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# Closure of tropical seaways favors the climate and vegetation in tropical Africa and South America approaching their present conditions

Ning Tan, Huan Li, Zhongshi Zhang, Haibin Wu, Gilles Ramstein, Yong Sun, Zhilin He, Baohuang Su, Zijian Zhang, Zhengtang Guo

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## To cite this version:

Ning Tan, Huan Li, Zhongshi Zhang, Haibin Wu, Gilles Ramstein, et al.. Closure of tropical seaways favors the climate and vegetation in tropical Africa and South America approaching their present conditions. Global and Planetary Change, 2024, pp.104351. 10.1016/j.gloplacha.2023.104351. hal-04379824

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Closure of tropical seaways favors the climate and vegetation
in tropical Africa and South America approaching their present
conditions

Ning Tan, Huan Li, Zhongshi Zhang, Haibin Wu, Gilles Ramstein,
Yong Sun, Zhilin He, Baohuang Su, Zijian Zhang, Zhengtang Guo

<figure><img src="image_4.png" /></figure>

PII: S0921-8181(23)00325-9

DOI: https://doi.org/10.1016/j.gloplacha.2023.104351

Reference: GLOBAL 104351

To appear in: *Global and Planetary Change*

Received date: 12 June 2023

Revised date: 23 December 2023

Accepted date: 26 December 2023

Please cite this article as: N. Tan, H. Li, Z. Zhang, et al., Closure of tropical seaways favors the climate and vegetation in tropical Africa and South America approaching their present conditions, *Global and Planetary Change* (2023), https://doi.org/10.1016/j.gloplacha.2023.104351

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
# Closure of tropical seaways favors the climate and vegetation in tropical Africa and South America approaching their present conditions

Ning Tan<sup>a,b</sup>, Huan Li<sup>c</sup>, Zhongshi Zhang<sup>d,e</sup>, Haibin Wu<sup>a,f,g</sup>, Gilles Ramstein<sup>h</sup>, Yong Sun<sup>j</sup>, Zhilin

He<sup>a,b</sup>, Baohuang Su<sup>j</sup>, Zijian Zhang<sup>a,b</sup>, Zhengtang Guo<sup>a,f,g</sup>

*Corresponding author: Ning Tan (ning.tan@mail.iggcas.ac.cn) and Huan Li (huan.li.lzu@gmail.com)

## Abstract

The tectonically induced closure/constriction of the Central American Seaway (CAS) and Indonesian Seaway (Indo) during the early to mid-Pliocene has been associated with many climatic events, such as the onset of Northern Hemisphere glaciation, the intensification of the Atlantic meridional overturning circulation (AMOC) and Australian aridification. However, studies on how the closure/constriction of tropical seaways affects the tropical climate system are still sparse and not systematic. Previous studies have linked the constriction of Indo to the aridification over East Africa and discussed the role of CAS closure in affecting the moisture supply over South America, but the underlying mechanism and combined effect of both tropical seaways are not well studied. In this study, we evaluate the impacts of tropical seaways' closure/constriction and distinguish the relative roles of CAS

<footnote><sup>a</sup>Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China</footnote>

<footnote><sup>b</sup>Innovation Academy for Earth Science, CAS, Beijing 100029, China</footnote>

<footnote><sup>c</sup>School of Geographic Science, Nantong University, Tongjingdadao 99, Nantong 226007, China</footnote>

<footnote><sup>d</sup>Department of Atmospheric Science, School of Environmental Studies, China University of Geosciences, Wuhan 430074, China</footnote>

<footnote><sup>e</sup>Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China</footnote>

<footnote><sup>f</sup>University of Chinese Academy of Sciences, Beijing 100049, China</footnote>

<footnote><sup>g</sup>CAS Center for Excellence in Life and Paleoenvironment, Beijing 100044, China</footnote>

<footnote><sup>h</sup>Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay 91191 Gif-sur-Yvette, France</footnote>

<footnote><sup>i</sup>State Key Laboratory of Tibetan Plateau Eartl System, Environment and Resources (TPESER), Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China</footnote>

<footnote><sup>j</sup>State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, 46 Zhong-Guan-Cun South Avenue, Beijing 100081, China</footnote>
and Indo on climate in tropical Africa and South America using the NorESM-L Atmosphere-Ocean General Circulation Model (AOGCM) and a dynamic vegetation model (LPJ-GUESS). Our results show that the closure of the CAS leads mainly to aridification in northeastern Brazil, resulting in an expansion of tropical xerophytic shrubland and savanna in this region. The narrowing of the Indo mainly leads to enhanced aridification in eastern tropical Africa, and reduces the extent of tropical forests in eastern and northern tropical Africa, which is generally consistent with the data. The closure/narrowing of the two tropical seaways results in a superposition of the individual seaway's effect, particularly over northeastern Brazil region, which exhibits enhanced aridification compared to the closure of the individual CAS. The seaways' changes are shown to be pivotal for the evolution of climate and vegetation over East Africa and northeastern South America to contemporary conditions.

