تشعشعات خورشیدی گرمای اضافی را در مناطق استوایی نسبت به قطب ها ارسال می کند، و اقیانوس نقش کلیدی در توزیع مجدد گرما در اطراف زمین برای جبران این عدم تعادل انرژی ایفا می کند ^{1 ، 2 ، 3} . در اقیانوس، این توزیع مجدد انرژی تا حد زیادی از طریق گردش جهانی واژگونی انجام می‌شود، سیستمی از جریان‌های اقیانوسی که آب دریا را با علائم منحصربه‌فرد ترموهالین (دما-نمک) و بیوژئوشیمیایی (به عنوان مثال، کربن، اکسیژن، مواد مغذی) در داخل و بین حوضه‌های اقیانوسی حمل می‌کند ^{. , 5 , 6 , 7 , 8} . در طول مسیر، توده‌های آب سطحی، میانی، عمیق و پرتگاهی ایجاد، مخلوط، ترکیب و تبدیل ^{می‌شوند .10 ، 11 ، 12 مسیرهای} سه ^{بعدی پیچیده و نامنظم را دنبال می‌کنند که می‌توانند در مقیاس‌های زمانی فصلی تا هزاره‌ای متفاوت باشند . _ _} _  _ _ شکل  1 و فیلم تکمیلی  1 ).

یکی از اجزای متمایز سیستم جهانی واژگونی، سلول گردشی واژگونی نصف النهار اقیانوس اطلس فوقانی (AMOC) است که آب های گرم و شور را از نیمه گرمسیری اقیانوس اطلس جنوبی در سراسر خط استوا به سمت عرض های جغرافیایی بالا در اقیانوس اطلس شمالی منتقل می کند، جایی که آنها غرق می شوند و به عنوان آب های عمیق سرد به سمت استوا می روند. . در زیر سلول AMOC بالایی، یک سلول مغاک ضعیف‌تر وجود دارد که از غرق شدن آب‌های سردتر و شورتر در امتداد لبه یخی قطب جنوب ²⁴ منشأ می‌گیرد (شکل  1 ). این سلول ها با هم حداکثر 25 درصد از خالص انتقال انرژی (گرما) اقیانوس و جو جهانی را حمل می کنند ^{1 ، 21 ، 25 ، 26.}. سلول AMOC فوقانی تأثیر فوق العاده ای بر آب و هوای منطقه و آب و هوای جهانی از طریق تبادل هوا-دریا ^{27 ، 28 ، 29 ، 30 ، 31 ، 32 ، 33 ، 34 ، 35 ، 36} و به عنوان مثال از طریق تبادل با کرایوسفر، از طریق، اعمال می کند. ذوب یخ دریا ³⁷ . AMOC گرمای انسانی و ذخیره کربن اضافی را از سطح به اعماق اقیانوس تنظیم می کند ^{21 ، 38 ، 39 ، 40}، که زمان و اثرات منطقه ای تغییرات آب و هوایی انسانی را تعدیل می کند. همه سناریوهای انتشار گازهای گلخانه‌ای پیش‌بینی می‌کنند که انتظار می‌رود سلول AMOC بالایی در قرن بیست و یکم در واکنش به شرایط آب و هوایی انسانی تضعیف ^شود . گردش خون، رویدادهای شدید، و اکوسیستم های دریایی ^{17 ، 22 ، 42 ، 43 ، 44 ، 45 ، 46 ، 47}. شادابی شدید در اقیانوس اطلس شمالی می تواند باعث تضعیف یا “خاموش شدن” سلول AMOC فوقانی شود، که مطالعات دیرینه اقلیم نشان می دهد می تواند عواقب گسترده ای بر آب و هوای جهانی داشته باشد ^{48 ، 49 ، 50} .

The schematic represents the pathways of surface (red), intermediate (yellow), deep (blue), and abyssal (purple) waters over the bottom topography (blue shading). Transitions between these colors indicate water mass transformations. Important currents and topographic features mentioned in the text are labeled, and dashed white lines indicate the nominal latitudes of the five Atlantic Meridional Overturning Circulation (AMOC) monitoring arrays (Box 1). AC Agulhas Current, ACC Antarctic Circumpolar Current, ARs Agulhas Rings, BC Brazil Current, BCS Benguela Current System, DWBC Deep Western Boundary Current, MC Malvinas Current, NBC North Brazil Current, SEC South Equatorial Current.

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The South Atlantic Ocean plays a unique role in the global overturning circulation because it is the only ocean basin with a net equatorward heat transport^{2,7,16,51,52,53,54,55,56,57}, and it is the basin where the AMOC upper and abyssal cells interact the most (Fig. 1). Furthermore, the South Atlantic acts as the primary conduit for the deeper cold dense water masses formed in the North Atlantic to exit the Atlantic and interact with water masses in other ocean basins^{8,10,58,59,60,61,62,63,64,65} (blue pathway in Fig. 1). Interocean exchanges from the Pacific and Indian basins alter the thermohaline and chemical properties of the surface and intermediate water masses (red and yellow pathways, respectively, in Fig. 1) as they are formed, transformed, and transported through the South Atlantic on their way to the North Atlantic^65,66. Cold and fresh water masses from the Pacific Ocean flow through Drake Passage^6,58,65,67 and largely traverse eastward to meet the warm and salty water masses from the Indian Ocean flowing through the gap between South Africa and the Antarctic continent (Fig. 1). These flows primarily enter the South Atlantic through the Cape Basin region via mesoscale eddies referred to as Agulhas Rings and the Benguela Current System (BCS)^59,63,68,69 to feed into the upper limb of the AMOC^{59,60,65,66,67,70,71}. Interocean exchanges induce two regions of high mesoscale variability in the South Atlantic, the Confluence of the Brazil Current (BC) and Malvinas Current (MC) on the western boundary and the Agulhas Leakage on the eastern boundary (areas with elevated sea surface height variance in Box 1). These regions of high mesoscale activity and air-sea interactions contribute to the formation of new water masses^9,10,11,65. At the deepest layers (purple pathway in Fig. 1), newly formed abyssal waters of Antarctic origin flow northward into the Argentine and the Brazil Basins^{72,73,74,75,76}, cross the equator, and ultimately mix with other deep and abyssal waters.

While many of the constituent flows of AMOC are meridional (north-south) in orientation, zonally oriented (east-west) flows can play an important role in linking different AMOC pathways. This is particularly apparent in the South Atlantic, where the lower limb of the upper AMOC cell, marked by North Atlantic Deep Water (NADW) carried southward by the Deep Western Boundary Current (DWBC), splits around 20°S and loses a small portion of its flow eastward across the basin toward the African coast^12,64,77,78 (blue pathway in Fig. 1), although it remains in question whether this eastward NADW flow continues into the Cape Basin or recirculates around the Angola Basin⁷⁹. The upper limb of the AMOC transports surface and intermediate water masses northwestward across the basin via Agulhas Rings and through the BCS and South Equatorial Current (SEC)^{10,62,80,81,82,83,84,85} (red and yellow pathways, respectively, in Fig. 1).

Deep water formation associated with the AMOC at high latitudes in the North Atlantic has been linked to changes in local stratification and in the thermohaline properties of the upper limb of the AMOC in the South Atlantic^10,16,65. Theory and models have suggested the direction of the meridional freshwater transport by the upper AMOC cell at the southernmost border of the South Atlantic (i.e., 34.5°S) is a control parameter on the mono- or bi-stability of AMOC^{86,87,88,89,90,91}. In a monostable system, AMOC imports freshwater into the South Atlantic. In a bistable system, where AMOC exports freshwater poleward out of the South Atlantic, it is possible to abruptly switch from an “AMOC on” to “AMOC off” state causing abrupt regime shifts in the water cycle and regional weather patterns leading to planetary climate impacts⁴⁷.

