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Evaluating the Formation Mechanisms of the Equatorial Pacific SST Warming Pattern in CMIP5 Models

doi: 10.1007/s00376-015-5184-6

  • Based on the historical and RCP8.5 runs of the multi-model ensemble of 32 models participating in CMIP5, the present study evaluates the formation mechanisms for the patterns of changes in equatorial Pacific SST under global warming. Two features with complex formation processes, the zonal El Niño-like pattern and the meridional equatorial peak warming (EPW), are investigated. The climatological evaporation is the main contributor to the El Niño-like pattern, while the ocean dynamical thermostat effect plays a comparable negative role. The cloud-shortwave-radiation-SST feedback and the weakened Walker circulation play a small positive role in the El Niño-like pattern. The processes associated with ocean dynamics are confined to the equator. The climatological evaporation is also the dominant contributor to the EPW pattern, as suggested in previous studies. However, the effects of some processes are inconsistent with previous studies. For example, changes in the zonal heat advection due to the weakened Walker circulation have a remarkable positive contribution to the EPW pattern, and changes in the shortwave radiation play a negative role in the EPW pattern.
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    Huang P., 2015: Seasonal changes in tropical SST and the surface energy budget under global warming projected by CMIP5 models. J. Climate, 28, 6503- 6515.10.1175/ The seasonal changes in tropical SST under global warming are investigated based on the RCP 8.5 and historical runs in 31 CMIP5 models. The tropical SST changes show three pronounced seasonal patterns, the peak locking to the equator throughout the year, the weaker equatorial changes and stronger hemispheric asymmetric changes (HAC) in boreal autumn. The magnitude of the seasonal patterns is comparable to the tropical-mean warming and the annual-mean patterns, implying great impacts on global climate changes. The peak locking to the equator is a result of the equatorial locking of the minimum damping of climatological latent heat flux and the ocean heat transport changes. Except the role of ocean heat transport suggested in previous studies, the weaker equatorial warming in boreal autumn is contributed by the stronger evaporation damping due to the stronger climatological evaporation and the increased surface wind speed. The seasonal variations of the HAC are driven by the variations of the damping effect of climatological evaporation. In boreal summer, the damping effect of climatological evaporation, greater in the Southern Hemisphere, promotes the development of the HAC. Consequently, the HAC peaks in boreal autumn when the damping effect of climatological evaporation transforms to a reverse meridional pattern, greater in the Northern Hemisphere. The wind-evaporation-SST feedback, as the key process of the annual-mean HAC, plays an amplifying role to the seasonal variations of the HAC in tropical SST.
    Huang P., J. Ying, 2015: A multimodel ensemble pattern regression method to correct the tropical Pacific SST change patterns under global warming. J. Climate, 28, 4706- 4723.10.1175/ This study develops a new observational constraint method, called multi-model ensemble pattern regression (EPR), to correct the projections of regional climate change by conventional unweighted multi-model mean (MMM). The EPR method first extracts leading modes of historical bias using inter-model EOF analysis, then builds up the linear correlated modes between historical bias and change bias using multi-variant linear regression, and finally estimates the common change bias induced by common historical bias. Along with correcting common change bias, the EPR method implicitly removes the inter-model uncertainty in the change projection deriving from the inter-model diversity in background simulation. The EPR method is applied to correct the patterns of tropical Pacific SST changes using the historical and RCP 8.5 runs in 30 CMIP5 models and observed SSTs. The common bias patterns of the tropical Pacific SSTs in historical runs, including the excessive cold tongue, the southeastern warm bias and the narrower warm pool, are estimated to induce La Niña-like change biases. After the estimated common change biases are removed, the corrected SST changes display a pronounced El Niño-like pattern and have much greater zonal gradients. The bias correction decreases by around half of the inter-model uncertainties in the MMM SST projections. The patterns of corrected tropical precipitation and circulation change are dominated by the enhanced SST change patterns, displaying a pronounced warmer-get-wetter pattern and a decreased Walker circulation with decreased uncertainties.
    Huang P., S.-P. Xie, K. M. Hu, G. Huang, and R. H. Huang, 2013: Patterns of the seasonal response of tropical rainfall to global warming. Nature Geoscience, 6, 357- 361.10.1038/ convection is an important factor in regional climate variability and change around the globe . The response of regional precipitation to global warming is spatially variable, and state-of-the-art model projections suffer large uncertainties in the geographic distribution of precipitation changes . Two views exist regarding tropical rainfall change: one predicts increased rainfall in presently rainy regions (wet-get-wetter) , and the other suggests increased rainfall where the rise in sea surface temperature exceeds the mean surface warming in the tropics (warmer-get-wetter). Here we analyse simulations with 18 models from the Coupled Model Intercomparison Project (CMIP5), and present a unifying view for seasonal rainfall change. We find that the pattern of ocean warming induces ascending atmospheric flow at the Equator and subsidence on the flanks, anchoring a band of annual mean rainfall increase near the Equator that reflects the warmer-get-wetter view. However, this climatological ascending motion marches back and forth across the Equator with the Sun, pumping moisture upwards from the boundary layer and causing seasonal rainfall anomalies to follow a wet-get-wetter pattern. The seasonal mean rainfall, which is the sum of the annual mean and seasonal anomalies, thus combines the wet-get-wetter and warmer-get-wetter trends. Given that precipitation climatology is well observed whereas the pattern of ocean surface warming is poorly constrained , our results suggest that projections of tropical seasonal mean rainfall are more reliable than the annual mean.
