Evaluating the large-scale hydrological cycle response within the PlioMIP2 ensemble

The mid-Pliocene (~3 million years ago) is one of the most recent warm periods with high CO2 concentrations in the atmosphere and resulting high temperatures and is often cited as an analog for near-term future climate change. Here, we apply a moisture budget analysis to investigate the response of the large-scale hydrological cycle at low latitudes within a 40 https://doi.org/10.5194/cp-2021-72 Preprint. Discussion started: 13 July 2021 c © Author(s) 2021. CC BY 4.0 License.


Introduction
Earth's hydrological cycle can induce regional and global climate anomalies, thereby regulating the balance of global water resources (Eltahir and Bras, 1996). Many studies have indicated that pronounced climate change can occur as anthropogenic 55 CO2 rises, including an increase in surface temperature (Xie et al., 2010;Long et al., 2014), Arctic amplification (Stuecker et al., 2018;Smith et al., 2019), and impacts on animal and plant populations (Root et al., 2003). Under current global warming, both observations and model simulations suggest a tendency for the wet-regions-getting-wetter-and-dry-regions-getting-drier phenomenon (Held and Soden, 2006;Wentz et al., 2007;Chou et al., 2009;Wang et al., 2012;Li et al., 2013). That is, rainfall minus evaporation increases (decreases) in regions of climatological convergence (divergence). Note that these 60 changes in the large-scale hydrological cycle could induce severe climatic disasters worldwide, leading to considerable impacts on economies, ecosystems, and agriculture (Asokan and Destouni, 2014;Bengtsson, 2014). Therefore, understanding the potential processes responsible for large-scale hydrological cycle changes in a warmer climate is of great importance.
Previous studies have suggested that the thermodynamic effect caused by increased atmospheric moisture content in a warmer climate is one of the primary contributors to a tendency of wet gets wetter and dry gets drier (Chou et al., 2009;Seager et al., 2010). This mechanism directly follows the nonlinearity of the Clausius-Clapeyron relationship, which acts to enhance the increase in atmospheric moisture content over regions with the warmest surface temperatures (Allen and Ingram, 2002;Stephens and Ellis, 2008). On the other hand, large-scale atmospheric circulation can change substantially due to nonuniform temperature changes under global warming and hence induce changes in the hydrological cycle via the so-called 70 dynamic effect (Han et al., 2019a). The dynamic effect is relatively more complicated than the thermodynamic effect among climate models. Seager et al. (2010)  circulation and Walker circulation. Increased CO2 concentration could directly increase atmospheric static stability over tropical oceans, favoring a slowdown of these atmospheric overturning circulations (Vallis et al., 2015). Other studies have indicated that local Hadley circulation shifts poleward due to the decreased meridional temperature gradient in response to 75 increased CO2 concentrations (Sharmila and Walsh, 2018;Hu et al., 2018a). These circulation anomalies widen the subtropical dry zones (Previdi and Liepert, 2007;Sun et al., 2013b). In addition, Long et al. (2016) highlighted that model uncertainty in tropical rainfall comes from the discrepancies of the atmospheric circulation anomalies among models. Thus, the spread of circulation changes in response to global warming across climate models leads to a diversity of responses in the hydrological cycle.

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Proxy data indicate that the mid-Pliocene (~3 million years ago) was one of the most recent warm periods with CO2 levels similar to the current anthropogenically elevated value of 400 ppm and can be considered an analog for future climate change (Dowsett et al., 2012;Burke et al., 2018;Tierney et al., 2019). Pliocene Model Intercomparison Project Phase 1 (PlioMIP1) simulations have been used to investigate how the climate system responded to mid-Pliocene boundary conditions, including elevated atmospheric CO2 concentrations. These past warm climate simulations exhibit many 85 similarities with future climate projections. For example, one robust characteristic is increased temperature from 1.8 to 3.6℃ during the Pliocene compared with the preindustrial period (PI) , with Arctic amplification in response to a significant sea-ice extent decline (Howell et al., 2016;Zheng et al., 2019). These features could have reduced the meridional surface temperature gradient, inducing weaker tropical circulation (i.e., local Hadley circulation) during the Pliocene (Sun et al., 2013a;Li et al., 2015;Corvec and Fletcher, 2017). Additionally, some studies have suggested a 90 weakened zonal sea surface temperature (SST) gradient in the Pacific during the Pliocene (Wara et al., 2005;Scroxton et al., 2011), which would have favored weaker Walker circulation. These features could have induced large-scale changes in Pliocene hydroclimate. Using a climate simulation that captures the warming patterns seen in Early Pliocene sea surface temperature proxies, Burls and Fedorov (2017) suggested that the dynamic process might play a key role in driving wetter subtropics due to this weaker tropical circulation during the Early Pliocene warm climate compared with the future climate.