Over the past two decades, the international scientific community has made numerous advances in the study of the AMOC due to the growth of long-term observing programs at various latitudes in the Atlantic basin^{15,16,19,92,93,94,95,96,97,98}. These observational programs combined with models have revolutionized our understanding of the mean structure and pathways of the AMOC, its variability on time-scales ranging from days to decades, and its drivers. While the development of a comprehensive AMOC observing system in the South Atlantic has lagged behind the North Atlantic, there is now a concerted international effort focused on studying the role of the South Atlantic dynamics in driving AMOC changes and regional climate variability. This international South Atlantic Meridional Overturning Circulation (SAMOC) initiative began in 2007^{10,99,100,101}. The SAMOC observing system consists primarily of purposely designed trans-basin and boundary current mooring arrays and synoptic ship-based hydrographic observations in the South Atlantic and in the regions of interocean exchanges (Box 1; Box 2). SAMOC also leverages global satellites, Argo profiling floats, and surface drifting buoys, which provide key observations that are useful for overturning studies. The goal of the SAMOC initiative is to monitor climatically relevant oceanic fluxes of mass, heat, and freshwater, and provide observations critical to validate and improve numerical models and climate predictions^102,103,104.

Several recent review articles have summarized what we have learned from AMOC observing systems, but they primarily focused on the North Atlantic^19,95,96. The only review article focused solely on the South Atlantic circulation precedes major findings from the SAMOC initiative¹⁰. The present review will highlight recent advances revealed by the SAMOC observing system along with newly developed observational analysis techniques. The review will serve to demonstrate the critical role of long-term sustained observations for detecting and understanding circulation variability in this region across a multitude of time-scales, and our present ability to model some of the changes. Specifically, we will focus on the insights gained into changes in circulation, water masses, transports, boundary currents, and eddy-rich interocean exchanges. These changes must be monitored and studied to continue to assess and understand their known (and still unknown) impacts on climate phenomena such as precipitation patterns and extreme events (e.g., marine heat waves, hurricanes), sea level rise, and temperature and salinity trends. Finally, we present a summary and our perspective on the remaining key outstanding research questions and identify observational gaps that challenge our current ability to answer these questions.

^{ویژگی‌های توده‌های آب اقیانوس} اطلس جنوبی از اولین بررسی‌های اقیانوس‌شناسی، که توسط آلمان ^انجام شده ^{است ، مورد مطالعه قرار} گرفته ^است . از آن زمان، آزمایش گردش جهانی اقیانوس (WOCE) در دهه 1990، پروژه آب و هوا و اقیانوس-تغییرپذیری، پیش بینی پذیری و تغییر (CLIVAR) در دهه 2000، و اخیراً، برنامه تحقیقات هیدروگرافیک مبتنی بر کشتی اقیانوس جهانی (GO-SHIP) ) با نمونه برداری کامل و ایجاد بهترین شیوه ها برای ترانسکت های هیدروگرافی ده ساله مکرر ¹¹⁰ (جعبه  1) ادامه داده اند.). با گسترش برنامه بین المللی آرگو در دهه 2000 ^111 ، 112 ، و همچنین سفرهای دریایی تحقیقاتی در حمایت از ابتکار SAMOC (جعبه  1 )، مشاهدات هیدروگرافیک در منطقه در 20 سال گذشته به طور چشمگیری افزایش یافته است ^{12 ، 56 ، 85 ، 113 ، 114 ، 115} . تا مارس 2022، بیش از 540 شناور آرگو فعال (شامل 210 شناور بیوژئوشیمیایی آرگو و 12 شناور عمیق آرگو) وجود داشت که پروفیل های آب (بیشتر در 2000 متر بالایی) را در بخش اقیانوس اطلس جنوبی بین استوا و 60 درجه جنوبی جمع آوری می کردند. . فراتر از تخمین حمل و نقل AMOC (جعبه  1این مشاهدات در ارزیابی توزیع‌ها، ویژگی‌ها و دینامیک اقیانوسی در مقیاس بزرگ تا مزو مقیاس (1000 تا 50 کیلومتر) در حوضه کلیدی بوده است. علاوه بر این، با به دست آوردن داده های هیدروگرافی مکرر که چندین دهه را در بر می گیرد، می توان ارزیابی کرد که چگونه این آب ها تحت تأثیر تغییرات آب و هوایی قرار می گیرند.

یک تصویر ساده از مسیرهای توده آب AMOC در اقیانوس اطلس جنوبی شامل جریان های متناوب به سمت شمال و جنوب در امتداد لایه های سطحی / میانی، عمیق و پرتگاهی است که یکدیگر را جبران می کنند، به طوری که جریان کل در هر عمق کامل و خط فرا حوضه ای تقریباً متعادل می شود. در مقیاس های زمانی طولانی این جریان ها یا در لایه های چگالی یکسان حرکت می کنند (آدیاباتیک) یا در نهایت از آنها عبور می کنند (دیاباتیک). سایر سیستم‌های گردش اقیانوسی می‌توانند بر گردش واژگونی تأثیر بگذارند و می‌توانند تحت تأثیر قرار گیرند، مانند چرخش نیمه گرمسیری اقیانوس اطلس جنوبی ^32 ، 116 و سلول‌های نیمه گرمسیری ^115.. به دلیل فرآیندهای متقابل متنوع هوا و دریا، لایه بالایی از توده های آبی تشکیل شده است که در سطح از مناطق استوایی (آب سطحی)، نیمه گرمسیری (آب مرکزی) و دور قطبی (آب میانی قطب جنوب، AAIW) سرچشمه می گیرند. این آب ها می توانند به صورت محلی تشکیل شوند و می توانند تحت تأثیر آب های اقیانوس آرام، هند و اقیانوس جنوبی قرار گیرند و توزیع ها و مسیرهای پیچیده آنها توسط گردش واژگونی تنظیم می شود (شکل 1)  .). این پس‌زمینه میانگین زمانی را تشکیل می‌دهد که اندام فوقانی سلول AMOC با آن تعامل دارد و در مسیر خود به سمت اقیانوس اطلس شمالی قرار می‌گیرد. اندام تحتانی سلول AMOC فوقانی شامل صادرات NADW به اقیانوس جنوبی است که در آنجا به جریان قطبی قطب جنوب (ACC) می پیوندد. ^{صادرات NADW توسط جریان آبهای سطحی، مرکزی و} میانی به سمت شمال از طریق لایه های بالایی در اقیانوس اطلس جنوبی و در سراسر استوا، و با جریان شمالی پایین آب قطب جنوب (AABW) که توسط سلول ابیسی حمل می شود، جبران ^{می شود . 74 , 76 , 114 , 117 , 118 , 119}. گرما و آب شیرین منتقل شده توسط این تبادلات بین اقیانوسی به طور قابل ملاحظه ای بر اقیانوس اطلس به عنوان یک کل تأثیر می گذارد ^{58 ، 65 ، 66 ، 69 ، 120} .

How AMOC water mass pathways in the South and North Atlantic Ocean are linked, and in particular how water masses cross the equator, is also a topic of considerable interest. For deep water masses, the main route from the North Atlantic to the South Atlantic is thought to be via the DWBC with some zonal detours within the deep equatorial current system^67,121 (Fig. 1). For warm surface and intermediate waters flowing northward from the South to the North Atlantic, the dominant pathway along the western boundary via the North Brazil Under Current (NBUC) and North Brazil Current (NBC) system is often referred to as the “direct” pathway^{122,123,124,125}. The “direct” pathway reaches the Caribbean Sea via the NBC retroflection region, where NBC rings are spun off (Fig. 1). The “indirect” interior pathway consists of the complex zonal equatorial current system, fed in part by the NBC retroflection, and interior northward transport in the subtropical cells in the tropical Atlantic which also plays an important role in the AMOC¹¹⁵ (Fig. 1).

The connection to the neighboring ocean basins adds complexity to the South Atlantic’s water mass configuration. In the subtropical South Atlantic, between 35°S and 11°S, recent hydrographic surveys have provided more detailed descriptions and sources of the water masses^{12,56,74,84,85,114,126} (Fig. 2c; Table 1). South Atlantic Central Waters (SACW) are produced locally as they originate from the subduction of subtropical Mode Waters in hotspots of air-sea interaction and eddy activity in the southwestern Atlantic at the Brazil-Malvinas Confluence (BMC) and the southeastern Atlantic^127,128 (Box 1). These two regional hotspots also encompass pathways whereby relatively fresh and highly oxygenated intermediate waters enter the South Atlantic. In the BMC, a regional variety of Subantarctic Mode Water is subsequently transformed into a fresh Atlantic variety of AAIW^129,130,131, and a saltier AAIW variety that has Indian origin enters the South Atlantic through the Agulhas Leakage and BCS^{56,68,80,131,132} (Fig. 2a–c).

a Conservative temperature (Θ, °C), b Absolute salinity (S_A, g kg⁻¹), and c Dissolved oxygen (O₂, μmol kg⁻¹) along the SAMBA section at 34.5°S in January 2017⁵⁶. The neutral density layers (γⁿ, kg m⁻³) bounding each water mass are shown in white dotted curves in all panels. A map showing the location of anticyclonic and cyclonic eddies crossed during the cruise in magenta and cyan, respectively, is shown on top of panel a. The diamonds indicate the location of the 128 CTD profiles, colored in black, magenta and cyan depending if they were occupied outside, or within an anticyclonic or a cyclonic eddy, respectively. The upper 1000 dbar are vertically stretched. SW Surface Water, SACW South Atlantic Central Water, AAIW Antarctic Intermediate Water, UCDW Upper Circumpolar Deep Water, NADW North Atlantic Deep Water, LCDW Lower Circumpolar Deep Water, AABW Antarctic Bottom Water.

