line contours is 10 m s−1.
Citation: Journal of Climate 32, 4; 10.1175/JCLI-D-18-0434.1
The responses of the ITCZ width, Hadley cell edge, and eddy-driven jet in each of the six experiments are summarized in Fig. 7 and Table 1. This clearly demonstrates the nonmonotonicity of the ITCZ response to the +4 K perturbation: The initially wide double-ITCZ and narrow single-ITCZ circulations (
The climatological (a) ITCZ width, (b) Hadley cell strength, (c) Hadley cell edge, and (d) eddy-driven jet position for the control simulations (black crosses) and +4 K simulations (red circles) for all six experiments. The quantitative response of each variable for each experiment is shown in text (see also Table 1). The experiments are labeled by 
Citation: Journal of Climate 32, 4; 10.1175/JCLI-D-18-0434.1
On the other hand, the eddy-driven jet (Fig. 7c) has its smallest response for the 
c. Eddy momentum flux responses
The position of the eddy-driven jet is determined by the latitude of maximum eddy momentum flux convergence. In turn, this depends on where midlatitude eddies are generated, that is, where the baroclinicity is largest, and how the eddies propagate through the atmosphere. The Hadley cell edge and strength are also partly controlled by the stresses of eddies generated in the midlatitudes (Walker and Schneider 2006). Examining the upper-tropospheric eddy momentum flux across the control experiments (Fig. 8), it is apparent that as 
Eddy momentum flux (solid) and eddy momentum flux convergence (dashed) averaged between the 150 and 225 hPa levels for (a) three of the control experiments and (b) the response to the +4 K perturbation.
Citation: Journal of Climate 32, 4; 10.1175/JCLI-D-18-0434.1
In terms of the response of the eddy momentum flux to the +4 K perturbation (Fig. 8b), the 
d. ITCZ contraction and descent area expansion
Recent work on the contraction of the ITCZ under global warming has highlighted the compensation between the ascending and descending branches of the Hadley cell: Models that simulate a greater ITCZ contraction also tend to simulate a greater expansion of the descent region of the Hadley cell (Fig. 1 of Byrne and Schneider 2016a). Given that the Hadley cell extent is the sum of the ITCZ and descent widths, this may appear to suggest that ITCZ narrowing would not impact Hadley cell width because of the compensating effect of descent region expansion. However, for the GFDL-AM2.1 simulations considered in this study, although there is a strong negative linear relationship between ITCZ width and descent width (Fig. 9a), it is not a one-to-one compensation. This is made explicit in Fig. 9a by plotting lines of constant Hadley cell width (dashed lines), which are simply lines with a slope of −1 with different intercepts. It is clear that, for a given ITCZ contraction, the descending region does not widen as much and this results in a narrowing of the overall Hadley cell (Fig. 9b). This means that for the control experiments, there is a strong linear relationship between the ITCZ width and Hadley cell extent (Pearson correlation 
The climatological (a) ITCZ width vs descent width and (b) ITCZ width vs Hadley cell extent for the six GFDL-AM2.1 experiments with varying 
Citation: Journal of Climate 32, 4; 10.1175/JCLI-D-18-0434.1
A similar effect of compensation occurs when considering the responses of the atmospheric circulation to the +4 K forcing (Fig. 9c). For these cases, the descent expansion is always larger than the ITCZ contraction, resulting in an expansion of the Hadley cell with global warming. However, the amount of descent area expansion is dependent on the ITCZ contraction such that there is a clear linear relationship between the ITCZ width response and Hadley cell extent response (Fig. 9d; 
The connections between the ITCZ narrowing and eddy-driven jet shifts under the global warming perturbation are not as robust as the Hadley cell changes. Thus, although the correlation between 
e. Causes of different deep tropical circulations
Although not the primary focus of this work, it is interesting to briefly consider why there are such strikingly different responses in the tropical precipitation for the different control experiments (Fig. 4). As we vary the control SST profile with the 
Less expected is the large difference in response to global warming with these different control states (Fig. 4). SST gradients cannot be invoked to explain this difference among the +4 K responses, and so we consider the zonal-mean atmospheric energy budget to better understand the tropical response (Fig. 10). Even though the equator always has the warmest SST, the energy input into the atmosphere there is negative in the 
Zonal-mean atmospheric energy budget in the tropics for the (a) 
Citation: Journal of Climate 32, 4; 10.1175/JCLI-D-18-0434.1
For the QOBS case, it is remarkable that the mean mass circulation changes so dramatically under the +4 K perturbation, while the atmospheric heating (and hence moist static energy flux) only has very small changes equatorward of 15°. This implies that either changes in gross moist stability or eddy fluxes must be occurring for this simulation (Kang et al. 2009; Byrne and Schneider 2016b). A more detailed examination of the causes of the tropical circulation changes (or lack thereof) under the +4 K SST perturbation is outside the scope of this study and left for future work.
