# Climate Models Exaggerate the Enhanced Double-ITCZ in the Warming Tropical Pacific Due To Preexisting Precipitation Bias

Zhiyuan Li, Alexey Fedorov

► To cite this version:

Zhiyuan Li, Alexey Fedorov. Climate Models Exaggerate the Enhanced Double-ITCZ in the Warming Tropical Pacific Due To Preexisting Precipitation Bias. Geophysical Research Letters, 2025, 52 (11), 10.1029/2025gl115445. http://dx.doi.org/10.1029/2025gl115445

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Distributed under a Creative Commons Attribution 4.0 International License
# Geophysical Research Letters®

## RESEARCH LETTER

10.1029/2025GL115445

**Key Points:**

* Climate models predict an enhanced double-Intertropical Convergence Zone (ITCZ) in a warming climate, while they already have a preexisting double-ITCZ bias

* Bias-corrected simulations show a much weaker intensification of the double-ITCZ under CO₂ rise

* The projected increase in southeastern Pacific rainfall is likely overestimated due to the models' preexisting bias

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

Z. Li,
zhiyuan.li@yale.edu

**Citation:**

Li, Z., & Fedorov, A. V. (2025). Climate models exaggerate the enhanced double-ITCZ in the warming tropical Pacific due to preexisting precipitation bias. *Geophysical Research Letters*, *52*, e2025GL115445. https://doi.org/10.1029/2025GL115445

Received 19 FEB 2025
Accepted 12 MAY 2025

**Author Contributions:**

**Conceptualization:** Alexey V. Fedorov

**Formal analysis:** Zhiyuan Li, Alexey V. Fedorov

**Funding acquisition:** Alexey V. Fedorov

**Investigation:** Zhiyuan Li, Alexey V. Fedorov

**Methodology:** Zhiyuan Li, Alexey V. Fedorov

**Project administration:** Alexey V. Fedorov

**Software:** Zhiyuan Li
**Supervision:** Alexey V. Fedorov

**Visualization:** Zhiyuan Li
**Writing – original draft:** Zhiyuan Li

**Writing – review & editing:** Alexey V. Fedorov

© 2025. The Author(s).
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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# Climate Models Exaggerate the Enhanced Double-ITCZ in the Warming Tropical Pacific Due To Preexisting Precipitation Bias

Zhiyuan Li¹ and Alexey V. Fedorov¹,²

¹Department of Earth and Planetary Science, Yale University, New Haven, CT, USA, ²LOCEAN/IPSL, Sorbonne University, Paris, France

**Abstract** Future greenhouse warming projections from the CMIP6 multi-model ensemble indicate increased precipitation in the southeastern Pacific Ocean and a reduction in interhemispheric precipitation asymmetry, suggesting an enhanced double-Intertropical Convergence Zone (ITCZ) in the tropical Pacific. However, these models exhibit a persistent annual-mean double-ITCZ bias in preindustrial and historical simulations, casting doubt on their projections. Indeed, models with the smallest preexisting double-ITCZ bias —aside from a few outliers—typically show the least enhancement of the double-ITCZ feature under global warming. To investigate this further, we use CESM2 and conduct flux-adjusted simulations that nearly eliminate the precipitation bias in preindustrial experiments, followed by CO₂-doubling simulations. Our bias-reduced simulations show a much smaller reduction in the precipitation asymmetry index and only half the increase in rainfall in the southeastern Pacific compared to the original model. We conclude that climate models likely overestimate the enhanced double-ITCZ feature under global warming due to the preexisting precipitation bias.

