More Like this

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. 

   more » « less
2. The Role of Atlantic Basin Geometry in Meridional Overturning Circulation

   https://doi.org/10.1175/JPO-D-21-0036.1  

   Ragen, Sarah ; Armour, Kyle C. ; Thompson, LuAnne ; Shao, Andrew ; Darr, David ( March 2022 , Journal of Physical Oceanography)

   We present idealized simulations to explore how the shape of eastern and western continental boundaries along the Atlantic Ocean influences the Atlantic meridional overturning circulation (AMOC). We use a state-of-the art ocean–sea ice model (MOM6 and SIS2) with idealized, zonally symmetric surface forcing and a range of idealized continental configurations with a large, Pacific-like basin and a small, Atlantic-like basin. We perform simulations with five coastline geometries along the Atlantic-like basin that range from coastlines that are straight to coastlines that are shaped like the coasts of the American and African continents. Changing the Atlantic basin coastline shape influences AMOC strength in a manner distinct from simply increasing basin width: widening the basin while maintaining straight coastlines leads to a 10-Sv (1 Sv ≡ 10⁶m³s⁻¹) increase in AMOC strength, whereas widening the basin with the geometry of the American and African continents leads to a 6-Sv increase in AMOC strength, despite both cases representing the same average basin-width increase relative to a control case. The structure of AMOC changes are different between these two cases as well: a more realistic basin geometry results in a shoaled AMOC while widening the basin with straight boundaries deepens AMOC. We test the influence of the shape of the both boundaries independently and find that AMOC is more sensitive to the American coastline while the African coastline impacts the abyssal circulation. We also find that AMOC strength and depth scales well with basin-scale meridional density difference, even with different Atlantic basin geometries, illuminating a robust physical link between AMOC and the North Atlantic western boundary density gradient.

    

   more » « less
3. The Role of the Southern Ocean in Abrupt Transitions and Hysteresis in Glacial Ocean Circulation

   https://doi.org/10.1029/2018PA003415  

   Hines, Sophia K. V. ; Thompson, Andrew F. ; Adkins, Jess F. ( April 2019 , Paleoceanography and Paleoclimatology)

   High‐latitude Northern Hemisphere climate during the last glacial period was characterized by a series of abrupt climate changes, known as Dansgaard‐Oeschger events, which were recorded in Greenland ice cores as shifts in the oxygen isotopic composition of the ice. These shifts in inferred Northern Hemisphere high‐latitude temperature have been linked to changes in Atlantic meridional overturning strength. The response of ocean overturning circulation to forcing is nonlinear and a hierarchy of models have suggested that it may exist in multiple steady state configurations. Here, we use a time‐dependent coarse‐resolution isopycnal model with four density classes and two basins, linked by a Southern Ocean to explore overturning states and their stability to changes in external parameters. The model exhibits hysteresis in both the steady state stratification and overturning strength as a function of the magnitude of North Atlantic Deep Water formation. Hysteresis occurs as a result of two nonlinearities in the model—the surface buoyancy distribution in the Southern Ocean and the vertical diffusivity profile in the Atlantic and Indo‐Pacific basins. We construct a metric to assess circulation configuration in the model, motivated by observations from the Last Glacial Maximum, which show a different circulation structure from the modern. We find that circulation configuration is primarily determined by North Atlantic Deep Water density. The model results are used to suggest how ocean conditions may have influenced the pattern of Dansgaard‐Oeschger events across the last glacial cycle.

    

   more » « less
4. Transient Overturning Compensation between Atlantic and Indo-Pacific Basins

   https://doi.org/10.1175/JPO-D-20-0060.1  

   Sun, Shantong ; Thompson, Andrew F. ; Eisenman, Ian ( August 2020 , Journal of Physical Oceanography)

   null (Ed.)

   Abstract Climate models consistently project (i) a decline in the formation of North Atlantic Deep Water (NADW) and (ii) a strengthening of the Southern Hemisphere westerly winds in response to anthropogenic greenhouse gas forcing. These two processes suggest potentially conflicting tendencies of the Atlantic meridional overturning circulation (AMOC): a weakening AMOC due to changes in the North Atlantic but a strengthening AMOC due to changes in the Southern Ocean. Here we focus on the transient evolution of the global ocean overturning circulation in response to a perturbation to the NADW formation rate. We propose that the adjustment of the Indo-Pacific overturning circulation is a critical component in mediating AMOC changes. Using a hierarchy of ocean and climate models, we show that the Indo-Pacific overturning circulation provides the first response to AMOC changes through wave processes, whereas the Southern Ocean overturning circulation responds on longer (centennial to millennial) time scales that are determined by eddy diffusion processes. Changes in the Indo-Pacific overturning circulation compensate AMOC changes, which allows the Southern Ocean overturning circulation to evolve independently of the AMOC, at least over time scales up to many decades. In a warming climate, the Indo-Pacific develops an overturning circulation anomaly associated with the weakening AMOC that is characterized by a northward transport close to the surface and a southward transport in the deep ocean, which could effectively redistribute heat between the basins. Our results highlight the importance of interbasin exchange in the response of the global ocean overturning circulation to a changing climate. 

   more » « less
5. Glacial Ice Sheet Extent Effects on Modeled Tidal Mixing and the Global Overturning Circulation

   https://doi.org/10.1029/2019PA003644  

   Wilmes, S. ‐B. ; Schmittner, A. ; Green, J. A. M. ( August 2019 , Paleoceanography and Paleoclimatology)

   At present, tides supply approximately half (1 TW) of the energy necessary to sustain the global deep meridional overturning circulation (MOC) through diapycnal mixing. During the Last Glacial Maximum (LGM; 19,000–26,500 years BP), tidal dissipation in the open ocean may have strongly increased due to the 120‐ to 130‐m global mean sea level drop and changes in ocean basin shape. However, few investigations into LGM climate and ocean circulation consider LGM tidal mixing changes. Here, using an intermediate complexity climate model, we present a detailed investigation on how changes in tidal dissipation would affect the global MOC. Present‐day and LGM tidal constituents M₂, S₂, K₁, and O₁are simulated using a tide model and accounting for LGM bathymetric changes. The tide model results suggest that the LGM energy supply to the internal wave field was 1.8–3 times larger than at present and highly sensitive to Antarctic and Laurentide ice sheet extent. Including realistic LGM tide forcing in the LGM climate simulations leads to large increases in Atlantic diapycnal diffusivities and strengthens (by 14–64% at 32°S) and deepens the Atlantic MOC. Increased input of tidal energy leads to a greater drawdown of North Atlantic Deep Water and mixing with Antarctic Bottom Water altering Atlantic temperature and salinity distributions. Our results imply that changes in tidal dissipation need be accounted for in paleoclimate simulation setup as they can lead to large differences in ocean mixing, the global MOC, and presumably also ocean carbon and other biogeochemical cycles.

    

   more » « less