**Keywords:** Indonesian seaway, Central American Seaway, Tropical Africa and South America, Vegetation change

# 1. Introduction

Tropical Africa and South America have the largest rainforests in the world with high biodiversity (Saatchi et al., 2011; Soref et al., 2017). The long-term evolution of climate and environment on these tropical continents is of great importance, as it can provide a better understanding of the greatest biodiversity and even the dispersal of our human ancestors (Barboni et al., 2019; Gibert et al., 2022; WoldeGabriel et al., 1994). Under present conditions, a contrasting "wet-city" pattern prevails in west-eastern tropical Africa (Gibba et al., 2019), consequently, most of western tropical Africa is covered by rainforest, while eastern tropical Africa consists of savanna and montane grasslands (Beck et al., 2018). This open and C4-dominated vegetation in East Africa may date back to the early Miocene as indicated by the multiproxy reconstructions from the new fossil mammal sites in Kenya and Uganda (MacLatchy et al., 2023; Peppe et al., 2023). However, due to the paucity of records prior to the Late Miocene, many previous studies have implied that the tropical forest and wetter conditions might be observed over East Africa prior to the late Miocene (Acosta et al., 2022; Couvreur et al., 2021; Feakins et al., 2013). Despite the early origin of C4 vegetation, the variations of vegetation also existed since the early-to-mid Miocene in the long-term records (Bonnefille, 2010; Feakins et al., 2013; Peppe et al., 2023). Some data studies also suggest that the tropical rainforests may have re-expanded and savannas may have shrunk in the early to mid Pliocene. At some fossil sites in East Africa, moisture taxa and forests have
been recorded between 5-3 Ma (Bonnefille, 2010; Joordens et al., 2019; Morley, 2000). Notable aridification in East Africa was observed from the mid-Pliocene onwards (see summary by Cane and Molnar, 2001) and intensified in the late Pliocene in association with the occurrence of Northern Hemisphere ice sheets (deMenocal, 1995; Trauth et al., 2021). In tropical South America, the earliest occurrence of modern rainforest probably dates to the Eocene (Burnham and Johnson, 2004), but the modern-like distribution of rainforest in this region might occur in the middle to late Miocene in association with the uplift of the Andes and the emergence of the modern drainage system (Figueiredo et al., 2009; Hoorn et al., 2010). The northeastern Brazil (also referred to as the "Nordeste") now exhibits semi-arid to arid conditions that delineate the Amazon rainforest from the Atlantic coastal rainforest of southeastern South America (Salazar et al., 2007). The extent of aridity in the Nordeste region can vary due to fluctuations in glacial-interglacial climate (Wang et al., 2004) and modern climate variability (Marengo et al., 2016).

Though many factors, e.g., orbital parameters' changes (Gibert et al., 2022), CO₂ evolution (Acosta et al., 2022) and modern climate variability (Marengo et al., 2016), can make the dry/moisture variations around the tropical regions, the main cause of long-term evolution of the climate and vegetation in both tropical East Africa and Nordeste is undoubtedly the geographic distribution (including highland-lowland contrast, land-sea distribution, etc.) due to tectonic activities in Earth's history. A recent modelling study by Acosta et al. (2022) examined the influence of changes in paleogeography and CO₂ concentration over the past 95 Myr on the evolution of African and South American monsoons. Their results show that the widening of the South Atlantic and the Andean orogeny can enhance the South American monsoon, while the East African rift zone orogeny can enhance the East African monsoon. In contrast, an earlier modelling study (Sepulchre et al., 2006) indicated that the East African Rift System can directly reduce precipitation in East Africa. Changes in critical seaways are also important for the environment' evolution. Zhang et al. (2014) demonstrated that the retreat of the Tethys Sea is the main tectonic driving force for the formation of the Sahara Desert. The gradual shoaling and narrowing of the Central American and the Indonesian seaways (hereafter referred to as “CAS” and “Indo”) that occurred from the Miocene to the Pliocene (Auer et al., 2019; Gaina and Müller, 2007; Hall, 2002; Haug et al., 2001; O'Dea et al., 2016) are extensively discussed due to their potential to alter the ocean circulation and the global climate (Brierley and Fedorov, 2016; Jochum et al., 2009; Keigwin, 1982; Lunt et al., 2008; Tan et al., 2022). The tectonic-induced closure of the CAS is estimated to be around 4
Ma, indicated by the geochemical proxies showing the large contrast condition between the tropical Pacific and Atlantic (Haug et al., 2001). The constriction and shoaling of Indo took place along with the slow convergence of the Australia and New Guinea plate to the Sundaland and Eurasia plate, resulting in the formation of various small islands (Auer et al., 2019; Gaina and Müller, 2007). A modern-like state of Indo probably formed around 3-4 Ma as summerized by Cane and Molnar (2001). The different states of the CAS can lead to different strengths of Atlantic meridional overturning circulation (AMOC) (Nof et al., 2006; Sepulchre et al., 2014). The closure of CAS may link to the cooling over northen hemisphere (NH) high latitudes (Brierley and Fedorov, 2016; Lunt et al., 2008; Tan et al., 2017) and affect moisture supply in the South American region (Jaramillo et al., 2020). The narrowing of the Indo shows obvious influences on the Pacific ocean circulation (Jochum et al., 2009; Tan et al., 2022) and may lead to aridifications in Australia (Christensen et al., 2017; Krebs et al., 2011) and in East Africa (Cane and Molnar, 2001) in the latter study (Cane and Molnar, 2001), the authors have invoked the constriction of Indo as the precursor for the East African aridification around 3-4 Ma. This aridification would potentially affect the evolution of hominids during this period (Bonnefille, 2010; Gibert et al., 2022). However, this modelling study is based on an Ocean General Circulation Model (OGCM) and the underlied mechanism is still unclear. In the absence of geological records, very few studies have systematically quantified the potential impact of tropical seaways' changes on tropical South American climate, while the change to the ocean circulation and ocean temperature around tropical Atlantic due to the closure of the CAS (Molnar et al., 2008; Sepulchre et al., 2014) can indeed affect the local environment. The impact of tropical seaways on the climate and environment of tropical Africa and South America remains unclear. To understand that, it requires more studies based on more sophisticated climate models.