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Between ~1100 and 1600 dbar (~1100–1600 m depth), an oxygen minimum layer of Upper Circumpolar Deep Water (UCDW; Fig. 2c) originating from Drake Passage flows equatorward in the Cape Basin and southward in the Argentine Basin⁵⁶, while the trans-basin zonally-integrated UCDW transport is southward at 34.5°S. At greater depths, two different veins of NADW flow southward. Near the western boundary, NADW has more pristine properties (saltier and warmer) due to a more direct pathway from its North Atlantic source region, than the vein hugging the African continental slope (fresher and colder), which has followed a more complex and longer route connected through an interior pathway^{64,67,77,78,85,117}.

Lower Circumpolar Deep Water (LCDW) and Weddell Sea AABW both flow northward along the western side of the Brazil Basin. However, the northward spreading of LCDW and AABW is blocked on the eastern side of the Mid-Atlantic Ridge by the Walvis Ridge, and only older varieties of abyssal waters are found with no clear interior pathway^{7,56,60,80,114,129} (Figs. 1, 2). Abyssal water masses are mixed on their way to the north and become harder to track and distinguish from other water masses near the tropical region. Although there are some inter-study differences as to which isopycnal marks the boundary between NADW, LCDW, and AABW^12,56,133, waters with neutral densities larger than 28.15 kg m⁻³ are classified as AABW, with younger waters in the western portion of the basin and older waters in the eastern portion (Table 1).

Recent observational and modeling studies suggest that a reduction in the strength of the AMOC could lead to an additional warming of South Atlantic sea surface temperatures relative to cooling of North Atlantic sea surface temperatures^35,134,135 and a weakened salinity divergence between the South and North Atlantic, resulting in a net salt gain in the South Atlantic¹³⁶.

Tracking water mass characteristics and their changes over space and time can help to determine water mass pathways and understand their origins and impacts. As an example, a 21st-century climate projection predicts increased Agulhas leakage in response to the poleward shift of the Southern Hemisphere westerlies, which will cause an increased transport of warm and salty water from the Indian into the South Atlantic Ocean⁸¹. An upper ocean warming and salinification trend have been traced by observations through the South Atlantic¹³⁷ (Fig. 3a, c) up to the western boundary at 11°S¹²² from the early 2000s to the early 2010s, apparently consistent with the above-mentioned scenario. Adding more nuance, a more recent modeling study⁶⁵ showed that the combined effect of a weakening fresh South Pacific inflow and increasing salty Indian inflow on the northward AMOC transport in the NBUC region might be driving the observed salinification from 2000–2009 (Fig. 3c).

a, b Trends of temperature averaged over the top 700 dbar and between 700 and 2000 dbar. c, d Trends of salinity averaged over the top 700 dbar and between 700 and 2000 dbar. The decadal linear trends in (a–d) are computed from annual Argo climatology²⁴⁶ for the years 2004–2018. Black contour lines indicate trends that are significant at the 90% confidence level. The black horizontal line indicates the position of the western SAMBA moorings at 34.5°S. e In situ hourly bottom temperature data from western SAMBA moorings⁷⁵ at 47.5°W (Site C; brown curve) and 44.5°W (Site D; blue curve) from March 2009 to June 2019. Squares and stars are corresponding hydrographic measurements during research cruises used to calibrate the mooring data at 47.5°W and 44.5°W, respectively.

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Near the surface, the observed ocean warming associated with ongoing planetary warming is fairly ubiquitous in the South Atlantic from 1980 to 2020¹³⁸ (Fig. 3a), with maximum warming, on the order of 1.2 °C decade⁻¹ in the upper 700 dbar (~700 m depth) (Fig. 3a), found along the BC and in the BMC region^138,139,140. Upper ocean salinity has also increased from the 1950s to the 2000s (Fig. 3c), with large increases in high salinity regions such as in the sea surface salinity maximum found between 0° and 30°S^141,142,143 and in the BC and BMC region where an increase on the order of 0.15 psu decade⁻¹ (comparable to 0.15 g kg⁻¹ decade⁻¹) in the upper 700 m is observed (Fig. 3c).

The thermocline waters most affected by long-term warming signals are the mode and intermediate waters since they are formed at the surface in both the subpolar and subtropical regions and then penetrate the South Atlantic subtropical gyre at depth^{144,145,146,147}. A recent study found that about 1.5 × 10²² J of heat was converging in the South Atlantic in the σ_θ = 26.0–27.0 kg m⁻³ potential density layer (roughly between 200 and 700 m depth) from 2006 to 2015¹⁴⁸ (Fig. 3a). Despite its small area, upper ocean warming in the South Atlantic accounts for roughly 10% of global upper ocean warming. Some of this excess heat has been taken up by subtropical Atlantic mode waters since nearly 1980, which then ventilate the main thermocline and transport heat away from the surface^{128,138,148,149}. South Atlantic AAIW properties also show significant warming, and their salinity seems to be decreasing over the past few decades^148,150 (Fig. 3b, d), due to competing influences from increased Agulhas Leakage and declining South Pacific inflow (salinification) vs. the intensification of the hydrological cycle (freshening) (Fig. 3d). In the mid-depth layer between AAIW and NADW, warming on the order of 0.044 °C decade⁻¹ and freshening on the order of 0.006 g kg⁻¹ decade⁻¹ has been observed from 2006 to 2015¹⁵¹ (Fig. 3b, d). Although temperature and salinity changes in the ocean interior can be due to isopycnal heave (i.e., vertical movement of density layers), this was not the case for this mid-depth layer. Rather, changes were attributed to anomalous southward advection by the AMOC¹⁵¹.

Decadal and longer time-scale temperature and salinity trends associated with deep (2000–4000 m) and abyssal (4000–6000 m) water masses are relatively small and can be obscured by sub-decadal time-scale variability^{38,152,153,154,155,156}. However, in these deep and abyssal layers, noticeable warming and freshening trends are emerging in the South Atlantic that is either expression of decadal and multi-decadal variability or of anthropogenic climate change^{12,73,157,158,159,160}. More studies are needed to determine whether these trends, should they persist, are due to changes in the properties of NADW and AABW, or their relative abundance and their vertical distributions^158,161. The observed AABW warming is strongest in the Southwest Atlantic where the younger variety of AABW has to cross through the Vema Channel near 31°S, 39°W. Steady and significant AABW warming has been observed from moored observations and hydrographic data in the Vema Channel of about 0.02 °C decade⁻¹ since the 1980s, which has accelerated to approximately 0.026 °C decade⁻¹ since 2005^76,162. Abyssal warming of 0.02 ± 0.01 °C decade⁻¹ was also observed over the period 2009–2019 from continuous hourly measurements from two moored instruments located near the 4500 m and 4800 m isobaths in the northwestern Argentine Basin along the SAMBA line at 34.5°S⁷⁵ (Fig. 3e). Closer to the tropics, abyssal warming on the order of 0.02 to 0.03 °C decade⁻¹ was observed in the Brazil Basin between 0°–30°S, 40°W–10°W from recent deep Argo data (2019–2020) and hydrographic data spanning multiple decades¹⁶³, and in the western tropical Atlantic from multi-decadal repeat hydrographic cruises¹².

As described in Box 1, estimates of AMOC and associated meridional heat transport (MHT), and/or freshwater transport from the surface to the seafloor across the South Atlantic have been derived from repeated CTD sections, XBT transects, inverse models, blended in situ and satellite data, and moorings at multiple latitudes in the South Atlantic from 35 to 11°S with different record lengths, time and space scales, and accuracies. Key recent findings are synthesized and described below (Table 2, Fig. 4).

