Dynamics of Southern Hemisphere Super Gyre Response to External Forcings

doi:10.21203/rs.3.rs-4971848/v1

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Dynamics of Southern Hemisphere Super Gyre Response to External Forcings

https://doi.org/10.21203/rs.3.rs-4971848/v1

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The subtropical gyres in the Southern Hemisphere are interlinked through the Tasman and Agulhas Leakages and extend throughout the mid-latitude ocean basins of all major oceans. This vast ocean circulation system is called the Southern Hemisphere Super Gyre (SHSG). Previous studies have found a significant strengthening and poleward shift of the SHSG in recent decades. By analyzing multi-member ensembles from the Canadian Earth System Model and the Community Earth System Model, as well as a multi-model ensemble from the Detection and Attribution Model Intercomparison Project, this study investigates the relative contributions of greenhouse gas, aerosol, and ozone depletion forcings to changes in the SHSG since the 1950s. Results show that the strengthening and poleward shift of the SHSG have been dominated by the greenhouse gas forcing, which induces an intensification and poleward shift of the westerlies in the Southern Hemisphere, leading to a southward migration of the zero wind stress curl line and an increase in the positive wind stress curl over the southern SHSG. In contrast, the ozone depletion forcing plays a secondary role in changing the SHSG because its effect on the westerlies is further south than the greenhouse gas forcing. The aerosol forcing has little influence on the SHSG due to its weak effect on the winds in the Southern Hemisphere. The dominance of changes in wind stress curl is further validated through a set of partially coupled experiments in which the contribution of buoyancy and wind stress forcings are separated.

Southern Hemisphere Super Gyre

Tasman Leakage

Agulhas Leakage

external forcings

wind stress curl

In the Southern Hemisphere, each of the three ocean sections has its own distinct subtropical gyre, but they are interconnected through flows, forming a coherent system (Reid 1986; Davis 2005; Cai et al. 2005). This interconnected system of gyres, known as the Southern Hemisphere Super Gyre (SHSG), plays a role in the global ocean conveyor belt and can impact circulation and water mass distribution, thereby influencing global climate (Ridgway and Dunn 2007; Roemmich 2007). The Pacific and Indian Ocean sections are primarily connected through two pathways. In the upper ocean, the Indonesian Throughflow serves as the main pathway (Godfrey 1989; Gordon 2005), while the Tasman Leakage operates at lower depths (Metzl et al. 1990; Fine 1993; Speich et al. 2002). The connection between the Indian Ocean and the Atlantic Ocean is primarily through the Agulhas Leakage, which transports water from the Indian Ocean to the South Atlantic (De Ruijter et al. 1999; Weijer et al. 2001; Van Sebille et al. 2009; Durgadoo et al. 2013). Because the South American continent is too far south, it blocks the connection between the Pacific Ocean and the Atlantic Ocean and the SHSG does not extend through the Drake Passage (Speich et al. 2002; Ridgway and Dunn 2007; Speich et al. 2007; Duan et al. 2016). The southern eastward flow of the SHSG is challenging to discern. In some regions, it can be considered as a meandering eastward component of the flow along the northern boundary of the Antarctic Circumpolar Current (Ridgway and Dunn 2007).

The influence of human activities on climate has become increasingly important since the industrial age (Mitchell 1989; Thompson and Solomon 2002; Cai and Cowan 2007). Anthropogenic climate forcings are typically classified into greenhouse gas (GHG), aerosol (AER), and ozone depletion (OD) forcings, and the impacts of these forcings on wind and ocean circulation have substantial differences (Hurrell and Van Loon 1994; Shindell 2004; Cai et al. 2006; Yang et al. 2007b; Duan et al. 2016; Liu et al. 2022; Li et al. 2023). Numerous studies indicate that the strengthening and poleward shift of the westerly winds in the Southern Hemisphere are due to the influences of both GHG and OD forcings in recent decades (Thompson et al. 2000; Jacobs 2006; Yang et al. 2007b). The AER forcing, which is located mainly in the Northern Hemisphere, has been found to cause global cooling and subsequent weakening of the subpolar jet and the subtropical jet in the Southern Hemisphere since the early 20th century (Rotstayn et al. 2013; Wang et al. 2020).

Previous studies have found a southward shift of the southern boundary of the SHSG and increased transport in the inter-ocean water exchange through the Tasman and Agulhas Leakages, indicating a spin-up of the SHSG over the past decades and under the future warming climate. For example, Cai et al. (2005) showed that global warming induces an intensification and southward shift of the SHSG, which is associated with a strengthening and poleward shift of the westerly winds in the Southern Hemisphere. Furthermore, Cai (2006) found that Antarctic ozone depletion causes an intensification of the SHSG. In addition, Wang et al. (2014) also showed that increasing greenhouse gases and ozone depletion contribute to the intensification and poleward shift of the SHSG. By combining satellite and Argo measurements with numerical model solutions, Qu et al. (2019) revealed that the SHSG has intensified and shifted southward since the 1990s. However, what causes such changes in the SHSG during the historical record is not well understood. In particular, the relative contributions of different radiative forcings in influencing the strength and position of the SHSG are largely unknown.

Through the Detection and Attribution Model Intercomparison Project (DAMIP) of the Coupled Model Intercomparison Project Phase 6 (CMIP6; Gillett et al. 2016; Eyring et al. 2016), it becomes possible to distinguish the individual contributions of different external forcings to changes in ocean circulation. In this study, we use models with individual climate forcings to analyze the zero mass transport streamfunction line and inter-ocean water exchange since the 1950s. These analyses reflect changes in the position and strength of the SHSG. The rest of this paper is organized as follows: Section 2 introduces the observational and model datasets used in the study; Section 3 discusses how the SHSG varies in the historical simulation and three types of single-forcing simulations, as well as the role of wind stress curl in driving this variation; and Section 4 presents the conclusions and discussion.

2.1 Observations

The present study uses a gridded monthly altimetric absolute dynamic topography product from 1993 to 2020 with a horizontal resolution of 0.25°, derived from TOPEX/POSEIDON, Jason-1/2/3, ERS-1/2, ENVISAT, and Sentinel-3/6 altimeter observations and distributed by Copernicus Marine and Environment Monitoring Services, which is obtained by subtracting the geoid height from the altimetric mean sea surface height (SSH) above the reference ellipsoid (Rio 2010). To further examine the changes in ocean circulation, we also utilize the observed monthly gridded fields of temperature and salinity from the Argo floats data set to calculate the geostrophic velocities. These data cover the period from 2005 to 2020 and have a horizontal resolution of 1° × 1° and 27 (standard) levels from the surface to 2,000 m depth from the Asian Pacific Data Research Center of the International Pacific Research Center, University of Hawaii.

2. 2 Single forcing experiments

To investigate the influence of climate forcings on the SHSG over the 20th century, we primarily examine a large ensemble and a nine-model ensemble mean from DAMIP (Table 1), each consisting of four types of simulations: historical, GHG-only, AER-only, and OD-only (Gillett et al. 2016). The historical simulation is driven by changes in all anthropogenic and natural forcings, and covers the period from 1850 to 2014. The remaining three single-forcing simulations are driven by changes only in GHG, AER, and OD from 1850 to 2020, respectively. To maintain temporal consistency and to focus on the period with the most significant climate forcing, we primarily investigate the 1920–2014 period in the following analyses. The change in the DAMIP is defined as the difference of each type of simulation from the climatological mean between 1850–1919.

