Advances in Research on the ITCZ: Mean Position, Model Bias, and Anthropogenic Aerosol Influences

赤道辐合带相关研究进展:平均位置、模式偏差和人为气溶胶影响

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  • Corresponding author: Shuyun ZHAO, zhaosy@cug.edu.cn
  • Funds:

    Supported by the National Natural Science Foundation of China (42005128) and National Key Research and Development Program of China (2017YFA0603502)

  • doi: 10.1007/s13351-021-0203-2
  • Note: This paper will appear in the forthcoming issue. It is not the finalized version yet. Please use with caution.

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  • The zonal-mean position of the intertropical convergence zone (ITCZ) and its shift in the meridional direction significantly influence both the tropical and even global climate. This work reviews three aspects of the progress in ITCZ-relevant research: 1) the mechanism behind the asymmetry of the ITCZ annual- and zonal-mean positions relative to the equator; 2) causes of the double-ITCZ problem (pervasive in climate models) and the efforts to solve it; and 3) the physical mechanisms by which anthropogenic aerosols affect the location of the zonal-mean ITCZ. According to recent studies, the north-of-the-equator location of the annual- and zonal-mean ITCZ is mainly driven by the cross-equatorial energy transports in the ocean, induced by the Atlantic overturning circulation. A quantitative relationship between the ITCZ shift and the anomalous cross-equatorial energy transport in the atmosphere has been found. Presently, the double-ITCZ problem is still the most common and pronounced bias in tropical precipitation simulations with climate models. Recently, some studies have found that simply correcting the biases in hemispheric energy contrast does not improve the simulation of the ITCZ with climate models; whereas others have found that improving model resolutions and convective parameterizations in climate models, such as entrainment rate, rain-droplet re-evaporation, and convection triggering function, can alleviate the double-ITCZ bias. Therefore, it seems that the double-ITCZ problem in climate models is rooted in the complex physics of the models, which is not yet well-understood. In addition, anthropogenic aerosols are suggested to be able to induce meridional shifts of the ITCZ, but through various physical mechanisms. Absorbing aerosols like black carbon influence the ITCZ position basically via instantaneous absorption of shortwave radiation in the atmosphere, whereas scattering aerosols like sulfate affect the location of the ITCZ through the cloud lifetime effect and the subsequent response of surface evaporation.
    本文从三个方面回顾了赤道辐合带(ITCZ)的相关研究进展。(1)ITCZ位移的原因。ITCZ年平均与纬向平均位置均位于赤道以北,这主要是由大西洋翻转环流引起的海洋越赤道能量传输驱动的。ITCZ位移与大气中异常越赤道能量传输存在定量关系。(2)双赤道辐合带(Double-ITCZ)问题(即气候模式中普遍存在的热带降水模拟偏差)的成因和解决途径。有研究发现,改进模式的分辨率和对流参数,如夹卷率、雨滴再蒸发和对流触发机制等可以缓解Double-ITCZ偏差。Double-ITCZ问题可能根源于气候模式中物理机制的不完善(当今大部分气候模式仍然没有解决这一问题)。(3)人为气溶胶影响纬向平均ITCZ南北位置的物理机制。黑碳等吸收性气溶胶影响ITCZ位置的原因在于其对大气短波辐射的瞬时吸收,而硫酸盐等散射性气溶胶则通过云生命时间效应引起的蒸发响应来影响ITCZ的位置。
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  • Fig. 1.  The climatological mean position of the intertropical convergence zone (ITCZ) in each month (black line: global; red line: Indo–western Pacific, 50°E–150°W; cyan line: eastern Pacific, 150°–77.5°W; green line: Atlantic, 50°W–0°). Data are from the Global Precipitation Climatology Project (GPCP) monthly mean precipitation from 1981 to 2010, and the ITCZ position at each longitude is represented by the precipitation centroid between 15°S and 15°N. Similar plots denoting the seasonal cycle of the ITCZ position can be found in many previous studies, e.g., Voigt et al. (2016).

    Fig. 2.  Schematic diagram of the relationship between the atmospheric cross-equatorial energy (moisture) transport and the position of the ITCZ.

    Fig. 3.  Long-term annual-mean precipitation (mm day−1) over the tropics (30°S–30°N) from (a) the Tropical Rainfall Measuring Mission (TRMM) satellite observation, and (b) the phase 6 of the Coupled Model Intercomparison Project (CMIP6) multi-model ensemble mean. (c) Long-term annual-mean precipitation bias (model − observation; mm day−1) over the tropics (30°S–30°N). The black dashed boxes in (c) show where the south branch of the double-ITCZ bias is located in model results. Data are from the annual-mean precipitation data of TRMM3b43 from 1998 to 2010, and the historical run of CMIP6 from 1981 to 2010. Similar plots denoting the double-ITCZ bias can be found in many previous studies, e.g., Samanta et al. (2019) and Tian and Dong (2020).

    Fig. 4.  Schematic plots of the physical mechanisms by which (a) black carbon and (b) sulfate aerosols impact the position of the ITCZ, originating from Zhao and Suzuki (2019). In the plots, cross-equatorial energy and moisture transports are both anomalous terms, and evaporation is the interhemispheric contrast of anomalous terms (Southern Hemisphere − Northern Hemisphere). Red fonts mark the most important initial forcing in the physical mechanisms of the ITCZ displacement caused by the two types of aerosols.