**Plain Language Summary** The Intertropical Convergence Zone (ITCZ) is a critical feature of tropical precipitation and climate. It is characterized by a band of intense rainfall and thunderstorms that in the tropical Pacific Ocean remains north of the equator throughout the year. However, climate models often struggle to simulate the ITCZ accurately, commonly producing an additional annual-mean zonal rainband south of the equator in the Pacific—a phenomenon known as the double-ITCZ bias. Under global warming, models predict an even stronger southern precipitation band, resulting in an enhanced double-ITCZ. This study examines whether this enhanced double-ITCZ is a genuine feature of climate change or an artifact of the models' precipitation bias. Using climate model experiments, we demonstrate that reducing the preexisting double-ITCZ bias significantly weakens the projected enhancement of the double-ITCZ in a warming climate. Similarly, we find that models with the smallest precipitation biases, relative to observations, tend to simulate the weakest double-ITCZ feature in the future. This suggests that current models may misrepresent future changes in tropical precipitation patterns due to preexisting biases, which underscores the need for improved climate simulations to ensure more reliable projections of future rainfall.

## 1. Introduction

The Intertropical Convergence Zone (ITCZ) is a critical feature of the tropical atmosphere, characterized by a band of clouds and intense precipitation that forms where the trade winds from the Northern and Southern Hemispheres converge (e.g., Schneider et al., 2014; Waliser & Gautier, 1993, and references therein). In the Pacific Ocean, the ITCZ stays north of the equator throughout the year (e.g., Adler et al., 2003). However, occasionally it shifts to the Southern Hemisphere during strong El Niño events (e.g., Hu & Fedorov, 2018; Wallace et al., 1998). Over the Maritime Continent and the Indian Ocean, the ITCZ migrates across the equator seasonally, while in the Atlantic, it remains mostly confined to the Northern Hemisphere (Xie & Carton, 2004).

To explain why the ITCZ annual mean position lies in the Northern Hemisphere, several hypotheses have been proposed: the latitudinal asymmetries of land-sea distribution between the hemispheres (Broccoli et al., 2006; Chiang & Friedman, 2012; Manabe & Broccoli, 1985); the effect of the Atlantic Meridional Overturning Circulation, which transports heat to the Northern Hemisphere (Frierson et al., 2013; Marshall et al., 2014); and the wind—evaporation—sea surface temperature (SST) (WES) feedback contributing to the northerly position of the ITCZ in the Pacific (Kang et al., 2008; Timmermann et al., 2005; Xie & Philander, 1994). Most recently, Z. Li and
Fedorov (2022) proposed a positive feedback mechanism between the North Equatorial Countercurrent and the ITCZ, which may also help maintain the ITCZ northerly position.

However, climate models often struggle to accurately simulate the position and strength of tropical rainfall bands, leading to a phenomenon known as the double-ITCZ bias in the eastern tropical Pacific (Wang et al., 2008; Xie & Philander, 1994). In coupled General Circulation Models (GCMs), this bias manifests as a secondary annual-mean zonal ITCZ that forms south of the equator, distorting simulations of tropical Pacific cloud, wind, and precipitation patterns. This bias may also affect rainfall projections in the tropics and climate sensitivity estimates, leading to substantial uncertainties in future climate predictions (Tian & Dong, 2020). The double-ITCZ bias is typically stronger in coupled GCMs but is also present in atmospheric GCMs forced by observed SSTs (G. Li & Xie, 2014; Oueslati & Bellon, 2013).

There are many physical factors that may contribute to the double-ITCZ problem (Burls et al., 2017; Hwang & Frierson, 2013; G. Li & Xie, 2014; G. J. Zhang et al., 2019; Zhao & Fedorov, 2020). For example, Hwang and Frierson (2013) highlight how cloud bias, particularly in the Southern Ocean, reinforces the double ITCZ structure by affecting the tropical energy budget. Xiang et al. (2017), Zhou and Xie (2017), and Zhou et al. (2022) argue that SST biases contributing to the double-ITCZ problem in coupled models originate from surface heat (energy) flux biases in the atmospheric model. Hirota and Takayabu (2012) also discuss how energy imbalances and large-scale circulation patterns, affected by ocean-atmosphere coupling, can maintain this bias. Moreover, regional and seasonal manifestation of the double-ITCZ bias is known to depend on geographic and seasonal factors (Adam et al., 2018).