Monthly values of AMOC upper cell transport (black solid curves) and MHT (red curves, when available) time series from mooring observing arrays: a TRACOS in the Tropical Atlantic^98,115 at 11°S from July 2013 to February 2018, b SAMBA in the Subtropical South Atlantic^55,97 at 34.5°S from September 2013 to July 2017. The AMOC abyssal-cell transport below 3000 m at 34.5°S is also shown⁹⁷ (black dashed curve). c Monthly AMOC upper cell transport and MHT time series constructed from blended products based on combinations of in situ hydrographic profiles and satellite altimetry⁵⁷ at two selected latitudes in the South Atlantic (20°S and 34.5°S) during 1993–2020. The correlation values between AMOC and MHT (r_AMOC,MHT) time series are included when available. d, e Temporal mean (circles) ± one standard deviation (horizontal error bars) of the available monthly AMOC and MHT full-length records, respectively, from moorings^55,97,98,115 (black) and blended in situ altimetry products⁵⁷ (blue), and from the AX18 XBT quarterly repeated transect⁵⁷ during 2002–2019 (dark green), as a function of latitude. Positive values indicate northward transport, and negative values indicate southward transport. AMOC Atlantic Meridional Overturning Circulation, MHT Meridional Heat Transport.

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Time-mean estimates of the upper AMOC cell and full-depth MHT at various latitudes determined from trans-basin shipboard data and also a ~30-year (1993-present) blended product of in situ and satellite observations (Table 2) exhibit a wide range of northward volume transport values ranging from 11.7 to 28.0 Sv^126,164 with MHT ranging from 0.27 to 0.92 PW^56,164 and large standard deviations ranging from 1.4 to 4.6 Sv and from 0.10 to 0.22 PW, respectively^56,164. Concurrent AMOC estimates at multiple latitudes reveal that both the variability of AMOC and MHT decrease northward from 34.5 to 20°S^{19,57,164,165} (Fig. 4d, e).

Using data from the TRACOS mooring array at 11°S in the tropical Atlantic, the seasonal and shorter time-scale variability of the upper AMOC cell was investigated over the period 2013–2018⁹⁸ (Table 2; Fig. 4a). A pronounced annual (seasonal) cycle has been observed at 11°S with an amplitude of 8 Sv attributed to the variability of both the upper ocean geostrophic and Ekman transport. Based on Argo data and repeated shipboard velocity measurements combined with satellite winds, the time-mean upper AMOC flow at 11°S was estimated as 16.4 Sv¹¹⁵.

In the subtropical South Atlantic, the first estimates of the daily upper cell AMOC transport were obtained using measurements from a pair of PIES moorings near the 1300 m isobath on either flank of the South Atlantic at 34.5°S deployed as part of the pilot configuration of SAMBA^15,100. These analyses have demonstrated that the AMOC is highly variable at time-scales ranging from a few days to a year. Using nine of the SAMBA PIES moorings that had continuous daily observations from September 2013 to July 2017, a more recent study resolved the daily AMOC transports from the surface down to ~4700 dbar (~4700 m)⁹⁷, and also found a highly variable upper AMOC cell (black solid curve in Fig. 4b). This improved and updated AMOC estimate allows for better accuracy of the volume transport estimates as well as the ability to attribute the sources of the signals. A key new result from this study was the estimation of the daily time-varying strength of the abyssal cell (defined as the maximum southward flow between 3000 dbar and the seafloor beneath the upper AMOC cell; black dashed curve in Fig. 4b); previously, this cell has only been determined from once-a-decade snapshot estimates from basin-wide sections^{126,166,167,168} or combinations of hydrographic sections with inverse models^61,169. The flows in the upper and abyssal cells vary independently, and the upper cell circulation (time-mean of 17.3 Sv northward transport, standard deviation of 15.5 Sv) is more energetic than the abyssal-cell circulation (time-mean of −7.8 Sv southward transport, standard deviation of 6.2 Sv) at all time-scales resolved over this 4-year observational period⁹⁷ (Table 2; Fig. 4b). Of interest, historical observations yielded ~7 Sv of northward-flowing AABW¹⁷⁰ (~4 Sv through the Vema Channel¹⁷¹ and ~3 Sv through the Hunter Channel¹⁷²), which matches remarkably well with the independent SAMBA estimate of −7.8 ± 2.7 Sv (time-mean ± bias error) for the southward upper limb of the abyssal cell⁹⁷, assuming that this flow is compensated by the AABW in the lower limb of the abyssal flow. In addition, the amplitude of the upper AMOC seasonal cycle was 7 Sv using nine moorings⁹⁷, compared to an amplitude of only 4 Sv when using two moorings¹⁵. SAMBA has continued to expand over time and now has on the order of twenty moorings⁹⁷, but an AMOC estimate has yet to be generated using more than nine mooring sites.

Measurements from SAMBA have revealed that the variations in the upper AMOC cell at 34.5°S are somewhat more complex than what has previously been observed in the subtropical North Atlantic on intraseasonal, seasonal, and interannual time-scales^{13,18,19,92,93,94,153,173,174,175}. This is because density and pressure variations on both sides of the Atlantic at 34.5°S are important to the AMOC variations at all time-scales, from daily to interannual, based on the observations collected to date. Furthermore, at 34.5°S it is critical to independently observe the Ekman, baroclinic, and barotropic components of the AMOC volume and heat transports, as all contribute roughly equally to the semi-annual AMOC cycle, and both the baroclinic and barotropic components can dominate interannual variations in different years^15,55,97. In contrast to the importance of both the Ekman and geostrophic contributions at both boundaries to the AMOC at 34.5°S, at 11°S the annual (seasonal) AMOC variations are primarily forced by geostrophic variations with the largest seasonal pressure variability on the eastern boundary, and secondarily by winds⁹⁸. Despite some dynamical differences at 11°S and 34.5°S, the amplitude of the seasonal cycles observed by the two arrays is comparable^97,98. The ~30-year AMOC and MHT records produced from the blended in situ and satellite observations show that the relative importance of the geostrophic and Ekman components driving AMOC and MHT fluctuations vary with both time-scale and latitude between 20 and 34.5°S^57,165. This suggests that different mechanisms (wind forcing, buoyancy forcing, and internal ocean dynamics) are responsible for driving the observed fluctuations at different latitudes. A well-defined AMOC and MHT seasonal cycle was found between 20 and 34.5°S, with the largest amplitudes (4 Sv and 0.3 PW, respectively) at 34.5°S^57,165. While these records indicate that there is no MHT coherence between the South and North Atlantic on interannual time-scales, northward propagation of large MHT anomalies from 34.5 to 20°S with a 9-month lead time were found, thus expanding our understanding of the latitudinal connectivity of the AMOC system⁵⁷. It is plausible that MHT variations on interannual and shorter time-scales are dominated by regional processes with low coherence across the gyres and hemispheres, as has been suggested by numerical simulations¹⁷⁶. Longer observational records will improve analyses of AMOC/MHT meridional coherence within the Atlantic basin on seasonal, interannual, decadal, and longer time-scales and across a range of processes.

On shorter time-scales ranging from a few days to weeks, moored observations from TRACOS (11°S) and SAMBA (34.5°S) have also revealed large amplitudes of variability and rapid changes of the boundary current transports by the NBC/NBUC, the BC, and the DWBC near the western boundary^113,122,177 and by the Angola Current, BCS, and Deep Eastern Boundary Current (DEBC) near the eastern boundary^85,178,179. These results highlight the strength of continuous observing systems to resolve these highly variable flows near the continental slope, which in turn are key contributors to the upper and abyssal AMOC transports variability across the basin.

Moored instruments and regular hydrographic sections provide unprecedented temperature, salinity, velocity and/or bottom pressure information near the western and eastern boundaries of the South Atlantic^{74,84,85,98,113,114,122,177,178,179,180,181}. However, observations from which meridional heat or freshwater transports can be computed in the interior of the basin are still very sparse. A new technique for estimating interior full-depth temperature profiles by combining satellite, CTD, and Argo profile measurements determined an average MHT during 2013–2017 of 0.5 PW, with a daily standard deviation of 0.8 PW, for the SAMBA array at 34.5°S⁵⁵. This time-mean estimate falls within the range of the previous results from snapshot shipboard measurements and blended products (Table 2) and demonstrates that moored arrays on the boundary combined with ancillary data in the interior can be used to continuously observe heat transport.