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Advances in Research on the ITCZ: Mean Position, Model Bias, and Anthropogenic Aerosol Influences

    Corresponding author: Shuyun ZHAO, zhaosy@cug.edu.cn
  • 1. Department of Atmospheric Science, School of Environmental Studies, China University of Geosciences, Wuhan 430078
  • 2. Key Laboratory of Meteorological Disaster, Ministry of Education/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044
  • 3. State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing 100081
Funds: Supported by the National Natural Science Foundation of China (42005128) and National Key Research and Development Program of China (2017YFA0603502)

Abstract: The zonal-mean position of the intertropical convergence zone (ITCZ) and its shift in the meridional direction significantly influence both the tropical and even global climate. This work reviews three aspects of the progress in ITCZ-relevant research: 1) the mechanism behind the asymmetry of the ITCZ annual- and zonal-mean positions relative to the equator; 2) causes of the double-ITCZ problem (pervasive in climate models) and the efforts to solve it; and 3) the physical mechanisms by which anthropogenic aerosols affect the location of the zonal-mean ITCZ. According to recent studies, the north-of-the-equator location of the annual- and zonal-mean ITCZ is mainly driven by the cross-equatorial energy transports in the ocean, induced by the Atlantic overturning circulation. A quantitative relationship between the ITCZ shift and the anomalous cross-equatorial energy transport in the atmosphere has been found. Presently, the double-ITCZ problem is still the most common and pronounced bias in tropical precipitation simulations with climate models. Recently, some studies have found that simply correcting the biases in hemispheric energy contrast does not improve the simulation of the ITCZ with climate models; whereas others have found that improving model resolutions and convective parameterizations in climate models, such as entrainment rate, rain-droplet re-evaporation, and convection triggering function, can alleviate the double-ITCZ bias. Therefore, it seems that the double-ITCZ problem in climate models is rooted in the complex physics of the models, which is not yet well-understood. In addition, anthropogenic aerosols are suggested to be able to induce meridional shifts of the ITCZ, but through various physical mechanisms. Absorbing aerosols like black carbon influence the ITCZ position basically via instantaneous absorption of shortwave radiation in the atmosphere, whereas scattering aerosols like sulfate affect the location of the ITCZ through the cloud lifetime effect and the subsequent response of surface evaporation.

赤道辐合带相关研究进展:平均位置、模式偏差和人为气溶胶影响

本文从三个方面回顾了赤道辐合带(ITCZ)的相关研究进展。(1)ITCZ位移的原因。ITCZ年平均与纬向平均位置均位于赤道以北,这主要是由大西洋翻转环流引起的海洋越赤道能量传输驱动的。ITCZ位移与大气中异常越赤道能量传输存在定量关系。(2)双赤道辐合带(Double-ITCZ)问题(即气候模式中普遍存在的热带降水模拟偏差)的成因和解决途径。有研究发现,改进模式的分辨率和对流参数,如夹卷率、雨滴再蒸发和对流触发机制等可以缓解Double-ITCZ偏差。Double-ITCZ问题可能根源于气候模式中物理机制的不完善(当今大部分气候模式仍然没有解决这一问题)。(3)人为气溶胶影响纬向平均ITCZ南北位置的物理机制。黑碳等吸收性气溶胶影响ITCZ位置的原因在于其对大气短波辐射的瞬时吸收,而硫酸盐等散射性气溶胶则通过云生命时间效应引起的蒸发响应来影响ITCZ的位置。
    • The intertropical convergence zone (ITCZ), also termed the equatorial trough, is a narrow band in which the tropical trade winds of the Northern Hemisphere (NH) and Southern Hemisphere (SH) converge. The zonal-mean ITCZ is characterized by low-level convergence and high-level divergence in its horizontal circulation. In vertical motion, it manifests a strong ascending current; and in precipitation, it manifests a narrow strong precipitation belt around the equator (Avery et al., 2001; Bellucci et al., 2010; Krishnamurti et al., 2013; Kang et al., 2018b).

      Meridional shifts of the ITCZ are vital for precipitation in the monsoon regions on both sides of the equator (Wang et al., 2014; Biasutti et al., 2018). Riehl (1979) revealed that the monsoon precipitation in southern and south-central Mexico is affected by the meridional shift of the ITCZ in the eastern tropical Pacific. A northward (southward) displacement of the ITCZ in the equatorial eastern Pacific corresponds to strong (weak) monsoon rainfall in south-central Mexico (Hu and Feng, 2002). Paleoclimatic studies show a robust correlation between the ITCZ position and the East Asian monsoon (Cheng et al., 2012). For instance, in the early Holocene, the ITCZ moved northward during the NH warm period, at which time the East Asian summer monsoon strengthened and the winter monsoon weakened (Yancheva et al., 2007). Schneider et al. (2014) also indicated that the Indian summer monsoon rainfall was weak when the boreal-summer ITCZ shifted southward from the Holocene thermal maximum to the beginning of the Little Ice Age. During the latter half of the 20th century, with the ITCZ migrating southward, there was a corresponding arid tendency in North China, South Asia, and the Africa Sahel (Biasutti and Giannini, 2006; Bollasina et al., 2011; Qian and Zhou, 2014; Song et al., 2014; Wodzicki and Rapp, 2016). Xu et al. (2019) reviewed previous studies and demonstrated that frequent occurrences of El Niño phenomena and the southward movement of the ITCZ during the last century led to variations in atmospheric circulation, and caused an overall decreasing trend in the precipitation in the tropical/subtropical monsoon regions of the NH. The ITCZ also marks the position of the Hadley Cell, which may extend the influence of ITCZ shift to the whole globe through the interaction of the three-cell circulation.

      At present, factors affecting the ITCZ position and the ascription of ITCZ shift are still research hotspots. The annual-mean ITCZ is positioned north of the equator, particularly over the eastern equatorial Pacific and Atlantic (Zhang, 2001). Since as early as the 1990s, it has been recognized that the phenomenon is related to the shapes of the American and African continents, the cold tongue over the equatorial oceans, and the air–sea interactions (Xie, 2004). A series of experiments later revealed that the ITCZ always migrates toward the relatively warmer hemisphere, and ITCZ shift is sensitive to cloud and water vapor feedbacks in climate models (Chiang and Bitz, 2005; Broccoli et al., 2006; Kang et al., 2008, 2009). Recently, some researchers have investigated the position and displacement of the ITCZ from the perspective of energy transport. For example, Frierson et al. (2013) and Marshall et al. (2014) demonstrated that the cross-equatorial energy transports driven by the Atlantic meridional overturning circulation are key factors in determining the north-of-the-equator annual-mean position of the ITCZ. A quantitative relationship between the meridional shifts of the ITCZ and the cross-equatorial energy transports in the atmosphere has been found by many studies (Donohoe et al., 2013; Marshall et al., 2014; Schneider et al., 2014; Voigt et al., 2017; Zhao and Suzuki, 2019).