Despite improvements in the resolution and complexity across several generations of CMIP models (CMIP3, CMIP5, and CMIP6), the double-ITCZ bias still persists in coupled models (Oueslati & Bellon, 2015; X. Zhang et al., 2015), suggesting that further refinements in how models simulate SSTs and their interactions with atmospheric circulation, clouds, and precipitation are necessary. Collectively, these studies highlight the importance of reducing/eliminating the double-ITCZ bias to enhance the reliability of climate model projections and improve the representation of tropical climate dynamics.

In this study we investigate how the double-ITCZ bias affects future projections of tropical precipitation in coupled GCMs. We show that in global warming simulations under the SSP585 scenario, climate models develop a significantly stronger zonal rainfall band in the southeastern Pacific, compared to the preindustrial climate, while exhibiting a weaker interhemispheric precipitation asymmetry across the equator. This indicates an enhanced double ITCZ in a warming climate, consistent with previous studies (Z. Li & Fedorov, 2024). This finding raises a critical question: Is this change in tropical Pacific precipitation a genuine climate response to warming, or is it an artifact of the preexisting model double-ITCZ bias?

To address this question, we conduct a multi-model CMIP6 analysis and investigate the role of the double-ITCZ bias in the tropical precipitation response to global warming using a suite of numerical simulations with heat flux adjustment of different strengths. The flux adjustment allows us to reduce the precipitation bias and then examine the climate response in a warmer climate, specifically under the doubling CO₂ scenario. This approach helps refine the future climate projections in the Pacific region and better understand the impact of the double-ITCZ bias under anthropogenic warming.

We note that improving the physics and resolution of models is the ultimate means to reduce tropical precipitation bias. However, recent work on flux adjustments, even with state-of-the-art climate GCMs, has shown that—despite some earlier reservations (Neelin & Dijkstra, 1995)—flux adjusted experiments can provide powerful insights into the behavior of the climate system (e.g., Ray et al., 2018; Vecchi et al., 2014; Wittenberg et al., 2016; Zhao & Fedorov, 2020). For example, the last authors demonstrated that applying sufficiently strong, antisymmetric with respect to the equator heat flux anomalies in the tropical Pacific could generate a variety of annual mean precipitation patterns—from a single ITCZ in the north, to a double-ITCZ and a single ITCZ in the south.

## 2. Methods

### 2.1. CMIP6 Multi-Model Ensemble

We use the CMIP6 data set and compare Historical simulations and the Shared Socioeconomic Pathway 5-8.5 (SSP585 for brevity) scenario, representing the historical climate and a high-emissions global warming
projection, respectively. The SSP585 is comparable to the RCP8.5 global forcing pathway used in CMIP5. On average, global mean surface temperature under SSP585 rises by approximately 4.7°C–5.1°C relative to pre-industrial levels by the end of the 21st century (O'Neill et al., 2016), corresponding to about 8.5 W/m² of radiative forcing due to elevated concentrations of greenhouse gases in the atmosphere.

There are 23 CMIP6 models available with both historical and SSP585 experiments. Each model has only one ensemble member with the variant-id "r1i1p1f1" in the CMIP6 data set. The four letters refer to realization, initialization, physics, and forcing (Taylor et al., 2018). The numbers indicate specific model configurations. Following Z. Li and Fedorov (2024), the annual mean variables are calculated from 1930 to 1980 for the Historical experiments and from 2050 to 2100 for the SSP585 experiments.

## 2.2. GCM Experiments

To perform numerical experiments, we employ the NCAR Community Earth System Model v2 (CESM2.1.4)—a fully coupled model widely used for climate projections and sensitivity studies. This model has an approved representation of atmospheric, oceanic, and land processes compared to the previous version, CESM1. CESM2 integrates an updated version of the Community Atmosphere Model (CAM6) with enhanced cloud microphysics and convection schemes, leading to more accurate simulations of tropical precipitation and cloud properties (Danabasoglu et al., 2020; Gettelman et al., 2019). Additionally, CESM2 includes improvements in its oceanic component, the Community Ocean Model (POP2), which offers more realistic simulations of ocean circulation and SSTs (Kay et al., 2015).