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Although PlioMIP1 can reproduce similar patterns of the change in surface temperature to the reconstructed SST, models cannot capture the magnitude of warming at higher latitudes. For example, Dowsett et al. (2012) indicated that the ensemble of PlioMIP1 models underestimates the warming in the North Atlantic compared with the reconstructed SST. This might be induced by the uncertainties in PlioMIP1, including the uncertainty in atmospheric CO2 concentrations Howell et al., 2016;Samakinwa, 2018) and paleogeography and bathymetry Feng et 100 al., 2017;Samakinwa, 2018). In PlioMIP2 models, the boundary conditions have been updated using the new version of the U.S. Geological Survey PRISM4 dataset Haywood et al., 2016). These include the closed Canadian Archipelago and Bering Strait and a reduced Greenland ice sheet relative to PlioMIP1. In addition, PlioMIP2 focuses on a specific time slice during the mid-Pliocene at approximately 3.025 Ma, which could reduce the uncertainties in reconstructions (Mcclymont et al., 2020). Researchers have been investigating the mid-Pliocene climate by using PlioMIP2, 105 including Arctic warming (De Nooijer et al., 2020), Atlantic meridional overturning circulation (Zhang et al., 2021b), https://doi.org/10.5194/cp-2021-72 Preprint. Discussion started: 13 July 2021 c Author(s) 2021. CC BY 4.0 License. climate sensitivity (Haywood et al., 2020), global monsoons (Zhang et al., 2021a), and subtropical rainfall changes (Pontes et al., 2020). However, it is difficult to distinguish the relative impact of the Hadley circulation and Walker circulation on Pliocene hydrological cycling at low latitudes. Fortunately, the three-pattern decomposition of global atmospheric circulation (3P-DGAC; Hu et al., 2017Hu et al., , 2018b method can help us to decompose atmospheric circulation into zonal (i.e., local 110 Walker circulation) and meridional (i.e., local Hadley circulation) circulation at low latitudes. We apply this method to develop moisture budget analyses, which might provide some insight into the mechanisms of hydrological cycling during the mid-Pliocene. This paper is in the framework of updated PlioMIP2 models to quantitatively distinguish the relative contribution from zonal and meridional circulation anomalies to hydrological cycle changes. In the following section, we first introduce 115 the PlioMIP2 models and moisture budget decomposition. We then evaluate the simulated large-scale hydroclimate cycle response within the PlioMIP2 ensemble in section 3. Section 4 provides each moisture budget component's relative contribution to investigate the potential mechanisms driving the simulated changes in the mid-Pliocene hydrological cycle.
The corresponding mechanisms are discussed in section 5. The last section contains the conclusion and discussion.

Climate model simulations
In this study, we use the simulations from 13 models participating in PlioMIP2 (Table 1). All models include a preindustrial (PI) simulation and a Pliocene climate simulation. To calculate the ensemble mean, we interpolate all data onto a common grid with a 1°×1°resolution using bilinear interpolation.

Development of moisture budget decomposition
To examine the changes in precipitation (P) minus evaporation (E) in the PlioMIP2 mid-Pliocene experiments relative to their respective PI simulation, we decompose the moisture budget equation based on Seager et al. (2010), i.e., 145 R dp V q g dp Here, g is gravity, w  is the density of water, V  is the horizontal wind, and q is the specific humidity.
) (  is the annual mean difference of variables between the warmer climate state (mid-Pliocene) and PI simulation. Subscript 0 represents the variables in the PI simulation. In the warmer climate, the change in P minus E [PmE, the left-hand side of Eq.
(1)] is balanced by the thermodynamic (δTH, induced by increased specific humidity) and dynamic (δMCD, induced by 150 circulation anomalies) contributions and residual term (R, which is mainly involved in the contributions from high-frequency variability of transient eddies, nonlinear effects and surface boundary terms).
Since we are interested in understanding the relative contribution from zonal circulation (i.e., local Walker (2) and the following continuity equations are satisfied: Because three-pattern circulations (horizontal, meridional and zonal circulations) exist in both the low and middlehigh latitudes, the global atmospheric circulation can be expressed as the superposition of the horizontal, meridional and 170 zonal circulations, that is, with the following components: (6) is called the three-pattern decomposition model.