Thus, the objectives of this modelling study include 1) to explore the impacts of changes to tropical seaways on the climate and vegetation over tropical Africa/South America; 2) to distinguish the contribution of the closure of CAS and Indo on the tropical African/South American climate. For this, we performed a series of sensitivity experiments based on the low-resolution version of the Norwegian Earth System Model (NorESM-L; Zhang et al., 2012) and an off-line dynamic vegetation model LPJ-GUESS (Smith et al., 2001; Smith et al., 2014).
## 2. Method

### 2.1 Climate model

The NorESM-L is a low resolution version of NorESM, that is designed specifically for paleoclimate studies. In this model, the atmosphere model adopted the spectral Community Atmosphere model CAM4 (Neale et al., 2013) from the National Centre for Atmospheric Research (NCAR). The ocean component is originated from the Miami Isopycnic Coordinate Ocean Model (MICOM; Bleck et al., 1992) with a resolution of g37 (~3°) in the horizontal and 32 layers in the vertical. Additional details of the model can be found in Bentsen et al. (2013) and Zhang et al. (2012). The model NorESM-L has been applied in many paleoclimate studies and has shown good performance in the pre-industrial simulation (Li et al., 2020; Tan et al., 2022; Zhang et al., 2012, 2014).

### 2.2 Vegetation model

LPJ-GUESS (Smith et al., 2001; Smith et al., 2014) is Dynamic Global Vegetation Model (DGVM) with explicit scaling of individual-level processes among several patches in each grid cell. The model employs biophysical and physiological process parameterizations identical to the equilibrium model BJOME3 (Haxeltine and Prentice, 1996). In addition, LPJ-GUESS includes dynamic representations of establishment, mortality, growth, carbon allocation, plant allometry and dynamic competition among 11 plant functional types (PFTs), and can simulates photosynthetic plant distribution and competition among them. The physiological processes are simulated on a daily time step. The net primary production (NPP) is allocated to the leaves, sapwood and roots for each PFT at the end of simulation year (Smith et al., 2001). The model has been widely used to investigate past and future vegetation dynamics (e.g., Sitch et al., 2008; Smith et al., 2014; Zhou et al., 2020, etc.) and to estimate their interactions with climate and CO₂ concentrations (e.g., Chen et al., 2019; Li et al., 2019, etc.)

### 2.3 Experimental Design

We designed four climate experiments for this study. In these experiments, there are one Pliocene control experiment (PlioMIP2 core experiment from NorESM-L group (Li et al., 2020), hereafter referred as "PlioCtrl") and three seaway sensitivity experiments. They are respectively the dual opening Indo and CAS experiment with sill depth of 100 m (hereafter referred to as "IndoCAS100"), only opening CAS experiment ("CAS100") and only
widening Indo experiment (“Indo100”). The boundary conditions of these experiments are identical to PlioCtrl, except for the modification in the tropical seaways. The Indonesian seaway in IndoCAS100 and Indo100 has been enlarged by removing the land grid points over northern New Guinea and the central-eastern part of Java island and setting them 100 m in depth. The opening CAS in IndoCAS100 and CAS100 is located in the Panama region by removing three model land grid cells and setting them 100 m in depth. We modified these two tropical seaways broadly following the reconstruction of the period between the late Miocene and the early Pliocene (Hall, 2002; Molnar, 2008), though some details are not included because of the coarse resolution of the model and the large uncertainty in the reconstruction. These experiments have been firstly applied to discuss the role of tropical seaways in modulating the Pacific ocean circulation in Tan et al. (2022). Thus more details about the seaways’ setting can be found there and in Table S1 and Fig. S1.