    Huang P., I. -I. Lin, C. Chou, and R. H. Huang, 2015: Change in ocean subsurface environment to suppress tropical cyclone intensification under global warming. Nature Communications, 6, 7188.10.1038/ cyclones (TCs) are hazardous natural disasters. Because TC intensification is significantly controlled by atmosphere and ocean environments, changes in these environments may cause changes in TC intensity. Changes in surface and subsurface ocean conditions can both influence a TC's intensification. Regarding global warming, minimal exploration of the subsurface ocean has been undertaken. Here we investigate future subsurface ocean environment changes projected by 22 state-of-the-art climate models and suggest a suppressive effect of subsurface oceans on the intensification of future TCs. Under global warming, the subsurface vertical temperature profile can be sharpened in important TC regions, which may contribute to a stronger ocean coupling (cooling) effect during the intensification of future TCs. Regarding a TC, future subsurface ocean environments may be more suppressive than the existing subsurface ocean environments. This suppressive effect is not spatially uniform and may be weak in certain local areas.
    Knutson T. R., S. Manabe, 1995: Time-mean response over the tropical Pacific to increased CO2 in a coupled ocean-atmosphere model. J. Climate, 8, 2181- 2199.10.1175/1520-0442(1995)0082.0.CO; The time-mean response over the tropical Pacific region to a quadrupling of COâ is investigated using a global coupled ocean-atmosphere general circulation model. Tropical Pacific sea surface temperatures (SSTs) rise by about 4Â掳-5Â掳C. The zonal SST gradient along the equator decreases by about 20%, although it takes about one century (with COâ increasing at 1% per year compounded) for this change to become clearly evident in the model. Over the central equatorial Pacific, the decreased SST gradient is accompanied by similar decreases in the easterly wind stress and westward ocean surface currents and by a local maximum in precipitation increase. Over the entire rising branch region of the Walker circulation, precipitation is enhanced by 15%, but the time-mean upward motion decreases slightly in intensity. The failure of the zonal overturning atmospheric circulation to intensify with quadrupling of COâ is surprising in light of the increased time-mean condensation heating over the {open_quotes}warm pool{close_quotes} region. Three aspects of the model response are important for interpreting this result. (1) The time-mean radiative cooling of the upper troposphere is enhanced, due to both the pronounced upper-tropospheric warming and to the large fractional increase of upper-tropospheric water vapor. (2) The dynamical cooling term, -Ïâθ/â(mo), is enhanced due to increased time-mean static stability (-âθ/â(mo)). This is an effect of moist convection, which keeps the lapse rate close to the moist adiabatic rate, thereby making -âθ/â(mo) larger in a warmer climate. The enhanced radiative cooling and increased static stability allow for the enhanced time-mean heating by moist convection and condensation to be balanced without stronger time-mean upward motions. 37 refs., 13 figs., 3 tabs.
    Knutson T. R., J. J. Sirutis, S. T. Garner, G. A. Vecchi, and I. M. Held, 2008: Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geoscience, 1, 359- 364.10.1038/ sea surface temperatures in the tropical Atlantic Ocean and measures of Atlantic hurricane activity have been reported to be strongly correlated since at least 1950 (refs 1 , 2 , 3 , 4 , 5 ), raising concerns that future greenhouse-gas-induced warming 6 could lead to pronounced increases in hurricane activity. Models that explicitly simulate hurricanes are needed to study the influence of warming ocean temperatures on Atlantic hurricane activity, complementing empirical approaches. Our regional climate model of the Atlantic basin reproduces the observed rise in hurricane counts between 1980 and 2006, along with much of the interannual variability, when forced with observed sea surface temperatures and atmospheric conditions 7 . Here we assess, in our model system 7 , the changes in large-scale climate that are projected to occur by the end of the twenty-first century by an ensemble of global climate models 8 , and find that Atlantic hurricane and tropical storm frequencies are reduced. At the same time, near-storm rainfall rates increase substantially. Our results do not support the notion of large increasing trends in either tropical storm or hurricane frequency driven by increases in atmospheric greenhouse-gas concentrations.
    Liu Z. Y., S. Vavrus, F. He, N. Wen, and Y. F. Zhong, 2005: Rethinking tropical ocean response to global warming: The enhanced equatorial warming. J. Climate, 18, 4684-
    Ma J., S.-P. Xie, and Y. Kosaka, 2012: Mechanisms for tropical tropospheric circulation change in response to global warming. J. Climate, 25, 2979- 2994.10.1175/ The annual-mean tropospheric circulation change in global warming is studied by comparing the response of an atmospheric general circulation model (GCM) to a spatial-uniform sea surface temperature (SST) increase (SUSI) with the response of a coupled ocean鈥揳tmosphere GCM to increased greenhouse gas concentrations following the A1B scenario. In both simulations, tropospheric warming follows the moist adiabat in the tropics, and static stability increases globally in response to SST warming. A diagnostic framework is developed based on a linear baroclinic model (LBM) of the atmosphere. The mean advection of stratification change (MASC) by climatological vertical motion, often neglected in interannual variability, is an important thermodynamic term for global warming. Once MASC effect is included, LBM shows skills in reproducing GCM results by prescribing latent heating diagnosed from the GCMs. MASC acts to slow down the tropical circulation. This is most clear in the SUSI run where the Walker circulation slows down over the Pacific without any change in SST gradient. MASC is used to decelerate the Hadley circulation, but spatial patterns of SST warming play an important role. Specifically, the SST warming is greater in the Northern than Southern Hemisphere, an interhemispheric asymmetry that decelerates the Hadley cell north, but accelerates it south of the equator. The MASC and SST-pattern effects are on the same order of magnitude in our LBM simulations. The former is presumably comparable across GCMs, while SST warming patterns show variations among models in both shape and magnitude. Uncertainties in SST patterns account for intermodel variability in Hadley circulation response to global warming (especially on and south of the equator).
    Ma J., S.-P. Xie, 2013: Regional patterns of Sea surface temperature change: A source of uncertainty in future projections of precipitation and atmospheric circulation. J. Climate, 26, 2482- 2501.10.1175/ change in response to global warming has profound impacts on environment for life but is highly uncertain. Effects of sea surface temperature (SST) warming on the response of rainfall and atmospheric overturning circulation are investigated using Coupled Model Intercomparison Project simulations. The SST warming is decomposed into a spatially uniform SST increase (SUSI) and deviations from it. The SST pattern effect is found to be important in explaining both the multimodel ensemble mean distribution and intermodel variability of rainfall change over tropical oceans. In the ensemble mean, the annual rainfall change follows a 'warmer-get-wetter' pattern, increasing where the SST warming exceeds the tropical mean, and vice versa. Two SST patterns stand out both in the ensemble mean and intermodel variability: an equatorial peak anchoring a local precipitation increase and a meridional dipole mode with increased rainfall and weakened trade winds over the warmer hemisphere. These two modes of intermodel variability in SST account for one-third of intermodel spread in rainfall projection. The SST patterns can explain up to four-fifths of the intermodel variability in intensity changes of overturning circulations. SUSI causes both the Hadley and Walker circulations to slow down, as articulated by previous studies. The weakening of the Walker circulation is robust across models as the SST pattern effect is weak. The Hadley circulation change, by contrast, is significantly affected by SST warming patterns. As a result, near and south of the equator, the Hadley circulation strength change is weak in the multimodel ensemble mean and subject to large intermodel variability due to the differences in SST warming patterns.