      There has been a common and pervasive ITCZ bias, i.e., the double-ITCZ problem, in most climate models. The double-ITCZ problem is characterized by the positive simulated precipitation bias on both sides of the equator, especially on the south side. The exact reasons behind the double-ITCZ problem remain unclear. Hwang and Frierson (2013) proposed that the cloud albedo bias (not bright enough) over the Southern Ocean in climate models, which led to excessive absorption of shortwave radiation in the SH, broke the energy balance between the two hemispheres and finally resulted in the south-of-the-equator spurious precipitation belt. In light of this perspective, a set of studies attempted to alleviate the ITCZ simulation bias by correcting the energy balance bias between the two hemispheres (e.g., increasing the amount of supercooled liquid water in midlatitude clouds over the SH, or increasing the south-of-the-equator ocean albedo), but achieved little correction to the double-ITCZ problem (Kay et al., 2016; Stephens et al., 2016; Hawcroft et al., 2017). This suggests that the ITCZ simulation cannot be improved simply by changing the interhemispheric energy contrast. Studies found that improving the horizontal and vertical resolutions of climate models could reduce the double-ITCZ problem to some extent (Xu and Cheng, 2013; Landu et al., 2014; Song and Zhang, 2020). However, recent studies assessed the overall ITCZ simulations in models participating in the latest phase 6 of the Coupled Model Intercomparison Project (CMIP6) and found that the double-ITCZ problem was improved very little in comparison to CMIP5, suggesting the ITCZ bias remains a persistent problem (Samanta et al., 2019; Tian and Dong, 2020).

      The impacts of anthropogenic aerosols on the ITCZ location have drawn widespread attention. Some researches attributed the southward-shifting tendency of the ITCZ in the second half of the 20th century to the rapid increase of anthropogenic aerosol emissions through multi-model analyses (Chung and Seinfeld, 2005; Hwang et al., 2013; Ocko et al., 2014; Allen et al., 2015; Zhang et al., 2016; Chung and Soden, 2017; Zhou et al., 2018). Compared with greenhouse gases, anthropogenic aerosol particles have a shorter lifetime in the atmosphere (Garrett et al., 2010; Wang, 2011; Zhao, 2015; Kristiansen et al., 2016), so a uniform terrestrial distribution can not be achieved, with the anthropogenic aerosols in the NH far more abundant than those in the SH. The asymmetric distribution of anthropogenic aerosols can break the energy balance of the two hemispheres, causing anomalous cross-equatorial energy transport and the ITCZ latitudinal shifts. The various types of aerosols have quite different impacts on the ITCZ location (Ocko et al., 2014; Zhang et al., 2016; Zhou et al., 2018), and the mechanisms of influencing the ITCZ location also differ (Zhao and Suzuki, 2019). Studies have illustrated that the northward migration of the ITCZ caused by black carbon is primarily attributable to the aerosol’s absorption of shortwave radiation in the atmosphere (Zhao and Suzuki, 2019). For sulfate, an important anthropogenic scattering and hygroscopic aerosol, the aerosol–cloud interaction plays a crucial role in its influence on the ITCZ meridional movement (Chung and Soden, 2017). Therefore, an accurate simulation of the impact of aerosols on the ITCZ requires a comprehensive understanding and consideration of the aerosol–radiation and aerosol–cloud interactions in climate models.

      This work mainly reviews the progress in ITCZ-related research in terms of its annual- and zonal-mean positions, meridional shift, and model simulations in the past decades. It should be noted that this review is primarily on the zonal-mean ITCZ and its features in latitudinal direction. On this basis, we summarize questions that remain to be answered and give some thoughts on future research directions about the ITCZ. Section 2 discusses the relationship between the ITCZ annual-mean position and cross-equatorial energy transports. Section 3 reviews the efforts and achievements in eliminating the double-ITCZ problem in climate models. Section 4 introduces the research findings on the mechanism by which anthropogenic aerosols impact the ITCZ position. Summaries and discussions are given in Section 5.

    2.   Annual-mean position of the ITCZ and cross-equatorial energy transport
    • On the seasonal scale, the zonal-mean position of the ITCZ follows the seasonal migration of the sun, shifting to the NH in boreal summer and the SH in boreal winter (Fig. 1). The ITCZ inclines to shift toward a warmer hemisphere, not only on the seasonal scale, but also on longer timescales, from interannual to decadal. For example, paleoclimate records of various timescales reveal that the ITCZ moves southward (northward) during the cold (warm) periods in the NH (Broccoli et al., 2006). Although the north–south movement of the subsolar point is symmetrical about the equator, the annual-mean position of the ITCZ is not at the equator, but to its north (Donohoe et al., 2013; Marshall et al., 2014; Schneider et al., 2014). It should be noted that different studies may calculate the position of the ITCZ with different methods. Some studies use precipitation maximum (e.g., Marshall et al., 2014; Schneider et al., 2014) and others use precipitation centroid to identify the position of the ITCZ for each longitude (e.g., Donohoe et al., 2013; Zhao and Suzuki, 2019). Here, we use the precipitation centroid between 15°S and 15°N to represent the ITCZ position. The asymmetry of the ITCZ is pronounced over the eastern Pacific and Atlantic. As shown in Fig. 1, the ITCZ resides in the NH over the Atlantic and eastern Pacific all year round except for a short period in spring.

      Figure 1.  The climatological mean position of the intertropical convergence zone (ITCZ) in each month (black line: global; red line: Indo–western Pacific, 50°E–150°W; cyan line: eastern Pacific, 150°–77.5°W; green line: Atlantic, 50°W–0°). Data are from the Global Precipitation Climatology Project (GPCP) monthly mean precipitation from 1981 to 2010, and the ITCZ position at each longitude is represented by the precipitation centroid between 15°S and 15°N. Similar plots denoting the seasonal cycle of the ITCZ position can be found in many previous studies, e.g., Voigt et al. (2016).

