1. Introduction
The thermal structure of the atmosphere, and its circulation, are projected to change over the next few decades (IPCC 2013; Vallis et al. 2015). One of the most robust signals to emerge from climate models is the poleward shift of the latitudinal edges of the tropical belt (e.g., Hu et al. 2013; Kang et al. 2013). Such a widening of the tropical belt would directly affect the hydrological cycle in semiarid regions (IPCC 2014), as it pushes the dry zone (where most of the world’s deserts are concentrated) poleward, further drying the subtropics (e.g., Lu et al. 2007; Scheff and Frierson 2012).
In spite of the robust tropical expansion projected by climate models, large uncertainty remains in the observed trends over the last several decades (Davis and Rosenlof 2012; Allen et al. 2014; Lucas et al. 2014). Studies have reported that the tropical belt has been expanding poleward at a rate of 0.3°–2° decade−1 (Fu et al. 2006; Hudson et al. 2006; Hu and Fu 2007; Archer and Caldeira 2008; Seidel et al. 2008; Nguyen et al. 2013; D’Agostino and Lionello 2017). The wide range of observed trends is mostly due to the wide range of metrics used to define the tropical belt (e.g., tropopause height, subtropical jet, streamfunction, precipitation and evaporation, and outgoing longwave radiation). Furthermore, except for changes in mean meridional mass streamfunction, all other metrics were found to produce trends that are statistically insignificant (Davis and Rosenlof 2012). More recently, it has been shown that many of the metrics that were originally used do not correlate with the streamfunction expansion (Solomon et al. 2016; Davis and Birner 2017) and hence are not representative of a widening Hadley cell.
One of the main difficulties in observing a significant trend is the presence of both the internal variability of the climate system (e.g., Pacific decadal oscillation), which obscures the ongoing anthropogenic forced signal (Allen et al. 2014; Quan et al. 2014; Lucas and Nguyen 2015; Allen and Kovilakam 2017; Mantsis et al. 2017), and the multiplicity of forcing agents (Allen et al. 2012, 2014; Waugh et al. 2015) that are able to affect the width of the tropical circulation. For example, while enhanced greenhouse gas emissions were found to expand the tropical circulation in recent decades (e.g., Bony et al. 2013; Hu et al. 2013), the buildup of anthropogenic aerosols in the Northern Hemisphere were found to contract the circulation, and thus offset the expansion signal (Allen and Ajoku 2016). In the Southern Hemisphere, ozone depletion was found to be the main forcing of tropical expansion in the second half of the twentieth century (Son et al. 2009; Polvani et al. 2011b), and the ongoing recovery of ozone hole will therefore cancel the effect of greenhouse gases throughout the first half of the twenty-first century (Polvani et al. 2011a; Barnes et al. 2014).
The unabated emissions of greenhouse gases into the atmosphere is one of the main contributors to the Hadley cell expansion over the twenty-first century. Therefore, it is important to understand what controls tropical expansion under increased CO2 concentrations. To avoid complications from the presence of other forcings, we will here confine our attention to the 4 × CO2 scenario, which produces a large tropical expansion with unambiguous attribution. Furthermore, because the quadrupling of CO2 is instantaneous, it conveniently separates different components of the climate system, which respond with different time scales: this scenario, therefore, allows us to learn a great deal regarding the mechanism of tropical expansion.
Over the last decades, several mechanisms have been proposed to explain the tropical expansion, which we now briefly review. Enhanced CO2 concentrations warm both the troposphere and sea surface, while cooling the stratosphere. While several studies have claimed that changes in surface temperature Ts are mostly responsible for widening the tropical belt (e.g., Frierson et al. 2007; Adam et al. 2014; Son et al. 2018), others have argued that it is the atmospheric warming that matters the most (e.g., Lu et al. 2009; Bony et al. 2013). Part of this confusion stems from the fact that different time periods were chosen in different studies, as well as the different ozone forcings (Waugh et al. 2015).
Other studies (e.g., Lu et al. 2008; Ceppi and Hartmann 2013) have suggested that changes in eddy phase speed may drive the expansion of the Hadley cell under global warming. The rise of the tropopause height in warmer climates (Thuburn and Craig 1997; Santer et al. 2003) may affect the meridional temperature gradient aloft (Lorenz and DeWeaver 2007), and thus the eddy phase speed (Wittman et al. 2007). Because the mean zonal winds decrease from the midlatitudes equatorward, higher eddy phase speeds may push the subtropical critical latitude poleward (Chen and Held 2007). The dissipation of eddies at critical latitudes implies a poleward shift of the latitude of zero eddy momentum flux divergence. Since the latter balances the mean flow in steady state (e.g., Vallis 2006), this would push the latitudinal extent of the Hadley cell poleward.
