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1. Expedition 361 Scientific Prospectus: South African Climates (Agulhas LGM Density Profile)

   https://doi.org/10.14379/iodp.sp.361.2015  

   Hall, I.R. ; Hemming, S.R. ; LeVay, L.J. ( August 2015 , Scientific prospectus)

   null (Ed.)

   The Agulhas Current is the strongest western boundary current in the Southern Hemisphere, transporting some 70 Sv of warm and saline surface waters from the tropical Indian Ocean along the East African margin to the tip of Africa. Exchanges of heat and moisture with the atmosphere influence southern African climates, including individual weather systems such as extratropical cyclone formation in the region and rainfall patterns. Recent ocean models and paleoceanographic data further point at a potential role of the Agulhas Current in controlling the strength and mode of the Atlantic Meridional Overturning Circulation (AMOC) during the Late Pleistocene. Spillage of saline Agulhas water into the South Atlantic stimulates buoyancy anomalies that act as a control mechanism on the basin-wide AMOC, with implications for convective activity in the North Atlantic and Northern Hemisphere climate. International Ocean Discovery Program (IODP) Expedition 361 aims to extend this work to periods of major ocean and climate restructuring during the Pliocene/Pleistocene to assess the role that the Agulhas Current and ensuing (interocean) marine heat and salt transports have played in shaping the regional- and global-scale ocean and climate development. This expedition will core six sites on the southeast African margin and Indian–Atlantic ocean gateway. The primary sites are located between 416 and 3040 m water depths. The specific scientific objectives are • To assess the sensitivity of the Agulhas Current to changing climates of the Pliocene/Pleistocene, in association with transient to long-term changes of high-latitude climates, tropical heat budgets, and the monsoon system; • To reconstruct the dynamics of the Indian–Atlantic gateway circulation during such climate changes, in association with changing wind fields and migrating ocean fronts; • To examine the connection between Agulhas leakage and ensuing buoyancy transfer and shifts of the AMOC during major ocean and climate reorganizations during at least the last 5 My; and • To address the impact of Agulhas variability on southern Africa terrestrial climates and, notably, rainfall patterns and river runoff. Additionally, Expedition 361 will complete an intensive interstitial fluids program at four of the sites aimed at constraining the temperature, salinity, and density structure of the Last Glacial Maximum (LGM) deep ocean, from the bottom of the ocean to the base of the main thermocline, to address the processes that could fill the LGM ocean and control its circulation. Expedition 361 will seek to recover ~5200 m of sediment in total. The coring strategy will include the triple advanced piston corer system along with the extended core barrel coring system where required to reach target depths. Given the significant transit time required during the expedition (15.5 days), the coring schedule is tight and will require detailed operational planning and flexibility from the scientific party. The final operations plan, including the number of sites to be cored and/or logged, is contingent upon the R/V JOIDES Resolution operations schedule, operational risks, and the outcome of requests for territorial permission to occupy particular sites. All relevant IODP sampling and data policies will be adhered to during the expedition. Beyond the interstitial fluids program, shipboard sampling will be restricted to acquiring ephemeral data and to limited low-resolution sampling of parameters that may be critically affected by short-term core storage. Most sampling will be deferred to a postcruise sampling party that will take place at the Gulf Coast Repository in College Station, Texas (USA). A substantial onshore X-ray fluorescence scanning plan is anticipated and will be further developed in consultation with scientific participants. 

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2. Observed decline of the Atlantic meridional overturning circulation 2004–2012

   https://doi.org/10.5194/os-10-29-2014  

   Smeed, D. A. ; McCarthy, G. D. ; Cunningham, S. A. ; Frajka-Williams, E. ; Rayner, D. ; Johns, W. E. ; Meinen, C. S. ; Baringer, M. O. ; Moat, B. I. ; Duchez, A. ; et al ( January 2014 , Ocean Science)

   null (Ed.)