Regardless of the observational platform, studies consistently show that the upper cell AMOC volume and heat transports vary in a coherent manner (i.e., the AMOC and MHT time series are strongly correlated with one another with significant correlation coefficients r greater than 0.7 (Table 2; Fig. 4b, c). Furthermore, the characteristics of their variability are very similar on intraseasonal, seasonal, and interannual time-scales. Linear regressions of AMOC volume and heat transport find that a + 1 Sv change in AMOC at 34.5°S is equivalent to approximately a + 0.05 PW increase in MHT^55,182.

From SAMBA, the AMOC upper cell and MHT appear to be strengthening during 2013–2017 by 2.5 ± 3.8 Sv year⁻¹ and at a rate of 0.14 ± 0.18 PW year⁻¹, respectively⁵⁵. These short-term trends, while large, are not statistically significant relative to the large interannual modulation observed during this 4-year time period at 34.5°S. However, other studies suggest significantly smaller trends in transport when a longer time period is considered. For example, significant positive AMOC upper cell transport trends are observed from 1993 to 2020 at 25°S and 34.5°S (0.44 ± 0.38 Sv decade⁻¹ and 0.48 ± 0.36 Sv decade⁻¹, respectively), while a significant MHT trend is detected only at 34.5°S (0.03 ± 0.01 PW decade⁻¹) from the blended in situ and satellite altimetry data⁵⁷ (Fig. 4c). More years of continuous measurements are likely necessary to detect significant longer period signals and trends in the moored transport records given the large variability on short time-scales (e.g., Fig. 4b).

Cross-comparisons of the in situ/satellite AMOC/MHT estimates with that from moored arrays indicate that while the time means agree well, they usually yield low correlations and different seasonal-to-interannual variability even after subsampling the daily mooring records at monthly resolution^57,183 (Fig. 4b–e). The lack of coherence suggests that the estimates are sensitive to the methodology and/or sampling characteristics of the observations used to derive them. While the CTD, XBT and blended records have unquestionably expanded our understanding of the oceanic fluxes across multiple latitudes, the lower temporal sampling rates from synoptic (snapshot) observations and/or monthly gridded products (which combine observations with different temporal resolutions) may miss the high-frequency AMOC and MHT fluctuations captured by the mooring arrays. Moreover, moored arrays that use bottom pressure information to provide information about the barotropic contribution produce more energetic AMOC and MHT time series^15,55,97 than the other observational methods that use an assumption of net-zero volume (mass) transport (Box 1). To better determine the state of the AMOC/MHT and understand its variability, it is necessary to understand the physical reasons for these observational disparities (taking into account the uncertainties associated with each technique) when longer time series from all of these observing systems become available.

Estimates of the meridional freshwater transport by the overturning circulation, computed either from the sea surface to the seafloor or from the sea surface to the base of the upper AMOC cell (nominally 3000 dbar), have been limited to full-depth CTD sections^{56,166,167,168}, XBT transects¹⁶, and inverse models^61,184,185. Most trans-basin observations collected to date have shown that the freshwater transport associated with the overturning circulation (M_ov) between 11°S and 34.5°S is southward^{16,166,167,184}, and may have increased in strength over the past 30 years associated with increasing salinity trends in the upper ocean¹⁸⁴ (Fig. 3c). In contrast, a recent study found northward M_ov from a 2017 trans-basin section along 34.5°S⁵⁶. As mentioned earlier, the sign (i.e., direction) of the freshwater transport by the AMOC upper cell at this latitude may control whether AMOC exists in a bistable or monostable state, with far-reaching climate implications. Given their importance, more M_ov estimates from observations are clearly needed. Continuous M_ov estimates from the RAPID/MOCHA/WBTS array in the subtropical North Atlantic (26.5°N; Fig. 1; Box 1) and Argo data suggest that a linear relationship describing 91% of the variance exists between AMOC and M_ov¹⁸⁶ and so a + 1 Sv increase in AMOC volume transport at 26.5°N is roughly equivalent to a −0.05 Sv decrease in M_ov. Thus it seems feasible that future blended in situ and satellite products and moored observations can be used to construct M_ov time series similar to approaches used for MHT^{55,57,164,165}.

In the South Atlantic, branches of the AMOC are carried by currents at the western and eastern boundaries of the basin^6,65,69,119 (Fig. 1). In the west, cold fresh waters carried by the MC, a branch of the ACC, enter the basin through Drake Passage to flow northward along the coast of South America and then turn eastward as they encounter the warm salty southward flowing BC. Together, these waters flow eastward and join the warm water masses in the BCS, entering via the Agulhas Leakage, to flow northward off South Africa and Namibia. Both of these water mass routes feed the SEC, which crosses the South Atlantic south-east to north-west. Ultimately, these waters reach the equatorial regions, feeding into the NBC.

The Antarctic Circumpolar Current in Drake Passage

As well as influencing Atlantic circulation, the ACC transport through Drake Passage serves as a valuable metric for validating ocean and climate models and constraining inverse models. Few numbers have been more cited for this reason than the estimate of 134 Sv of time-mean ACC transport from the International Southern Ocean Studies (ISOS) program conducted in the 1970s^187,188. However, a reassessment of the ISOS data estimated the uncertainty of that mean ACC transport as 27 Sv¹⁸⁹. Recent observational estimates of the total ACC transport using different methodologies can differ by more than 30 Sv, with 141 Sv estimated from the DRAKE experiment¹⁹⁰, 173 Sv estimated from the cDrake experiment^191,192, and 175 Sv determined from an analysis of Argo float displacements and historical hydrography¹⁹³. The attribution of the differences between the Drake Passage observational estimates is primarily due to differences in the estimates of the barotropic component of the ACC transport^102,192.

Recent high-resolution global numerical simulations are providing useful context in interpreting these observational transport estimates. A multi-decadal global ocean and sea-ice simulation based on the HYbrid Coordinate Ocean Model (HYCOM)¹⁹⁴ with 1/12° horizontal resolution was analyzed to diagnose differences between the DRAKE and cDrake transport estimates¹⁰². The total time-mean HYCOM-modeled transport was 157 Sv which falls between the DRAKE and cDrake estimates, and also closely agrees with estimates from other recent data-constrained models (e.g., 155 Sv;¹⁹⁵ 149 Sv¹⁹⁶). In general, good agreement between the model and observations is found for the baroclinic component of transport; the HYCOM estimates¹⁰² (Fig. 5a) track the 20-year time series of baroclinic transport estimated from annually repeated GO-SHIP hydrographic sections along SR1b¹⁹⁷. The daily continuous observational estimates from cDrake permit the study of the variability of the full-depth ACC baroclinic transport in detail¹⁹¹. The energetic baroclinic transport had a standard deviation of 8.1 Sv with 65% of the transport variance concentrated at periods shorter than 60 days, highlighting the high-frequency variability in the record, while the annual means were steady during 2007–2011¹⁹¹ (magenta curve in Fig. 5a). The contemporaneous five samples of baroclinic components from SR1b and cDrake agree well¹⁹¹ (black dots and magenta curve in Fig. 5a). The main challenge for observations appears to be the barotropic component, which requires accurate velocity measurements across the entire passage at high spatial resolution. Shipboard ADCP (SADCP) measurements, although limited to the upper 1000 m, provide direct velocity measurements at the required horizontal resolution, on the order of 5 km¹⁹⁸. The time-mean ACC transport in the upper 1000 m estimated from repeat SADCP transects of Drake Passage is 95 Sv¹⁹⁸, in agreement with the 93 Sv estimated from HYCOM in the upper 1000 m (Fig. 5c). Finally, although the structure and strength of winds over the Southern Ocean have been changing over the past several decades^199,200, there is at present little evidence of a significant trend in the recent ACC transport estimates, and no long-term trend was found in the HYCOM Drake Passage transport over 40 years¹⁰².

a Monthly values of total (red curve) and baroclinic (blue curve) transports across the repeat SR1b hydrographic section in Drake Passage computed from the HYbrid Coordinate Ocean Model (HYCOM) in comparison with baroclinic transport estimated from 24 SR1b hydrographic surveys¹⁰² (black circles) over 1993–2018 and with monthly baroclinic transport (magenta curve) estimated from the continuous cDrake observations¹⁹¹ from December 2007 to October 2011. The HYCOM transports (available between 1979–2020) are shown for the period 1993–2018 (coincident with the 24 SR1b observations). b Cumulative barotropic Antarctic Circumpolar Current (ACC) transports from south to north across Drake Passage¹⁰². The circled purple line is the cumulative barotropic transport based on cDrake observations¹⁹²; the circled and thick orange lines are the HYCOM-modeled cumulative barotropic transports calculated using velocities subsampled at the cDrake mooring locations and at the full model resolution, respectively. The dashed vertical lines and circles indicate the cDrake mooring locations. c, d Vertical profile of transport per unit depth as a function of depth at the Drake Passage repeat SADCP section and repeat hydrographic SR1b section from HYCOM and observations¹⁰². The red horizontal bars are the HYCOM-modeled total transport. In c, the black circles are SADCP observations¹⁹⁸. In d, the black circles are baroclinic transports based on the long-term SR1b hydrographic surveys. Positive transports correspond to eastward flow.