Using idealized GCMs (e.g., Walker and Schneider 2006; Frierson et al. 2007; Levine and Schneider 2015), full atmospheric-only GCMs (e.g., Frierson et al. 2007), and coupled GCMs (e.g., Lu et al. 2007, 2008), it has been shown that, under global warming, the latitudinal extent of the Hadley cell follows the Held (2000) scaling. The tropical expansion was found to correlate with both the increase in subtropical static stability (e.g., Lu et al. 2008; Son et al. 2018) and the subtropical tropopause height, which pushes the baroclinic zone poleward (e.g., Lu et al. 2007).
In all above studies, the long-term response of the tropical expansion was studied (e.g., under statistically steady-state or slowly varying forcing) by correlating it with the above suggested components (e.g., Ts, tropopause height, static stability, and eddy phase speed). Since correlation alone need not imply a casual relation, especially in a highly coupled system, the relative importance of the different components that may drive the widening of the tropical belt could not be estimated.
Studying the evolution of the atmospheric circulation to abrupt change in CO2, on the other hand, will allow us to elucidate the relative importance of the different components in affecting the steady-state response. For instance, Wu et al. (2012) showed that under abrupt double CO2 forcing the stratosphere is first to respond, leading to changes in tropospheric eddies that alter the zonal mean circulation. While eddies were found to play an important role in these simulations, changes in eddy phase speed were found not to correlate with circulation changes (Wu et al. 2013; Staten et al. 2014). Using abrupt 4 × CO2 forcing in CMIP5 models, Grise and Polvani (2017) showed that while the Hadley cell expands and reaches 90% of its final location after the first few years, global-mean Ts monotonically increases reaching 90% of its final value only after several decades, implying that global-mean Ts is not the main driver of the Hadley cell expansion.
While the Hadley cell edge and global-mean Ts show different response times to quadrupling CO2, the poleward expansion of the dry zone (the latitude where precipitation equal evaporation) was found to follow the slower changes in global-mean Ts, rather than the rapid changes in the Hadley cell width (Grise and Polvani 2017). This implies that in spite of the high correlation between the edge of the Hadley cell (streamfunction expansion) and the edge of the dry zone (Quan et al. 2014; Solomon et al. 2016), different mechanisms control them. At steady state, changes in precipitation and evaporation are in balance with moisture flux divergence changes. Both long-term (Seager et al. 2010) and transient (Grise and Polvani 2017) responses point to the importance of the time-mean circulation and transient eddies in shifting the edge of the dry zone poleward. However, the effects of stationary eddy moisture flux, as well as the zonal mean flow, remain unclear.
Here, following Grise and Polvani (2017), the evolution of the Hadley cell expansion, the dry zone’s edge, and the above suggested mechanisms, are examined under abrupt 4 × CO2 forcing in CMIP5 models. The different time scales of the response of the relevant components will allow us to determine the relative importance of the different proposed mechanisms in widening the tropics. Since the steady-state result in these simulations stems from the transient response to the abrupt 4 × CO2 forcing, studying the transient response is a key for understanding the equilibrium widening of the Hadley. Because all mechanisms discussed above assume zonal symmetry, and because most of the continents reside in the Northern Hemisphere, the tropical response to increased CO2 is here studied only in the Southern Hemisphere, which provides the most appropriate conditions for validating the proposed mechanisms.
The paper is structured as follows. Section 2 describes the different simulations used to study the widening of the tropics, and the different metrics in question. The roles of the different atmospheric components and the two scaling theories on the Hadley cell expansion are examined in sections 3 and 4, respectively. The analysis of the mechanism behind the poleward shift of the dry zone edge is conducted in section 5. Finally, section 6 summarizes the results.
2. Methods
To study the evolution of the different components that affect the Hadley cell width, and the edge of the dry zone, three sets of the “r1i1p1” experiments from the CMIP5 multimodel ensemble (Taylor et al. 2012) are analyzed: first, 200 years of preindustrial control run with a constant 1850 forcing; second, 150 years of abrupt (set as initial conditions) 4 × CO2 forcing (relative to preindustrial values); and third, 140 years of 1% yr−1 increased CO2 run (starting from preindustrial values). For each experiment we analyze 23 coupled models, which are listed in Table 1.