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In contrast to the traditional two-dimensional decomposition of the atmospheric motion into vortex and divergent parts, the continuity Eq. (5) cannot guarantee the uniqueness of the stream functions ( , , ) have three spatial dimensions, respectively (Hu et al., 2017(Hu et al., , 2018a. The following restriction condition is needed to pick up the correct decomposition (Theorems 1 and 2 in Hu Equation (7) guarantees both the uniqueness of the stream functions R , H and W and the physical rationality of the 3P-DGAC method.
Using the 3P-DGAC method, we can rephrase the moisture budget in Eq. (1) to involve the contributions from zonal and meridional circulation. Here, we neglect the relatively smaller terms at low latitudes, including transient eddies, nonlinear effects and surface boundary terms. Thus, we mainly explore the contributions from δTH and δMCD to changes in PmE in this study. Then, the δTH and δMCD can be rewritten as: where subscripts D and A represent the terms that are related to divergence and moisture advection, respectively. In addition, 190 the subscripts R, Z, and M indicate the terms that are related to the horizontal, zonal and meridional circulations, respectively.
Note that R V  represents the horizontal vortex winds, which are nondivergent, which indicates that the terms that are related ) are zero. These terms can be clearly seen in Figs. 3(h) and 4(h). In addition, we ignore these two terms in this study.  Ocean], as well as at mid-high latitudes ( Fig. 1(a)). However, some models, i.e., the CESM2, GISS-E2-1-G, COSMOS and HadGEM3 models, show a drier Maritime Continent (Fig. 2), which might be related to the changes in Walker circulation 205 (we will discuss this latter in Section 5.3). In addition, the North African and Southeast Asian monsoon regions also show significant moistening signals, which are consistent with faunal remains and palynological transfer functions (Sanyal et al., 2004;Trauth et al., 2007;Xie et al., 2012), as well as with other modeling studies (Zhang et al., 2019a;Li et al., 2020;Feng et al., 2021). This change over Southeast Asia is robust among PlioMIP2 models, and only the COSMOS model shows a drier change over East Asia (Fig. 2(e)). Furthermore, the MMM PmE changes over Southeast Asia are mainly focused on the 210 summer time (not shown), suggesting a consequence of strengthened East Asian summer monsoon circulation (Salzmann et al., 2008;Wan et al., 2010;Yan et al., 2012;Li et al., 2018;Lu et al., 2021). Note that the mid-to highlatitude North Atlantic becomes drier (Fig. 1), and this change might be caused by the simulated largely increased SST in the North Atlantic ( Fig. 5(b)).

Changes in precipitation minus evaporation (PmE) in the PlioMIP2 models
Generally, the response of the hydrological cycle during the mid-Pliocene generally shows a wet-regions-getting-215 wetter-and-dry-regions-getting-drier pattern, especially over the ocean. These features are apparent in the zonal average of the PmE change ( Fig. 1(b)), except in the GISS-E2-1-G model ( Fig. 2(f)). The tropical regions become wetter, and subtropical regions become drier, which are similar to the results from future high-CO2 scenario experiments (Chou et al., 2009).