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While the ACC transport has remained steady, trends in ACC eddy energy appear to be correlated with interannual and longer time-scale changes in the prevailing winds^201,202. This phenomenon, predicted by theory and observed in eddy-permitting models, is termed eddy saturation, whereby increased winds result in increased eddy kinetic energy rather than increased circumpolar transport²⁰³. A related phenomenon for the meridional overturning circulation is eddy compensation, whereby poleward eddy fluxes compensate for increases in the equatorward wind-driven component of overturning transport²⁰². Thus, poleward eddy fluxes are a key dynamical process for closing the AMOC in the Southern Ocean. Drake Passage is one of the few polar regions where simultaneous measurements of temperature and velocity have made it possible to estimate eddy heat fluxes. Moored observations during the ISOS program yielded consistently poleward eddy heat fluxes, although estimates varied with depth and location and were also sensitive to time series length^204,205. Longer time series from the recent cDrake experiment²⁰⁶ provided eddy heat flux estimates that were statistically stable over 2-year subsets. These fluxes were predominantly poleward, with 50% of the heat flux surface intensified above 1000 m and uniformly distributed below²⁰⁷. The strongest heat fluxes observed by cDrake were between the Subantarctic and Polar Fronts, but the 50 km spacing in southern Drake Passage may not have captured the full eddy heat flux of the narrow Southern ACC Front. The high horizontal resolution is required in southern Drake Passage where the radius of deformation is of the order of 15 km. Using seven years of high-resolution SADCP and AX25 XBT temperature data from spatially near-repeated underway surveys in Drake Passage (Box 1; Box 2), poleward and surface-intensified eddy heat fluxes were identified not only in the surface layer of the Polar Front but also in the Southern ACC Front²⁰⁸. These results were recently updated using the now 20-year-long time series, calculating both time-mean and time-varying streamlines to isolate the dynamically important across-ACC heat flux component²⁰⁹. The time-varying streamlines provide the best estimate of the across-ACC component because they track the shifting and meandering of the ACC fronts. The depth-integrated (0–900 m) across-stream eddy heat flux is poleward and maximum in the south flank of the Subantarctic Front −0.10 ± 0.05 GW m⁻¹ (1 GW equals 10⁻⁶ PW) and decreases to become statistically insignificant toward the south. The long time series provides an uncommon opportunity to explore the seasonal cycle of eddy heat flux. The poleward eddy heat flux in the Polar Front Zone is enhanced during austral autumn–winter, suggesting a seasonal variation in eddy-driven upwelling and thus the overturning circulation²⁰⁹.

Agulhas Leakage in the Cape Basin

An interocean exchange route that is fundamental for the AMOC lies in the gap between the southern shores of Africa and the ACC. Lagrangian experiments using eddy-resolving ocean and, more recently, climate, models are able to quantify the simulated Agulhas Leakage from the Indian Ocean and allow estimates of associated water mass properties and their transformation^{62,65,67,81,119,210}. In the real ocean, however, Agulhas Leakage waters are difficult to trace and the interocean fluxes, which occur as eddies and filaments spun off the Agulhas retroflection, represent an extreme challenge to quantify because of the highly energetic environment^{131,211 ، 212 ، 213 ، 214} .

اخیراً می توان برخی از عناصر پیچیدگی نشت آگولهاس را با استفاده از مشاهدات مرز شرقی از سیستم مشاهده SAMBA ⁸⁵ (جعبه  1 ) جدا کرد. در حالی که کل نشت آگولهاس بین ~ 5-15 Sv ¹³² تا 21.3 ± 4.7 Sv ²¹⁵ تخمین زده می شود، مشاهدات SAMBA میانگین زمانی انتقال آب های ترموکلاین به سمت شمال را در عرض 34.5 درجه جنوبی در حوضه کیپ 24 Sv با اندازه بزرگ تعیین می کند. واریانس (انحراف استاندارد 17 Sv) به دلیل عبور از حلقه های آگولهاس ⁸⁵ . این انتقال با اندام فوقانی AMOC مرتبط است و به انتقال گرما و نمک به سمت شمال کمک می کند (شکل  1 ).). مشاهدات SAMBA نشان می‌دهد که تبادل جرم آب در حوضه کیپ عمدتاً از طریق فرارفت توسط گرداب‌های مقیاس متوسط ​​و از طریق نفوذ توده‌های آب گسترده که توسط دوقطبی‌های شدید ایجاد می‌شوند، رخ ^{می‌دهد} . نکته مهم، این مطالعه همچنین نیاز به نمونه برداری مداوم از ستون آب کامل را برجسته می کند، زیرا تبادلات بین حوضه به طور متناوب رخ می دهد و می تواند بر شناوری و جریان ها در اعماق و مقیاس های زمانی متعدد تأثیر بگذارد.

حلقه‌های آگولهاس به‌عنوان مکان‌های تبدیل توده‌های آب شدید عمل می‌کنند. ^{مقادیر زیادی از آب‌های مرکزی اقیانوس هند که در} هسته‌های حلقه‌ای گرفته می‌شوند، در محدوده دمایی نسبتاً بزرگی به آب‌های حالت نیمه گرمسیری ^{تبدیل می‌شوند} . تخمین های حمل و نقل قبلی 2-10 Sv با مشاهده حلقه های آگولهاس با عمر طولانی از کشتی ها یا ماهواره ها انجام شده است ^{59 ، 217 ، 218 ، 219 ، 220 ، 221 ، 222} . با این حال، شبیه‌سازی‌ها به‌طور مداوم مقدار نشتی را که در حلقه‌ها حمل می‌شود دست‌کم می‌گیرند و عمدتاً بخش غیر گردابی شار است که نقش غالب را در حمل‌ونقل نشت آگولهاس مدل‌سازی شده بازی می‌کند.^{210 223 224} . ^_ _ این به احتمال زیاد به این دلیل است که مدل‌ها به اندازه کافی طیف کاملی از تنوع در مقیاس مزو و زیرمزو (کمتر از 50 کیلومتر) را نشان نمی‌دهند، از این رو تخمین‌های خود را کم سوگیری می‌کنند و عدم قطعیت زیادی را در تخمین‌ها وارد می‌کنند. ^{مطالعات اخیر که ارتفاع‌سنجی ماهواره‌ای را با داده‌های هیدروگرافی درجا ترکیب می‌کند ،} ارزیابی بهتری از تأثیر حلقه‌های آگولهاس در اتصال مؤثر سیستم‌های مرزی شرقی و غربی اقیانوس اطلس جنوبی ^{را امکان‌پذیر کرده است .}. علیرغم پویایی پیچیده مرتبط با عبور حلقه‌های آگولهاس (با شکاف‌ها و ادغام‌های فراوان در حوضه کیپ)، برخی از حلقه‌ها در حین عبور از اقیانوس اطلس جنوبی به‌عنوان ساختارهای زیرسطحی منسجم باقی می‌مانند و به طور متوسط ​​حدود سه سال طول می‌کشد تا به سمت دیگر حوضه برسند. ^{216 222 225 226} . ^{_ _ _ پتانسیل مشاهدات جریان آگولهاس در اقیانوس} هند برای روشن کردن تنوع نشتی وجود دارد که گمان می‌رود با کنترل بالادست نشت توسط باد و پیچ و خم 227 هدایت می‌شود، اگرچه اخیراً این کنترل‌ها ^{زیر سوال} رفته‌اند .