List of the 23 CMIP5 models, and their atmospheric resolutions, analyzed in this study.
Two key metrics are used for estimating the width of the tropical belt, and are illustrated in Figs. 1a and 1b, respectively, from the preindustrial control run of the GFDL-ESM2G (Dunne et al. 2012):
- The edge of the Hadley cell
first changes sign poleward of its maximum value at 500 hPa (green cross in Fig. 1a); ϕ is latitude, p is pressure, υ is the meridional velocity, the overbar represents the annual mean, and the square brackets represent the zonal mean. The edge of the dry zone
Metrics for evaluating tropical expansion. (a) Zonal- and time-mean meridional mass streamfunction (kg s−1; colors) and eddy momentum flux (
Citation: Journal of Climate 32, 3; 10.1175/JCLI-D-18-0330.1
The above metrics are calculated after a 0.1° latitudinal cubic interpolation (which preserves the metrics’ latitudinal structure) of 
In addition, to properly analyze the different mechanisms of tropical expansion discussed in section 1, two other quantities, related to eddy components, are calculated as follows:
The latitude of maximum eddy momentum flux
The phase speed of maximum eddy momentum flux
The above eddy quantities are calculated after a 0.1° latitudinal quadratic interpolation (to capture the maximum value of eddy momentum fluxes). Only four models in Table 1 (GFDL-ESM2G, GFDL-ESM2M, MIROC5, and IPSL-CM5B-LR) have available daily data, which are required for calculating eddy fluxes. Thus, the eddy analysis in this study is performed for only these four models.
Finally, 
The different components in Eq. (1) [the Held and Hou (1980) scaling] are defined as follows;
The different components in Eq. (2) [the Held (2000) scaling] are defined as follows: The subtropical static stability N2 is averaged between 400–850 hPa, and the subtropical tropopause height He follows the above WMO definition. Vertically averaging the static stability up to a constant level, and not up to the tropopause height, allows disentangling the changes in both of these metrics, as discussed below.
The multimodel mean of each of the above quantities is calculated by first taking its annual and zonal average for each model separately, and then averaging across all models. All subtropical quantities are calculated between 35° and 45°S, and tropical quantities between 0° and 20°S. The subtropical latitudes are defined outside of the Hadley cell in order to properly examine the Held (2000) scaling, which includes extratropical quantitates. To estimate the response time of each of the above metrics and components to the abrupt 4 × CO2 forcing, we fit the following function, 
3. Assessing the importance of surface temperature and eddy phase speed
As discussed in section 1, two atmospheric components have been proposed to exert a strong influence on the widening of the Hadley cell: 1) the surface temperature Ts and 2) the eddy phase speed 
a. Surface temperature changes
Several previous studies argued for the importance of Ts in affecting the Hadley cell width. Both the global-mean Ts (e.g., Staten et al. 2012; Quan et al. 2014; Son et al. 2018) and the meridional Ts gradient (e.g., Frierson et al. 2007; Adam et al. 2014) have been suggested to affect the Hadley cell width. However, as these suggestions are based on correlations in the long-term response across many models, no causality arguments can be made. To examine whether changes in Ts vary together with the widening of the Hadley circulation, the evolutions of 
The annual and zonal multimodel-mean evolution relative to preindustrial values of the Hadley cell edge 
Citation: Journal of Climate 32, 3; 10.1175/JCLI-D-18-0330.1
The evolution of 
Changes in 
b. Eddy phase speed
Several studies (e.g., Lu et al. 2008; Ceppi and Hartmann 2013) have suggested that the increased eddy phase speed found under global warming (Chen and Held 2007) results in the poleward shift of the Hadley cell edge. Larger eddy phase speeds push the critical latitude poleward, so that eddies dissipate at higher latitudes. The associated shift of the latitude of zero eddy momentum flux divergence (i.e., the latitude where the mean meridional flow is zero) results in the widening of the Hadley cell. Again, since the above mechanism was examined by correlating the long-term response fields, the cause and effect relation remains unclear.