Thermodynamic and dynamic contributions to changes in PmE
Moisture budget analyses are conducted to shed light on the mechanisms driving the changes in PmE during the mid-230 Pliocene. Based on this decomposition, the changes in PmE are mainly influenced by the change in humidity with unaltered atmospheric circulation (called the thermodynamic term, δTH) and change in atmospheric circulation with no change in humidity (called the dynamic term, δMCD) at low latitudes. The thermodynamic term (δTH) and its decomposition are plotted in Fig. 3. It is clear that δTH captures the main features of hydrological cycle change (Figs. 3(a) vs 1(a)). That is, the positive and negative contributions over the already convergent (i.e., ITCZ and SPCZ) and divergent (subsidence of local 235 Hadley circulation) regions, respectively. This term does not alter the spatial distribution of climatological PmE (contours in Fig. 1(a)) but amplifies the intensity of the existing pattern of PmE, reflecting the wet-getting-wetter-and-dry-getting-drier https://doi.org/10.5194/cp-2021-72 Preprint. Discussion started: 13 July 2021 c Author(s) 2021. CC BY 4.0 License. mechanisms (Held and Soden, 2006). The results are consistent with future global warming scenarios (Chou et al., 2009;Wang et al., 2012;Li et al., 2013).
From the perspective of global atmospheric circulation, previous studies have indicated that global atmospheric 240 circulation can be decomposed into a superposition of horizontal, meridional, and zonal circulations (Hu et al., 2017(Hu et al., , 2018a. δTH is further decomposed by using the 3P-DGAC method (Fig. 3(c)-(k)). The estimated δTH in Fig. 3(b), calculated by the sum of the right-hand side in Eq. (8) of the 3P-DGAC decomposition method shows a similar distribution to the δTH field shown in Fig. 3(a) with a pattern correlation coefficient (PCC) of 0.80. This result indicates that the decomposition is representative. At low latitudes, the δTH mainly comes from terms that are related to climate mean meridional and zonal 245 circulation (Fig. 3(c) and (d)); while at mid-high latitudes, the δTH mainly comes from horizontal circulation (Fig. 3(e)). It is clear that the thermodynamic changes associated with meridional circulation can explain the large portion of δTH (PCC of 0.9) at low latitudes, which is caused by increased specific humidity within the divergence of climate mean meridional circulation ( M D TH _  ; Fig. 3(f)). The zonal circulation can also explain δTH to some extent, with a positive contribution mainly over Maritime Continent extending eastward to the equatorial central Pacific and eastern coast of North/South 250 America, and a negative contribution over the eastern Pacific extending from the western Indian Ocean to the Greater Horn of Africa and the eastern tropical Atlantic (Fig. 3(d)). These changes associated with zonal circulation are linked to the increased specific humidity with unaltered divergence of the mean zonal circulation ( Z D TH _  ; Fig. 3(g)). At mid-high latitudes, the δTH induced by climate mean horizontal circulation is caused by changes in moisture advection (Fig. 3(k)).  It is evident that the δTH component does not describe the full contribution to the changes in PmE, especially over the North African and Southeast Asian monsoon regions, SPCZ and North Indian Ocean, where we must consider the 265 dynamic effect. The dynamic effect (δMCD), reflecting the impact of circulation changes, partially offsets the δTH at low latitudes ( Fig. 4(a)). In particular, δMCD reduces PmE in the deep tropics, i.e., the ITCZ, SPCZ and Maritime Continent. In contrast, δMCD can moisten subtropical regions, especially over the subtropical eastern Pacific, southern Indian Ocean and Atlantic Ocean of both hemispheres. Compared with δTH, the overwhelming contribution from δMCD to changes in PmE lies adjacent to the North Indian Ocean, SPCZ and the North African and Southeast Asian monsoon regions ( Fig. 4(a)).

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The estimated δMCD in Fig. 4(b), calculated by the sum of the right-hand side terms in Eq. (9) of the 3P-DGAC decomposition method, is consistent with the δMCD in Fig. 4(a)  to dry the southern part of the deep tropics but moisten its northern part, which is associated with the northward shift of the ITCZ (Fig. 7(c)). In particular, the northward shift of the ITCZ is clear from 150°E to the east in the Pacific (Fig. 4(f)). In 275 addition, the component contributes a large portion to enhance PmE over the North African and Southeast Asian monsoon regions. However, the tropical southern Pacific is even more complicated. There is a tendency to reduce PmE via the M D MCD _  term but increase its southern part in the tropical southern Pacific. A previous study suggested that these changes in PmE followed the southward shift of the SPCZ, which was mainly modulated by the intensified and westward shift of the South Pacific subtropical high for the mid-Pliocene compared with the PI simulation (Pontes et al., 2020). For the 280 adjacent north Indian Ocean, the convergence of zonal circulation anomalies (