آرایه SAMBA همچنین منجر به ارزیابی بهتری از انتقال میانگین زمانی به سمت جنوب 12- Sv (انحراف استاندارد 17 Sv) مرتبط با DEBC در اقیانوس اطلس جنوبی ⁸⁵ شده است . این جریان عمیق شامل جریان جنوبی یک رگه شرقی NADW ^{64 ، 77 ، 78 ، 230} است . مشاهدات SAMBA همچنین نشان می‌دهد که بخش عمده‌ای از DEBC باید در حوضه کیپ در گردش باشد، با بخش کوچک‌تری از جریان در گردش جهانی واژگونی شرکت ^{می‌کند} .

گسترش قابل توجه شبکه رصدی SAMOC در محل در بیش از 15 سال گذشته (جعبه  1 )، به ویژه در امتداد خط SAMBA در 34.5 درجه جنوبی و خط TRACOS در 11 درجه جنوبی، اطلاعات بی سابقه ای در مورد ساختار و تغییرپذیری ارائه کرده است. AMOC در این دو عرض جغرافیایی حمل و نقل می کند. با این حال، سوابق رصدی پیوسته AMOC در محل هنوز از نظر مکان و زمان محدود هستند و مدل‌هایی برای مطالعه مکانیسم‌های AMOC و تأثیر AMOC بر آب و هوا و آب و هوا مورد نیاز است. مطالعات مبتنی بر مدل ^{30 ، 32 ، 103 ، 104 ، 125 ، 231 ، 232}بینش های مهمی در مورد رفتار و تأثیرات AMOC در اقیانوس اطلس جنوبی ارائه کرده اند، اما ارزش این بینش ها به وفاداری مدل ها به اقیانوس واقعی بستگی دارد. به منظور ارزیابی تأثیر آب و هوای AMOC و پیش‌بینی تغییرپذیری آینده، بسیار مهم است که مدل‌های آب و هوایی مورد استفاده برای پیش‌بینی آب و هوا و آب و هوا، حساسیت AMOC را در اقیانوس اطلس جنوبی در پاسخ به اجبار به دقت شبیه‌سازی کنند.

Observations from both the North and South Atlantic Ocean suggest that the geostrophic contribution to the volume and heat transports of the AMOC plays an equal or greater role compared to that played by the wind-driven Ekman transport in terms of the total AMOC and MHT seasonal to interannual variability^{13,26,57,153,174,233,234}. However, studies based on earlier eddy-permitting (¼° to 1° horizontal resolution) numerical models^{235,236,237,238} indicated that the seasonal to interannual variations in the modeled AMOC and MHT are predominantly controlled by the Ekman transport. The AMOC estimates at 34.5°S from monthly climatologies of temperature and salinity constructed from observations and numerical models suggested that the seasonality of the geostrophic transport from observations is largely controlled by density variations at the western boundary, but in the models, the eastern boundary dominates due to poor representation of the MC strength at the western boundary²³⁹. The weak seasonal cycle in the modeled geostrophic transport was primarily attributed to excessively strong baroclinicity below the surface mixed layer, whereas the observations show a strong vertical coherence in the velocity down to 1200 m.

Another well-known disagreement between observations and models is the direction of the freshwater transport induced by the overturning circulation (M_ov) near the southern boundary of the Atlantic^{16,90,91,120,166,167}. Contrary to the positive values of the M_ov from numerical models^{35,50,240,241,242}, most recent observation-based estimates of the M_ov in the South Atlantic^{16,120,166,167,184} gave negative values ranging from −0.28 to −0.05 Sv (with the exception of the recent snapshot estimate from a trans-basin cruise at 34.5°S⁵⁶), suggesting that changes in the AMOC would amplify the freshwater anomalies in the convection sites. As a result, contrary to the monostable state of the AMOC from models, observations suggest that the AMOC is bistable and might collapse with a large enough freshwater perturbation. The model-data difference in freshwater transports has been attributed to the biases in the modeled salinity fields: too fresh in the northward-flowing upper layer and too salty in the southward flowing lower layer^{35,50,91,240,241,242}.

Eddies play an important role in transporting heat and freshwater in the South Atlantic, and improved eddy parameterizations and/or higher resolution numerical models are required to properly simulate eddy characteristics and their impacts on regional and global climate^66,201,243. A recent study to investigate the impact of model resolution on the subtropical South Atlantic variability demonstrated the dominant role geostrophic transport plays in controlling the MHT variability in an eddy-resolving (0.1° horizontal resolution) simulation while both the geostrophic and Ekman transports influence MHT variability in a non-eddy resolving (1° horizontal resolution) simulation²⁴⁴. Interestingly, while model MHT at the northern boundary of the South Atlantic (8°S) drives heat content variability in the non-eddy-resolving simulation, in the eddy-resolving simulation ocean heat content variability is predominantly driven by MHT at the southern boundary of the South Atlantic (35°S). Recently, a 1/20° horizontal resolution ocean model was compared to AMOC observations at multiple latitudes in the North and South Atlantic, and was able to reproduce well the AMOC volume transport variations at 34.5°S and AMOC, NBUC, and DWBC transport variations at 11°S¹⁰⁴. Thus, better agreement between models and observations may be found as more models move towards eddy-resolving horizontal resolution.

The increased observations in the last 15+ years in the South Atlantic are invaluable for use in the initialization, validation, and evaluation of newer generations of ocean general circulation and coupled climate models^103,104. Models can provide information on multi-decadal to millennial time scales; however, the differences in the AMOC volume, heat and freshwater transports between numerical models and observations suggest that the AMOC variations in models respond differently to forcing than in the real ocean. It is challenging to implement model improvements based on comparatively short observational records. Thus, existing model-data differences point once again to the need for longer-term measurements in the South Atlantic from moorings to better assess the realism and representation of processes that control AMOC strength and variability in numerical models.

Through its long-term storage and transport of heat, freshwater, and carbon, ocean circulation strongly influences the magnitude, pace, and regional impacts of anthropogenic climate change. These processes are of particular importance in the South Atlantic Ocean, as they influence, and are influenced by, AMOC variability, the development of key water masses, and interocean exchanges. This review provides a comprehensive overview of recent advances in understanding AMOC variability in the South Atlantic, enabled by the tremendous growth of the SAMOC observing system since its inception in 2007.

As a result of the excess heat absorbed from the atmosphere associated with ongoing planetary warming, AMOC variability, interocean exchanges from the Pacific and Indian basins, and other factors elucidated in this review, the South Atlantic Ocean has warmed from the surface to the deep and abyssal ocean^{12,75,76,147,148,151,163} (Fig. 3a, b, e). Upper ocean South Atlantic salinity has increased, while intermediate, deep, and abyssal water masses are freshening^148,151,158 (Fig. 3c, d). Recent observational and modeling evidence suggests that a reduction in the strength of the AMOC associated with anthropogenic warming is linked to additional surface South Atlantic warming and salinification^135,136. Continuing to monitor these water mass changes and their causes is an ongoing priority for South Atlantic research.

Tracking water masses and their changes by combining models and observations has allowed us to refine our understanding of AMOC pathways in the South Atlantic. Quantifying the relative contributions of interocean exchanges from the Indian and Pacific basins into the South Atlantic, allows for a better understanding of the warming and salinification trends observed in upper layers^{65,81,122,137} (Fig. 3a, c). Observational evidence of NADW in the DEBC⁸⁵ and in the interior^64,77,78,230 in the South Atlantic has elucidated new deep water mass pathways. However, many open questions about AMOC water mass pathways remain, particularly the zonal interior paths, and the pathways between the well-observed latitudes. The South Atlantic Gateway Array (SAGA) array deployed along 10°W in 2021 measures the zonal flows carried by the BCS and Agulhas Rings and the underlying NADW interior pathway from the west to east in the mid-latitude central South Atlantic (Box 1) and will help to answer some of these questions in the near future.