Table 1

Experiment sets used in this study.
Name	Experiment	Members	Time	Description
DAMIP (CanESM5 & multi-model)	historical	10 / 9models^*	1850–2014	Historical simulations forced by evolving external forcings
	hist-GHG	10 / 9models^*	1850–2020	Greenhouse gas only simulations
	hist-aer	10 / 9models^*	1850–2020	Anthropogenic aerosol only simulations
	hist-stratO₃	10 /×^*	1850–2020	Stratospheric ozone only simulations
CESM	hist	20	1920–2005	Historical simulation forced by evolving external forcings
	xghg	20	1920–2005	Greenhouse gas fixed at 1920 condition
	xaer	20	1920–2005	Anthropogenic aerosol fixed at 1920 condition
	fixedO₃	8	1955–2005	Stratospheric ozone fixed at 1955 condition
Partially coupled	4xCO₂	1	150 years	Quadrupled CO₂
	4xCO₂_1xws	1	150 years	Same as 4xCO₂, expect wind stress fixed at 1850 condition
	CTRL	1	150 years	Control with preindustrial CO₂
^* The CanESM5 ensemble includes 10 members, and the multi-model ensemble includes 9 models from GHG-only and AER-only experiments. “×” represents no models used in OD-only experiments from the multi-model ensemble.

The large ensemble is the Canadian Earth System Model version 5 Large Ensemble (CanESM5-LENS; Swart et al. 2019). CanESM5 is a coupled model using the Canadian Coupler (CanCPL), with its atmosphere component represented by the Canadian Atmosphere Model (CanAM5) and its ocean component represented by a customized version of the Nucleus for European Modelling of the Ocean (NEMO) model. This study analyzes 10 ensemble members in the historical and three single-forcing simulations from CanESM5. The ensemble members have the same physics and same forcings, differing only in initial conditions.

The nine-model ensemble consists of ACCESS-CM2 (Bi et al. 2020), ACCESS-ESM1-5 (Ziehn et al. 2020), CESM2 (Danabasoglu et al. 2020), FGOALS-g3 (Li et al. 2020), GISS-E2-1-G (Kelley et al. 2020), HadGEM3-GC31-LL (Williams et al. 2018), IPSL-CM6A-LR (Boucher et al. 2020), MRI-ESM2-0 (Kawai et al. 2019; Yukimoto et al. 2019), and NorESM2-LM (Seland et al. 2020). Only the first realization members (r1i1p1f1) of models are used to ensure equal weights in inter-model analysis. This study utilizes the ensemble means of these models to subtract the internal variability. Note that the OD-only simulation in multi-model ensemble is not analyzed due to the limited number of models conducting it.

To assess the robustness and consistency of the results, we also examine another ensemble from the Community Earth System Model version 1 (CESM version 1). Its atmospheric component is the Community Atmosphere Model version 5 (Kay et al. 2015), its land component is the Community Land Model version 4 (CLM4; Lawrence et al. 2012), and its ocean component is the Parallel Ocean Program version 2 (POP2; Danabasoglu et al. 2012). The historical and anthropogenic forcing (GHG & AER) simulations consist of 20 ensemble members from 1920 to 2005, and the OD simulation consists of 8 ensemble members from 1920 to 2005 (England et al. 2016; Lenaerts et al. 2018).

2.3 Partially coupled experiments

A partial coupling technique (Lu and Zhao 2012; Liu et al. 2015; Luo et al. 2015) is employed using CESM to isolate and quantify the contributions of different processes to the changes in SHSG. The baseline experiments consist of a preindustrial control (CTRL) that is forced with preindustrial carbon dioxide levels (367 ppm), and an abrupt quadrupled CO₂ simulation (4xCO₂), in which the atmospheric CO₂ concentration is instantly quadrupled to 1468 ppm. The 4xCO₂_1xws is the same as the 4xCO₂, but the wind stress is overridden by the daily outputs from CTRL. With this set of experiments, we can decompose the total SHSG response to CO₂ quadrupling into contributions from the wind stress effect (4xCO₂ minus 4xCO₂_1xws) and the buoyancy forcing effect (4xCO₂_1xws minus CTRL). All of the simulations are integrated for 150 years, with analyses focused on the average over the last 50 years. See Liu et al. (2020) for more details about the design of the experiments.

3.1 Sea surface height

Due to the scarcity of direct measurements on ocean currents and the rarity of extensive time series data, we use satellite SSH products to derive changes in ocean circulations (Hill et al. 2011; Behrens et al. 2020; Graham and De Boer 2013; Rühs et al. 2022). We first examine the similarity between observed and simulated SSH in the Southern Hemisphere. Figure 1 shows the time-averaged SSH patterns during 1993–2020 from the observation (Fig. 1a), the CanESM5 ensemble (Fig. 1b), the CESM ensemble (Fig. 1c), and the multi-model ensemble (Fig. 1d). It can be seen that all model simulations faithfully reproduce the structures of the South Pacific Gyre (SPG), the South Indian Gyre (SIG), and the South Atlantic Gyre (SAG), as well as their interconnections, and that the three ocean gyres are connected by the Tasman and Agulhas Leakages. Since satellites only measure variability at the sea surface, we also examine the similarity between Argo-derived and simulated velocity fields. The similarity of vertically averaged (0–2,000 m) horizontal flow fields again confirms the reliability of the models (Fig. 1). However, in contrast to the detailed representation in the model data, Argo does not adequately sample the narrow Western Boundary Currents (WBCs) due to the limited sampling and heavy smoothing used in the Argo product (Send et al. 2010; Riser et al. 2016; Chandler et al. 2022).

Since the center of the gyre is identifiable as the SSH maximum, we focus on regions of high SSH (black boxes in Fig. 1a) and compare the change in each ocean basin (Fig. 2). In the South Indian and South Atlantic Oceans, the observation shows significant and increasing trends in SSH from 1993–2020, and the three large ensemble means show essentially the same upward trends from 1955–2020 (CanESM5 & multi-model/CESM historical simulation before 2014/2005 and then Shared Socioeconomic Pathway 585 (SSP585)/Representative Concentration Pathway 8.5 (RCP8.5) thereafter until 2020 to ensure time consistency with the observation). However, the SPG center has been remarkably stable in recent decades, with the growth rate being small and insignificant from both observations and model simulations. We conduct a correlation analysis on the historical SSH trends of the three model data from 1955–2020 and the observed data from 1993–2020 (Supporting information Fig. S1). The correlation coefficients between these data results indicate significant consistency, suggesting that all types of models are effective in simulating the changes of SSH in the Southern Hemisphere, which is consistent with previous studies (Landerer et al. 2014; Ferrero et al. 2021).

The use of satellite and Argo float data displays the trend of subtropical gyres in the Southern Hemisphere, but they have inherent limitations that affect their usefulness for certain aspects of oceanic research. Satellite data are restricted to surface observations, Argo floats sample primarily the upper 2,000 meters of the global ocean, but are limited near ocean boundaries and in narrow currents. In addition, the time span covered by both is relatively short. Since model simulations can faithfully reproduce the major patterns and temporal evolution of observations, the following analysis will primarily focus on analyzing model results to comprehensively understand the SHSG variability and to investigate the influence of various external forcings since the 1920s.

3.2 Streamfunction

To quantify the response of the SHSG to different climate forcings, we examine the total mass transport streamfunction. As shown in Fig. 3a, the historical simulation in the CanESM5 ensemble captures the mean pattern of the SHSG well. Figures 3b&c show the time series during 1920–2014 and the linear trends during 1955–2014 of the change in the zonal mean southern boundary of SHSG in the historical simulation and the contributions of each individual forcing. Here, the zero mass transport streamfunction line is utilized as the southern boundary of the SHSG (black line in Fig. 3a). The historical simulation reveals a remarkable poleward shift of the southern boundary of the SHSG at a rate of about 0.11° per decade since the 1950s (black line and bar in Figs. 3b&c). This movement is primarily attributed to the influence of GHG (red line and bar in Figs. 3b&c), while the OD forcing makes a secondary contribution (yellow line and bar in Figs. 3b&c). Under AER forcing, however, there is a minor equatorward shift in the southern boundary of the SHSG (blue line and bar in Figs. 3b&c).