Z D MCD _ 
) is the first-order contribution to strengthen the dynamic effect and hence enhance the PmE (Fig. 4(g)). In summary, the thermodynamic term induced by the divergence of the mean meridional circulation is the dominant process driving changes in PmE at low latitudes (Fig. 3(f)). However, the dynamic term partially offsets δTH, especially over the ITCZ, SPCZ and Maritime Continent, via changes in the divergence of meridional circulation. Even the dynamic term overwhelmingly contributes to the increased changes in PmE over North African and Southeast Asian monsoon regions and the North Indian Ocean. Note that the former two are mainly caused by meridional circulation anomalies, but the latter is 295 dominated by zonal circulation anomalies.
We further decompose the meridional moisture transport into terms that reflect the changes in specific humidity (meridional moisture transport induced by the thermodynamic effect; MMTT) and circulation (meridional moisture transport induced by the dynamic effect; MMTD) in Fig. 5(a). As expected, all models show that the MMTT is responsible for the wetter tropics and drier subtropics in the mid-Pliocene simulation, indicating a dry-gets-drier-and-wet-gets-wetter 300 mechanism. These features are robust among models and are associated with the increased specific humidity combined with the mean meridional circulation from the PI control ( Fig. 5(b)), as mentioned above. This is because the zonal-mean wind depicts southerly (northerly) wind between the equator and subtropical SH (Northern Hemisphere; NH) for the climate mean meridional circulation in the PI simulations. When the climatological wind is combined with increased specific humidity in the low-level troposphere ( Fig. 6(b)), more moisture is transported from the subtropics to the tropics, resulting in drier 305 subtropics and wetter tropics. In contrast, the MMTD shows a large spread across PlioMIP2 models. On average, the anomalous MMTD appears to weaken thermodynamic contributions in the subtropical NH but strengthen it in the subtropical SH via meridional circulation anomalies (Fig. 5(c)). This indicates that the changes in MMTD favor the transport of more (less) moisture from the tropics to the NH (SH) subtropics, which is caused by the northward shift of the meridional circulation (as detailed further in section 5.2). The equatorward moisture transport anomalies of SH's dynamic component 310 are due to anomalous southerly winds in the subtropical southern Pacific (Fig. 8). This feature acts to dry the SPCZ and moisten the south SPCZ and the equatorial central-eastern Pacific (Fig. 4(f)).

Mechanisms for the changes in moisture budget components
Thus far, we have shown that the anomalous hydroclimate within the mid-Pliocene simulations involves anomalies of both 320 thermodynamic and dynamic effects at low latitudes. In this section, we further examine the corresponding mechanisms in turn.

Changes in specific humidity
Figure 6(a) shows the changes in MMM SST superimposed on the reconstructed SST anomalies (Mcclymont et al., 2020). In the MMM, SSTs range from between 1 and 6℃ warmer in the mid-Pliocene simulations than in the PI simulations. The 325 most pronounced SST warming is located in the Southern Ocean, North Atlantic, and North Pacific, consistent with current studies (Haywood et al., 2020;Williams et al., 2021). Following the Clausius-Clapeyron relationship, this global warming https://doi.org/10.5194/cp-2021-72 Preprint. Discussion started: 13 July 2021 c Author(s) 2021. CC BY 4.0 License.
increases specific humidity in the low-level troposphere (Fig. 5(b)). On the other hand, the sinking branch of meridional circulation in the control climate is located in subtropical regions, showing divergent circulation  Fig. 6(d) ;Hastenrath, 1991;Peixoto and Oort, 1992).

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With increased specific humidity ( shows positive contribution and hence increase PmE in these regions (Figure 3g). In the contrary, the Z D TH _  favors to decrease PmE over the western Indian Ocean, eastern Pacific and tropical Atlantic (Figure 3g), where the climate mean zonal circulation is divergent (Figure 6d).