   Abstract. The Atlantic meridional overturning circulation (AMOC) has been observed continuously at 26° N since April 2004. The AMOC and its component parts are monitored by combining a transatlantic array of moored instruments with submarine-cable-based measurements of the Gulf Stream and satellite derived Ekman transport. The time series has recently been extended to October 2012 and the results show a downward trend since 2004. From April 2008 to March 2012, the AMOC was an average of 2.7 Sv (1 Sv = 106 m3 s−1) weaker than in the first four years of observation (95% confidence that the reduction is 0.3 Sv or more). Ekman transport reduced by about 0.2 Sv and the Gulf Stream by 0.5 Sv but most of the change (2.0 Sv) is due to the mid-ocean geostrophic flow. The change of the mid-ocean geostrophic flow represents a strengthening of the southward flow above the thermocline. The increased southward flow of warm waters is balanced by a decrease in the southward flow of lower North Atlantic deep water below 3000 m. The transport of lower North Atlantic deep water slowed by 7% per year (95% confidence that the rate of slowing is greater than 2.5% per year). 

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3. Spontaneous Activation of the Pacific Meridional Overturning Circulation (PMOC) in Long-Term Ocean Response to Greenhouse Forcing

   https://doi.org/10.1175/JCLI-D-23-0393.1  

   Curtis, Paul Edwin ; Fedorov, Alexey V. ( March 2024 , Journal of Climate)

   The present-day deep ocean global meridional overturning circulation is dominated by the Atlantic meridional overturning circulation (AMOC), with dense water sinking in the high-latitude North Atlantic Ocean. In contrast, deep-water formation in the subarctic North Pacific is inhibited by a strong upper-ocean halocline, which prevents the development of an analogous Pacific meridional overturning circulation (PMOC). Nevertheless, paleoclimate evidence suggests that a PMOC with deep-water formation in the North Pacific was active, for instance, during the warm Pliocene epoch and possibly during the most recent deglaciation. In the present study, we describe a spontaneous activation of the PMOC in a multimillennial abrupt 4 × CO₂experiment using one of the configurations of the Community Earth System Model (CESM1). Soon after the imposed CO₂increase, the model’s AMOC collapses and remains in a weakened state for several thousand years. The PMOC emerges after some 2500 years of integration, persists for about 1000 years, reaching nearly 10 Sv (1 Sv ≡ 10⁶m³s⁻¹), but eventually declines to about 5 Sv. The PMOC decline follows the AMOC recovery in the model, consistent with an Atlantic–Pacific interbasin seesaw. The PMOC activation relies on two factors: (i) gradual warming and freshening of the North Pacific deep ocean, which reduces ocean vertical stratification on millennial time scales, and (ii) upper-ocean salinity increase in the subarctic North Pacific over several centuries, followed by a rapid erosion of the pycnocline and activation of deep-water formation. Ultimately, our results provide insights on the characteristics of global ocean overturning in warm climates.

    

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4. South African Climates (Agulhas LGM Density Profile)

   https://doi.org/10.14379/iodp.proc.361.2017  

   Hall, I.R. ; Hemming, S.R. ; LeVay, L.J. ( September 2017 , Proceedings of the International Ocean Discovery Program)