f. The +8 K perturbation
The dependency of the atmospheric circulation to the basic state in the tropics has intriguing implications for the linearity of the response to global warming perturbations of increasing magnitudes. For example, after increasing the SSTs of the QOBS case by +4 K, the tropical precipitation is maximized in a single peak on the equator (Fig. 4c). It might be expected that the response of the circulation to a further +4 K increase in surface temperature may be more analogous to the 
4. Slab ocean simulation results
By increasing the amplitude of ocean heat transport from the deep tropics to the subtropics [Eq. (3)] the SSTs become less peaked near the equator and the precipitation widens from a single maximum to a double-ITCZ pattern (not shown, but see 
Climatological values (first number in each entry) of key atmospheric circulation parameters for the three slab ocean control experiments, and the response (in parentheses) to the 4 × CO2 perturbation.
In contrast to the specified SST experiments, for the slab ocean experiments, changes in the ITCZ have a larger impact on the shift of the eddy-driven jet than the Hadley cell edge. The reason for this is left for future study, but it may be related to the fact that in the slab ocean model changes in surface temperature, and hence near-surface baroclinicity, are possible, allowing for larger changes in the eddy-driven jet position.
5. CMIP5 aquaplanet experiments
It was suggested in the introduction that the large spread of deep tropical responses to the globally uniform +4 K perturbation in the CMIP5 aquaplanet experiments might explain part of the spread of higher-latitude responses. Here this idea is tested, focusing on the Hadley cell extent. Figure 11 shows the relationships between the ITCZ width, descent area, and total Hadley cell extent for the eight aquaplanet experiments, as well as their response to the +4 K perturbation. As for the GFDL-AM2.1 experiments, there is a strong negative relationship between the ITCZ width and descent area (Fig. 11a), and its slope is such that a narrow ITCZ tends to lead to a narrower Hadley cell (Fig. 11b). However, there is more scatter in the relationships compared to the GFDL-AM2.1 results (
As in Fig. 9, but for the eight aquaplanet simulations shown in Fig. 1.
Citation: Journal of Climate 32, 4; 10.1175/JCLI-D-18-0434.1
Although with such a small sample size (eight), the correlations between the ITCZ width and Hadley cell edge are not statistically significant; they are nevertheless suggestive that the mechanism demonstrated in section 3 is relevant for explaining the spread across the aquaplanet experiments. It is not surprising that the relationships are somewhat weaker compared to the targeted GFDL-AM2.1 experiments given the many differences in the formulations of the CMIP5 models including resolution and physical parameterization choices. It would be worthwhile to examine whether differing ITCZ width changes are related to Hadley cell extent changes in fully coupled simulations of the climate response to greenhouse gas forcing.
6. Summary and discussion
Theories for the expansion of the tropics and poleward shift of the eddy-driven jet are generally based on the amplified warming expected in the tropical upper troposphere, which leads to increased subtropical static stability (Lu et al. 2007; Frierson et al. 2007) and equator-to-pole temperature gradients (Kushner et al. 2001; Butler et al. 2010). Less attention has been given to the impacts that momentum budget changes associated with narrowing of the region of ascent in the deep tropics have onto the shifts of the Hadley cell edge and eddy-driven jet position. This study demonstrates that differing responses in the ITCZ width and Hadley cell strength to global warming can change the Hadley cell edge response by more than a factor of 2. This is shown with aquaplanet simulations in which the response of the ITCZ and Hadley cell strength depends strongly on its initial state. For those in which there is a substantial ITCZ narrowing and Hadley cell strength increase under warming, the Hadley cell expansion—and to a somewhat lesser degree, eddy-driven jet shift—is less than those in which there is not. This holds in both specified SST and slab ocean experiments.
There are various implications of this effect for the expected real-world response to greenhouse gas forcing. Comprehensive models suggest a significant spread in the response of the ITCZ width to warming [e.g., Fig. 1 of Byrne and Schneider (2016a)], although the multimodel mean does indicate a contraction. The complexity of the tropical circulation response to warming is apparent even in our single-model experiments (Fig. 4). Regardless of the reason for the varying tropical responses, some of the spread of the response of the Hadley cell edge and eddy-driven jet position across models may be attributable to this variability in the deep tropical response. Work to assess whether this is true in fully coupled CMIP5 experiments is ongoing. Another factor to consider is the prevalence of the double-ITCZ bias across comprehensive models. The idealized aquaplanet experiments analyzed here suggest that the sensitivity of the ITCZ to surface temperature increases is highest for a particular double-ITCZ configuration, obtained with the GFDL-AM2.1 model with the QOBS SST profile. Although the response is somewhat muted using a slab ocean configuration (Table 2), this does indicate that it is possible that comprehensive models may be overly sensitive in terms of their tropical responses of precipitation if they are simulating overly wide (i.e., double) ITCZs. Furthermore, our results suggest that the varying responses of both the ITCZ width and the Hadley cell strength are nonlinearly dependent on the control state. Again, it would be worth testing whether this holds across coupled models with realistic topography.