Similar results are found in the CESM ensemble and the multi-model ensemble (Supporting information Figs. S2 and S3). Taken together, these results show that the SHSG exhibits a significant trend of poleward shift in the historical simulations primarily due to GHG forcing, with a secondary influence from OD forcing.

3.3 Inter-ocean connection

The Tasman and Agulhas Leakages are the two major components of the SHSG that connect the ocean basins. In this subsection, we examine how the water transport through these two leakages changes as the SHSG shifts toward the polar regions.

The WBC in the South Pacific, known as the East Australian Current (EAC), is a component of the SPG and bifurcates into two distinct pathways at 30°-34°S (Cetina-Heredia et al. 2015; Bull et al. 2018; Oke et al. 2019). After departing from the northern coast of New Zealand, a portion of the water flows westward from the Tasman Front, traverses the Tasman Sea, and forms the East Auckland Current. At the same time, another portion of the water flows along the Australian coast and continues until it reaches the eastern side of Tasmania Island. This is referred to as "the EAC extension", which follows the Tasmania coastline and ultimately enters the southeastern Indian Ocean through the Tasman Leakage (Ridgway and Dunn 2003; Cai et al. 2005; Sutton and Bowen 2014; Chiswell et al. 2015; Sloyan et al. 2016). This unique process allows water to flow westward along the coast and contributes to the formation of the SHSG in the southeastern Indian Ocean. Building on this understanding of the complex pathways of the EAC, the following analysis leverages Argo and model data to capture these oceanic pathways.

To the west of Tasmania, the main path of the Tasman Leakage is quite stable. Based on the 141–143°E section, the Tasman Leakage is defined as the meridional integral of the westward flow in the upper 2,000 m between 40°S and 50°S from Argo observations (Fig. 4a). Similarly, the pattern of zonal velocity along the same section in the CanESM5 ensemble shows a resemblance to the observations (Fig. 4b). Recent studies suggested that the time-averaged transport of the Tasman Leakage is between 8 and 11 Sv (Ridgway and Dunn 2007; Qu et al. 2019; Behrens et al. 2020), in our study the transport is about 9.7 Sv in Argo and 8.9 Sv in the CanESM5 ensemble. The CanESM5 ensemble results provide insights into the time series of the Tasman Leakage transport during 1920 to 2014, showing a significant upward trend since the mid-1950s at a rate of about 0.3 Sv per decade (black line in Fig. 4c). In the GHG-only simulation, the Tasman Leakage transport also exhibits an increasing trend (red line in Fig. 4c), the magnitude of which is largely consistent with the historical simulation. Conversely, in the AER-only simulation, the Tasman Leakage transport shows a slight weakening, comparable to the subdued enhancement in the OD-only simulation (blue and yellow lines in Fig. 4c).

The water exchange between the Southern Indian Ocean and the Southern Atlantic Ocean is primarily attributed to the Agulhas Leakage, with the primary transport mechanism being the Agulhas rings, anticyclonic formations resulting from the retroflection of the Agulhas Current (AC; Gordon and Haxby 1990; Schouten et al. 2000; Rouault et al. 2009; Dencausse et al. 2010). The highly nonlinear and complex system involves how the WBC in the South Indian Ocean can either exit eastward through the Agulhas Return Current and reconnect to the Indian Ocean subtropical gyre, or westward into the Atlantic Ocean through the Agulhas Leakage (Biastoch et al. 2009; Dencausse et al. 2010; Beal et al. 2011; Durgadoo et al. 2013; Zhang et al. 2023). In contrast to the Tasman Leakage, this intricate pathway predominantly involves meso- to small-scale features (Reason et al. 2003; Van Sebille et al. 2009), which are difficult to capture comprehensively.

We select the mean zonal velocity across the section between 15–17°E, located west of the Agulhas Current retroflection point at 19°E, which exhibits a rapid decline in eddy-driven transport and provides a stable measurement of the Agulhas Leakage (Van Sebille et al. 2010). The mean transport of the Agulhas Leakage obtained from Argo is defined as the meridional integral of westward flow in the upper 2,000 m of the 15–17°E section, spanning latitudes 33–44°S (Fig. 4d). Similarly, the CanESM5 ensemble successfully captures this inter-ocean connection of the SHSG (Fig. 4e) in the same section. Figure 4f shows the time series of the Agulhas Leakage transport in the historical simulation and three climate forcing simulations. Under GHG forcing, there is an increasing trend since the 1950s that is significant at the 95% confidence level (red line in Fig. 4f), similar to the historical simulation (black line in Fig. 4f). AER and OD forcings have relatively minor contributions (blue and yellow lines in Fig. 4f).

Therefore, it is the GHG forcing that plays a dominant role in driving the historical changes of transports at both the Tasman and Agulhas Leakages, while the AER and OD forcings have much smaller effects. Significantly, this increase in inter-ocean connections coincides with the southward shift of the SHSG, which is also primarily attributed to the GHG forcing. Such consistency suggests that a underlying coherent process is at work, possibly related to changes in the wind stress curl (Cai et al. 2005; Bostock et al. 2006; Hill et al. 2008; Oliver and Holbrook 2014)

In summary, both inter-ocean connections within the SHSG show the increasing transport trend since the 1950s, and the consistent patterns are also reproduced in the CESM ensemble and the multi-model ensemble (Supporting information Figs. S4 and S5). The climate model analyses indicate that the GHG forcing primarily induces the spin-up of SHSG, while the influences of OD and AER are less significant. Next, we will perform a comprehensive analysis of the internal ocean basin circulation to better understand the influences of different external forcings and discuss the mechanisms behind these changes.

3.4 Internal ocean basin circulation

To better understand the variability of the inter-ocean connection, in this subsection we examine the circulation within each ocean basin. We focus first on the northern limbs of the subtropical gyres, which turn southward after reaching the western boundaries and supply water to the Tasman and Agulhas Leakages in the Pacific and Indian Oceans, respectively.

Previous studies have indicated that the South Equatorial Current (SEC) bifurcates around 17°S-18°S in the western ocean basins, with its southern branch transporting water poleward to close the subtropical gyres and contribute to inter-basin exchange, and its northern branch moving equatorward to close the tropical gyres (Rodrigues et al. 2007; Chen and Wu 2015; Yamagami and Tozuka 2015). Following Qu et al. (2019), in this study the northern limb of the subtropical gyre is represented by the southern branch of the SEC, which is defined by integrating the meridional westward flow between 20°S and 40°S in the upper 2,000 m in the 170°W section of the Pacific Ocean, the 60°E section of the Indian Ocean, and the 30°W section of the Atlantic Ocean (Fig. 5). The results indicate that in both the Indian and Atlantic Oceans (black lines Figs. 5a&c), transports in the northern limbs of the subtropical gyres show increasing trends since the 1950s, contributing additional water to the higher latitudes, and these trends are driven by the GHG forcing (red lines in Figs. 5a&c). In contrast, the AER forcing drives decreasing trends in transports (blue lines in Figs. 5a&c), but its magnitudes are significantly smaller than the GHG forcing. The OD forcing does not produce significant changes in transports for the northern limbs of the subtropical gyres (yellow lines in Figs. 5a&c).