Response in meridional circulation
In section 4, we have demonstrated that the primary dynamic contribution to changes in PmE is a consequence of anomalous meridional circulation ( Figure 7(a) shows the annual mean mass stream function (MSF) of meridional circulation for PI simulation (contours), which is similar to present-day meridional circulation (Cheng et al., 2020). During the mid-Pliocene, the meridional circulation changes are characterized by enhanced meridional circulation in the SH tropics 350 and weakened meridional circulation in the NH tropics (shading in Fig. 7), which is caused by the northward shift of meridional circulation in the SH, as indicated in our later discussion. To quantify meridional circulation changes, we further calculated the intensity in Fig. 7(b). The intensity is defined as the maximum of the absolute average MSF between 200 hPa and 925 hPa in the range of 30°S to 30°N (Oort and Yienger, 1996) in Fig. 7(a). Models simulate a consistently weakened meridional circulation intensity in the NH and a slightly strengthened intensity in the SH (Fig. 7(b)), which is related to the 355 hemispheric asymmetry of the atmospheric energy budget .
As a result, meridional circulation anomalies could induce divergent/convergent circulation anomalies in the lowlevel troposphere (Fig. 8). The weakened local meridional circulation leads to anomalous southerly winds spanning northeastern South America eastward to the northwestern Pacific region. These meridional circulation anomalies induce the anomalous divergence (convergence) of circulation over the Indo-Pacific warm pool (adjacent to subtropic regions), 360 resulting in a negative (positive) contribution from M D MCD _  (Figs. 8 vs 4(f)). In fact, this anomalous meridional circulation is closely related to the strengthened Asian summer monsoon (not shown), consistent with previous studies Prescott et al., 2019). In addition, anomalous northerly winds exist in the western tropical Pacific, but southerly winds are located in the central Pacific. These circulation anomalies could induce M D MCD _  , which favors moistening of the equatorial central Pacific and southern part of the SPCZ region but dries the SPCZ (Fig. 4(f)). Previous 365 studies have indicated that these circulation anomalies are caused by the southward shift of the SPCZ, which is mainly modulated by the intensified and westward shift of the South Pacific subtropical high for the mid-Pliocene compared with the PI simulation (Pontes et al., 2020).
One question arises as to what causes the meridional circulation changes in mid-Pliocene conditions. At low latitudes, it is worth noting that the ITCZ lies at the foot of the ascending branch of the meridional circulation, which is 370 highly linked to the hemispheric asymmetry of the atmospheric energy budget (Frierson et al., 2013). We further quantify the shift of the ITCZ in Fig. 7(b) and Earth's energy budget in Fig. 7(c) and (d). The definition of the ITCZ location is the latitude of the maximal annual mean precipitation between 20°S and 20°S (Frierson and Hwang, 2012;Donohoe et al., 2013).
On average, ensemble models show that the NH atmosphere receives 1.5 W˙m -2 more net radiation than the SH (Fig. 7(d)), which could induce an increased cross-equatorial southward energy flux of 0.22 PW (Fig. 7(e)). Thus, this imbalance in the atmospheric energy budget causes a 1.1°northward shift in the zonal-mean ITCZ latitude. Consequently, this shift of the ITCZ reorganizes atmospheric circulation (Watt-Meyer and Frierson, 2019), leading to the northward movement of the meridional circulation in the SH (Fig. 7(a)). This meridional circulation shift could result in a weakened (strengthened) meridional circulation in the NH (SH) (Fig. 7(a)) and hence drive M D MCD _  ( Fig. 4(f)) and MMTD (Fig. 5(c)).

Response in zonal circulation
As mentioned above, Z D MCD _  plays a key role in the changes in PmE over the northern Indian Ocean. As this term is linked to Walker circulation anomalies, we further discuss Walker circulation changes in the mid-Pliocene warm climate.
There is a noticeable diversity in the simulated Pacific Walker circulation (PWC) intensity across the models, with 395 the mean PWC intensity strengthened by 1 × 10 10 kg/m 2 compared with the PI simulation ( Fig. 9(a)). In addition, previous work has suggested that the PWC intensity is closely tied to the zonal SST and SLP gradient during the mid-Piacenzian (Tierney et al., 2019). In this paper, the dSLP and dSST are defined as the difference in SLP and SST across the equatorial Indo-Pacific (160°W-80°W, 5°S-5°N minus 80°E-160°E, 5°S-5°N). The PlioMIP2 models produce a large spread in simulating the changes in dSST (Fig. 9(c)) and dSLP ( Fig. 9(d)), which is consistent with the results in Fig. 9(a). Previous 400 studies have suggested that the east-west SST gradient was reduced in SST proxies (Tierney et al., 2019). This feature is captured by the CESM2, GISS-E2-1-G and HadGEM3 models ( Fig. 9(a)-(c)). However, other models, i.e., the CCSM4, CCSM4-Utrecht, EC-Earth3-LR, and NorESM-L models, consistently simulated stronger PWC intensity ( Fig. 9(a)-(c)). That is, the results suggest that the model-simulated changes in the strength of PWC are probably highly model dependent, which might be affected by the different parameterizations (Tierney et al., 2019).  However, the westward shift of PWC is a robust feature among these models except the COSMOS and GISS-E2-1-G models ( Fig. 9(b)). To discuss the impact of the PWC shift on atmospheric circulation in the tropics, we further calculate 415 the changes in the zonal mass stream function (ZMS) for the mid-Pliocene with respect to the PI simulation in Fig. 10. As suggested in Fig. 6(d), the ZMS in the PI simulation (contours in Fig. 10(a)) is characterized by ascending in the tropical western Pacific and Maritime Continent and descending in the western Indian Ocean and eastern Pacific, consistent with previous studies (Kamae et al., 2011;Bayr et al., 2014;Ma and Zhou, 2016;Han et al., 2020). Compared with the PI simulation, the most striking features in the mid-Pliocene simulation are weakened ascending over the Maritime Continent 420 and tropical western Pacific and strengthened descending on the western Indian Ocean, indicating a westward movement of the PWC (Fig. 10(b)).