A caveat of this work is that it focuses on simulations forced by perpetual equinoctial, that is, hemispherically symmetric, conditions. The observed Hadley circulation has a strong seasonal cycle and through much of the year does not exhibit the purely antisymmetric state one would expect with permanent equinoctial forcing (Lindzen and Hou 1988; Dima and Wallace 2003). In practice, this means that apparent changes in annual-mean ITCZ width could arise from changes in the amplitude of the seasonal cycle of ITCZ shifts, not changes in the width of the ascent during any particular season. Thus, when examining models that include a seasonal cycle, or observations, it would be necessary to examine the dynamics of individual seasons in addition to the annual mean. Furthermore, in individual seasons, shifts in the position of the ITCZ can cause changes in the angular momentum budget of the tropical atmosphere, and hence Hadley cell extent, in a similar fashion to the width changes discussed in this work (Kang and Lu 2012; Hilgenbrink and Hartmann 2018).
Because of the hemispheric asymmetry in energy input to the atmosphere in current Earth’s climate, the mean position of the ITCZ is in the Northern Hemisphere instead of being centered on the equator (Frierson et al. 2013; Marshall et al. 2014). Shifts in the position of the ITCZ are a dominant response to many different forcings that alter the hemispheric energy balance (e.g., Chiang and Bitz 2005; Hwang et al. 2013). This is distinct from the hemispherically symmetric boundary conditions imposed in both our specified SST and slab ocean experiments, in which the ascent is constrained to be symmetric about the equator, and changes in the width of the ascent are the dominant response to global warming. In the real world, changes in the hemispheric energy balance under global warming may force shifts in the position of the ITCZ in addition to changes in its width. However, as stated above, the ITCZ moving closer to or farther from the equator can have analogous impacts onto the momentum budget of the upper troposphere in the tropics as width changes. Specifically, if the mean position of the ITCZ were to move even farther from the equator under warming (e.g., Seo et al. 2017), this may have the effect of increasing the Hadley cell expansion compared to what would occur with the ITCZ shift.
This work focuses on the zonal-mean changes of the atmospheric circulation. In reality, tropical precipitation has a strong zonally asymmetric component, and El Niño signal that we have used as an example of ITCZ contraction is highly asymmetric in the zonal direction. Thus, there may be additional effects on the higher-latitude circulation due to zonally asymmetric effects such as a weakening of the Walker circulation (Vecchi and Soden 2007). Furthermore, as discussed in the introduction, both ITCZ narrowing and Hadley cell strengthening can play a role in moderating the expansion of the Hadley cell and the eddy-driven jet shift under warming. Recent work examining comprehensive coupled models has found that the multimodel median indicates a narrowing and slight weakening of the ITCZ under global warming (Byrne et al. 2018). In addition, there is a strong correlation between the degree of narrowing and strengthening expected (models with more narrowing tend to have more strengthening). Thus, it would be helpful to know which mechanism is more important. However, it is difficult to disentangle the impacts of a narrowing ITCZ from a strengthening overturning circulation in the experiments performed in this study because there is a strong relationship between the width and strength changes: The cases with most ITCZ narrowing 
Future work will examine the connection between ITCZ width or position and Hadley cell extent across comprehensive climate model ensembles. Previous work has found a connection across CMIP3 models: Those in which the ITCZ shift farther from the equator under global warming tended to have larger Hadley cell expansion (Kang and Lu 2012). As discussed in section 5, in the idealized aquaplanet simulations shown in Fig. 1, there is a correlation between the response in ITCZ width 
Acknowledgments
O. W. was supported by the NOAA Climate and Global Change Postdoctoral Fellowship Program, administered by UCAR’s Cooperative Programs for the Advancement of Earth System Science. D. M. W. F. was supported by National Science Foundation Grant AGS-1665247. The authors thank Spencer Hill and two anonymous reviewers for insightful comments that significantly improved the manuscript. O. W. thanks Aaron Donohoe, Casey Hilgenbrink, Isla Simpson, and Robb Wills for helpful discussions on this work.
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