However, the transport through the northern limb of the SPG does not show a clear upward trend (Fig. 5b). The EAC bifurcates into a flow along the Tasman Front and a flow that continues south as the EAC extension. Although the EAC does not have an increasing trend in transport, the EAC extension supplies more water to the Tasman Leakage due to a reduction in transport through the flow along the Tasman Front (black line in supporting information Fig. S6a), which is defined as a meridional integral eastward flow in the upper 2,000 m between 30°S and 40°S across the 165°E section. The strengthening of the EAC extension - a part of the SHSG - facilitates an increased inflow of water from the southwest Pacific Ocean into the southeast Indian Ocean. Similar to the increase in transport through the Tasman Leakage (black line in Fig. 4c), the decrease in flow along the Tasman Front is also due to GHG forcing (red line in supporting information Fig. S6a). The above results remain consistent across the CESM and multi-model ensembles (Supporting information Figs. S6b&c and S7).

Figure 6 examines the intermodal correlations between the SEC and WBC transport trends in different ocean basins. There is a strong positive correlation for the Indian and Atlantic Oceans, with correlation coefficients of 0.78 for the former and 0.73 for the latter, emphasizing that the strengthened southward WBCs in the Indian and Atlantic Ocean basins are closely related to the acceleration in the northern limbs of the subtropical gyres. However, there is a much weaker correlation (r = 0.15) for the Pacific Ocean and it does not pass the 95% test, which is consistent with the above analysis.

In summary, the increasing inter-basin water exchange in the connecting regions of the SHSG between 35°S-45°S is primarily driven by changes in transport at lower latitudes. The GHG forcing leads to a strengthening of the northern limbs of the subtropical gyres in the Indian and Atlantic Oceans and a weakening of the flow along the Tasman Front, supporting the strengthening of the WBCs as part of the SHSG. Since the 1950s, all three ocean basins have shown upward trends in the WBCs, consistent with increased transports through the Tasman and Agulhas Leakages, providing evidence for the spin-up of the SHSG.

3.5 Wind stress curl

The flows across the ocean basins are primarily driven by wind stress, which has been extensively studied (Huang 1991; Cai et al. 2005; Bostock et al. 2006; Ridgway and Dunn 2007; Yang et al. 2007b; Hill et al. 2008; Wang et al. 2014). According to Sverdrup balance, large-scale ocean circulation patterns are mainly determined by the spatial distribution of wind stress curl (Godfrey 1989). Increased positive wind stress curl in the subtropics of the Southern Hemisphere leads to an enhancement in Ekman pumping and a spin-up of the SHSG (Rintoul and England 2002; Yang et al. 2007a; Duan et al. 2016). The transports through the Tasman and Agulhas Leakages are also influenced by the wind (Hill et al. 2008; Sasaki et al. 2008; Biastoch et al. 2009; Rouault et al. 2009; Durgadoo et al. 2013; Oliver and Holbrook 2014; Tim et al. 2019). In the following, we will focus on changes in the wind stress curl to illustrate the impact of single forcing on the SHSG.

Figure 7 shows the climatological zonal average of the wind stress curl from 10°S to 60°S and its trends from 1955 to 2014 under the historical and single forcing simulations in the CanESM5 ensemble. Climatologically, the positive wind stress curl extends from 18°S to 50°S (cyan shading in Fig. 7) and the zero wind stress curl line is located at 50°S. Since the 1950s, there has been a significant positive anomaly in the wind stress curl between 36°S and 57°S (black line in Fig. 7), resulting in an increase in the positive wind stress curl over the southern SHSG and a southward migration of the zero wind stress curl line.

These changes in the wind stress curl are primarily attributed to the GHG forcing (red line in Fig. 7). The OD forcing also produces a positive anomaly in wind stress curl, but it appears to be smaller and further south and thus contributes less to the increase in the positive wind stress curl over the southern SHSG than the GHG forcing (yellow line in Fig. 7). In contrast, the wind stress curl changes induced by the AER forcing are much smaller (blue line in Fig. 7). The strength of the SHSG is more significantly impacted by the wind field within the positive wind stress curl region, rather than the entire westerly jet zone. Thus, the GHG forcing is the main driver for the spin-up of the SHSG. For the poleward shift of the SHSG, the southward migration of the zero wind stress curl line is driven by both GHG and OD forcings, with the GHG forcing showing a more pronounced trend of southward shift compared to the OD forcing (Supporting information Fig. S8). The result is consistent with previous studies (Cai 2006; Cai and Cowan 2007; Wang et al. 2014). The zero wind stress curl line is situated south of the southern boundary of the SHSG (i.e., the zero mass transport streamfunction line) and is more easily influenced by the OD forcing. In contrast, the zero mass transport streamfunction line is less responsive to the OD forcing than the zero wind stress curl line. Therefore, the poleward shift of the SHSG is found to be primarily influenced by the GHG forcing, with the OD forcing playing a secondary role. While both the GHG and OD forcings contribute almost equally to the southward migration of the westerlies, it is the former that plays a primary role in increasing the positive wind stress curl over the southern SHSG and thus driving the poleward shift and spin-up of the SHSG.

In the Pacific basin, the increasing transport of the Tasman Leakage from the EAC extension is attributed from the weakening of flow along the Tasman Front rather than the northern limbs of the SPG. The strengths of the flow along the Tasman Front and the EAC extension are anticorrelated, and both are consistently influenced by the wind stress curl over the South Pacific (Hill et al. 2008). Changes in the wind stress curl in the South Pacific (20–50°S, 180–280°E) drive these changes in the EAC system. According to the CanESM5 ensemble, the trend of wind stress curl in this region has increased over recent decades, primarily due to the influence of GHG forcing (Supporting information Fig. S9). This strengthening of basin-wide wind stress curl drives a southward expansion of the SPG (Sasaki et al. 2008; Hill et al. 2011). As the gyre shifts south, the EAC extension pathway receives more water at the expense of the Tasman Front, resulting in increased Tasman Leakage, as discussed in Section 3.4.

In conclusion, the poleward shift and spin-up of the SHSG can be attributed to the positive anomaly of the wind stress curl in the mid-latitudes of the Southern Hemisphere, driven primarily by the GHG forcing. The effect of OD on the winds is further south and thus plays a secondary role in the changes of the SHSG since the 1950s. The effect of AER on the changes in wind stress curl is much smaller.

3.6 Validation with the partially coupled experiments

To verify that changes in the SHSG are indeed driven by GHG-induced changes in wind stress, we employ the partially coupled experiments, in which the ocean circulation changes in response to CO₂ quadrupling are decomposed into a dynamic component driven by wind stress and a thermodynamic component induced by buoyancy forcing. Comparing Figs. 8a with 3a and 8b with 7, it can be seen that the patterns of mean ocean circulation and wind stress curl changes simulated in the CESM CO₂ quadrupling experiments are in good agreement with those in the CanESM5 ensemble. In particular, the CO₂ quadrupling experiment simulates an increase in the positive wind stress curl over the southern SHSG region, except that the magnitude of this change is greater due to its higher CO₂ concentration compared to the GHG-only simulation (black lines in Figs. 7 and 8b).

The difference between the two lines in Fig. 8a clearly shows the poleward shift of the southern boundary of the SHSG under the wind stress effect of CO₂ quadrupling, which is similar to the change in the GHG-only simulation (Fig. 3). In addition, the wind stress effect of CO₂ quadrupling leads to a significant increase in transport through both the Tasman Leakage (~ 5.52 Sv increase) and the Agulhas Leakage (~ 1.06 Sv increase). Furthermore, a comparison of the patterns between Figs. 8c, 8d, and 8f suggests that the circulation changes in the SHSG under CO₂ quadrupling are primarily due to its wind stress effect, while its buoyancy forcing effect plays only a secondary role.