      Early studies found that the ITCZ location corresponds to the distribution of sea surface temperature (SST), as the annual-mean ITCZ is tilted to the north of the equator in the eastern Pacific and Atlantic, coincident with the warmer SST to the north of the equator compared with that south of the equator (Lindzen and Nigam, 1987; Waliser and Gautier, 1993; Tomas and Webster, 1997). Moreover, the seasonal variations in SST also affect the strength of the cold tongue and the position of the ITCZ (Mitchell and Wallace, 1992). Xie and Philander (1994) indicated that the cold tongue in the east of the equatorial oceans was critical to the shape and location of the ITCZ. They demonstrated that, unless the cold tongue disappeared, it was difficult to simulate a single ITCZ at the equator even if the underlying surface was perfectly symmetrical. Instead, there were always two ITCZs on the two sides of the equator or a single ITCZ off the equator. Philander et al. (1996) first proposed the importance of the shapes of the American and African continents in setting the ITCZ location in the equatorial eastern Pacific and Atlantic. Xie (2004) summarized previous studies and suggested that the equatorial asymmetry of the American and African continents was an initial forcing; the asymmetrical signal intensified and propagated westward to the equatorial eastern Pacific and Atlantic with air–sea interaction and Rossby wave; as a result, the SST north of the equator is higher than its southern counterpart and the ITCZ deviates from the equator to the NH.

      Zhang and Delworth (2005) revealed that if the Atlantic overturning circulation collapsed, the ITCZ mean position would move to the equator. Frierson et al. (2013) explicitly linked the north-of-the-equator annual-mean position of the ITCZ with cross-equatorial energy transport in the ocean, and suggested that the northward cross-equatorial energy transport in the ocean heats the NH atmosphere, which nails the annual-mean ITCZ north of the equator. Marshall et al. (2014) summarized and proposed that the north-of-the-equator average position of the ITCZ was driven by the Atlantic overturning circulation, and suggested that it was the Atlantic overturning circulation that led to a northward cross-equatorial energy transport in the ocean (about 0.4 PW). The northward energy transport further led to the warmer NH than the SH and a compensating southward cross-equatorial energy transport in the atmosphere, directly determining the north-of-the-equator ITCZ average position. Kang et al. (2015) made a similar argument. Moreno-Chamarro et al. (2020) confirmed the link between the Atlantic multidecadal variation and the ITCZ migration based on model simulations, and suggested that the Pacific decadal oscillation did not force ITCZ migrations as it did not modulate the interhemispheric energy balance.

      Marshall et al. (2014) and Kang et al. (2015) unified the interpretations of the north-of-the-equator annual-mean ITCZ position from the perspectives of interhemispheric temperature contrast and energy transport. It is easier to understand the mean position of the ITCZ and its variation from the energy transport perspective. Cross-equatorial heat transport mainly occurs at the upper level, while cross-equatorial moisture transport primarily occurs at a lower level as shown in Fig. 2. They compose the upper and lower branches of an anomalous cross-equatorial circulation. When an anomalous southward (northward) energy transport crosses the equator in the atmosphere, a northward (southward) cross-equatorial moisture transport at low-level is enhanced, and hence the ITCZ moves northward (southward). Studies have quantitatively assessed the relationship between the ITCZ displacement and the anomalous atmospheric cross-equatorial energy transport, which is approximately between −7.5° and −2.4° PW−1 (e.g., Donohoe et al., 2013, 2014; Mahajan et al., 2013; Schneider et al., 2014; Voigt et al., 2017; Zhao and Suzuki, 2019). The quantitative relationship exists not only on the seasonal scale but also on the interannual and interdecadal timescales (Broccoli et al., 2006; Donohoe et al., 2013).

      Figure 2.  Schematic diagram of the relationship between the atmospheric cross-equatorial energy (moisture) transport and the position of the ITCZ.

      The energetic constraint on the zonal-mean ITCZ position can be understood through the concept of the energy flux equator (EFE; Kang et al., 2008), where the atmospheric meridional energy flux changes sign. The zonal-mean ITCZ lies near the EFE. By expanding the atmospheric meridional energy flux to the first order at the EFE, the latitude of EFE is proportional to the atmospheric cross-equatorial energy transport and inversely proportional to the net energy input to the atmosphere at the equator (Bischoff and Schneider, 2014, 2016; Schneider et al., 2014; Adam et al., 2016a, b; Kang et al., 2018b; Wei and Bordoni, 2018). Therefore, it was demonstrated by Bischoff and Schneider (2014) that the sensitivity of the ITCZ position to cross-equatorial energy transport depended on the net energy input to the equatorial atmosphere. The tropical rain belts with an annual cycle and a continent model intercomparison project (TRACMIP; Voigt et al., 2016) was organized to study the dynamics of the ITCZ and its response to the past and future radiative forcing. Many models that participated in TRACMIP could not capture the linear relationship between the ITCZ position and the atmospheric cross-equatorial energy transport on the seasonal scale (Biasutti and Voigt, 2020). Wei and Bordoni (2018) found that, on the seasonal scale, the ITCZ always lagged behind the EFE, making it possible for the EFE and the ITCZ to reside on opposite sides of the equator. But Song et al. (2018a, b) found that under global warming, the ITCZ and cross-equatorial energy transport corresponded very well at seasonal cycle, i.e., the seasonal delay of ITCZ relative to the EFE is well explained by the delay of cross-equatorial energy transport.

      Under current climate, the moist static energy tendency term (d<h>/dt) on the left-hand side of energy equation, which represents the energy storage in the atmospheric column, is often neglected even at the seasonal timescale in many previous studies (e.g., Adam et al., 2016b; Wei and Bordoni, 2018). However, Song et al. (2018a, b) showed that under global warming, this moist static energy tendency term will dominate the cross-equatorial energy transport changes at the seasonal timescales (Song et al., 2018a), causing the seasonal delay of ITCZ. This is because under global warming, the effective atmospheric heat capacity is greatly enhanced (Song et al., 2020), which dictates that the atmospheric response to the seasonal solar forcing will be delayed.

      Recently, the energetic framework has been used to regional ITCZ (Adam et al., 2016b; Boos and Korty, 2016), land and ocean separately (Song et al., 2020), which should be paid attention to in the future studies. Besides the ITCZ, a framework based on the global energy constraint for understanding the complexities of monsoon dynamics, i.e., distilling the energy flow in and out of continents and between the surface and the tropopause, was also proposed by Biasutti et al. (2018).

      Kang et al. (2018b) noted the limitation of the energy framework in predicting the meridional shift of the ITCZ. That is, if the change of total gross moist stability, rather than the change of the mass transport by the lower branch of the Hadley cell, dominates the meridional atmospheric energy transport variation, the energy framework would fail to predict ITCZ shift.