We perform four main experiments: a preindustrial control experiment following by a standard experiment with abrupt CO₂ doubling. These simulations demonstrate how a typical climate model with the double-ITCZ bias respond to warming. Then, we conduct a flux-adjusted preindustrial experiment designed to simulate a more realistic precipitation pattern and reduce precipitation bias in the tropical Pacific Ocean; this flux-adjusted model is then used to conduct an experiment with an abrupt doubling of CO₂ to study the impact of removing the double-ITCZ bias on precipitation response to warming. Hereafter, these four experiments are referred to as Control, Control_2xCO₂, FluxAdjusted, and FluxAdjusted_2xCO₂.

While we primarily focus in this paper on these four experiments, we also conduct an additional pair of experiments, referred to as Extreme_ITCZ and Extreme_ITCZ_2xCO₂, characterized by a stronger northern ITCZ and weaker precipitation in the southeastern Pacific, as compared to the observations. These two experiments are designed to demonstrate the robustness and monotonic changes observed in our sensitivity analysis. All sensitivity experiments are listed in Table S1 in Supporting Information S1.

The bias reduction is achieved by imposing a heat flux anomaly in the tropical eastern Pacific (Figure S1 in Supporting Information S1) between 150°W and 90°W in the ocean component POP2. The anomaly has a piecewise linear structure in the latitudinal direction, and its magnitude is set through trial and error to decrease precipitation over the southern band while strengthening and shifting the ITCZ toward the equator as observed. The Extreme_ITCZ experiment uses a simpler anomaly pattern with +30 W/m² from 5°N to 25°N and -10 W/m² from 5°S to 25°S.

All experiments are conducted at the horizontal resolution of f19_gx1v7: 1.9° × 2.5° for the atmosphere and land components, and approximately 1° for the ocean and ice components. The grid of the ocean model becomes finer approaching the equator with its latitudinal resolution reaching about 0.3°. All experiments are run for 100 years and the last 50 years of data in each experiment is used for analysis.

We note that several previous studies (e.g., Vecchi et al., 2014) aimed to simultaneously reduce major climatological biases in SST, salinity, and winds. To achieve this, adjustments were made to the fluxes of heat, freshwater, and momentum, specifically targeting these three fields. In contrast, we find that applying heat flux adjustments that do not directly target the SST field is more effective when the goal is to reduce precipitation bias.

## 2.3. Observational Data

We utilize the ERA5 reanalysis data, which provides high-resolution, global climate data with comprehensive temporal coverage. ERA5 is provided by the European Centre for Medium-Range Weather Forecasts and is based on a state-of-the-art atmospheric model that assimilates a wide range of observational data, including satellite,
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Future projections for tropical precipitation. A multi-model mean precipitation map for the (a) Historical and (b) SSP585 experiment, and (c) their difference. (d) Zonal mean profiles of multi-model mean precipitation in the eastern tropical Pacific averaged between 150°W and 90°W for the historical (black) and SSP585 (red) scenarios and the ERA5 reanalysis data (gray). Note the enhanced double-Intertropical Convergence Zone in the future simulation.</figcaption></figure>

surface, and atmospheric measurements (Hersbach, 2016; Hersbach et al., 2020). Here, we use the ERA5 reanalysis data spanning from 1979 to 2023. While ERA5 now extends its record back to 1940, we have chosen to focus on the period from 1979 onward (Bell et al., 2021). This time frame corresponds to the period when satellite data became widely available, improving the reliability and accuracy of the observational input for reanalysis.

## 2.4. Metrics to Assess the Double-ITCZ Feature

We use two main metrics to assess the strength of the southern precipitation band and the interhemispheric asymmetry of precipitation in the eastern tropical Pacific: the Interhemispheric Precipitation Asymmetry Index (PAI) and the Southeast Pacific Precipitation Index.