    Ma J., J.-Y. Yu, 2014: Linking centennial surface warming patterns in the equatorial Pacific to the relative strengths of the Walker and Hadley circulations. J. Atmos. Sci., 71, 3454- 3464.10.1175/ This study analyzes representative concentration pathway 4.5 projections by 18 models from phase 5 of the Coupled Model Intercomparison Project to show that surface warming patterns in the equatorial Pacific during the twenty-first century (centennial warming) are influenced by the relative strengths of the Walker and Hadley circulations. The stronger the Hadley (Walker) circulation is, the greater the surface warming in the central Pacific (CP) [eastern Pacific (EP)]. The EP warming is associated with the Bjerknes feedback, while the CP warming is associated with the wind–evaporation–sea surface temperature feedback. This atmospheric circulation influence on the centennial warming is similar to that found for the EP and CP El Ni09o. This suggests a methodology to constrain the estimate of the projected surface warming patterns in the equatorial Pacific using recent El Ni09o activity. The constraint indicates that the “most likely” centennial warming patterns have a maximum in the EP and are 39% weaker than the warming projected by the 18-model mean. The most-likely projection also shows alternating stronger and weaker warming in the subtropical North Pacific, which is not predicted by the 18-model mean projection. Nevertheless, the two projections agree on the minimum warming in the southeastern subtropical Pacific.
    Meehl G. A., W. M. Washington, 1996: El Niño-like climate change in a model with increased atmospheric CO2 concentrations. Nature, 382, 56- 60.
    Ramanathan V., W. Collins, 1991: Thermodynamic regulation of ocean warming by cirrus clouds deduced from observations of the 1987 El Niño. Nature, 351, 27- 32.10.1038/ observations made during the 1987 El Nino that show that in the upper range of sea surface temperatures, the greenhouse effect increases with surface temperature at a rate which exceeds the rate at which radiation is being emitted from the surface. Highly reflective cirrus clouds are produced which act like a thermostat, shielding the ocean from solar radiation.
    Seager R., R. Murtugudde, 1997: Ocean dynamics, thermocline adjustment, and regulation of tropical SST. J. Climate, 10, 521- 534.
    Song, X. L, G. J. Zhang, 2014: Role of climate feedback in El Niño-like SST response to global warming. J. Climate, 27, 7301- 7318.10.1175/ Under global warming from the doubling of CO 2 , the equatorial Pacific experiences an El Ni09o–like warming, as simulated by most global climate models. A new climate feedback and response analysis method (CFRAM) is applied to 10 years of hourly output of the slab ocean model (SOM) version of the NCAR Community Climate System Model, version 3.0, (CCSM3-SOM) to determine the processes responsible for this warming. Unlike the traditional surface heat budget analysis, the CFRAM can explicitly quantify the contributions of each radiative climate feedback and of each physical and dynamical process of a GCM to temperature changes. The mean bias in the sum of partial SST changes due to each feedback derived with CFRAM in the tropical Pacific is negligible (0.5%) compared to the mean SST change from the CCSM3-SOM simulations, with a spatial pattern correlation of 0.97 between the two. The analysis shows that the factors contributing to the El Ni09o–like SST warming in the central Pacific are different from those in the eastern Pacific. In the central Pacific, the largest contributor to El Ni09o–like SST warming is dynamical advection, followed by PBL diffusion, water vapor feedback, and surface evaporation. In contrast, in the eastern Pacific the dominant contributor to El Ni09o–like SST warming is cloud feedback, with water vapor feedback further amplifying the warming.
    Sun D.-Z., J. Fasullo, T. Zhang, and A. Roubicek, 2003: On the radiative and dynamical feedbacks over the equatorial Pacific cold tongue. J. Climate, 16, 2425- 2432.10.1175/ analysis of the climatic feedbacks in the NCAR Community Climate Model, version 3 (CCM3) over the equatorial Pacific cold tongue is presented. Using interannual signals in the underlying SST, the radiative and dynamical feedbacks have been calculated using both observations and outputs from the NCAR CCM3. The results show that the positive feedback from the greenhouse effect of water vapor in the model largely agrees with that from observations. The dynamical feedback from the atmospheric transport in the model is also comparable to that from observations. However, the negative feedback from the solar forcing of clouds in the model is significantly weaker than the observed, while the positive feedback from the greenhouse effect of clouds is significantly larger. Consequently, the net atmospheric feedback in the CCM3 over the equatorial cold tongue region is strongly positive (5.1 W m[SUP-2]K[SUP-1]), while the net atmospheric feedback in the real atmosphere is strongly negative (26.4 W m[SUP-2]K[SUP-1]). A further analysis with the aid of the International Satellite Cloud Climatology Project (ISCCP) data suggests that cloud cover response to changes in the SST may be a significant error source for the cloud feedbacks. It is also noted that the surface heating over the cold tongue in CCM3 is considerably weaker than in observations. In light of results from a linear feedback system, as well as those from a more sophisticated coupled model, it is suggested that the discrepancy in the net atmospheric feedback may have contributed significantly to the cold bias in the equatorial Pacific in the NCAR Climate System Model (CSM).
    Sun D.Z., Coauthors , 2006: Radiative and dynamical feedbacks over the equatorial cold tongue: Results from nine atmospheric GCMs. J. Climate, 19, 4059-