      Nevertheless, it is generally recognized that the north-of-the-equator annual-mean position of the ITCZ is induced by the oceanic south-to-north cross-equatorial energy transport driven by the Atlantic overturning circulation, and the compensation of the atmospheric cross-equatorial energy transport in the reverse direction. The atmospheric compensation for the cross-equatorial energy transport in the ocean is about 50% (Marshall et al., 2014; Stephens et al., 2016). Kang et al. (2008) performed an idealized experiment and found that the compensation of the atmosphere for the oceanic cross-equatorial energy transport was sensitive to a parameter in the convection scheme, which is related to the entrainment of dry air into clouds in the climate model. With the entrainment rate increasing, convective precipitation is suppressed, and the atmospheric compensation rate decreases rapidly. Correspondingly, the magnitude of the ITCZ displacement also declines. This indicates that convection is of great importance for the ITCZ position change. Möbis and Stevens (2012) also emphasized the importance of convection scheme in simulating the ITCZ location. Kang et al. (2009) further found that if there were no cloud and water vapor feedbacks, the percentage of the atmospheric compensation for the oceanic cross-equatorial energy transport would decrease by about 25%.

      The oceanic cross-equatorial energy transport ultimately manifests as an energy imbalance between the NH and SH at the surface. One part of the energy imbalance is compensated for by the atmosphere as cross-equatorial energy transport, while the other part is left at the top of the atmosphere (TOA). Kang et al. (2008, 2009) found that cloud feedback reduced the proportion of the surface energy imbalance remaining at the TOA and increased the atmospheric compensation for the oceanic cross-equatorial energy transport. This explains why the degree of atmospheric compensation for the oceanic cross-equatorial energy transport decreases rapidly when clouds are not allowed to feedback.

      In summary, the annual-mean ITCZ position north of the equator can be explained from the perspective of energy transport. The ITCZ annual-mean position is determined by the oceanic northward cross-equatorial heat transport induced by the Atlantic overturning circulation, as well as the compensating atmospheric southward cross-equatorial heat transport that is largely undertaken through cloud feedback. However, why would cloud feedback compensate the oceanic cross-equatorial heat transport (manifesting as surface interhemispheric energy imbalance) rather than radiate it out of the TOA? Should it be the case? More detailed physical mechanisms and processes on this topic are to be further explored in the future.

    3.   The double-ITCZ problem
    • Double-ITCZ itself is a natural phenomenon. Because of the cold tongue in eastern equatorial ocean, sometimes the ITCZ does not manifest as a single precipitation belt but as two branches, each at one side of the equator, especially over the Pacific and Atlantic. Zhang (2001) found that the most obvious double-ITCZ phenomenon normally occurs over the eastern equatorial Pacific during boreal spring; when El Niño events occurred, the boreal-spring double ITCZ over the eastern equatorial Pacific would be replaced by a single ITCZ. The covariation between the spring ITCZ structure (single- or double-ITCZ) and the El Niño–Southern Oscillation might be explained by the net energy input into the atmosphere at the equator (more in El Niño episodes and less in La Niña episodes; Adam et al., 2016b; Kang et al., 2018b; Wodzicki and Rapp, 2020). Masunaga and L’Ecuyer (2010) analyzed the development process of the SH branch of double-ITCZ over the Pacific, and found that warm SST and wind (moisture) convergence emerged in this region in January, shallow convective clusters developed in February, and organized and vigorous deep convection appeared in March forming the SH branch of double-ITCZ phenomenon.

      However, the double-ITCZ problem, or the double-ITCZ bias, is generally one of the most obvious and pervasive precipitation biases in climate models (including atmospheric general circulation models and coupled atmosphere–ocean models). The double-ITCZ problem is characterized by the excessive precipitation on both sides of the equator, particularly on the south side (Fig. 3; Lin, 2007; Hirota and Takayabu, 2013; Li and Xie, 2014; Oueslati and Bellon, 2015). Currently, improving the simulation of the ITCZ remains one of the key issues in model development. However, there is no fundamental conclusions as to the reason for the double-ITCZ problem.

      Figure 3.  Long-term annual-mean precipitation (mm day−1) over the tropics (30°S–30°N) from (a) the Tropical Rainfall Measuring Mission (TRMM) satellite observation, and (b) the phase 6 of the Coupled Model Intercomparison Project (CMIP6) multi-model ensemble mean. (c) Long-term annual-mean precipitation bias (model − observation; mm day−1) over the tropics (30°S–30°N). The black dashed boxes in (c) show where the south branch of the double-ITCZ bias is located in model results. Data are from the annual-mean precipitation data of TRMM3b43 from 1998 to 2010, and the historical run of CMIP6 from 1981 to 2010. Similar plots denoting the double-ITCZ bias can be found in many previous studies, e.g., Samanta et al. (2019) and Tian and Dong (2020).

      Hwang and Frierson (2013) found that the models with a more severe double-ITCZ problem have a net downward radiation bias both at the TOA and the surface in the SH. Furthermore, they found that the excessive absorption in the SH was due to the biases in clouds, as the simulated cloud fraction over the Southern Ocean is low and the albedo is insufficient, resulting in more shortwave incident radiation at the TOA over the SH midlatitudes (Li and Xie, 2014). They suggested that the excessive energy absorbed in the SH was transported to the NH via the upper branch of the anomalous Hadley cell over the equator, while the lower branch transported water vapor southward and caused a southward shift of the ITCZ, inducing the double-ITCZ problem. In light of the work of Hwang and Frierson (2013), some studies have attempted to eliminate the double-ITCZ problem by correcting the interhemispheric energy contrast bias, but the results have not been encouraging. For instance, Kay et al. (2016) artificially increased the content of super-cooled liquid water in low clouds over the Southern Ocean to increase the SH albedo within the Community Earth System Model version 1 (CESM1), which was developed by NCAR. Although they rectified the bias of shortwave absorption in the SH, the double-ITCZ bias was not significantly reduced. Hawcroft et al. (2017) directly increased the surface albedo of the Southern Ocean in HadGEM2-ES, a fully atmosphere–ocean coupled model, but again, did not improve the ITCZ simulation.