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The westward shift of the PWC can also be seen from the potential velocity (Fig. 11). This shows that the center of anomalous positive values is located in the northern Indian Ocean. In contrast, the center of a negative value exists in the equatorial eastern Pacific and western Atlantic in the low-level troposphere ( Fig. 11(a)). Concurrent, generally opposite anomalies can be seen in the upper-level troposphere ( Fig. 11(b)). Indeed, these features indicate an upward (downward) 435 motion shift from the tropical western Pacific (eastern Pacific) to the west of the Indian Ocean (central Pacific), resulting from the westward shift of the PWC (Figs. 9(b) and 10). That is, when divergent/convergent circulations are combined with the climate mean specific humidity ( 0  q ) in the lower troposphere, they can trigger a negative/positive contribution from the Z D MCD _  term to changes in PmE ( Fig. 4(g)).
https://doi.org/10.5194/cp-2021-72 Preprint. Discussion started: 13 July 2021 c Author(s) 2021. CC BY 4.0 License.  The PlioMIP2 models show large spatial differences in PmE. The MMM generally depicts a wet-regions-getting-450 wetter (i.e., ITCZ, Maritime Continent and monsoon regions) and-dry-regions-getting-drier (i.e., a sinking branch Hadley circulation) pattern during the mid-Pliocene warm climate. According to the moisture budget equation, a large part of the changes in PmE at low latitudes are due to the increased specific humidity. However, the thermodynamic component cannot fully explain the changes in PmE. The dynamic effects offset the thermodynamic effects to some extent and even determine a larger contribution to the changes in PmE in the southern tropical Pacific and northern Indian Ocean. We find increased 455 hemispheric asymmetries of the atmospheric energy budget (larger atmospheric energy over NH than SH) during the mid-Pliocene compared with PI, which could induce the northward shift of the ITCZ and reorganize atmospheric circulation.
These features can result in a weakening meridional circulation in the Northern Hemispheric monsoon regions and a https://doi.org/10.5194/cp-2021-72 Preprint. Discussion started: 13 July 2021 c Author(s) 2021. CC BY 4.0 License. strengthening meridional circulation in the SH. In addition, the anomalous meridional circulation can dry the deep tropics but moisten the northern part of the ITCZ. Furthermore, these anomalies dry the SPCZ region and wet its southern part, 460 which is associated with the southward shift of the SPCZ. We also find a robust westward shift in PWC, which appears to moisten the northern Indian Ocean via anomalous convergence of zonal circulation.
Our analyses provide a relatively complete understanding of the changes in the large-scale hydrological cycle within the PlioMIP2 ensemble. It is evident that in a warmer climate, the air could hold more moisture, and thus, the thermodynamic effects amplify the intensity of PmE but do not alter its spatial pattern ( Fig. 2(a); Held and Soden, 2006).

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Note that the hemispheric asymmetries of atmospheric energy could induce regional meridional circulation anomalies and thus alter the distribution of PmE anomalies during the mid-Pliocene via the M D MCD _  term at low latitudes. The PlioPMIP2 ensemble simulations suggest that hemispheric asymmetries of atmospheric energy are the key factor altering the spatial pattern of PmE via changes in the local meridional circulation. However, we should note that a noticeable intermodel spread exists in capturing the main features in the past warm climate, particularly for the changes in Walker circulation, such 470 as the large spread in the simulated changes in the intensity of PWC, dSST and dSLP in Fig. 9, consistent with previous studies (Arthur et al., 2021). Further effort to understand the intermodel uncertainty needs to be investigated in future work.
In addition, previous studies indicate that the storm track (transient eddy component) may play a key role in changes in PmE for mid-to high latitudes (Seager et al., 2010;Han et al., 2019a;Han et al., 2019b). Due to the lack of hourly model data, we mainly discuss the relative contributions from moisture budget components to changes in PmE at low latitudes in this paper.

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Much more work should be conducted to study the impact of storm tracks on changes in PmE during the mid-Pliocene using hourly data in the future at mid-to-high latitudes.