    Taylor K. E., R. J. Stouffer, and G. A. Meehl, 2012: An overview of CMIP5 and the experiment design. Bull. Amer. Meteor. Soc., 93, 485- 498.10.1175/ fifth phase of the Coupled Model Intercomparison Project (CMIP5) will produce a state-of-the- art multimodel dataset designed to advance our knowledge of climate variability and climate change. Researchers worldwide are analyzing the model output and will produce results likely to underlie the forthcoming Fifth Assessment Report by the Intergovernmental Panel on Climate Change. Unprecedented in scale and attracting interest from all major climate modeling groups, CMIP5 includes “long term” simulations of twentieth-century climate and projections for the twenty-first century and beyond. Conventional atmosphere–ocean global climate models and Earth system models of intermediate complexity are for the first time being joined by more recently developed Earth system models under an experiment design that allows both types of models to be compared to observations on an equal footing. Besides the longterm experiments, CMIP5 calls for an entirely new suite of “near term” simulations focusing on recent decades and the future to year 2035. These “decadal predictions” are initialized based on observations and will be used to explore the predictability of climate and to assess the forecast system's predictive skill. The CMIP5 experiment design also allows for participation of stand-alone atmospheric models and includes a variety of idealized experiments that will improve understanding of the range of model responses found in the more complex and realistic simulations. An exceptionally comprehensive set of model output is being collected and made freely available to researchers through an integrated but distributed data archive. For researchers unfamiliar with climate models, the limitations of the models and experiment design are described.
    Vecchi G. A., B. J. Soden, 2007: Global warming and the weakening of the tropical circulation. J. Climate, 20, 4316- 4340.10.1175/ This study examines the response of the tropical atmospheric and oceanic circulation to increasing greenhouse gases using a coordinated set of twenty-first-century climate model experiments performed for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The strength of the atmospheric overturning circulation decreases as the climate warms in all IPCC AR4 models, in a manner consistent with the thermodynamic scaling arguments of Held and Soden. The weakening occurs preferentially in the zonally asymmetric (i.e., Walker) rather than zonal-mean (i.e., Hadley) component of the tropical circulation and is shown to induce substantial changes to the thermal structure and circulation of the tropical oceans. Evidence suggests that the overall circulation weakens by decreasing the frequency of strong updrafts and increasing the frequency of weak updrafts, although the robustness of this behavior across all models cannot be confirmed because of the lack of data. As the climate warms, changes in both the atmospheric and ocean circulation over the tropical Pacific Ocean resemble “El Ni09o–like” conditions; however, the mechanisms are shown to be distinct from those of El Ni09o and are reproduced in both mixed layer and full ocean dynamics coupled climate models. The character of the Indian Ocean response to global warming resembles that of Indian Ocean dipole mode events. The consensus of model results presented here is also consistent with recently detected changes in sea level pressure since the mid–nineteenth century.
    Xie S.-P., S. G. H. Philander, 1994: A coupled ocean-atmosphere model of relevance to the ITCZ in the eastern Pacific. Tellus A, 46, 340- 350.10.1034/ The intertropical convergence zone (ITCZ) stays in the northern hemisphere over the Atlantic and eastern Pacific, even though the annual mean position of the sun is on the equator. To study some processes that contribute to this asymmetry about the equator, we use a two-dimensional model which neglects zonal variations and consists of an ocean model with a mixed layer coupled to a simple atmospheric model. In this coupled model, the atmosphere not only transports momentum into the ocean, but also directly affects sea surface temperature by means of wind stirring and surface latent heat flux. Under equatorially symmetric conditions, the model has, in addition to an equatorially symmetric solution, two asymmetric solutions with a single ITCZ that forms in only one hemisphere. Strong equatorial upwelling is essential for the asymmetry. Local oceanic turbulent processes involving vertical mixing and surface latent heat flux, which are dependent on wind speed, also contribute to the asymmetry.
    Xie S.-P., C. Deser, G. A. Vecchi, J. Ma, H. Y. Teng, and A. T. Wittenberg, 2010: Global warming pattern formation: Sea surface temperature and rainfall. J. Climate, 23, 966- 986.10.1175/ Spatial variations in sea surface temperature (SST) and rainfall changes over the tropics are investigated based on ensemble simulations for the first half of the twenty-first century under the greenhouse gas (GHG) emission scenario A1B with coupled ocean鈥揳tmosphere general circulation models of the Geophysical Fluid Dynamics Laboratory (GFDL) and National Center for Atmospheric Research (NCAR). Despite a GHG increase that is nearly uniform in space, pronounced patterns emerge in both SST and precipitation. Regional differences in SST warming can be as large as the tropical-mean warming. Specifically, the tropical Pacific warming features a conspicuous maximum along the equator and a minimum in the southeast subtropics. The former is associated with westerly wind anomalies whereas the latter is linked to intensified southeast trade winds, suggestive of wind鈥揺vaporation鈥揝ST feedback. There is a tendency for a greater warming in the northern subtropics than in the southern subtropics in accordance with asymmetries in trade wind changes. Over the equatorial Indian Ocean, surface wind anomalies are easterly, the thermocline shoals, and the warming is reduced in the east, indicative of Bjerknes feedback. In the midlatitudes, ocean circulation changes generate narrow banded structures in SST warming. The warming is negatively correlated with wind speed change over the tropics and positively correlated with ocean heat transport change in the northern extratropics. A diagnostic method based on the ocean mixed layer heat budget is developed to investigate mechanisms for SST pattern formation. Tropical precipitation changes are positively correlated with spatial deviations of SST warming from the tropical mean. In particular, the equatorial maximum in SST warming over the Pacific anchors a band of pronounced rainfall increase. The gross moist instability follows closely relative SST change as equatorial wave adjustments flatten upper-tropospheric warming. The comparison with atmospheric simulations in response to a spatially uniform SST warming illustrates the importance of SST patterns for rainfall change, an effect overlooked in current discussion of precipitation response to global warming. Implications for the global and regional response of tropical cyclones are discussed.
    Zhang L., T. Li, 2014: A simple analytical model for understanding the formation of Sea surface temperature patterns under global warming. J. Climate, 27, 8413-
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Manuscript revised: 04 October 2015
Manuscript accepted: 27 October 2015
通讯作者: 陈斌,
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    沈阳化工大学材料科学与工程学院 沈阳 110142