The first metric builds on the tropical precipitation asymmetry index defined by Hwang and Frierson (2013), but here we focus on the eastern tropical Pacific by constraining the area to 150°W–90°W. We also restrict the southern precipitation band to latitudes between 0°S and 10°S. This agrees with the ERA reanalysis (Figure 1d) and allows us to isolate the zonal precipitation band from the storms tracks in GCM simulations (Figures 1a and 1b). The northern precipitation boundary extends to 20°N as in Hwang and Frierson (2013):

$$PAI = \frac{\bar{P}_{EQ,20^\circ N} - \bar{P}_{EQ,10^\circ SS}}{\frac{2}{3} \times \bar{P}_{EQ,20^\circ N} + \frac{1}{3} \times \bar{P}_{EQ,10^\circ S}}$$

where $\bar{P}$ denotes the area-averaged annual-mean precipitation, and the subscripts refer to the latitudinal boundaries of each region. Weighting factors are introduced to account for different latitudinal boundaries of precipitation bands in each hemisphere.

The second metric, the Southeast Pacific Precipitation Index, is defined as the annual-mean climatological precipitation over the area spanning from 150°W to 90°W and from the equator to 10°S.

Our results are somewhat sensitive to the choice of the southern boundary because of the equatorial shift of the precipitation band and the potential influence of storm tracks. To demonstrate the robustness of our findings, an
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Southeast Pacific precipitation index and the interhemispheric precipitation asymmetry index. (a) Changes in the Southeast Pacific precipitation index (averaged 150°W–90°W, 0°S–10°S; Methods) in SSP585 experiments relative to the Historical experiment as a function of the index itself for 23 CMIP6 models. Each CMIP6 model is shown with a different color marker, and the blue open star refers to the multi-model mean. The open black, red, and brown stars, linked with a dashed line, refer to the Control, Flux_Adjusted, and Extreme_ITCZ experiments, respectively; for these experiments we compare the CO₂-doubling scenario to Preindustrial conditions. The vertical lines indicate the mean values of these indices for the ERA5 reanalysis data, while shading shows 1 standard deviation of 10-year running means; the horizontal line marks no change in the indices with global warming. (b) As in panel (a) but for the interhemispheric precipitation asymmetry index (Methods).</figcaption></figure>

alternative set of indices based on the maximum precipitation within each band has also been constructed and analyzed (Figure S2 in Supporting Information S1). Using maxima instead of mean values renders the results insensitive to the location of the southern boundary and shifts in the precipitation band. We find no qualitative differences between the two approaches.

### 3. Results

We first examine projected changes in tropical precipitation patterns using the CMIP6 models. The multi-model ensemble mean precipitation from the Historical experiments exhibits a distinct second precipitation band visible in the southeastern Pacific, between 150°W and 90°W (Figure 1a). The meridional profiles in Figure 1d further highlight this double-ITCZ bias, with a much stronger second peak in the Southern Hemisphere around 5°S in the Historical experiment (black line) compared to the ERA5 data (gray line). Additionally, the simulated ITCZ is slightly weaker in intensity and shifted more northward from the equator.

Figures 1b and 1c show the multi-model ensemble mean precipitation in the SSP585 experiment and its difference against Historical experiment. With warming, the Western Pacific Warm Pool and the northeastern Pacific where the ITCZ is located have the largest increase in precipitation, following the “wet gets wetter” mechanism (Held & Soden, 2006). At the same time, these experiments reveal an increase in precipitation in the southeastern Pacific Ocean, commonly linked to the double-ITCZ bias. This is also evident in Figure 1d, where both the northern and southern precipitation bands intensify and shift toward the equator under the SSP585 scenario. This strengthening and equatorward intensification of the ITCZ is, in part, related to the development of the eastern equatorial warming pattern in most models (e.g., Byrne & Schneider, 2016; Fu & Fedorov, 2023; Heede & Fedorov, 2021; Heede et al., 2020, 2021; Lau & Kim, 2015; Z. Li & Fedorov, 2024; Studholme et al., 2022; Zhou et al., 2019). Spatial maps of the precipitation anomalies for each model and their meridional profiles are shown in Figures S3 and S4 in Supporting Information S1, respectively.