Therefore, the partially coupled experiments confirm that it is the GHG forcing that induces to a southward shift of the zero wind stress curl line and an increase in the positive wind stress curl over the southern SHSG region, which in turn causes a strengthening and poleward shift of the SHSG.

Observational and reanalysis data have indicated a spin-up and southward migration of the SHSG since 1993 (Qu et al. 2019). This study analyzes climate model simulations and reveals that this change in the SHSG can be traced back to the mid-20th century, predating the satellite observational record and aligning with the rising in industrial activities. The dynamic mechanisms responsible for the change in the SHSG are identified and summarized in Fig. 9. Due to changes in the wind stress curl under the GHG forcing, the southern branches of the SECs as the northern limbs of the SIG (solid large orange arrow in Fig. 9) and the SAG (solid large red arrow in Fig. 9) have strengthened (hollow small orange and red arrows in Fig. 9, respectively), and the eastward flow along the Tasman Front (solid small blue arrow in Fig. 9) as the part of the SPG has weakened (hollow small blue arrow in Fig. 9). These changes contribute to the stronger southward flow of the WBCs, and supply more water to the transports through the Tasman and Agulhas Leakages (solid large and hollow small black arrows in Fig. 9 represent the climatological transports and changes, respectively). In addition, the zero mass transport streamfunction line (black curve in Fig. 9), defined as the southern boundary of the SHSG, has shifted poleward under the GHG forcing.

These changes in the ocean circulation are primarily a result of changes in the wind stress curl in the mid-latitudes of the Southern Hemisphere, as depicted by the orange shading in Fig. 9. Since the 1950s, increasing GHG has led to an intensification and southward shift of the westerly winds, which has strengthened the positive wind stress curl over the southern SHSG region. Consequently, the southern boundary of the SHSG has shifted southward, and strengthening the connections between the ocean basins. While the OD forcing also contributes to the strengthening and southward shift of the Southern Ocean westerlies, its impact is further south and thus plays a secondary role for the change in the SHSG. The wind change in the Southern Hemisphere due to the AER forcing is much smaller, making its impact on the ocean circulation comparatively unimportant.

This study reveals the extent to which different external forcings affect the SHSG, thereby improving our understanding of the relationship between the atmospheric and oceanic circulation changes. The contribution of the OD forcing to the southward shift of the SHSG may have been overestimated in previous studies (e.g., Cai 2006; Wang et al. 2014), because the influence of the wind stress curl on the southern boundary of the SHSG is not simply due to the change in the zero wind stress curl line, but to a combined effect of its intensity and change in spatial distribution. In addition, the upper ocean in reality is not in a state of Sverdrup balance, and the circulation boundary is also influenced by buoyancy factors and topographic features (De Boer et al. 2013; Wang et al. 2014; Behrens and Bostock 2023), the influence of which on the SHSG is not yet fully understood. Further research is needed to better understand how ocean circulation responds to climate forcings.

Conflict of interest

The authors have no relevant financial or non-financial interests to disclose.

Funding

This work was supported by the National Natural Science Foundation of China (NSFC; 42230405) and the Laoshan Laboratory (No. LSKJ202202401).

Author Contributions

JL and YL were responsible for design of the research. Material preparation, data collection, and analysis were performed by JL. YL and FL helped interpret the results. JL wrote the first draft of the manuscript and YL, FL and JY helped with the revision of the manuscript. All authors read and approved the revised manuscript.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC; 42230405) and the Laoshan Laboratory (No. LSKJ202202401). F. L. is supported by the Fundamental Research Funds for the Central Universities (No. 202341016) and “Youth Innovation Team Program” Team in Colleges and Universities of Shandong Province (No. 2022KJ042).

Data Availability

The CMIP6 data were downloaded from https://esgf-node.llnl.gov/projects/cmip6/. The CESM1 data were downloaded from https://www.earthsystemgrid.org/. The satellite-observed dataset and Argo floats data were downloaded from http://apdrc.soest.hawaii.edu/. The data about partially coupled experiments are available from the corresponding author upon request.