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Evaluating the Formation Mechanisms of the Equatorial Pacific SST Warming Pattern in CMIP5 Models

  • 1. Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100190
  • 2. University of Chinese Academy of Sciences, Beijing 100049
  • 3. Joint Center for Global Change Studies, Beijing 100875

Abstract: Based on the historical and RCP8.5 runs of the multi-model ensemble of 32 models participating in CMIP5, the present study evaluates the formation mechanisms for the patterns of changes in equatorial Pacific SST under global warming. Two features with complex formation processes, the zonal El Niño-like pattern and the meridional equatorial peak warming (EPW), are investigated. The climatological evaporation is the main contributor to the El Niño-like pattern, while the ocean dynamical thermostat effect plays a comparable negative role. The cloud-shortwave-radiation-SST feedback and the weakened Walker circulation play a small positive role in the El Niño-like pattern. The processes associated with ocean dynamics are confined to the equator. The climatological evaporation is also the dominant contributor to the EPW pattern, as suggested in previous studies. However, the effects of some processes are inconsistent with previous studies. For example, changes in the zonal heat advection due to the weakened Walker circulation have a remarkable positive contribution to the EPW pattern, and changes in the shortwave radiation play a negative role in the EPW pattern.

1. Introduction
  • The warming patterns of equatorial Pacific SST due to rising greenhouse gas concentrations is one of the most important problems in projecting regional climate change and has thus been paid considerable attention in the research community for decades (Clement et al., 1996; Collins, 2005; Liu et al., 2005; Xie et al., 2010; Ma and Yu, 2014). The patterns of equatorial Pacific SST warming (EPSW) affect various aspects of regional and global climate change. For example, they dominate the changes in annual-mean precipitation, with increased (decreased) rainfall over the areas of large (small) SST warming, and play a more important role in the changes in tropical cyclone intensity than the local absolute SST increases (Vecchi and Soden, 2007; Knutson et al., 2008; Xie et al., 2010; Huang et al., 2015). Moreover, the uncertainties of EPSW also dominate the uncertainties of the changes in atmospheric circulation over the equatorial Pacific (Ma et al., 2012; Ma and Xie, 2013).

    Two well-known features of EPSW patterns have been obtained from the multi-model ensemble (MME) of CMIP3 and CMIP5, and from individual model simulations (Fig. 1a): the zonal El Niño-like warming pattern (simply referred to as the El Niño-like pattern hereafter), with more warming in the eastern than western Pacific (Ramanathan and Collins, 1991; Meehl and Washington, 1996; Collins, 2005; Vecchi and Soden, 2007; Song and Zhang, 2014); and the meridional equatorial peak warming (EPW) pattern (Liu et al., 2005; Xie et al., 2010). However, these patterns remain controversial in different scenarios (DiNezio et al., 2009; Zhang and Li, 2014) and different models (Huang and Ying, 2015). For instance, a few studies have suggested a La Niña-like warming (Clement et al., 1996; Cane et al., 1997) or a zonal uniform warming (DiNezio et al., 2009) for the zonal structure of the SST warming over the equatorial Pacific.

    Several distinct mechanisms have been proposed to explain the discrepant SST warming patterns. For the zonal structure, the weakened Walker circulation associated with a slower increase in rainfall than in moisture (Held and Soden, 2006) can reduce the zonal SST gradient to promote an El Niño-like pattern by reducing the westward surface wind stress and the westward oceanic current as well as the cold upwelling in the eastern Pacific (Vecchi and Soden, 2007). The zonal SST gradient can also be weakened by a greater evaporative cooling in the western Pacific than in the eastern Pacific (Knutson and Manabe, 1995) and by the stronger cloud radiation regulation in the western Pacific (Ramanathan and Collins, 1991). On the other hand, the zonal SST gradient can be enlarged by the increased ocean vertical temperature gradient in the eastern Pacific with upwelling colder subsurface water, known as the ocean dynamical thermostat effect (Clement et al., 1996; Cane et al., 1997), favoring a La Niña-like warming pattern. Moreover, the zonal warming pattern can be enlarged by the Bjerknes feedback of zonal air-sea coupling (Bjerknes, 1969; Song and Zhang, 2014).

    Figure 1.  The (a) MME SST warming pattern and (b) mixed layer ocean temperature warming pattern in the equatorial Pacific. Stippling indicates that more than 80% of models have the same sign.

    In terms of the meridional pattern, (Seager and Murtugudde, 1997) attributed the EPW pattern to the weaker trade wind at the equator than that in the subtropics, and (Liu et al., 2005) to the changes in latent heat, shortwave cloud forcing and ocean vertical mixing. (Xie et al., 2010) further emphasized the dominant role of the climatological minimum of evaporative cooling at the equator.

    All of these formation mechanisms seem theoretically reasonable. However, some mechanisms can be found merely in individual model experiments. For example, the ocean dynamical thermostat as a damping effect to the El Niño-like pattern was found in the Zebiak-Cane CGCM with a uniform heat flux forcing situation (Clement et al., 1996). Based on hybrid CGCM experiments, the EPW pattern was attributed to the stronger trade wind speed in the subtropics than at the equator (Seager and Murtugudde, 1997), whereas the effect of evaporative cooling was suggested based on the simulations of the GFDL's CGCM (Knutson and Manabe, 1995). However, the performances of these mechanisms in a large group of models remain unclear.

    In the present study, we analyze the changes in the ocean mixed layer energy budgets in 32 CMIP5 models to evaluate the importance of these mechanisms on the formation of the equatorial Pacific SST warming pattern. To quantify the importance of these mechanisms, we decompose the ocean mixed layer energy budgets into various terms to represent the respective mechanisms. The paper is organized as follows: Section 2 describes the models, variables and methods. Section 3 presents the results. Conclusions are given in section 4.

2. Models and methods
  • Outputs from 32 CMIP5 models are used in the present study. Table 1 lists the names and relevant organizations of the 32 models. The details of the models can be found at (Taylor et al., 2012). The historical runs for the period 1981-2000 and the RCP8.5 runs for 2081-2100 are used to represent the current and future climate, respectively.

    The variables include the monthly mean SST, total cloud fraction (its standard variable name in CMIP5 is clt), surface latent heat flux (Q E), sensible heat flux (Q H), net longwave radiation (Q LW), net shortwave radiation (Q SW), surface zonal (uas) and meridional (vas) wind velocity, surface scalar wind speed (sfcWind), ocean temperature (thetao), and ocean 3D mass transport (umo, vmo, and wmo). The net longwave/shortwave radiation is defined as the difference between upward and downward longwave/shortwave radiation. The sign of the flux is defined such that a positive flux warms the ocean. Some variables not archived in a few models are marked in Table 1. Moreover, the ocean vertical mass transport not well described in CSIRO Mk3.6.0, BNU-ESM and MIROC5 is also excluded ( errata/cmip5errata.html). Ocean 3D currents are obtained from the ocean 3D mass transports. All of the model outputs are interpolated onto a 2.5°× 2.5° grid.