The Southeast Pacific Precipitation Index under the Historical scenario and its changes under the SSP585 scenario are shown in Figure 2a. Out of 23 models analyzed, 18 models simulate a too strong precipitation band in the
southern hemisphere, indicative of the double ITCZ, except AWI-CM-1-1-MR, AWI-ESM-1-REcoM, CESM2-WACCM, MPI-ESM1-2-LR, and NorESM2-MM. In the warming scenario, all models simulate a stronger precipitation in the southern hemisphere than in their Historical simulations.

Next, we introduce a weighted precipitation asymmetry index to further quantify the double-ITCZ bias, where a higher index indicates a stronger difference between the two precipitation bands and a less severe double-ITCZ bias. Out of 23, 19 models have the double-ITCZ problem with a too small asymmetry index compared to the observations, except AWI-CM-1-1-MR, AWI-ESM-1-REcoM, CESM2-WACCM, and NorESM2-MM. This is consistent with their weaker than the observed precipitation in the southeastern Pacific. Twenty-one models simulate a decrease in the asymmetry index in the SSP585 scenario relative to the Historical experiment, suggesting a more pronounced double ITCZ in the future projections.

Since the CMIP6 models consistently exhibit a double-ITCZ bias—characterized by a stronger than observed second precipitation band—and project an even more enhanced double ITCZ in future scenarios, the question arises: does the decrease in the asymmetry index and intensification of southeastern Pacific precipitation reflect a true climate change signal, or are these merely artifacts of the preexisting bias.

Across the models, we find no robust correlations between the indices and their changes (Figure 2). For instance, while both CESM2-WACCM and NorESM2-MM exhibit realistic precipitation profiles, their responses to warming differ significantly. If one removes an outlier, CESM2-WACCM, the correlation between the Southeast Pacific Mean Precipitation Index and its change becomes 0.39 with *p* = 0.08, indicating a marginal statistical significance. These low correlations can be attributed to various factors, including differences in the simulated structure of tropical precipitation south of the equator (Figures S3 and S4 in Supporting Information S1) and differences in parameterizations across GCMs (Bony et al., 2015; Flato et al., 2014; Hourdin et al., 2017; Stephens et al., 2010).

The low correlations complicate drawing conclusions based solely on the model intercomparison. Nevertheless, models with smallest preexisting double-ITCZ bias in both precipitation indices (Figure 2) typically show the smallest enhancement, if any, of the double-ITCZ feature in future projections, with few outliers. To investigate this further, we conduct several sensitivity experiments with flux adjustments and investigate the role of the preexisting precipitation bias in enhancing the double-ITCZ under global warming.

The precipitation map in our preindustrial Control experiment and the difference with ERA5 are shown in Figures S5a and S5d in Supporting Information S1. A dipole-like precipitation change pattern north of the equator indicates a weaker ITCZ slightly shifted northward from the equator in the Control experiment as compared to ERA5. Meanwhile, the positive precipitation anomaly south the equator is characteristic of the double-ITCZ bias.

To reduce this bias, we now impose additional heat flux anomalies within the model's ocean component (the Flux_Adjusted experiment, Methods). Although this experiment simulates a slightly dryer West Pacific Warm Pool, the precipitation bias has been significantly reduced across most of the Pacific (Figures S5e and S6 in Supporting Information S1). In particular, the meridional profile of the precipitation bias in the eastern Pacific, which is of our primary interest, shows a dramatic improvement compared to the Control experiment.

Abrupt CO₂ doubling experiments are then conducted for both the Control and Flux_Adjusted experiments. For the Control experiment, precipitation increases in the central and eastern tropical Pacific but decreases in the west and in the subtropics (Figure 3g). This is consistent with the development of the eastern equatorial Pacific (El Niño-like) warming pattern. Spatial maps of the SST anomalies for each model are shown in Figure S7 in Supporting Information S1. In the east precipitation increases in both hemispheres, leading to a more pronounced double ITCZ in the future simulation relative to the Control (Figures 1 and 4a).

However, the Flux_Adjusted model exhibits a notably different response to warming, with the rainfall anomaly extending farther west toward the West Pacific Warm Pool and displaying an asymmetric pattern in the east (Figure 3h). The difference between changes in the two warming experiments is illustrated in Figure 3i. Increased precipitation is observed near the ITCZ and the SPCZ storm tracks, while precipitation decreases in the southeastern Pacific. Thus, the reduction of the original double-ITCZ bias acts to decrease the tendency toward the enhanced double-ITCZ feature in the Flux_Adjusted_2xCO₂ experiment.