We then set up four vegetation experiments to investigate the impacts of tropical seaways’ closure on the distribution of tropical African/Southern American vegetation. In the four vegetation experiments, we forced LPJ-GUESS with the simulated climatic anomalies from climate experiments PlioCtrl, IndoCAS100, CAS100 and Indo100. In these experiments, LPJ-GUESS was forced "off-line" with monthly mean temperature, precipitation and cloud cover from climate experiments. The CO₂ concentration was kept constant at a Pliocene control level of 400 ppmv, and nitrogen deposition is kept constant at a pre-industrial level of 1 kgN/ha/year. The simulations started after a 1000-year spin-up phase starting from bare soil. The duration of 1000 years spin up is found to be long enough to reach an equilibrium vegetation state (Smith et al. 2001). These PlioCtrl, IndoCAS100, CAS100 and Indo100 vegetation experiments thus give information about the vegetation dynamics in response to the tropical African/Southern American climate under different tropical seaways’ scenarios.

# 3. Results

## 3.1 Climate change

With the closure/narrowing of tropical seaways, tropical African and South American continents become warmer, whereas the tropical south Atlantic gets cooler (Fig. 1a). This temperature anomaly results from a superposition of each seaway’s effect. The constriction of Indo contributes more to warming both tropical continents (Fig. 1b), and the closure of the CAS contributes more to the cooling of the tropical south Atlantic (Fig. 1c). The mean annual
precipitation anomaly is more complex than the temperature anomaly after the closure of the
tropical seaways (Fig. 1d). In tropical Africa, it shows wetting in the west region and drying
in the east area. In the tropical Atlantic, more precipitation appears in the northern part, and
much less precipitation occurs in the southern part. In South America, the arid condition
happens in the Nordeste region. The results of single seaway's experiments show that the
constriction of Indo mainly controls the precipitation change in tropical Africa and plays a
second-order role in the tropical Atlantic and South America. In contrast, the closure of the
CAS is the primary cause for the precipitation change over tropical Atlantic and South
America.

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Mean annual temperature (a, b, c) and precipitation (d, e, f) anomalies due to the closure/narrowing of tropical seaways. The stippling indicates regions where results are statistically significant at 99% confidence criteria.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Annual precipitation in tropical Africa and South America under the Pliocene condition (a, b) and the annual precipitation contrast between East and West African/South American (c, d) region as defined by the black rectangle in (a) and (b). The black rectangles in c, d show the result of dual opening seaway experiment (IndoCAS100) and the result of single seaway experiment which is closer to that of IndoCAS100.</figcaption></figure>

Overall, the closure of the tropical seaways strengthens the contrast of annual precipitation rate between East and West Africa/South America and makes the West/East rainfall pattern more similar to the modern condition, mainly due to the reduction of rainfall in the eastern region (Fig. 2). In the case of opening tropical seaways, the difference in annual precipitation between East and West tropical Africa is about 985 mm. After the closure of the tropical seaways, the W-E Africa precipitation contrast increases by ~242 mm in PlioCtrl, which is mainly controlled by the narrowing of the Indo, as shown in Fig. 2c. In tropical South America (Fig. 2d), after the closure of the tropical seaways, the W-E precipitation contrast increases by ~518 mm in PlioCtrl, and the closure of the CAS plays the dominant role (Fig. 2d).

The changes in mean annual precipitation between the western and eastern regions result mainly from anomalous precipitation during the rainy season over the eastern regions in both
tropical continents. Comparison with modern observations shows that the low-resolution version of NorESM used in this study can generally simulate the seasonal variability of precipitation over tropical East Africa and the Nordeste region (Fig. 3), although biases occur to some extent. In East Africa, the simulated short rainy seasons (September - December) receive significantly more rainfall under Pliocene boundary conditions (PlioCtrl and Seaways' experiments) than under modern conditions, resulting in two heavy rainy seasons within a year, especially with the widening of Indo situation (Indo100 and IndoCAS100). In Nordeste, simulated seasonal precipitation under Pliocene boundary conditions is generally similar to that at PI. Only when CAS is opened (CAS100, IndoCAS100) does precipitation increase significantly during the rainy season.

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Seasonal precipitation in tropical East Africa (a) and in the Nordeste region (b). These two regions are defined as in Fig. 2. The horizontal axis denotes the month in a year (e.g., the number "1" denotes "January"). The dashed line in the diagram shows the modern observational data (GPCP 2.3). Solid lines show the results simulated in this study. Grey lines indicate the results obtained under modern conditions, other colored lines indicate the results simulated under the Pliocene boundary conditions.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Anomalies of precipitation and VIMF (Vertically integrated moisture flux) during the rainy seasons (March-April-May (MAM), October-November-December (OND)) after the closure/narrowing of tropical seaways (PlioCtrl minus IndoCAS100) (a-b) and the absolute VIMF in IndoCAS100 (c-d).</figcaption></figure>