  • The change under global warming is first defined as the difference between the 20-year long-term mean of the RCP8.5 run and that of the historical run. Changes in each model are normalized by their respective tropical SST warming averaged between 60°S to 60°N, in order to remove the influence of tropical mean SST change. Then, the regional mean SST increase is removed to define the EPSW pattern. As shown in Fig. 1a, the sign agreement test indicates that most of the CMIP5 models (more than 80% of the 32 models) show some universal patterns of EPSW.

  • The formation mechanisms of the EPSW patterns can be detected from the surface energy budget changes. For instance, the effect of evaporative cooling can be represented by the latent heat changes (Xie et al., 2010), the effect of cloud-shortwave-radiation-SST feedback by the shortwave radiation changes (Ramanathan and Collins, 1991), and the effect of the ocean dynamical thermostat is implied in the ocean heat transport changes (Clement et al., 1996; DiNezio et al., 2009).

    For the change in long-term mean, the energy budget balance in the ocean mixed layer can be expressed as (Xie et al., 2010) \begin{equation} \Delta Q_{E}+\Delta Q_{H}+\Delta Q_{LW}+\Delta Q_{SW}+\Delta D_{O}=0 ,(1) \end{equation} where denotes future change. ∆ Q E,∆ Q H,∆ Q LW,∆ Q SW and ∆ D O represent changes in latent heat flux, sensible heat flux, net longwave radiation, net shortwave radiation and ocean dynamical processes, respectively. The D O can be decomposed as \begin{equation} \Delta D_{O}=\Delta Q_{u}+\Delta Q_{v}+\Delta Q_{w}+\Delta R , (2)\end{equation} where ∆ Q u,∆ Q v and ∆ Q w represent changes in the ocean 3D heat transports, and ∆ R is a residual term representing changes in heat transports due to sub-grid scale processes such as vertical mixing and lateral entrainment (DiNezio et al., 2009).

    Because the ∆ Q u,∆ Q v and ∆ Q w include both the effects of changes in ocean currents and changes in ocean temperature gradients associated with different mechanisms, we decompose them into two components: \begin{eqnarray} \Delta Q_{u}&\approx&-\rho_{o}c_{\it p}\int\limits_{-H}^0\Delta u\dfrac{\partial T}{\partial x}dz-\rho_{o}c_{\it p}\int\limits_{-H}^0u \dfrac{\partial\Delta T}{\partial x}dz\nonumber\\ &=&\Delta Q_{u1}+\Delta Q_{u2} ,\nonumber\\ \Delta Q_{v}&\approx&-\rho_{o}c_{\it p}\int\limits_{-H}^0\Delta v\dfrac{\partial T}{\partial y}dz-\rho_{o}c_{\it p}\int\limits_{-H}^0v \dfrac{\partial\Delta T}{\partial y}dz\nonumber\\ &=&\Delta Q_{v1}+\Delta Q_{v2} ,\\ \Delta Q_{w}&\approx&-\rho_{o}c_{\it p}\int\limits_{-H}^0\Delta w\dfrac{\partial T}{\partial z}dz-\rho_{o}c_{\it p}\int\limits_{-H}^0w \dfrac{\partial\Delta T}{\partial z}dz\nonumber\\ &=&\Delta Q_{w1}+\Delta Q_{w2} ,(3)\nonumber \end{eqnarray}

    where ρ o is sea water density; c p is specific heat at constant pressure; H is mixed layer depth, chosen as a constant of 30 m; and u,v,w and T are ocean zonal, meridional and vertical current, and temperature, respectively. ∆ Q u1,∆ Q v1 and ∆ Q w1 represent the effect of changes in ocean currents, which mainly reflect the role of changes in surface wind stress and in atmospheric general circulation (Vecchi and Soden, 2007); and ∆ Q u2,∆ W v2 and ∆ Q w2 represent the effect of changes in ocean temperature gradients. The patterns of mixed layer temperature changes in Fig. 1b are close to the EPSW patterns (Fig. 1a), with a spatial correlation coefficient near 0.97, indicating that the mixed layer energy budget is reasonable for studying the SST change pattern and that the mixed layer depth (30 m) is properly chosen.

    Another important variable involving multiple processes is latent heat flux (Xie et al., 2010). The surface latent heat flux in models is calculated using the bulk formulas: \begin{equation} \label{eq1} Q_{E}=\rho _{a}LC_{E}Vq_{s}(T_{ss})(1-{RH}e^{-\alpha T'}) , (4)\end{equation} where ρ a is surface air temperature; L is latent heat of evaporation; C E is the exchange coefficient; V is surface wind speed; q s(T) is the saturated specific humidity, following the Clausius-Clapeyron relationship; T ss is SST; and T' is the difference between SST and surface air temperature, known as the stability parameter. RH is the relative humidity, α=L/(R vT2)≈ 0.06 K-1, and R v is the ideal gas constant for water vapor.

    From Eq. (2), changes in latent heat flux can be influenced by changes in SST, surface wind speed, surface stability and RH, related to different processes (Xie et al., 2010; Huang, 2015). Thus, ∆ Q E is decomposed into two parts: ∆ Q E=∆ Q EO+∆ Q EA, where ∆ Q EO=α Q E ∆ T ss is the response of SST change (Newtonian cooling) and ∆ Q EA contains the effects of changes in wind speed, RH and surface stability (Du and Xie, 2008; Xie et al., 2010). In ∆ Q EA, the effect due to surface wind speed change can be written as ∆ Q EW=Q E∆ V/V, which is the key aspect in the wind-evaporation-SSTfeedback (Xie and Philander, 1994) and important to the SST warming pattern formation (Xie et al., 2010). The residual of ∆ Q EA, ∆ Q ER=∆ Q EA-∆ Q EW, represents both the effect of changes in RH and surface stability.

    The ∆ Q EO=α Q E∆ T ss, including the effects of the climatological evaporation Q E and the SST change, can be divided into two terms, following (Huang, 2015): \begin{equation} \label{eq2} \Delta Q'_{EO}=\alpha\langle{Q_{E}}\rangle\Delta T'_{ss}+\alpha Q'_{E}\langle{\Delta T_{ss}}\rangle=\Delta Q'_{EO1}+ \Delta Q'_{EO2} , (5)\end{equation} where the angled brackets denote the tropical Pacific mean, the prime represents the deviations, the term ∆ Q' EO1 represents the response of the spatially non-uniform SST change, and ∆ Q' EO2 the effect of the spatial distribution of the climatological latent heat flux.