These changes are evident in the meridional precipitation profiles averaged between 150°W and 90°W (Figure 4). The Control experiments under global warming shows a shift and a widening of the ITCZ toward the equator, but
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Precipitation and its response to CO₂ doubling. (a, d, and g) Precipitation map for the Control and Control_2xCO₂ experiments, and their difference, respectively. (b, e, and h) Precipitation map for the FluxAdjusted, FluxAdjusted_2xCO₂, and their difference. (c, f, and i) Difference between the first and second column. For example, the last panel shows the difference between the response to warming in the flux-adjustment experiment relative to the standard experiment.</figcaption></figure>

its maximum precipitation remains nearly unchanged. Within the southern precipitation band, the maximum precipitation increases significantly, from 2.7 to 4.6 mm/day (a 1.9 mm/day increase). In contrast, in the FluxAdjusted experiment, after the double-ITCZ bias reduction, the ITCZ widens less, and the maximum precipitation in the Southern Hemisphere increases by a smaller amount, from 2.0 to 3.0 mm/day, or a 1 mm/day increase (Figure 4b). The net precipitation increase is smaller, and the southern precipitation peak remains relatively weak compared to the ITCZ (Figure 4c). The precipitation increase in the Southern Hemisphere in the FluxAdjusted experiments is only half of that in the Control experiments, while changes in the Northern Hemisphere are similar.

The precipitation asymmetry indices and their changes for the Control and FluxAdjusted experiments are presented in Figure 2 as black and red stars, respectively. The precipitation bias is nearly corrected by imposing the flux adjustments, with the Southeastern Pacific precipitation index decreasing from 2.13 to 1.65 mm/day (ERA5: 1.40 mm/day) and the asymmetry index increasing from 0.69 to 0.86 (ERA5: 0.94). Under the doubling of CO₂ scenario, the Control run shows a significant increase in the SPCZ index and a decrease in the asymmetry index, whereas the FluxAdjusted experiment exhibits only half of those changes.

Finally, we conduct another experiment, Extreme_ITCZ, to explore conditions characterized by a stronger than observed ITCZ and extremely weak precipitation in the southeastern Pacific (brown star in Figure 2). The monotonic progression of changes in the two indices with warming, from a weak-ITCZ to a more realistic ITCZ and finally to a strong-ITCZ case, underscores the robustness of our analysis and conclusions. Spatial maps and meridional profiles in the Extreme_ITCZ experiment (Figure S8 in Supporting Information S1), exhibit patterns similar to those in the FluxAdjusted experiment, albeit with stronger anomalies. One interesting observation is that, although the southern precipitation band is nearly eliminated in the eastern Pacific in the Extreme_ITCZ case, there is a small but noticeable precipitation increase there under global warming (Figure S8d in Supporting Information S1).
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Meridional profiles of precipitation and change. (a) The meridional profiles of precipitation (averaged between 150°W and 90°W) in the Control (black), Control_2xCO₂ (red) experiments, and ERA5 reanalysis, c.f. Figure 1d. (b) As in panel (a) but for the FluxAdjusted (black) and FluxAdjusted_2xCO₂ (red) experiments. (c) Changes in precipitation under abrupt CO₂ doubling for the Control (solid) and FluxAdjusted (dashed) experiments.</figcaption></figure>

4. Discussion

The double-ITCZ bias has persisted across multiple generations of CMIP models, from CMIP3 and CMIP5 to CMIP6 (e.g., Adam et al., 2018; G. J. Zhang et al., 2019; Zhao & Fedorov, 2020). This persistence highlights the challenges faced by climate models in accurately representing present and future tropical precipitation patterns.