Beal, L. M., and Coauthors, 2011: On the role of the Agulhas system in ocean circulation and climate. Nature, 472, 429–436, https://doi.org/10.1038/nature09983.
Behrens, E., and H. Bostock, 2023: The Response of the Subtropical Front to Changes in the Southern Hemisphere Westerly Winds—Evidence From Models and Observations. J. Geophys. Res. Oceans, 128, e2022JC019139, https://doi.org/10.1029/2022JC019139.
——, J. Williams, O. Morgenstern, P. Sutton, G. Rickard, and M. J. M. Williams, 2020: Local Grid Refinement in New Zealand’s Earth System Model: Tasman Sea Ocean Circulation Improvements and Super‐Gyre Circulation Implications. J. Adv. Model. Earth Syst., 12, https://doi.org/10.1029/2019MS001996.
Bi, D., and Coauthors, 2020: Configuration and spin-up of ACCESS-CM2, the new generation Australian Community Climate and Earth System Simulator Coupled Model. J. South. Hemisphere Earth Syst. Sci., 70, 225–251, https://doi.org/10.1071/ES19040.
Biastoch, A., C. W. Böning, F. U. Schwarzkopf, and J. R. E. Lutjeharms, 2009: Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies. Nature, 462, 495–498, https://doi.org/10.1038/nature08519.
Bostock, H. C., B. N. Opdyke, M. K. Gagan, A. E. Kiss, and L. K. Fifield, 2006: Glacial/interglacial changes in the East Australian current. Clim. Dyn., 26, 645–659, https://doi.org/10.1007/s00382-005-0103-7.
Boucher, O., and Coauthors, 2020: Presentation and Evaluation of the IPSL‐CM6A‐LR Climate Model. J. Adv. Model. Earth Syst., 12, e2019MS002010, https://doi.org/10.1029/2019MS002010.
Bull, C. Y. S., A. E. Kiss, E. Van Sebille, N. C. Jourdain, and M. H. England, 2018: The Role of the New Zealand Plateau in the Tasman Sea Circulation and Separation of the East Australian Current. J. Geophys. Res. Oceans, 123, 1457–1470, https://doi.org/10.1002/2017JC013412.
Cai, W., 2006: Antarctic ozone depletion causes an intensification of the Southern Ocean super-gyre circulation. Geophys. Res. Lett., 33, L03712, https://doi.org/10.1029/2005GL024911.
Cai, W., and T. Cowan, 2007: Trends in Southern Hemisphere Circulation in IPCC AR4 Models over 1950–99: Ozone Depletion versus Greenhouse Forcing. J. Clim., 20, 681–693, https://doi.org/10.1175/JCLI4028.1.
Cai, W., G. Shi, T. Cowan, D. Bi, and J. Ribbe, 2005: The response of the Southern Annular Mode, the East Australian Current, and the southern mid-latitude ocean circulation to global warming. Geophys. Res. Lett., 32, L23706, https://doi.org/10.1029/2005GL024701.
——, D. Bi, J. Church, T. Cowan, M. Dix, and L. Rotstayn, 2006: Pan-oceanic response to increasing anthropogenic aerosols: Impacts on the Southern Hemisphere oceanic circulation. Geophys. Res. Lett., 33, L21707, https://doi.org/10.1029/2006GL027513.
Cetina‐Heredia, P., M. Roughan, E. Van Sebille, M. Feng, and M. A. Coleman, 2015: Strengthened currents override the effect of warming on lobster larval dispersal and survival. Glob. Change Biol., 21, 4377–4386, https://doi.org/10.1111/gcb.13063.
Chandler, M., N. V. Zilberman, and J. Sprintall, 2022: Seasonal to Decadal Western Boundary Current Variability From Sustained Ocean Observations. Geophys. Res. Lett., 49, e2022GL097834, https://doi.org/10.1029/2022GL097834.
Chen, Z., and L. Wu, 2015: Seasonal Variation of the Pacific South Equatorial Current Bifurcation. J. Phys. Oceanogr., 45, 1757–1770, https://doi.org/10.1175/JPO-D-14-0085.1.
Chiswell, S. M., H. C. Bostock, P. J. Sutton, and M. J. Williams, 2015: Physical oceanography of the deep seas around New Zealand: a review. N. Z. J. Mar. Freshw. Res., 49, 286–317, https://doi.org/10.1080/00288330.2014.992918.
Danabasoglu, G., S. C. Bates, B. P. Briegleb, S. R. Jayne, M. Jochum, W. G. Large, S. Peacock, and S. G. Yeager, 2012: The CCSM4 Ocean Component. J. Clim., 25, 1361–1389, https://doi.org/10.1175/JCLI-D-11-00091.1.
Danabasoglu, G., and Coauthors, 2020: The Community Earth System Model Version 2 (CESM2). J. Adv. Model. Earth Syst., 12, e2019MS001916, https://doi.org/10.1029/2019MS001916.
Davis, R. E., 2005: Intermediate-Depth Circulation of the Indian and South Pacific Oceans Measured by Autonomous Floats. J. Phys. Oceanogr., 35, 683–707, https://doi.org/10.1175/JPO2702.1.
De Boer, A. M., R. M. Graham, M. D. Thomas, and K. E. Kohfeld, 2013: The control of the Southern Hemisphere Westerlies on the position of the Subtropical Front: The Subtropical Front and Wind Stress. J. Geophys. Res. Oceans, 118, 5669–5675, https://doi.org/10.1002/jgrc.20407.
De Ruijter, W. P. M., A. Biastoch, S. S. Drijfhout, J. R. E. Lutjeharms, R. P. Matano, T. Pichevin, P. J. Van Leeuwen, and W. Weijer, 1999: Indian‐Atlantic interocean exchange: Dynamics, estimation and impact. J. Geophys. Res. Oceans, 104, 20885–20910, https://doi.org/10.1029/1998JC900099.
Dencausse, G., M. Arhan, and S. Speich, 2010: Routes of Agulhas rings in the southeastern Cape Basin. Deep Sea Res. Part Oceanogr. Res. Pap., 57, 1406–1421, https://doi.org/10.1016/j.dsr.2010.07.008.
Duan, Y., H. Liu, W. Yu, and Y. Hou, 2016: The mean properties and variations of the Southern Hemisphere subpolar gyres estimated by Simple Ocean Data Assimilation (SODA) products. Acta Oceanol. Sin., 35, 8–13, https://doi.org/10.1007/s13131-016-0901-2.
Durgadoo, J. V., B. R. Loveday, C. J. C. Reason, P. Penven, and A. Biastoch, 2013: Agulhas Leakage Predominantly Responds to the Southern Hemisphere Westerlies. J. Phys. Oceanogr., 43, 2113–2131, https://doi.org/10.1175/JPO-D-13-047.1.
England, M. R., L. M. Polvani, K. L. Smith, L. Landrum, and M. M. Holland, 2016: Robust response of the Amundsen Sea Low to stratospheric ozone depletion. Geophys. Res. Lett., 43, 8207–8213, https://doi.org/10.1002/2016GL070055.
Eyring, V., S. Bony, G. A. Meehl, C. A. Senior, B. Stevens, R. J. Stouffer, and K. E. Taylor, 2016: Overview of the Coupled Model Intercomparison Project Phase 6 (CMIP6) experimental design and organization. Geosci. Model Dev., 9, 1937–1958, https://doi.org/10.5194/gmd-9-1937-2016.
Ferrero, B., M. Tonelli, F. Marcello, and I. Wainer, 2021: Long-term Regional Dynamic Sea Level Changes from CMIP6 Projections. Adv. Atmospheric Sci., 38, 157–167, https://doi.org/10.1007/s00376-020-0178-4.
Fine, R. A., 1993: Circulation of Antarctic intermediate water in the South Indian Ocean. Deep Sea Res. Part Oceanogr. Res. Pap., 40, 2021–2042, https://doi.org/10.1016/0967-0637(93)90043-3.
Gillett, N. P., and Coauthors, 2016: The Detection and Attribution Model Intercomparison Project (DAMIP v1.0)contribution to CMIP6. Geosci. Model Dev., 9, 3685–3697, https://doi.org/10.5194/gmd-9-3685-2016.
Godfrey, J. S., 1989: A sverdrup model of the depth-integrated flow for the world ocean allowing for island circulations. Geophys. Astrophys. Fluid Dyn., 45, 89–112, https://doi.org/10.1080/03091928908208894.
Gordon, A., 2005: Oceanography of the Indonesian Seas and Their Throughflow. Oceanography, 18, 14–27, https://doi.org/10.5670/oceanog.2005.01.
Gordon, A. L., and W. F. Haxby, 1990: Agulhas eddies invade the south Atlantic: Evidence From Geosat altimeter and shipboard conductivity‐temperature‐depth survey. J. Geophys. Res. Oceans, 95, 3117–3125, https://doi.org/10.1029/JC095iC03p03117.