    Figure 2.  Changes in (a) latent heat flux ($\Delta Q_E$), (b) net longwave radiation ($\Delta Q_LW$), and (c) net shortwave radiation ($\Delta Q_SW$). (d-f) As in (a-c) but with the respective tropical Pacific mean removed.

    Figure 3.  Regional changes in the (a) zonal, (b) meridional and (c) vertical heat transport, and (d) the residual term in Eq. (2).

3. Results
  • Figures 2a-c exhibit the changes in latent heat flux (∆ Q E), net longwave radiation (∆ Q LW) and net shortwave radiation (∆ Q SW). Changes in sensible heat flux (∆ Q H) are omitted due to relatively small values. SST warming is mainly contributed by increases in net downward longwave radiation, while changes in latent heat and net shortwave radiation suppress surface warming. The regional deviations of these surface energy budgets are shown in Figs. 2d-f. Changes in latent heat flux and net shortwave radiation exhibit pronounced spatial patterns (Figs. 2d and f), indicating more important influences on the EPSW pattern; whereas, the increases in net longwave radiation (Fig. 2e) are mainly spatially uniform, contributed by the near uniform increases in greenhouse gases.

    For the ocean dynamics (Fig. 3), the 3D heat transports are mainly located in the equatorial Pacific, except the meridional heat transport, which cools the NH and warms the SH off the equator. The horizontal heat advection (Figs. 3a and b) warms the surface of the equator, while the vertical heat advection (Fig. 3c) cools SST in the eastern Pacific. In addition, the residual term mainly warms the equatorial eastern Pacific and cools the off-equatorial flanks of the eastern Pacific (Fig. 3d).

  • In the MME, the SST warming in the eastern Pacific is larger than that in the western Pacific, exhibiting an El Niño-like pattern. The difference between the regional mean of (5°S-5°N, 145°-85°W) and (5°S-5°N, 125°E-175°W), denoted by the dashed green boxes in Fig. 1a, is around 0.12°C per 1°C of global warming.

    Figure 4.  Components of the regional changes in latent heat flux: (a) $\Delta Q_EO$, (b) $\Delta Q_EA$, (c) $\Delta Q_EO1$ and $\Delta Q_EO2$

    Figure 5.  (a) Cloud-shortwave-radiation-SST feedback index in the historical run. (b) Changes in total cloud fraction.

    Figure 6.  Regional changes in the ocean heat transports induced by changes in (a) zonal current ($\Delta Q'_u1$), (b) meridional current ($\Delta Q'_v1$), (c) vertical current ($\Delta Q'_w1$), (d) zonal gradients of temperature ($\Delta Q'_u2$), (e) meridional gradients of temperature ($\Delta Q'_v2$), and (f) vertical gradients of temperature ($\Delta Q'_w2$).

    Four mechanisms are suggested to influence the zonal pattern formation. The total effect of evaporative cooling, represented by the changes in latent heat flux, causes warmer SST in the eastern than the western Pacific, favoring an El Niño-like pattern (Fig. 2d). Figures 4a and b show the Newtonian cooling effect (∆ Q EO) and the atmospheric forcing effect (∆ Q EA). The ∆ E EO near the equator is similar to the EPSW pattern (Fig. 4a), indicating a favorable factor for the El Niño-like pattern. On the contrary, the atmospheric adjustment effect (Fig. 4b) appears to damp the El Niño-like warming. In ∆ E EO, the effect of the spatial distribution of climatological latent heat flux (∆ E EO2, Fig. 4d) is the dominant contributor to the total effect of evaporative cooling, favoring an El Niño-like pattern (Knutson and Manabe, 1995), while the effect of non-uniform SST change (∆ Q EO1, Fig. 4c) plays a damping role.

    The cloud-shortwave-radiation-SST feedback is suggested to be another factor favoring the El Niño-like pattern, which can be represented by the changes in shortwave radiation. As shown in Figs. 2c and f, there is more decreased net shortwave radiation over the western Pacific than the western Pacific, favoring an El Niño-like pattern. To illustrate the role of cloud-shortwave-radiation-SST feedback, a cloud-shortwave-radiation-SST feedback index (CSFI) is defined by regressing monthly net shortwave radiation anomalies to SST anomalies (Sun et al., 2003; Sun et al., 2006), to quantify the strength of shortwave feedbacks in the climate system. Figure 5a shows the spatial distribution of the CSFI in the historical run. The CSFI is negative in most parts near the equator, suggesting a negative convective cloud-shortwave-radiation-SST feedback, and positive over the eastern Pacific, indicating a positive stratus cloud-shortwave-radiation-SST feedback (Ramanathan and Collins, 1991; Song and Zhang, 2014). The negative (positive) cloud-SST feedback will suppress (enhance) the local SST warming. This process can be demonstrated by the changes in cloud amount (Fig. 5b). Thus, the cloud-shortwave-radiation-SST feedback weakens the zonal gradient of SST, contributing to an El Niño-like pattern.

    The changes in ocean heat transports associated with the ocean current changes (Figs. 6a-c) indirectly reflect the effect of the changes in atmospheric general circulation connected by the surface wind stress changes. The effects of changes in ocean zonal and vertical currents both warm the SST along the equator (Figs. 6a, c), which is associated with the weakened Walker circulation (Vecchi and Soden, 2007). However, the zonal current changes do not contribute much to the zonal gradient of SST changes (Fig. 6a) because of the near uniform zonal current changes (Fig. 7a). Meanwhile, the downwelling changes in the eastern Pacific (Fig. 7b)——weakening the cold upwelling and warming the SST——mainly represent the effect of weakened Walker circulation on the zonal gradient of SST changes (Fig. 6c and ∆ Q w1). The effect of changes in meridional current also warms the SST in the eastern Pacific around 5°N (Fig. 6b) with a relatively weak magnitude, which could be attributed to the weak weakening of the meridional overturning circulation (Vecchi and Soden, 2007; Ma and Xie, 2013).

    The ocean dynamical thermostat effect can be represented by changes in the ocean heat transports due to changes in ocean vertical temperature gradients (Figs. 6f) (Cane et al., 1997; Seager and Murtugudde, 1997; An and Im, 2014). Under global warming, the ocean vertical temperature gradients will increase (Fig. 7b), with less solar radiation absorbed in the subsurface than at the surface. Thus, the background upwelling pulls up cooler subsurface water to cool the surface in the eastern Pacific, damping the El Niño-like pattern (Fig. 6f).