To better understand the anomalous precipitation in the eastern tropical region after the closure/narrowing of the tropical seaways, we compare the vertically integrated moisture fluxes (VIMF) between PlioCtrl and IndoCAS100 (Fig. 4). The VIMF anomalies show that from March to May, the dryness over the Nordeste region after the closure/narrowing of tropical seaways mainly results from the anticyclonic anomalous moisture transport, inhibiting the moisture convergence and further reducing the precipitation. The dryness in the Nordeste region from October to December can be attributed to the weakening of the moisture convergence along the South Atlantic Convergence Zone. The VIMF changes in tropical East Africa are not obviously related to the precipitation changes. The divergent moisture transport at a lower level (750 hPa) (Fig. S2) helps to indicate further that the divergence of moisture transport intensifies around tropical East African and Nordeste region after the closure of the tropical seaways, associating with relatively less rainfall in these regions.
<figure><img src="image_9.png" /><figcaption>**Fig.5.** ITCZ positions during boreal summer (a, b) and boreal winter (c, d) in each experiment. (b) and (d) show the regional averaged ITCZ latitude position over three respective regions as defined in panel (a). The ITCZ position is calculated by the precipitation centroid between 20°S-20°N (as defined in Voigt et al., 2016). The positive values denote the north latitudes (°N), the negative values denote the south latitudes (°S).</figcaption></figure>

The closure/narrowing of tropical seaways can also modify the ITCZ movement over tropical regions. As shown in Fig. 5, the narrowing of Indo mainly causes a narrowing of the range of the ITCZ oscillation in the tropical Africa between the boreal summer and winter seasons. More precisely, the ITCZ in the tropical Africa shifts 1° southward in boreal summer and 1.8° northward in boreal winter. The closure of the CAS mainly affects the ITCZ motion over the tropical Atlantic and South America. Overall, the closure of the CAS pulls the ITCZ northward over these two regions during the boreal summer and winter seasons. The effects of the closure/narrowing of dual tropical seaways are more confounding and depend more on the dominant effect of a single seaway. The change in ITCZ shift over these regions may partially explain the change in rainfall patterns shown above. This is particularly true for the tropical Atlantic and South American regions. An earlier study by Giannini et al. (2004) showed that the southward shift of the ITCZ triggered by the positive sea-surface temperature (SST) anomaly in the southern tropical Atlantic might lead to anomalously larger rainfall over northeastern Brazil (Nordeste). Similarly, in this study, the closure of CAS leads to
SST cooling over the southern tropical Atlantic compared to the Plio_Ctrl (Fig. S3), pulls the ITCZ further north and then reduces precipitation in the Nordeste region as well as the tropical Atlantic. However, the rainfall changes in the tropical East Africa is more complex and cannot be explained directly by the ITCZ movement, whereas the regional atmospheric circulation (e.g. the divergence conditions over Eastern Africa discussed in the above section) and the patterns of Indian ocean temperature change are critical for the precipitation anomalies over this region (see reviews in Palmer et al., 2023).

## 3.2 Vegetation change

With the closure/narrowing of the tropical seaways, tropical forests are declining to varying degrees, together with expansions of C4 vegetation dominated by tropical grassland and savanna on both the African and South American continents (Fig. 1a in tropical Africa, vegetation responds to the narrowing of seaways mainly in the northeastern regions, and the narrowing of the Indo (Fig. 6a vs Fig. 6c) plays more pronounced role in the vegetation change in tropical Africa than the closure of the CAS (Fig. 6a vs Fig. 6b). In the eastern regions, tropical forests expand eastward in response to warm conditions (Fig. 1e), due to the opening of the Indo seaway (Fig. 6c). The expansion of tropical forests is at the expense of tropical xerophytic shrubland and savanna, which is mainly contributed by declines of C4 cover (Fig. S4). In addition, the southern boundary of the Sahara shifts northward, along with the expansion of savannas and grasslands in northern tropical Africa as the Indo opens (Fig. 6c). This expansion of savannas and grasslands is a result of northward C4 grass and slight increases of temperate broad-leaved evergreen trees (Fig. S4). Similar to tropical Africa, vegetation in tropical South America changes mainly in the eastern regions (Fig. 6d), but the dominant factor is different. Closure of the CAS has a greater impact on vegetation in this region than narrowing of the Indo (Fig. 6e vs Fig. 6f). Compared to vegetation under the climate conditions with the opening CAS (Fig. 6e), tropical forests in the Nordeste region shrink when either a single seaway or both the CAS and Indo are closed, and tropical xerophytic shrubland replaces these tropical forests. C4 grass expands in the Nordeste region (Fig. S4), leading to the larger distribution of tropical xerophytic shrubland. Tropical forest shrinkage and tropical xerophytic shrubland expansion are related to dry conditions in the Nordeste region when both seaways closed.
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Vegetation distributions in tropical Africa (a, b, c) and South America under different climate conditions. The tropical biomes here are converted from the PFT fractions simulated by the vegetation model LPJ-GUESS through climate limitation rules and assumptions on the maximum coverage of certain PFTs included in the grid cells (Dallmeyer et al., 2019).</figcaption></figure>