In this study, we have investigated whether the persistent double-ITCZ bias in CMIP6 models affects future climate projections for tropical precipitation. Our results demonstrate that the double-ITCZ bias is not only a longstanding issue across models, but the double ITCZ feature becomes significantly stronger under future warming scenarios across the majority of climate models. However, the key takeaway from our results is that the enhanced double-ITCZ problem in future projections is, at least in part, a consequence of preexisting biases in the models. The flux-adjusted experiments, specifically designed to mitigate the precipitation bias, lead to a significant reduction in double ITCZ intensity, particularly in the southeastern Pacific. In fact, our experiments suggest that eliminating the original double-ITCZ bias can largely eliminate the emerging double-ITCZ in the future simulations. Likewise, climate models with the smallest bias in both asymmetry indices show the smallest changes in these indices (except for one outlier).

This finding raises the question: Why does the preexisting precipitation bias exacerbate the double-ITCZ problem in global warming simulations? To the first order, this finding is consistent with the “wet gets wetter, dry gets drier” paradigm (e.g., Held & Soden, 2006), which follows from the Clausius-Clapeyron relationship and water vapor budget. If there is already a sufficiently strong precipitation band in the southern hemisphere, it becomes even stronger with warming.

To assess the “Wet Gets Wetter” contribution of the preexisting bias to the projected precipitation changes south of the equator, we follow the approach of Adam et al. (2023), which allows for separate treatment of changes in precipitation and evaporation (also see Siler et al. (2019)). We define the adjusted precipitation change as $\delta P_{adj} = \delta P - \alpha P_{bias}$. Here, $\delta P$ denotes the modeled precipitation change between the SSP585 and Historical experiments, and $-\alpha P_{bias}$ is the thermodynamic adjustment due to the preexisting precipitation bias $P_{bias}$, where $\alpha$ is the Clausius-Clapeyron parameter, that is, the relative increase in water vapor saturation pressure per degree of warming, and the evaporation bias has been neglected. We find that across the models, the average adjustment to the precipitation response is roughly 30% of the full response (Figure S9 in Supporting Information S1). This estimate is comparable to, but smaller than the ~50% reduction in precipitation change found in our flux-adjusted experiment. The difference may result from the large inter-model spread and/or changes in wind convergence toward the southern precipitation band neglected by the thermodynamic scaling.

The large model spread is one of the challenges in evaluating the impacts of the double-ITCZ bias on future projections. While our flux-adjusted warming experiments elucidate the issue, differences in responses across climate

models (e.g., CESM2-WACCM vs. NorESM2-MM) suggests that the mechanisms driving the bias can be model-specific. Differences in physical parameterizations likely contribute to variations in model responses to warming. We also hypothesize that different climate groups use different strategies to reduce the double-ITCZ bias in their preindustrial and historic simulations, possibly resulting in error compensation that affects both the mean state and climate variability (e.g., Bayr et al., 2019). With warming, in some of these models these errors grow more rapidly, quickly exacerbating the double-ITCZ bias.
The lack of a consistent relationship between the strength of the double-ITCZ bias and precipitation response to warming across models complicates the interpretation of future climate projections. However, while we cannot exclude that the double-ITCZ feature will be slightly enhanced in the future climate, our results suggest that the models clearly overestimate this change. Therefore, it is likely that more realistic simulations of the ITCZ in the present-day climate will reduce or possibly eliminate the double-ITCZ feature under future warming scenarios.

## Data Availability Statement

All CMIP6 data set is available from World Climate Research Programme at https://esgf-node.lnl.gov/projects/cmip6/. ERA5 reanalysis data for precipitation are available from Hersbach et al. (2023). All analysis code and figure scripts are publicly available at https://github.com/AYANAMYYY/Double_ITCZ_GRL_2025.git (Z. Li, 2025).

## Acknowledgments

This study has been supported by NASA (80NSSC21K0558 and 80NSSC20K1634) and NOAA (NA200AR4310377). Additional support has been provided by the ARCHANGE Project (ANR-18-MPGA-0001, France). We would like to acknowledge high-performance computing support from the Derecho system (doi: 10.5065/qx9a-pg09) provided by the NSF National Center for Atmospheric Research (NCAR), sponsored by the National Science Foundation.

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