Graham, R. M., and A. M. De Boer, 2013: The Dynamical Subtropical Front: The Dynamical Subtropical Front. J. Geophys. Res. Oceans, 118, 5676–5685, https://doi.org/10.1002/jgrc.20408.
Hill, K. L., S. R. Rintoul, R. Coleman, and K. R. Ridgway, 2008: Wind forced low frequency variability of the East Australia Current. Geophys. Res. Lett., 35, L08602, https://doi.org/10.1029/2007GL032912.
——, ——, K. R. Ridgway, and P. R. Oke, 2011: Decadal changes in the South Pacific western boundary current system revealed in observations and ocean state estimates. J. Geophys. Res., 116, C01009, https://doi.org/10.1029/2009JC005926.
Huang, R. X., 1991: The Three‐Dimensional Structure of Wind‐Driven Gyres: Ventilation and Subduction. Rev. Geophys., 29, 590–609, https://doi.org/10.1002/rog.1991.29.s2.590.
Hurrell, J. W., and H. Van Loon, 1994: A modulation of the atmospheric annual cycle in the Southern Hemisphere. Tellus A, 46, 325–338, https://doi.org/10.1034/j.1600-0870.1994.t01-1-00007.x.
Jacobs, S., 2006: Observations of change in the Southern Ocean. Philos. Trans. R. Soc. Math. Phys. Eng. Sci., 364, 1657–1681, https://doi.org/10.1098/rsta.2006.1794.
Kawai, H., S. Yukimoto, T. Koshiro, N. Oshima, T. Tanaka, H. Yoshimura, and R. Nagasawa, 2019: Significant improvement of cloud representation in the global climate model MRI-ESM2. Geosci. Model Dev., 12, 2875–2897, https://doi.org/10.5194/gmd-12-2875-2019.
Kay, J. E., and Coauthors, 2015: The Community Earth System Model (CESM) Large Ensemble Project: A Community Resource for Studying Climate Change in the Presence of Internal Climate Variability. Bull. Am. Meteorol. Soc., 96, 1333–1349, https://doi.org/10.1175/BAMS-D-13-00255.1.
Kelley, M., and Coauthors, 2020: GISS‐E2.1: Configurations and Climatology. J. Adv. Model. Earth Syst., 12, e2019MS002025, https://doi.org/10.1029/2019MS002025.
Landerer, F. W., P. J. Gleckler, and T. Lee, 2014: Evaluation of CMIP5 dynamic sea surface height multi-model simulations against satellite observations. Clim. Dyn., 43, 1271–1283, https://doi.org/10.1007/s00382-013-1939-x.
Lawrence, D. M., K. W. Oleson, M. G. Flanner, C. G. Fletcher, P. J. Lawrence, S. Levis, S. C. Swenson, and G. B. Bonan, 2012: The CCSM4 Land Simulation, 1850–2005: Assessment of Surface Climate and New Capabilities. J. Clim., 25, 2240–2260, https://doi.org/10.1175/JCLI-D-11-00103.1.
Lenaerts, J. T. M., J. Fyke, and B. Medley, 2018: The Signature of Ozone Depletion in Recent Antarctic Precipitation Change: A Study With the Community Earth System Model. Geophys. Res. Lett., 45, https://doi.org/10.1029/2018GL078608.
Li, L., and Coauthors, 2020: The Flexible Global Ocean‐Atmosphere‐Land System Model Grid‐Point Version 3 (FGOALS‐g3): Description and Evaluation. J. Adv. Model. Earth Syst., 12, e2019MS002012, https://doi.org/10.1029/2019MS002012.
Li, S., W. Liu, R. J. Allen, J.-R. Shi, and L. Li, 2023: Ocean heat uptake and interbasin redistribution driven by anthropogenic aerosols and greenhouse gases. Nat. Geosci., 16, 695–703, https://doi.org/10.1038/s41561-023-01219-x.
Liu, F., J. Lu, Y. Luo, Y. Huang, and F. Song, 2020: On the Oceanic Origin for the Enhanced Seasonal Cycle of SST in the Midlatitudes under Global Warming. J. Clim., 33, 8401–8413, https://doi.org/10.1175/JCLI-D-20-0114.1.
Liu, J., Y. Luo, and F. Liu, 2022: Externally forced symmetric warming in the Arctic and Antarctic during the second half of the twentieth century. Geosci. Lett., 9, 16, https://doi.org/10.1186/s40562-022-00226-x.
Liu, W., J. Lu, and S.-P. Xie, 2015: Understanding the Indian Ocean response to double CO2 forcing in a coupled model. Ocean Dyn., 65, 1037–1046, https://doi.org/10.1007/s10236-015-0854-6.
Lu, J., and B. Zhao, 2012: The Role of Oceanic Feedback in the Climate Response to Doubling CO ₂. J. Clim., 25, 7544–7563, https://doi.org/10.1175/JCLI-D-11-00712.1.
Luo, Y., J. Lu, F. Liu, and W. Liu, 2015: Understanding the El Niño-like oceanic response in the tropical Pacific to global warming. Clim. Dyn., 45, 1945–1964, https://doi.org/10.1007/s00382-014-2448-2.
Metzl, N., B. Moore, and A. Poisson, 1990: Resolving the intermediate and deep advective flows in the Indian Ocean by using temperature, salinity, oxygen and phosphate data: the interplay of biogeochemical and geophysical tracers. Glob. Planet. Change, 3, 81–111, https://doi.org/10.1016/0921-8181(90)90057-J.
Mitchell, J. F. B., 1989: The “Greenhouse” effect and climate change. Rev. Geophys., 27, 115–139, https://doi.org/10.1029/RG027i001p00115.
Oke, P. R., and Coauthors, 2019: Revisiting the circulation of the East Australian Current: Its path, separation, and eddy field. Prog. Oceanogr., 176, 102139, https://doi.org/10.1016/j.pocean.2019.102139.
Oliver, E. C. J., and N. J. Holbrook, 2014: Extending our understanding of South Pacific gyre “spin-up”: Modeling the East Australian Current in a future climate. J. Geophys. Res. Oceans, 119, 2788–2805, https://doi.org/10.1002/2013JC009591.
Qu, T., I. Fukumori, and R. A. Fine, 2019: Spin‐Up of the Southern Hemisphere Super Gyre. J. Geophys. Res. Oceans, 124, 154–170, https://doi.org/10.1029/2018JC014391.
Reason, C. J. C., J. R. E. Lutjeharms, J. Hermes, A. Biastoch, and R. E. Roman, 2003: Inter-ocean fluxes south of Africa in an eddy-permitting model. Deep Sea Res. Part II Top. Stud. Oceanogr., 50, 281–298, https://doi.org/10.1016/S0967-0645(02)00385-5.
Reid, J. L., 1986: On the total geostrophic circulation of the South Pacific Ocean: Flow patterns, tracers and transports. Prog. Oceanogr., 16, 1–61, https://doi.org/10.1016/0079-6611(86)90036-4.
Ridgway, K. R., and J. R. Dunn, 2003: Mesoscale structure of the mean East Australian Current System and its relationship with topography. Prog. Oceanogr., 56, 189–222, https://doi.org/10.1016/S0079-6611(03)00004-1.
Ridgway, K. R., and J. R. Dunn, 2007: Observational evidence for a Southern Hemisphere oceanic supergyre: SOUTHERN HEMISPHERE OCEANIC SUPERGYRE. Geophys. Res. Lett., 34, n/a-n/a, https://doi.org/10.1029/2007GL030392.
Rintoul, S. R., and M. H. England, 2002: Ekman Transport Dominates Local Air–Sea Fluxes in Driving Variability of Subantarctic Mode Water. J. Phys. Oceanogr., 32, 1308–1321, https://doi.org/10.1175/1520-0485(2002)032<1308:ETDLAS>2.0.CO;2.
Rio, M.-H., 2010: Absolute Dynamic Topography from Altimetry: Status and Prospects in the Upcoming GOCE Era. Oceanography from Space, V. Barale, J.F.R. Gower, and L. Alberotanza, Eds., Springer Netherlands, 165–179.
Riser, S. C., and Coauthors, 2016: Fifteen years of ocean observations with the global Argo array. Nat. Clim. Change, 6, 145–153, https://doi.org/10.1038/nclimate2872.
Rodrigues, R. R., L. M. Rothstein, and M. Wimbush, 2007: Seasonal Variability of the South Equatorial Current Bifurcation in the Atlantic Ocean: A Numerical Study. J. Phys. Oceanogr., 37, 16–30, https://doi.org/10.1175/JPO2983.1.
Roemmich, D., 2007: Super spin in the southern seas. Nature, 449, 34–35, https://doi.org/10.1038/449034a.