   International Ocean Discovery Program Expedition 361 drilled six sites on the southeast African margin (southwest Indian Ocean) and in the Indian-Atlantic Ocean gateway, from 30 January to 31 March 2016. In total, 5175 m of core was recovered, with an average recovery of 102%, during 29.7 days of on-site operations. The sites, situated in the Mozambique Channel at locations directly influenced by discharge from the Zambezi and Limpopo River catchments, the Natal Valley, the Agulhas Plateau, and Cape Basin, were targeted to reconstruct the history of the greater Agulhas Current system over the past ~5 My. The Agulhas Current is the strongest western boundary current in the Southern Hemisphere, transporting some 70 Sv of warm, saline surface water from the tropical Indian Ocean along the East African margin to the tip of Africa. Exchanges of heat and moisture with the atmosphere influence southern African climates, including individual weather systems such as extratropical cyclone formation in the region and rainfall patterns. Recent ocean model and paleoceanographic data further point at a potential role of the Agulhas Current in controlling the strength and mode of the Atlantic Meridional Overturning Circulation (AMOC) during the Late Pleistocene. Spillage of saline Agulhas water into the South Atlantic stimulates buoyancy anomalies that may influence basin-wide AMOC, with implications for convective activity in the North Atlantic and global climate change. The main objectives of the expedition were to establish the role of the Agulhas Current in climatic changes during the Pliocene–Pleistocene, specifically to document the dynamics of the Indian-Atlantic Ocean gateway circulation during this time, to examine the connection of the Agulhas leakage and AMOC, and to address the influence of the Agulhas Current on African terrestrial climates and coincidences with human evolution. Additionally, the expedition set out to fulfill the needs of Ancillary Project Letter number 845, consisting of high-resolution interstitial water sampling to help constrain the temperature and salinity profiles of the ocean during the Last Glacial Maximum. The expedition made major strides toward fulfilling each of these objectives. The recovered sequences allowed generation of complete spliced stratigraphic sections that range from 0 to between ~0.13 and 7 Ma. This sediment will provide decadal- to millennial-scale climatic records that will allow answering the paleoceanographic and paleoclimatic questions set out in the drilling proposal. 

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5. Expedition 361 Preliminary Report: South African Climates (Agulhas LGM Density Profile)

   https://doi.org/10.14379/iodp.pr.361.2016  

   Hall, I.R. ; Hemming, S.R. ; LeVay, L.J. ( May 2016 , Preliminary report)

   null (Ed.)

   International Ocean Discovery Program (IODP) Expedition 361 drilled six sites on the southeast African margin and in the Indian-Atlantic ocean gateway, southwest Indian Ocean, from 30 January to 31 March 2016. In total, 5175 m of core was recovered, with an average recovery of 102%, during 29.7 days of on-site operations. The sites, situated in the Mozambique Channel at locations directly influenced by discharge from the Zambezi and Limpopo River catchments, the Natal Valley, the Agulhas Plateau, and Cape Basin, were targeted to reconstruct the history of the greater Agulhas Current system over the past ~5 my. The Agulhas Current is the strongest western boundary current in the Southern Hemisphere, transporting some 70 Sv of warm, saline surface water from the tropical Indian Ocean along the East African margin to the tip of Africa. Exchanges of heat and moisture with the atmosphere influence southern African climates, including individual weather systems such as extratropical cyclone formation in the region and rainfall patterns. Recent ocean model and paleoceanographic data further point at a potential role of the Agulhas Current in controlling the strength and mode of the Atlantic Meridional Overturning Circulation (AMOC) during the Late Pleistocene. Spillage of saline Agulhas water into the South Atlantic stimulates buoyancy anomalies that act as control mechanisms on the basin-wide AMOC, with implications for convective activity in the North Atlantic and global climate change. The main objectives of the expedition were to establish the sensitivity of the Agulhas Current to climatic changes during the Pliocene–Pleistocene, to determine the dynamics of the Indian-Atlantic gateway circulation during this time, to examine the connection of the Agulhas leakage and AMOC, and to address the influence of the Agulhas Current on African terrestrial climates and coincidences with human evolution. Additionally, the expedition set out to fulfill the needs of the Ancillary Project Letter, consisting of high-resolution interstitial water samples that will constrain the temperature and salinity profiles of the ocean during the Last Glacial Maximum. The expedition made major strides toward fulfilling each of these objectives. The recovered sequences allowed generation of complete spliced stratigraphic sections that span from 0 to between ~0.13 and 7 Ma. This sediment will provide decadal- to millennial-scale climatic records that will allow answering the paleoceanographic and paleoclimatic questions set out in the drilling proposal. 

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