# 4. Discussions

## 4.1 Indian ocean temperature changes due to the closure/narrowing of the tropical seaways

In this paper, we investigated the impact of closure/narrowing of the tropical seaways on the environment of tropical Africa and South America based on a state of art AOGCM model (NorESM-L) and a dynamic vegetation model (LPJ-GUESS). Our results show that the closure of the CAS contributes to a drier climate over northeastern tropical South America, mainly by shifting sea surface temperature in the tropical Atlantic. The narrowing of the Indo primarily affects the degree of aridification of tropical East Africa. An earlier study by Cane and Molnar (2001) also deduced that the narrowing of the Indo could lead to a drier condition in East Africa. Their work relied on an ocean general circulation model and invoked the modern relationship between Indian Ocean temperature change and precipitation in East Africa as the controlling mechanism. Indeed, Indian Ocean temperature anomalies are critical for rainfall in East Africa under modern conditions (Black, 2005; Goddard and Graham, 1999). The decrease in precipitation over East Africa simulated in this study could also be
related to the temperature changes in the Indian Ocean, since the sea temperature in the Indian Ocean has been altered due to the changes in the tropical seaways. After the closure/narrowing of the tropical seaways, sea surface temperature decreases significantly along the tropical southern Indian Ocean, and subsurface temperature decreases throughout the Indian Ocean (Fig. 7). As shown in the geological data (SST reconstructed by Mg/Ca), sea surface temperature shows significant cooling (2-3 °C) in the Leeuwin Current region (ODP 763A) and weak cooling (~1 °C) in the tropical eastern Indian Ocean (ODP 214), the subsurface cooling is more pronounced (cooling of 4 °C in ODP 214) during the mid-Pliocene (Karas et al., 2009, 2011). Karas et al. (2009) consider the narrowing of the Indo as one of the important causes of this cooling. In this study, the simulated SST changes following the narrowing of the Indo are generally consistent with the data, especially for the changes in surface and subsurface temperature in the tropical eastern Indian Ocean (ODP 214). However, the simulated cooling in the Leeuwin Current region (ODP 763A) is relatively weak in comparison with the data. In the tropical western Indian Ocean, the reconstructed SST values during the mid-Pliocene (ODP 709C) show a relatively unchanged state, while in this study we observe a weak warming in this region.

It should be noted that the different configurations of the seaways' settings and different models can lead to distinct results. An earlier OGCM study by Rodgers et al. (2000) also shows a relatively higher amplitude cooling trend in the southern tropical Indian Ocean when the northern tip of New Guinea is shifted 5° northward. Community Earth System Model (CESM) results from Brierley and Fedorov (2016) show a very weak impact on the Indian Ocean after removing several ridges in the New Guinea region. Our results are comparable to the Kiel Climate Model (KCM) of Krebs et al. (2011), in which the modification of Indo are similar to that in this study. Nevertheless, the uncertainty in the paleogeometry of Indo makes it difficult to decide which are the best scenarios. Overall, it can be concluded that the closure/constriction of tropical seaways, especially the Indo, contributes significantly to the temperature changes in the Indian Ocean during the middle Pliocene, although global cooling should not be neglected either.
<figure><img src="image_11.png" /></figure>

**Fig. 7.** Temperature changes in the Indian Ocean due to the closure/narrowing of the tropical seaways. a-c show the temperature changes at the surface of the ocean and d-f show the temperature changes below the surface (400 m). The data sites (ODP 709C, DSDP 214, ODP 763A) in the panel show the ocean temperature changes during the mid-Pliocene (3-4 Ma) reconstructed using the Mg/Ca method based on the foraminifera species living at the surface and below the surface (300-450 m) (see more details in Karas et al., 2009, 2011). The white color in ODP 709C site denotes nearly “unchanged” state.

## 4.2 Vegetation changes in tropical east Africa and South America during the mid Pliocene

In tropical east Africa, although the very recent multiproxy study has shown that the origin of open and C4-dominated vegetation appeared as early as the early Miocene (Peppe et al., 2023), many pollen data in East Africa have documented the drastic aridification in the early-to-mid Pliocene in response to precipitation decline (Bonnefille, 2010; Cane and Molnar, 2001; Trauth et al., 2021). For example, pollen data from central Ethiopia (Bonnefille, 1995; Bonnefille et al., 1987; WoldeGabriel et al., 1994) indicate forested habitats and even a rainforest environment around 4.2 Ma under relatively moist and warm conditions, after which evergreen shrubland and montane forests disappeared around 3.3 and 2.9 Ma, respectively, due to the enhanced aridification. Similarly, vegetation in the Omo region of southern Ethiopia evolved from tropical forests to mosaic woodlands and then to a state dominated by xeric species, consistent with aridification during the early-to-mid Pliocene
(Bonnefille, 1995; Wesselman, 1995). In our study, the simulated tropical forest shrinkage and expansion of tropical xerophytic shrublands and savannas around East Africa after the constriction of the Indo indicate a consistent trend with the reconstructed vegetation during the early-to-mid Pliocene. In the tropics, the amount and distribution of precipitation throughout the year are the limiting factors for plant distribution in most regions, except in mountains, where temperature also plays an important role. The constriction of the Indo mainly controls the precipitation change in tropical Africa (Fig. 1), indicating wetting in western tropical Africa and drying in the eastern region. This drying in the eastern region leads to expansion of tropical xerophytic shrubland and savanna, to the detriment of tropical forests in this region (Fig. 6). The southern boundary of the Sahara shifts southward due to the constriction of Indo-induced desiccation.