Rotstayn, L. D., M. A. Collier, S. J. Jeffrey, J. Kidston, J. I. Syktus, and K. K. Wong, 2013: Anthropogenic effects on the subtropical jet in the Southern Hemisphere: aerosols versus long-lived greenhouse gases. Environ. Res. Lett., 8, 014030, https://doi.org/10.1088/1748-9326/8/1/014030.
Rouault, M., P. Penven, and B. Pohl, 2009: Warming in the Agulhas Current system since the 1980’s. Geophys. Res. Lett., 36, 2009GL037987, https://doi.org/10.1029/2009GL037987.
Rühs, S., C. Schmidt, R. Schubert, T. G. Schulzki, F. U. Schwarzkopf, D. Le Bars, and A. Biastoch, 2022: Robust estimates for the decadal evolution of Agulhas leakage from the 1960s to the 2010s. Commun. Earth Environ., 3, 318, https://doi.org/10.1038/s43247-022-00643-y.
Sasaki, Y. N., S. Minobe, N. Schneider, T. Kagimoto, M. Nonaka, and H. Sasaki, 2008: Decadal Sea Level Variability in the South Pacific in a Global Eddy-Resolving Ocean Model Hindcast. J. Phys. Oceanogr., 38, 1731–1747, https://doi.org/10.1175/2007JPO3915.1.
Schouten, M. W., W. P. M. De Ruijter, P. J. Van Leeuwen, and J. R. E. Lutjeharms, 2000: Translation, decay and splitting of Agulhas rings in the southeastern Atlantic Ocean. J. Geophys. Res. Oceans, 105, 21913–21925, https://doi.org/10.1029/1999JC000046.
Seland, Ø., and Coauthors, 2020: The Norwegian Earth System Model, NorESM2 – Evaluation of theCMIP6 DECK and historical simulations. Climate and Earth System Modeling,.
Send, U., and Coauthors, 2010: A Global Boundary Current Circulation Observing Network. Proceedings of OceanObs’09: Sustained Ocean Observations and Information for Society, OceanObs’09: Sustained Ocean Observations and Information for Society, European Space Agency, 841–853.
Shindell, D. T., 2004: Southern Hemisphere climate response to ozone changes and greenhouse gas increases. Geophys. Res. Lett., 31, L18209, https://doi.org/10.1029/2004GL020724.
Sloyan, B. M., K. R. Ridgway, and R. Cowley, 2016: The East Australian Current and Property Transport at 27°S from 2012 to 2013. J. Phys. Oceanogr., 46, 993–1008, https://doi.org/10.1175/JPO-D-15-0052.1.
Speich, S., B. Blanke, P. De Vries, S. Drijfhout, K. Döös, A. Ganachaud, and R. Marsh, 2002: Tasman leakage: A new route in the global ocean conveyor belt. Geophys. Res. Lett., 29, https://doi.org/10.1029/2001GL014586.
——, ——, and W. Cai, 2007: Atlantic meridional overturning circulation and the Southern Hemisphere supergyre: AMOC AND THE SH SUPERGYRE. Geophys. Res. Lett., 34, n/a-n/a, https://doi.org/10.1029/2007GL031583.
Sutton, P. J. H., and M. Bowen, 2014: Flows in the Tasman Front south of Norfolk Island. J. Geophys. Res. Oceans, 119, 3041–3053, https://doi.org/10.1002/2013JC009543.
Swart, N. C., and Coauthors, 2019: The Canadian Earth System Model version 5 (CanESM5.0.3). Geosci. Model Dev., 12, 4823–4873, https://doi.org/10.5194/gmd-12-4823-2019.
Thompson, D. W. J., and S. Solomon, 2002: Interpretation of Recent Southern Hemisphere Climate Change. Science, 296, 895–899, https://doi.org/10.1126/science.1069270.
——, J. M. Wallace, and G. C. Hegerl, 2000: Annular Modes in the Extratropical Circulation. Part II: Trends. J. Clim., 13, 1018–1036, https://doi.org/10.1175/1520-0442(2000)013<1018:AMITEC>2.0.CO;2.
Tim, N., E. Zorita, K.-C. Emeis, F. U. Schwarzkopf, A. Biastoch, and B. Hünicke, 2019: Analysis of the position and strength of westerlies and trades with implications for Agulhas leakage and South Benguela upwelling. Earth Syst. Dyn., 10, 847–858, https://doi.org/10.5194/esd-10-847-2019.
Van Sebille, E., A. Biastoch, P. J. Van Leeuwen, and W. P. M. De Ruijter, 2009: A weaker Agulhas Current leads to more Agulhas leakage: AGULHAS LEAKAGE SENSITIVITY. Geophys. Res. Lett., 36, n/a-n/a, https://doi.org/10.1029/2008GL036614.
Van Sebille, E., P. J. Van Leeuwen, A. Biastoch, and W. P. M. De Ruijter, 2010: Flux comparison of Eulerian and Lagrangian estimates of Agulhas leakage: A case study using a numerical model. Deep Sea Res. Part Oceanogr. Res. Pap., 57, 319–327, https://doi.org/10.1016/j.dsr.2009.12.006.
Wang, G., W. Cai, and A. Purich, 2014: Trends in Southern Hemisphere wind-driven circulation in CMIP5 models over the 21st century: Ozone recovery versus greenhouse forcing. J. Geophys. Res. Oceans, 119, 2974–2986, https://doi.org/10.1002/2013JC009589.
Wang, H., S. Xie, X. Zheng, Y. Kosaka, Y. Xu, and Y. Geng, 2020: Dynamics of Southern Hemisphere Atmospheric Circulation Response to Anthropogenic Aerosol Forcing. Geophys. Res. Lett., 47, https://doi.org/10.1029/2020GL089919.
Weijer, W., W. P. M. De Ruijter, and H. A. Dijkstra, 2001: Stability of the Atlantic Overturning Circulation: Competition between Bering Strait Freshwater Flux and Agulhas Heat and Salt Sources. J. Phys. Oceanogr., 31, 2385–2402, https://doi.org/10.1175/1520-0485(2001)031<2385:SOTAOC>2.0.CO;2.
Williams, K. D., and Coauthors, 2018: The Met Office Global Coupled Model 3.0 and 3.1 (GC3.0 and GC3.1) Configurations. J. Adv. Model. Earth Syst., 10, 357–380, https://doi.org/10.1002/2017MS001115.
Yamagami, Y., and T. Tozuka, 2015: Interannual variability of South Equatorial Current bifurcation and western boundary currents along the Madagascar coast. J. Geophys. Res. Oceans, 120, 8551–8570, https://doi.org/10.1002/2015JC011069.
Yang, X., D. Wang, J. Wang, and R. X. Huang, 2007a: Connection between the decadal variability in the Southern Ocean circulation and the Southern Annular Mode. Geophys. Res. Lett., 34, 2007GL030526, https://doi.org/10.1029/2007GL030526.
Yang, X.-Y., R. X. Huang, and D. X. Wang, 2007b: Decadal Changes of Wind Stress over the Southern Ocean Associated with Antarctic Ozone Depletion. J. Clim., 20, 3395–3410, https://doi.org/10.1175/JCLI4195.1.
Yukimoto, S., and Coauthors, 2019: The Meteorological Research Institute Earth System Model Version 2.0, MRI-ESM2.0: Description and Basic Evaluation of the Physical Component. J. Meteorol. Soc. Jpn. Ser II, 97, 931–965, https://doi.org/10.2151/jmsj.2019-051.
Zhang, R., S. Sun, Z. Chen, H. Yang, and L. Wu, 2023: On the Decadal and Multidecadal Variability of the Agulhas Current. J. Phys. Oceanogr., 53, 1011–1024, https://doi.org/10.1175/JPO-D-22-0123.1.
Ziehn, T., and Coauthors, 2020: The Australian Earth System Model: ACCESS-ESM1.5. J. South. Hemisphere Earth Syst. Sci., 70, 193–214, https://doi.org/10.1071/ES19035.

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Dynamics of Southern Hemisphere Super Gyre Response to External Forcings

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Version 1

Abstract

Figures

1. Introduction

2. Data sets

2.1 Observations

2. 2 Single forcing experiments

2.3 Partially coupled experiments

3. Results

3.1 Sea surface height

3.2 Streamfunction

3.3 Inter-ocean connection

3.4 Internal ocean basin circulation

3.5 Wind stress curl

3.6 Validation with the partially coupled experiments

4. Conclusion and discussion

Declarations

Conflict of interest

Funding

Author Contributions

Acknowledgements

Data Availability

References

Supplementary Files

Status:

Version 1