      Kay et al. (2016) and Hawcroft et al. (2017) both found that increasing the SH albedo could alter the interhemispheric energy contrast, but the resulting anomalous cross-equatorial energy transport mainly occurred in the ocean rather than in the atmosphere. Stephens et al. (2016) also arrived at similar conclusions. Green and Marshall (2017) explained why the ITCZ position was not sensitive to the interhemispheric energy contrast, as expected from the interaction between the atmospheric Hadley circulation and the oceanic subtropical circulation. The interaction is accomplished by an Ekman-driven oceanic cross-equatorial cell, the strength of which is determined by the interhemispheric asymmetries in the trade winds. This oceanic cross-equatorial cell is more efficient at transporting energy across the equator than the atmosphere and always damps the ITCZ shift. Schneider (2017) gave a muting factor of the Ekman coupling between the atmospheric and oceanic energy fluxes on the ITCZ shift, which was 3 in the current climate in the zonal and annual mean. Kang et al. (2018a, b) found that Ekman coupling alone could not fully explain the dominant role of the ocean in cross-equatorial energy transport, and deep circulation, subtropical gyres, as well as ocean heat uptake processes were in need to be examined. Green et al. (2019) found the damping effect of the oceanic cross-equatorial circulation on the ITCZ shift was dependent weakly on the forcing amplitude but strongly on the forcing distribution, i.e., the damping effect became stronger when the forcing is polar amplified. Afargan-Gerstman and Adam (2020) confirmed that the damping effect of the oceanic cross-equatorial circulation on the ITCZ shift was nonlinearly related to the hemispherically asymmetric heating and ocean stratification. Zhang et al. (2019) believed that a possible underlying reason was that some feedback mechanism, involving subtropical low-level clouds, was missing. These details indicate that the double-ITCZ problem cannot be eliminated by simply changing the interhemispheric energy contrast, and it might be rooted in the complex physical processes in climate models.

      In general, precipitation is heavier over warmer oceans, as more heat and moisture are transported from the sea surface to the lower troposphere over warmer oceans, facilitating convective activities. However, there are exceptions, namely, the southeastern Pacific, where only slight precipitation associated with shallow convection is observed, despite the relatively high SST there. Takayabu et al. (2010) explained that the mid and lower troposphere is very dry over the southeastern Pacific, and the entrainment of dry environmental air into convective parcels reduces buoyancy and inhibits deep convection.

      Several studies relate the double-ITCZ problem to the simulated SST bias in the ocean, which is also pervasive in climate models. Lin (2007) analyzed the double-ITCZ problem in IPCC Fourth Assessment Report (AR4) coupled general circulation models (GCMs), focusing on the ocean–atmosphere feedback. It was suggested in Lin (2007) that the insufficient equatorial Pacific precipitation between the two false anomalous rain-belts was associated with one or more of the three biases in ocean–atmosphere feedback over the equatorial Pacific: excessive zonal SST gradient–trade wind feedback, overly positive SST–latent heat flux feedback, and insufficient SST–surface shortwave flux feedback. Song and Zhang (2017) found that the tropical North Atlantic cold SST bias and southeastern Pacific warm bias played an important role in the double-ITCZ bias in spring. They prescribed SSTs in these regions using observational values and found that the double-ITCZ bias in spring was modified significantly. Bellucci et al. (2010) proposed an index to measure the difference between the SST threshold (THR) for deep convection and the most likely temperature (MLT) of the ocean surface in a specific region (called the THR–MLT index). Oueslati and Bellon (2015) used the index to investigate the local SST control on precipitation, in particular on the double-ITCZ bias in the southeastern Pacific. They found that the occurrence of convection required SST to exceed the corresponding THR in the southeastern Pacific, and models with lower THRs for convection generally had more severe double-ITCZ biases. Oueslati and Bellon (2015) further explored the interaction between SST, large-scale circulation, and the double-ITCZ problem and identified that the precipitation bias in the tropical southeastern Pacific was caused by local thermodynamic processes related to SST and dynamic processes related to precipitation–circulation coupling. Samanta et al. (2019) demonstrated that the cold equatorial SST biases at least exacerbate the double-ITCZ biases in the Pacific. Song and Zhang (2020) also highlighted the cold equatorial SST biases in the double-ITCZ problem and suggested that the cold equatorial SST biases was caused by the convection bias in the equatorial Amazon region and the consequent easterly wind bias in the equatorial Pacific.

      In many climate models, the double-ITCZ bias is severe over the equatorial eastern Pacific, and the false southern branch of the ITCZ lies exactly in the dry subsidence region of the southeastern Pacific (Takayabu et al., 2010). Bellucci et al. (2010) used a regime sorting methodology developed by Bony et al. (2004), in which the monthly-mean vertical pressure velocity in the mid-troposphere (500 hPa) was used as a proxy for large-scale ascent or descent motions. They showed that GCMs simulated too frequent deep convections associated with strong ascending motions in the southeastern Pacific, which is supposed to be a dry subsidence region. Hirota and Takayabu (2013) also found that although humidity was very low in the mid and lower troposphere over the dry subsidence area in the southeastern Pacific for many models, deep convections were not sufficiently suppressed, resulting in positive precipitation anomalies and a false precipitation band.

      It has been suggested from studies using various state-of-the-art models that the double-ITCZ bias can be reduced by improving the treatments in convective parameterizations (e.g., Bacmeister et al., 2006; Chikira, 2010; Song and Zhang, 2018; Dunne et al., 2020) and/or cloud micro- and macro-physics (Qin and Lin, 2018; Woelfle et al., 2019; Lin et al., 2020). For example, Oueslati and Bellon (2013) found that the simulated precipitation bias in the southern branch of the ITCZ could be reduced by increasing the entrainment rate in convective parameterization for all seasons except winter; Song and Zhang (2018) implemented several improvements (e.g., on convective trigger function, updraft module, and entrainment rates) in the Zhang–McFarlane convection scheme in CESM1.2.1, and found that the double-ITCZ problem could be eliminated by these improvements in convection scheme; Woelfle et al. (2019) found a reduced double-ITCZ bias from CESM1 to CESM2, which is basically due to the reduced warm SST bias in southeast Pacific that was achieved by altering the microphysics (e.g., condensation, the dependence of autoconversion and accretion rates on cloud water variance) in drizzling clouds and hence the cloud lifetime and radiation effect. The double-ITCZ problem involves a complex chain of interactions including convection, cloud microphysics, large-scale circulation, SST, upper ocean circulation, and heat transport. The above studies imply that convection and cloud–precipitation physics possibly play a key role in the chain to determine the double-ITCZ bias, and therefore have been increasingly accounted for in model development (Oueslati and Bellon, 2013, 2015; Song and Zhang, 2016; Song and Zhang, 2018; Zhang et al., 2019).