    The energy budget analyses basically verify that the previous suggested mechanisms are pronounced in the MME of the 32 CMIP5 models. However, they also exhibit great discrepancies in spatial structure and strength (Figs. 2f, 4c and d, and 6). The effects of weakened Walker circulation (Fig. 6c) and ocean dynamical thermostat (Fig. 6f) are confined near the equator (2.5°S-2.5°N), with great horizontal gradients, because of the narrow upwelling and stratification region in the eastern Pacific. Whereas, the effects of climatological evaporation and cloud radiation feedback extend to 5°S-5°N, close to the structure of the SST change pattern.

    The effect of climatological evaporation, cloud radiation feedback, the weakened Walker circulation, and the ocean dynamical thermostat can be represented by the zonal differences between the eastern (5°S-5°N, 145°-85°W) and western (5°S-5°N, 125°E-175°W) Pacific of ∆ Q EO2, ∆ Q SW, ∆ Q w1 and ∆ Q w2, respectively. The climatological evaporation contributes the most to the El Niño-like pattern with the east-west difference exceeding 2 W m-2 (around 2.03 W m-2), while the ocean dynamical thermostat contributes a comparable damping to the El Niño-like pattern formation (-1.96 W m-2). The cloud-shortwave radiation-SST feedback (0.92 W m-2) and the weakened Walker circulation (0.59 W m-2) play a positive but relatively small role.

    Figure 7.  (a) Changes in horizontal currents averaged in the mixed layer (vectors less than 0.02 m s$^-1$ are omitted). (b) Vertical gradients of changes in ocean temperature (color shading) and the zonal overturning current (vectors; m s$^-1$) at the equator (averaged between 2.5$^\circ$S and 2.5$^\circ$N). Changes in vertical velocity are multiplied by 100 for display, and vectors less than 0.05 are omitted.

    Figure 8.  (a) Zonal mean of $\Delta T'_ss$ (multiplied by 10; units: K) and the terms with positive contribution to the EPW pattern (units: W m$^-2$ K$^-1$). (b) The terms with negative contribution to the EPW pattern (units: W m$^-2$ K$^-1$).

  • The meridional EPSW exhibits a peak warming at the equator (Fig. 8a). Three terms of the zonal-mean heat budgets peak at the equator, favoring the EPW pattern (Fig. 8a): \(\alpha Q'_E\langle \Delta T_ss\rangle\), representing the effect of climatological evaporation; ∆ Q' u, representing the effect of changes in ocean zonal heat transport; and ∆ R', representing the effect of changes in the ocean residual term. On the other hand, the changes in the shortwave radiation (∆ Q' SW), RH and stability (∆ Q' ER), the meridional heat transport (∆ Q' v), and the vertical heat transport (∆ Q' w), damp the EPW pattern (Fig. 8b).

    Among the mechanisms, the latent heat changes due to the effect of the climatological evaporative cooling is the greatest positive contribution to the EPW pattern (Fig. 8a), which was first mentioned by (Liu et al., 2005) and emphasized by (Xie et al., 2010). Another important positive factor in the present analysis, which has not been emphasized, is the effect of changes in the ocean zonal heat transport due to the weakened Walker circulation (yellow curve in Fig. 8a), as demonstrated in Figs. 6a and 7a. This result is inconsistent with that in (Liu et al., 2005), suggesting the changes in oceanic circulation are not important. The residual term (∆ R') involving sub-grid scale processes, such as the ocean vertical mixing, also has a positive contribution to the EPW pattern, although its meridional range is relatively small. Meanwhile, these favorable mechanisms are balanced mainly by the effects of changes in the ocean vertical heat transports due to enhanced oceanic vertical temperature gradients and the latent heat changes due to changes in the atmospheric RH and stability (Fig. 8b). It should be noted that the effects of changes in shortwave radiation (Liu et al., 2005) and surface wind speed (Seager and Murtugudde, 1997), believed to be positive in forming the EPW pattern, do not contribute to the EPW pattern positively. The former damps the EPW pattern, while the latter mainly affects the off-equatorial patterns.

4. Conclusions
  • This paper analyzes the changes in the mixed-layer energy budget using 32 CMIP5 models, to investigate the formation mechanisms of the annual-mean equatorial Pacific SST warming patterns. Discussed are two patterns that are pronounced but whose mechanisms are unclear: the zonal El Niño-like pattern and the meridional equatorial peak pattern.

    For the El Niño-like pattern, we examined the effects of climatological evaporation, the cloud-shortwave-radiation-SST feedback, the weakening of the Walker circulation, and the ocean dynamical thermostat. The quantitative energy budget analyses, based on the MME of the CMIP5 models, revealed that the effect of climatological evaporation plays a major role, while the cloud-shortwave-radiation-SST feedback and the weakened Walker circulation play relatively small roles. On the contrary, the effect of the ocean dynamical thermostat plays a major negative role, damping the El Niño-like pattern formation, with comparable magnitude to the effect of climatological evaporation. The effects of climatological evaporation and the cloud-radiation feedback on the equator extend much wider meridionally than those of the effects associated with ocean dynamics.

    For the meridional EPW pattern, the dominant role of the climatological latent heat flux is also apparent in the MME of the 32 CMIP5 models, as in (Xie et al., 2010). Nevertheless, the performances of some mechanisms evaluated in the present study are different from those in some previous studies. The changes in the zonal heat transport due to the weakened Walker circulation make a considerable positive contribution to the EPW pattern, which is inconsistent with the result in (Liu et al., 2005). Moreover, the effect of changes in shortwave radiation damps the EPW pattern, which is inconsistent with the positive role proposed by (Liu et al., 2005), while the effect of surface wind speed mainly influences the off-equatorial patterns, which is also inconsistent with the positive role proposed in (Seager and Murtugudde, 1997).

    The present study is based on the MME of 32 CMIP5 models' outputs. The inter-model spreads in the EPSW are quite large in current CMIP models (DiNezio et al., 2009; Huang and Ying, 2015), with great impacts on the uncertainties in projecting regional climate changes (Huang et al., 2013; Ma and Xie, 2013). The present energy budget analysis provides a useful method to study the importance of the mechanisms to the inter-model uncertainty in the EPSW, which is worthy of study in the future.




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