To determine if indeed the eddy phase speed causes the changes in latitude of maximum eddy momentum flux (which corresponds to the latitude of zero eddy momentum flux divergence) and the poleward shift of the Hadley cell edge, the evolution of 
(a) The annual and zonal multimodel-mean (over all models with available daily data) evolution, relative to preindustrial values, of the Hadley cell edge 
Citation: Journal of Climate 32, 3; 10.1175/JCLI-D-18-0330.1
The annual and zonal multimodel-mean (over all models with available daily data) response, relative to preindustrial values, of the Hadley cell edge 
Citation: Journal of Climate 32, 3; 10.1175/JCLI-D-18-0330.1
4. The relative importance of the Hadley cell width scalings and their components
Having shown that Ts and 
a. Comparing Hadley cell width scalings
Previous studies found that while changes in 
The statistically steady-state Hadley cell edge 
Citation: Journal of Climate 32, 3; 10.1175/JCLI-D-18-0330.1
As the poleward shift of 
(a) The annual and zonal multimodel-mean evolution relative to preindustrial values of the Hadley cell edge 
Citation: Journal of Climate 32, 3; 10.1175/JCLI-D-18-0330.1
b. The roles of static stability and tropopause height
Previous studies found that both components of the Held (2000) scaling [the tropopause height and static stability; Eq. (2)] correlate with 
(a) The evolution of the log of the ratio of the annual and zonal multimodel-mean Hadley cell edge based on Held (2000) scaling 
Citation: Journal of Climate 32, 3; 10.1175/JCLI-D-18-0330.1
What changes in the temperature field are responsible for the rapid and slow responses of 
To more clearly illustrate the effect of the tropopause height on 
c. Changes in subtropical baroclinicity
How come the tropopause height causes the response of 
To examine this possibility, a linear normal-mode instability analysis is conducted following Smith (2007), which was found to capture the baroclinic properties of eddies in both GCMs and reanalysis data (Jansen and Ferrari 2012; Chemke and Kaspi 2015, 2016a,b; Chemke et al. 2016). In this analysis the eigenvalue problem of the linearized quasigeostrophic potential vorticity (QGPV) equation is numerically solved at each latitude, using each model’s mean zonal wind, static stability, and tropopause height. The tropopause height sets the upper boundary of the problem. The resulting growth rates of the eddies are used as a measure for baroclinicity. First, the subtropical growth rate (the mean of the growth rate over all wavenumbers averaged over the subtropics) is calculated for each model using the zonal- and annual-mean fields of the preindustrial and of the statistically steady-state abrupt 4 × CO2 runs (last 50 years). Then, the growth rate of the statistically steady-state abrupt 4 × CO2 run is recalculated, but with the tropopause height in preindustrial values.
The difference in the growth rate between the 4 × CO2 and preindustrial runs is plotted against the difference between the 4 × CO2 run using the preindustrial tropopause height and preindustrial run (Fig. 8a). First, most of the models (17) show a decrease in the growth rate relative to preindustrial values (y axis in Fig. 8), with a multimodel mean value of −1.1 × 10−7 s−1 (red dot). This indicates that the baroclinicity decreases under the abrupt 4 × CO2 forcing. Second, if the tropopause height affects the growth rate, the dots in Fig. 8a should not fall on the 1:1 line (black line). However, in most models changes in growth rate are similar (blue dots on the 1:1 line; 
Changes relative to preindustrial values in mean (averaged over all wavenumbers) subtropical growth rate Δσ (s−1) as a function of (a) changes in growth rate while holding tropopause height in preindustrial values 
Citation: Journal of Climate 32, 3; 10.1175/JCLI-D-18-0330.1
In contrast, static stability changes have a large effect: the difference in the growth rate between the 4 × CO2 and preindustrial runs is plotted against the difference between the 4 × CO2 and preindustrial runs using the 4 × CO2 static stability values (Fig. 8b). Using fixed 4 × CO2 static stability values rather than preindustrial values, as was done for the tropopause height, precludes including stratospheric static stability values for the 4 × CO2 calculation, when the tropopause height increases. Keeping the static stability at abrupt 4 × CO2 values yields changes in baroclinicity that are different than when allowing the static stability to change (dots do not fall on the 1:1 line in Fig. 8b; 
In summary, the rapid increase of static stability (i.e., rapid decrease of the subtropical lapse rate) in the first ~10 years decreases the baroclinicity, and explains most of the poleward shift of the Hadley cell and of the eddy momentum flux. The tropopause height, on the other hand, seems to have a minor effect on baroclinicity, and thus shows different response time than the Hadley cell width. This result is in agreement with the results of Son et al. (2018), who found high correlation between changes in subtropical static stability and Hadley cell shift in past and projected climates.