The AMOC arrays have expanded over time to include more moorings to better resolve boundary current processes associated with the AMOC variability, such as wind and buoyancy forcing, advection, and/or wave propagation^85,113,122. Even the relatively short trans-basin mooring records available to date have transformed our understanding of the energetic South Atlantic circulation, its dynamics, and its contribution to upper and abyssal AMOC variability on daily to interannual time scales^15,97,98, illustrating the need for high-frequency measurements to avoid aliasing rapid short time scale signals. The dominant mechanisms causing changes in the AMOC strength varies between wind forcing, buoyancy forcing, and internal ocean dynamics on seasonal, interannual, and decadal time scales between 11°S and 34.5°S^15,57,97,98 (Fig. 4; Table 2). Because upper cell AMOC and MHT vary in a consistent manner^16,55,57,97 (Fig. 4b, c), changes in AMOC can impact ocean heat content, pointing to the importance of sustained AMOC monitoring at various latitudes in the South and North Atlantic.

The AMOC volume and property (e.g., heat) transports appear to be somewhat sensitive to the methodology, assumptions, and/or sampling characteristics of the observations used to derive them. For example, the phasing (semi-annual vs. annual) and amplitude of the seasonal cycles currently vary between different studies using moored arrays or blended in situ and satellite measurements at the same latitude, with moored arrays producing records with larger transport variations than other methods. It is possible that the temporal lengths of some of the observational records are still too short to produce directly comparable estimates and/or ascertain whether any of the observing systems miss or alias important contributions to the AMOC transport variability. Future research plans include reconciling the different estimation methods when longer overlapping time series become available. It is important to acknowledge that while infrequent or monthly proxy or partial-basin observations will never be as accurate as full-depth trans-basin continuous moored measurements, these observations are cost-effective and available on a longer time-scale and at a wider range of locations, thus providing high scientific value despite the inherent limitations.

The observing systems at the gateways from the Pacific and Indian basins monitor interocean exchange processes such as buoyancy exchange, winter mixing, convection, and upwelling. Estimates of the total ACC transport through Drake Passage from a number of recent targeted field campaigns show higher ACC transports (141–173 Sv) than historical values, with increased transport largely due to improved estimation of the barotropic component of the flow^191,192. The annual mean ACC transport appears to have been steady from observations^191,192, and no long-term trend has been detected evaluating ACC transport in a data-constrained model¹⁰². Maintaining long-term observations of the ACC barotropic component remains a challenge (e.g., the highly-resolved cDrake experiment ended in 2011), making it hard to directly and continuously monitor the total ACC transport. Thus, it becomes important to have realistic data-constrained models of ACC transport and to continue monitoring via the annual full-depth SR1b hydrographic cruises and repeat XBT/SADCP transects. In addition, observations and models suggest that the baroclinic and barotropic ACC transports are not correlated; therefore, monitoring either transport component alone seems insufficient to assess the temporal variability of the total ACC transport¹⁰².

In comparison to Pacific-Atlantic, Indo-Atlantic ocean exchanges are less well observed because they occur intermittently since they are largely accomplished through eddies and/or filaments⁸⁴. The eastern SAMBA moorings measure highly variable northward transports of thermocline waters at 34.5°S, associated with the passage of Agulhas Rings that contributes to transports by the upper limb of the AMOC. Developing the techniques and methods to better estimate the eddy flux contributions to interocean exchange, and hence reconcile the disparate estimates of transport by the Agulhas Rings^85,132,215 remains an ongoing challenge. As a step in this direction, the number of eastern SAMBA PIES moorings was doubled in 2021 from five to ten sites between Walvis Ridge and the African coast in order to better resolve the variability associated with the Agulhas Leakage into the South Atlantic.

Topographically constrained narrow boundary current flows and mesoscale features and processes associated with eddies are often inadequately represented in low-resolution numerical models, which can lead to an inadequate representation of the AMOC. SAMOC observations in the past 15+ years have provided unprecedented information about the structure and variability of the AMOC and water mass distributions and pathways, which are invaluable for validating and constraining models^{65,103,104,234}. Model-data intercomparisons are helping to reconcile differences in key processes driving the AMOC in the South Atlantic, such as the role of eddies, AMOC pathways, the relative roles of geostrophic and wind-driven Ekman transports in setting AMOC variability, and the direction of the meridional freshwater transports. Recently, an ocean model with more realistic atmospheric forcing and eddy-resolving horizontal resolution was able to better reproduce the observed AMOC volume transport variations at 11°S and at 34.5°S, indicating the need for models with better representation of boundary flows over sloping topography and mesoscale eddies to accurately represent AMOC variability in the South Atlantic¹⁰⁴.

Continuous evaluation and improvements of the existing observing systems are essential for further scientific advances. For example, three additional moored instruments in the interior of the basin were added to the SAMBA array in 2019–2021 to further improve estimates of the interior and abyssal flows. Future plans include extending the western and eastern SAMBA measurements to the shelf-break regions to better observe processes at the continental boundaries, where boundary currents intensify, and to better constrain basin-wide AMOC estimates. At the western edge of SAMBA in the Southwestern Atlantic two tall dynamic height moorings will be deployed in 2022 at the continental shelf break and at the base of the continental slope to add observations of the vertical structure of the BC and of the DWBC, respectively. Similarly, the eastern SAMBA array will soon include additional moorings on the South African shelf.

Most continuous observing systems in the South Atlantic have focused on measuring physical properties (velocity, temperature, and salinity) to determine volume and heat transports. More time series estimates of AMOC freshwater transport are needed to determine whether volume and freshwater transport co-vary, and whether or not observed freshwater transports are consistently positive or negative, suggesting AMOC is monostable or bistable, respectively. Given the important role of the AMOC in modulating carbon uptake by the ocean from the atmosphere, future moored array improvements could include new biogeochemical sensors, which would allow for the estimation of oceanic nutrients and carbon fluxes. Some of the South Atlantic arrays are starting to incorporate dissolved oxygen sensors (e.g., some TRACOS and SAMBA moorings) to better track water masses and to study combined biogeochemical and physical properties changes relevant for marine ecosystems.

In addition to augmenting the measurements made by existing platforms, it is important to develop pilot observing systems to study AMOC-related processes in undersampled regions and shed light on other remaining unknowns. Such observational arrays could also include surveying from newer autonomous underwater vehicles and uncrewed surface vehicles. Pilot and process studies like the SAGA program can help to increase our understanding of the zonal component of the AMOC, and deploying more bottom-mounted moorings and deep Argo floats in the South Atlantic is important for quantifying ongoing deep and abyssal temperature and salinity changes. Expendable data pod technology systems are being tested at a few SAMBA and SAGA mooring sites to more routinely (once or twice a year) collect data recorded by bottom-mounted moorings, reducing ship time requirements and lowering the risk of data loss.

Pan-Atlantic observational efforts are crucial for improving climate models and tracking the projected decline of AMOC in the next century in response to anthropogenic forcing^19,41. Sustained and continuous monitoring of AMOC changes in the South Atlantic is needed to assess their impacts on climate patterns, such as shifts in regional surface temperature and precipitation patterns³² and extreme events (e.g., marine heat waves, hurricanes), the variability of the key boundary current systems^{85,113,122,177}, coastal sea level rise, warming/cooling or salinification/freshening of the ocean^75,76, deoxygenation and acidification processes. All of these phenomena have large societally-relevant planetary impacts on agriculture, fisheries, infrastructure, and health around the globe^103,245.

Sufficiently long in situ AMOC time series in the North and South Atlantic, together with high-resolution numerical models, will enable a synthesis of the observed transports to elucidate how each key component contributes to the AMOC’s mean state and variability, the forcing processes, the influence of meridional freshwater transport on AMOC stability, and the mechanisms responsible for the meridional coherence and propagation/pathways of AMOC signals on multi-decadal time scales. These studies require international coordination, collaboration, and data and code sharing among scientists within the entire Atlantic Ocean basin.

The SAMOC initiative is the result of a concerted international effort where observing components are combined towards a common objective taking advantage of the available resources. Sustaining a long-term AMOC observing system in the South Atlantic for decades will be challenging but is necessary to address the many outstanding research questions. The ability to maintain these observing systems in the South Atlantic depends not just on institutions and funding agencies in many countries, but also on the availability of resources (research infrastructure, trained personnel). Reducing the costs of these observing programs by determining which sites are key for sustained monitoring, and reducing the need for ship time to collect data from the moorings (i.e., through technological innovations), are high priorities for all of the ongoing AMOC arrays. The SAMOC observing system can learn from assessments ongoing in the North Atlantic⁹⁶, but must be mindful that a “one-size-fits-all” AMOC observing strategy may not be possible because of the many unique features of the South Atlantic that have been highlighted in this review.