      Another potential method for reducing the double-ITCZ problem is to increase model resolution. For example, Landu et al. (2014) found based on an aquaplanet model that the simulated ITCZ structure (single or double ITCZ) depended on the model horizontal resolution; Xu and Cheng (2013) found that improving the vertical resolution in the boundary layer could improve low clouds and the ITCZ simulation; and Song and Zhang (2020) found that when the horizontal resolution of CESM1 was doubled, both SST and precipitation were improved in the northeastern Pacific Ocean.

      Overall, the double-ITCZ problem prevalent in climate models may be rooted in the model complex physics, in particular the description of cloud and precipitation physics in the parameterization schemes in models. This is supported by the findings in Xiang et al. (2017) that the largest source of the double-ITCZ problem is from the tropics and atmospheric models. The biases in simulated clouds over the Southern Ocean and energy contrast between the two hemispheres are probably only other manifestations of the imperfect cloud and precipitation schemes in models. We cannot rectify the double-ITCZ problem by simply modulating the interhemispheric energy contrast. The fundamental solution to the double-ITCZ problem should be to improve the description of physical processes, including the horizontal and vertical resolutions, in climate models.

    4.   Impacts of anthropogenic aerosols on the ITCZ position
    • Aerosols generally refer to solid and liquid particles suspending in the atmosphere. Industrial development emits not only long-lived greenhouse gases such as CO2 but also huge amounts of aerosols and/or their precursors, rendering a rapid increase of anthropogenic aerosols in the atmosphere in recent decades. Aerosols have much shorter lifetimes in the atmosphere (typically between a few hours and a dozen days) compared to greenhouse gases, which makes it difficult for them to distribute homogenously like long-lived greenhouse gases.

      In the industrially developed NH, the contents of anthropogenic aerosols in the atmosphere are much larger than those in the SH. The asymmetric distribution of anthropogenic aerosols about the equator can alter the interhemispheric energy balance (Zhang et al., 2016).

      Therefore, the impacts of anthropogenic aerosols on the ITCZ location have attracted extensive attention. Some studies even attribute the southward displacement of the ITCZ during the second half of the 20th century primarily to the rapid increase of anthropogenic aerosol emissions (e.g., Hwang et al., 2013; Ocko et al., 2014; Allen et al., 2015; Wang, 2015; Chung and Soden, 2017). Correspondingly, the weakening tendency of the East Asian summer monsoon in this period is also attributed to anthropogenic aerosol increase in some studies (e.g., Song et al., 2014; Li et al., 2016). However, the studies attributing the southward shift of the ITCZ in the second half of the 20th century to anthropogenic aerosols are mainly based on model simulations (mainly CMIP3 and CMIP5), which need validation from other methods. In addition, both Hwang et al. (2013) and Allen et al. (2015) mentioned that the model-simulated southward shifts of the ITCZ during the second half of the 20th century were usually underestimated compared to observations. This is consistent with the findings in Song et al. (2014) that the weakening of the East Asian summer monsoon is underestimated in CMIP5 models, which suggests that there are uncertainties about the contribution of anthropogenic aerosols to the ITCZ southward shift during the latter half of the 20th century. Besides, a recent study found that the decrease of anthropogenic aerosols in the recent four decades has contributed to the seasonal delay of northward march of ITCZ (Song et al., 2021).

      Aerosols can be divided into radiatively absorbing aerosols (such as black carbon) and scattering aerosols (such as sulfate) according to their distinctively different optical properties (Yang et al., 2016; Zhao and Suzuki, 2019). The absorbing black carbon can increase the atmospheric absorption in the NH instantaneously, resulting in an anomalous southward atmospheric cross-equatorial energy transport and a northward cross-equatorial moisture transport. The black carbon induced anomalous southward atmospheric cross-equatorial energy transport and northward moisture transport form the upper and lower branches of an anomalous circulation (Fig. 4a), leading to a northward ITCZ shift (Mahajan et al., 2013; Ocko et al., 2014; Zhao and Suzuki, 2019). Therefore, the fundamental driving factor of the ITCZ displacement caused by black carbon is its instantaneous absorption of shortwave radiation in the atmosphere (Zhao and Suzuki, 2019). The scattering sulfate impacts the ITCZ latitudinal location through a completely different physical mechanism from black carbon. The instantaneous radiative forcing of sulfate at the TOA is almost equal to that at the surface, which means sulfate’s instantaneous radiation effect does not change the radiation budget in the atmosphere (Suzuki and Takemura, 2019). Zhao and Suzuki (2019) recognized that the northward anomalous cross-equatorial energy transport and the southward displacement of the ITCZ induced by sulfate are primarily attributable to its cloud lifetime effect (aerosols act as cloud condensation nuclei, reduce cloud droplet size, and increase cloud lifetime; Albrecht, 1989) and the surface evaporation response. Figure 4b shows that sulfate restrains evaporation in the NH, but still enhances the southward cross-equatorial moisture transport as the sulfate-induced cloud lifetime effect slows the hydrological cycle of the NH. On the south side of the equator, water vapor condenses, produces rainfall, and releases latent heat, which enhances the northward atmospheric cross-equatorial energy transport. The upper-level cross-equatorial energy transport and lower-level cross-equatorial moisture transport make up a closed anomalous cell.

      Figure 4.  Schematic plots of the physical mechanisms by which (a) black carbon and (b) sulfate aerosols impact the position of the ITCZ, originating from Zhao and Suzuki (2019). In the plots, cross-equatorial energy and moisture transports are both anomalous terms, and evaporation is the interhemispheric contrast of anomalous terms (Southern Hemisphere − Northern Hemisphere). Red fonts mark the most important initial forcing in the physical mechanisms of the ITCZ displacement caused by the two types of aerosols.