In tropical South America, modern-like climate and vegetation conditions can be dated to the middle to late Miocene, which is related to the continuous uplift of the Andes and thus the emergence of modern drainage systems in this region (Figueiredo et al., 2009; Hoorn et al., 2010). More regional and detailed records from the late Miocene to Pliocene are still lacking. A recent data study on the Guajira Peninsula in northern Colombia reveals that the modern dry biomes over this region probably occurred in the mid Pliocene. Interestingly, we observe a drying effect on these region when considering the constriction of the Indonesian seaway. It may to some extent help to promote the environment transition over this region. The most significant impact is the arification in the Nordeste region resulting from the CAS closure. Some similar result is also observed in other AOGCM experiment (See Figure 9 in Jaramillo et al., 2020; Sepulchre et al., 2014). Nevertheless, further data are needed to verify environmental changes in this region.

It should be noted that there are overestimates of tree cover from the vegetation model LPJ-GUESS (Smith et al., 2014), it can not perfectly simulate the tropical forest distribution over our studied regions. But we can still obtain the extent of relative vegetation change after the closure/constriction of tropical seaways. Nevertheless, this is still not sufficient to understand the overall evolution of the tropical environment. Some other important factors, such as the changes in pCO₂, which is important for global temperature and the transition from C3 to C4 vegetation, are not considered in our study. Therefore, we cannot discuss the impacts of pCO₂ on C4 vegetation change which has historically been a key factor in mammalian faunal adaptation and is dominant in modern tropical grassland and savanna ecosystems (Edwards et
al., 2010; Peppe et al., 2023). Orbital parameters (Contoux et al., 2013; Gibert et al., 2022), topography (Sepulchre et al., 2014) and the occurrence of NH ice sheets (Trauth et al., 2021) are also critical factors in tropical environmental change. Gibert et al. (2022) studied the response of East African climate to the precession change. Their results show that the dry/wet conditions over East Africa is much sensible to the orbital parameters. Sepulchre et al. (2014) investigated the impact of the uplift of eastern African topography and they found that the uplift of eastern African topography can obviously decrease the precipitation over the East Africa. As our results show here, the narrowing of the Indo is also important for environmental changes in East Africa. The tectonically induced slow evolution of the seaway may provide the background conditions for a gradual, not abrupt transition of climate and environment as expressed in the data (Trauth et al., 2021), which may further amplify the response to the orbital parameters' changes. In addition, the effects of tropical seaways' closure/narrowing may also promote the buildup of NH ice sheets in MIS M2 glacial period (De Schepper et al., 2013; Tan et al., 2017) and the expansion of NH ice sheets in the late Pliocene (Cane and Molnar, 2001; Lunt et al., 2008), leading to further transition in tropical regions. Moreover, the vegetation-climate feedback is also important but is not considered in this study (Lin et al., 2019; Thompson et al., 2022). If the biome type has an important change, the differences in surface albedo and evapotranspiration will alter the temperature, precipitation, humidity etc., which in turn may affect the vegetation development. Future studies should consider all these factors to capture and understand the overall evolution of the tropical environment.

**5. Conclusions**

The closure/narrowing of the Central American Seaway (CAS) and the Indonesian Seaway (Indo) are considered to be important factors that influenced climate during the Pliocene and have been intensively studied. Tropical climate and vegetation also changed significantly during this period, especially in the East African region. The closure/narrowing of the CAS and Indo can undoubtedly affect climate in tropical regions, but there are still too few studies based on AOGCMs that discuss the potential impact of changes to tropical seaways on environmental changes in tropical Africa and South America. In this work, we systematically investigate the effects of tropical seaways' closure/narrowing on climate and vegetation over these two regions. Our results show that closure of the CAS leads mainly to aridification and savanna expansion in northeastern Brazil. Narrowing of the Indo leads mainly to increased
aridification and shrinkage of tropical forests over tropical East Africa. These results show good agreement with both reconstructed SST values over the Indian Ocean and pollen records in the East African region. Our study highlights the importance of the changes of the tropical seaways in altering the tropical environment and shaping it to be closer to modern conditions.

## Acknowledgements

This study was funded by the national key research and development program (No. 2022YFF0801504) and National Natural Science Foundation of China (NSFC programs 41888101, 42125502, 41907371, 42101146). Ning Tan acknowledges support from the IGGCAS key program (No. IGGCAS-202201). Zhongshi Zhang acknowledges the SapienCE (Project No. 221712) from Norwegian Research Council. Zhilin He acknowledges NSFC programs 42007398. Many thanks to the editor (Dr. Chenglong Deng, Dr. Jimin Sun) and three reviewers (Dr. Alexandre Pohl and two anonymous reviewers) for their constructive comments. The authors thank Dr. Pierre Sepurcire and colleagues in the "Evolution of Monsoon and Environment" group at IGCAS for their helpful discussions.