d. Applicability to more realistic scenarios
The above analyses are conducted using the abrupt 4 × CO2 forcing. The abrupt change in CO2 helps disentangle the different response times of the different components that affect the Hadley cell expansion, and thus provides better understanding of the controlling mechanisms. In a more realistic scenario, however, CO2 increases continuously. To examine the above results in such a realistic scenario, the 1% CO2 increase per year scenario is now investigated. Figure 9a is similar to Fig. 6a, only showing the evolution under the 1% CO2 increase per year scenario, relative to preindustrial values, of the poleward shift of 
(a) The annual and zonal multimodel-mean evolution, from the 1% yr−1 increase in CO2, relative to preindustrial values of the Hadley cell edge 
Citation: Journal of Climate 32, 3; 10.1175/JCLI-D-18-0330.1
To see the different responses of static stability and tropopause height in the 1% CO2 increase per year scenario, the different terms of Eq. (3) are plotted in Fig. 9b. By construction, the sum of the change in static stability (green) and tropopause height (blue) equals the change in 
5. Edge of the dry zone
Another important metric for the edge of the tropics is the latitude where precipitation equals evaporation 
The evolution of 
The annual and zonal multimodel-mean evolution relative to preindustrial values of (a) the latitude where precipitation equals evaporation 
Citation: Journal of Climate 32, 3; 10.1175/JCLI-D-18-0330.1
Plotting the evolution of the contribution of the meridional divergence of the right-hand side of Eq. (5) to the shift of 
What causes the different behavior of transient and stationary eddy moisture fluxes? Figure 11 shows the vertically integrated, multimodel, zonal-mean latitudinal structure of the transient (Figs. 11a,c) and stationary (Figs. 11b,d) eddy moisture flux in the preindustrial run (red lines) and the statistically steady-state 4 × CO2 run (blue lines). Both transient and stationary eddy moisture flux are negative, as they act to transfer moisture from the tropics toward the South Pole (Figs. 11a,b). While the transient eddy moisture flux reaches maximum value at midlatitudes (~40°S), the stationary eddy moisture flux reaches maximum value in the lower subtropics (~25°S), where most of the Southern Hemisphere continents reside. Because of their different latitudinal structures, the transient eddy moisture flux tend to diverge moisture in the subtropics (positive values in Fig. 11c), while the stationary eddy moisture flux tends to converge moisture in the subtropics (negative values in Fig. 11d). Under increased CO2 concentrations, both the transient and stationary eddy moisture fluxes shift poleward (cf. red and blue lines in Figs. 11a and 11b). Thus, a poleward shift of the transient eddy moisture flux increases its divergence in the subtropics, while a poleward shift of the stationary eddy moisture flux increases its convergence in the subtropics (cf. red and blue lines in Figs. 11c and 11d), resulting in opposite tendencies, as shown in Fig. 10d, that lead to the total cancelation in the subtropics.
The annual and zonal vertically integrated multimodel-mean (over all models with available daily data) (a) transient eddy moisture flux (kg m−1 s−1), (b) stationary eddy moisture flux (kg m−1 s−1), (c) meridional divergence of transient eddy moisture flux (kg m−2 s−1), and (d) meridional divergence of stationary eddy moisture flux (kg m−2 s−1) as a function of latitude. In all panels, the red and blue lines respectively represent the preindustrial run and the average over last 50 years of the abrupt 4 × CO2 forcing run.
Citation: Journal of Climate 32, 3; 10.1175/JCLI-D-18-0330.1
6. Summary
The projected widening of the tropical circulation with increased greenhouse gases (e.g., Lu et al. 2007; Vallis et al. 2015) and the associated shift of the dry zone edge have important societal impacts. Several mechanisms have been suggested to explain this poleward shift, but the coupling among the different atmospheric components did not allow for a quantification of their relative importance in expanding the tropics. Here, using the abrupt quadruple-CO2 scenario runs from the CMIP5, we have been able to separate the relevant mechanisms of tropical expansion. The different response times of the various mechanisms has enabled us to disentangle the different components in question and elucidate their relative importance.