      As discussed in Section 3, many studies have tried to correct the double-ITCZ bias by simply increasing the SH cloud or surface albedo (Kay et al., 2016; Hawcroft et al., 2017), but have achieved little improvement because the ocean took the primary role in undertaking cross-equatorial energy transport. This means that disturbing the interhemispheric energy contrast does not necessarily result in ITCZ shift. But why can anthropogenic aerosols cause ITCZ shift? Chung and Soden (2017) statistically analyzed multiple CMIP5 model results and suggested that the basic reason is the impact of aerosols on cloud properties. Zhao and Suzuki (2019) found that for sulfate, the dominant component of anthropogenic aerosols, about one-third of the total anomalous cross-equatorial energy transport was undertaken by the atmosphere. By comparing the influence of different warm rain schemes on the sulfate climate effect, Zhao and Suzuki (2021) further revealed that the proportion of total anomalous cross-equatorial energy transport undertaken by the atmosphere would increase to about 50% with the cloud lifetime effect of sulfate increasing. Besides the cloud lifetime effect, the aerosol cloud albedo effect (increasing cloud droplet number concentration and albedo with fixed cloud water content) induces cooling effect (Twomey, 1977), whereas the aerosol cloud thermal effect (increasing thin cloud longwave emissivity) induces warming effect (Garrett and Zhao, 2006; Zhao and Garrett, 2015), which may render their overall influence on the ITCZ position very small. This means that the southward migration of the ITCZ induced by total anthropogenic aerosols is primarily attributable to aerosol–cloud interaction, the aerosol cloud lifetime effect in particular, rather than simply disturbing the energy contrast between the two hemispheres.

      To summarize, studies found total anthropogenic aerosols led to southward movements of the ITCZ in the latter half of the 20th century. The reason was not only because anthropogenic aerosols disturbed the energy balance between the NH and SH, but more importantly, because anthropogenic aerosols disturbed the interhemispheric energy balance through interacting with clouds. For sulfate, the most important anthropogenic aerosol, its cloud lifetime effect causes a considerable portion of the total anomalous cross-equatorial energy transport to occur in the atmosphere, inducing obvious southward migration of the ITCZ. At present, aerosol–cloud interaction remains one of the largest uncertainty sources in climate change research (IPCC, 2013). This reminds us that improving our understanding of the physical mechanisms of aerosol–cloud interactions is vital to the accurate simulation of the effects of aerosols on the ITCZ.

    5.   Summary and discussion
    • In this paper, we briefly introduced the research progress of the annual- and zonal-mean location and meridional shift of the ITCZ in recent years, focusing on three aspects: 1) the mechanism behind the asymmetry of the ITCZ annual- and zonal-mean position about the equator; 2) the double-ITCZ problem in climate models and efforts to correct it; and 3) the physical mechanisms of anthropogenic aerosols impacting the ITCZ position.

      1) Studies in the past dozen years have basically demonstrated that the driving factor of the north-of-the-equator ITCZ annual- and zonal-mean position is the Atlantic overturning circulation, which manifests a northward oceanic cross-equatorial energy transport and induces a subsequent compensating southward atmospheric cross-equatorial energy transport. A quantitative relationship (−7.5° to −2.4° PW−1) has been established between the displacement of the ITCZ mean position and the atmospheric cross-equatorial energy transport. Studies have found that cloud feedback is vital for the atmosphere to compensate for oceanic cross-equatorial energy transport, but the specific physical mechanisms (including dynamics and thermodynamics) require further study.

      2) Recent studies have demonstrated that simply correcting the interhemispheric energy contrast cannot effectively solve the double-ITCZ problem. Additionally, the double-ITCZ bias cannot be completely eliminated even when observational SST data are used in climate models. These findings indicate that the double-ITCZ problem in climate models is likely to be rooted in the imperfections of the physical schemes. In fact, some studies have already found that improving convection parameterizations, cloud microphysics, and resolutions in climate models, could reduce the double-ITCZ bias to some degree. However, models participating in CMIP6 generally still have systematic double-ITCZ biases, which show very little improvement compared to those participating in CMIP5. This means that the majority of climate models still have not solved the double-ITCZ problem, and future work is still needed.

      3) Recent studies have demonstrated that total anthropogenic aerosols cause the ITCZ to shift southward not only because they can disturb the interhemispheric energy balance but more likely because they can interact with clouds. This makes them more efficient at causing the ITCZ shift. It is found that increasing the cloud lifetime effect of anthropogenic aerosols can enhance the ITCZ sensitivity to anthropogenic aerosols. At present, aerosol–cloud–precipitation interactions are still not well represented in most climate models, and thus are deemed one of the largest uncertainty sources in climate change predictions. Accurate simulation of the impact of anthropogenic aerosols on the ITCZ position ​​requires the improvement of aerosol–cloud–precipitation physical processes in models.

      The review from the above three aspects generally demonstrates that the energy constraints associated with the ITCZ zonal-mean position and shift have been gradually established in recent years, and the improvements in cloud schemes (including parameterizations of convection and microphysics) appear to be of great importance for solving the systematic double-ITCZ problem and simulating the influence of human activities on the zonal-mean ITCZ position. However, caveats still exist in these studies and should be addressed with caution in future studies on ITCZ. First, it should be noted that the ITCZ shift and the cross-equatorial energy transport are not linearly related. Their relation may depend on forcing types and their distributions. Second, all efforts that have been successful in correcting the double-ITCZ bias in atmospheric models (i.e., SST is fixed) should be re-examined in atmosphere–ocean fully coupled models, as the heat capacity and circulation of the coupled ocean may reduce the sensitivity of the ITCZ to model modifications to, for example, convections (Talib et al., 2020). Moreover, attentions should be paid to the possible side-effects of correcting the double-ITCZ bias, i.e., the better ITCZ should not be at the cost of a deterioration in precipitation simulation over other areas, say North China.

      Acknowledgments. We would like to thank Dr. Xianwen Jing for his kind help and suggestions in organizing some sentences and paragraphs.

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