## Declaration of Interest Statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data Availability Statement

The NorESM-L model description can be found in Bentsen et al (2013) and Zhang et al (2012). Modelling data applied for the result analysis in this manuscript will be made available on request. Figures were plotted with the NCAR Command Language (version NCL 6.4.0; http://dx.doi.org/10.5065/D6WD3XH5) and Python version 2.7.15 (https://www.python.org/download/releases/2.7/).

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Supporting Information for

# Closure of tropical seaways favors the climate and vegetation in tropical Africa and South America approaching their present conditions

Ning Tan<sup>a,b</sup>, Huan Li<sup>c</sup>, Zhongshi Zhang<sup>d,e</sup>, Haibin Wu<sup>a,f,g</sup>, Gilles Ramstein<sup>h</sup>, Yong Sun<sup>i</sup>, Zhilin He<sup>a,b</sup>, Baohuang Su<sup>j</sup>, Zijian Zhang<sup>a,b</sup>, Zhengtang Guo<sup>a,f,g</sup>

\*Corresponding author Ning Tan (ning.tan@mail.iggcas.ac.cn) and Huan Li (huan.li.lzu@gmail.com)

## Contents of this file

Table S1, Figure S1 - S4

## Introduction

This Supporting Information includes one table and four additional figures for the modelling results, which can further support our conclusions in the manuscript.

<footnote><sup>a</sup>Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China</footnote>

<footnote><sup>b</sup>Innovation Academy for Earth Science, CAS, Beijing 100029, China</footnote>

<footnote><sup>c</sup>School of Geographic Science, Nantong University, Tongjin 241000, Nantong 226007, China</footnote>

<footnote><sup>d</sup>Department of Atmospheric Science, School of Environmental Studies, China University of Geosciences, Wuhan 430074 China</footnote>

<footnote><sup>e</sup>Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China</footnote>

<footnote><sup>f</sup>University of Chinese Academy of Sciences, Beijing 100049, China</footnote>

<footnote><sup>g</sup>CAS Center for Excellence in Life and Pat environment, Beijing 100044, China</footnote>

<footnote><sup>h</sup>Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay 91191 Gif-sur-Yvette, France</footnote>

<footnote><sup>i</sup>State Key Laboratory of Tibetan Plateau Earth System, Environment and Resources (TPESER), Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China</footnote>

<footnote><sup>j</sup>State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, 46 Zhong-Guan-Cun South Avenue, Beijing 100081, China</footnote>
<table><caption>**Table. S1** Setting configurations for each experiment (modified from Tan et al., 2022).</caption><thead><tr><td>Experiment name</td><td>Boundary Conditions</td><td>Indonesian</td><td>CAS</td><td>Integration length</td><td>Climatologies</td></tr></thead><tbody><tr><td>PlioCtrl</td><td>PRISM4</td><td>Modern</td><td>Closed</td><td>1600yrs</td><td>Last 100yrs</td></tr><tr><td>IndoCAS100</td><td>PRISM4</td><td>Enlarged (100m)</td><td>Open (100m)</td><td>1600yrs</td><td>Last 100yrs</td></tr><tr><td>Indo100</td><td>PRISM4</td><td>Enlarged (100m)</td><td>Closed</td><td>1600yrs</td><td>Last 100yrs</td></tr><tr><td>CAS100</td><td>PRISM4</td><td>Modern</td><td>Open (100m)</td><td>1600yrs</td><td>Last 100yrs</td></tr></tbody></table>

<figure><img src="image_12.png" /><figcaption>**Fig. S1.** Schematic of tropical seaways' changes in this study (modified after Tan et al., 2022). The single seaway change (Indo100, CAS100) is identical to its condition in the IndoCAS100, with keeping the other seaway unchanged from the PlioCtrl condition.</figcaption></figure>

<figure><img src="image_13.png" /></figure>
**Fig. S2.** Divergent moisture transport at 750 hPa in PlioCtrl (a, b) and IndoCAS100 (c, d) for both rainy seasons (March-April-May, October-November-December).

<figure><img src="image_14.png" /></figure>

**Fig. S3.** Sea surface temperature changes in seaways' experiments relative to PlioCtrl.

<figure><img src="image_15.png" /></figure>

**Fig. S4.** Tropical vegetation cover (PFTs fraction) under different climate conditions.
<figure><img src="image_16.png" /></figure>
**Declaration of Interest Statement**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

<figure><img src="image_17.png" /></figure>
**Highlights**

* Changes to tropical seaways greatly affect the tropical climate and environment.

* Central American seaway closure leads to savanna expansion in northeastern Brazil.

* Indonesian seaway constriction enhances aridification over tropical East Africa.

<figure><img src="image_18.png" /></figure>