Upon abrupt 4 × CO2, the Hadley cell rapidly expands poleward, with a response time of ~7 years (Grise and Polvani 2017). Confirming several earlier studies (e.g., Frierson et al. 2007; Lu et al. 2007, 2008), we find that this expansion can be explained with the Held (2000) theory, where changes in subtropical baroclinicity control the width of the Hadley cell. In addition, we have shown that a rapid increase in static stability (i.e., decrease in the subtropical lapse rate) in the first few years of the simulation leads to the rapid poleward shift of the Hadley cell edge. This was found to occur in both the abrupt 4 × CO2 and the 1% yr−1 increase forcing. Changes in tropopause height, however, were found to divert the poleward shift of the Held (2000) theory from that of the Hadley cell edge. Unlike the static stability, the increase in tropopause height was found to have a minor effect on subtropical baroclinicity, and thus on the widening of the tropical belt.
Changes in subtropical baroclinicity also affect subtropical eddy fields. As in Ceppi and Hartmann (2013), we find that the poleward shift of the Hadley cell is indeed highly correlated with the poleward shift of subtropical eddy momentum flux. Several studies have suggested that this correlation might not be solely due to changes in eddy generation (i.e., baroclinicity) outside the tropics, but also due to an increase in eddy phase speed, which pushes the critical latitude poleward (where eddies dissipate) together with the subtropical eddy momentum flux. Here, however, we have shown that the increase of the eddy phase speed under abrupt 4 × CO2 (e.g., Chen and Held 2007) does not correlate with the poleward shift of the Hadley cell edge. And, more importantly, changes in global-mean surface temperature and meridional surface temperature gradient were also found not to accompany changes in Hadley cell width.
Unlike the poleward shift of the Hadley cell edge, which is relatively fast, the poleward shift of the dry zone edge has a slower time response, and monotonically increases throughout the simulation (Grise and Polvani 2017; Seviour et al. 2018). In the first few years after CO2 quadrupling the rapid poleward movement of the Hadley cell edge shifts the dry zone edge poleward. This accounts for most of the poleward shift of the dry zone edge. The slower increase in moisture further pushes the dry zone edge poleward through the rest of the run. Eddy moisture flux was found to have a minor contribution to shifting the dry zone edge poleward, due to the cancelling effects of transient and stationary eddies, which, respectively, diverge and converge moisture in the subtropics.
The widening of the Hadley cell is only one example of circulation changes under increased CO2 concentrations. For each of these circulation changes, one can find several mechanisms in the literature. Having several mechanisms for one physical phenomenon stems from the difficulty of isolating any causality in such a coupled and complex system, which leads to develop mechanisms based on correlating the long-term fields. However, studying the evolution of the different components in the atmosphere to an abrupt and strong forcing allows taking a step toward causality, and thus having better physical understanding of the climate system.
It is important to note that the key role of static stability that we have found in our study, that of modulating the Hadley cell edge under increased greenhouse gases, does not necessarily apply under different forcings. For example, analyzing different idealized warming patterns, Tandon et al. (2013) argued that the wind shear, and not static stability, is most important for changing subtropical baroclinicity and the edge of the Hadley cell. Thus, the widening of the circulation in recent years, which is likely driven by both internal variability (Allen et al. 2014; Quan et al. 2014; Lucas and Nguyen 2015; Allen and Kovilakam 2017; Mantsis et al. 2017) and different forcing agents (Allen et al. 2012, 2014; Waugh et al. 2015), may stem from other components and not only from static stability. Nonetheless, the main result of our study is that subtropical static stability is key to tropical expansion under increased CO2 concentrations. In accordance with our results, Shaw and Tan (2018) have recently demonstrated the importance of subtropical changes in modulating the width of the Hadley cell under increased greenhouse gases.
Finally, the next step for fully understanding the widening of the Hadley cell under increased greenhouse gases should focus on elucidating the different warming rates in the lower and upper subtropical troposphere (i.e., the increase in static stability). To accomplish that, as was done in Chemke and Polvani (2018), one has to analyze the thermodynamic equation and quantify for the different terms that affect the temperature field. Currently, only one of the CMIP5 models provides all the terms needed to analyze the thermodynamic equation, but this is not sufficient for disentangling the natural variability and the model biases from the forced response.
Acknowledgments
This research 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 (CPAESS). We acknowledge the WCRP’s Working Group on Coupled Modeling, which is responsible for CMIP, and we thank the climate modeling groups (listed in Table 1) for producing and making available their model output. For CMIP the U.S. Department of Energy’s PCMDI provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. LMP is grateful for the continued support of the U.S. National Science Foundation.
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