An official website of the United States government
Abstract. Agulhas Leakage transports warm and salty Indian Ocean waters into the Atlantic Ocean and as such is an important component of the global ocean circulation. These waters are part of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC), and Agulhas Leakage variability has been linked to AMOC variability. Agulhas Leakage is expected to increase under a warming climate due to a southward shift in the South Hemisphere westerlies, which could further influence the AMOC dynamics. This study uses a set of high-resolution pre-industrial control and historical and transient simulations with the Community Earth System Model (CESM) with a nominal horizontal resolution of 0.1° for the ocean and sea-ice and 0.25° for the atmosphere and land. At these resolutions, the model represents the necessary scales to investigate the Agulhas Leakage transport variability and its relation to the AMOC. The simulated Agulhas Leakage transport of 19.7 ± 3 Sv lies well within the observed range of 21.3 ± 4.7 Sv. A positive correlation between the Agulhas Current and the Agulhas Leakage is shown, meaning that an increase of the Agulhas Current transport leads to an increase in Agulhas Leakage. The Agulhas Leakage impacts the strength of the AMOC through Rossby wave dynamics that alter the cross-basin geostrophic balance with a time-lag of 2–3 years. Furthermore, the salt flux associated with the Agulhas Leakage influences AMOC dynamics through the salt-advection feedback by reducing the AMOC’s freshwater transport at 34° S. The Agulhas Leakage transport indeed increases under a warming climate due to strengthened and southward shifting winds. In contrast, the Agulhas Current transport decreases, both due to a decrease in the Indonesian Throughflow as well as the strength of the wind-driven subtropical gyre. The increase in Agulhas Leakage is accompanied by a higher salt flux into the Atlantic Ocean, which suggests a destabilisation of the AMOC by salt-advection-feedback.
more »
« less
The Role of Atlantic Basin Geometry in Meridional Overturning Circulation
Abstract 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 ≡ 10 6 m 3 s −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
- Award ID(s):
- 1850900
- PAR ID:
- 10339447
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- Volume:
- 52
- Issue:
- 3
- ISSN:
- 0022-3670
- Page Range / eLocation ID:
- 475 to 492
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The Miocene (∼23–5 Ma) is a past warm epoch when global surface temperatures varied between ∼5 and 8°C warmer than today, and CO2concentration was ∼400–800 ppm. The narrowing/closing of the tropical ocean gateways and widening of high‐latitude gateways throughout the Miocene is likely responsible for the evolution of the ocean's overturning circulation to its modern structure, though the mechanisms remain unclear. Here, we investigate early and middle Miocene ocean circulation in an opportunistic climate model intercomparison (MioMIP1), using 14 simulations with different paleogeography, CO2, and vegetation. The strength of the Southern Ocean‐driven Meridional Overturning Circulation (SOMOC) bottom cell is similar in the Miocene and Pre‐Industrial (PI) but dominates the Miocene global MOC due to weaker Northern Hemisphere overturning. The Miocene Atlantic MOC (AMOC) is weaker than PI in all the simulations (by 2–21 Sv), possibly due to its connection with an Arctic that is considerably fresher than today. Deep overturning in the North Pacific (PMOC) is present in three simulations (∼5–10 Sv), of which two have a weaker AMOC, and one has a stronger AMOC (compared to its PMOC). Surface freshwater fluxes control northern overturning such that the basin with the least freshwater gain has stronger overturning. While the orography, which impacts runoff direction (Pacific vs. Atlantic), has an inconsistent impact on northern overturning across simulations, overall, features associated with the early Miocene—such as a lower Tibetan Plateau, the Rocky Mountains, and a deeper Panama Seaway—seem to favor PMOC over AMOC.more » « less
-
The factors controlling the present-day pattern of temperature variance are poorly understood. In particular, it is unclear why the variance of wintertime near-surface temperatures on daily and synoptic time scales is roughly twice as high over North America as over Eurasia. In this study, continental geometry’s role in shaping regional wintertime temperature variance is investigated using idealized climate model simulations run with midlatitude continents of different shapes. An isolated, rectangular midlatitude continent suggests that in the absence of other geographic features, the highest temperature variance will be located in the northwest of the continent, roughly collocated with the region of largest meridional temperature gradients, and just north of the maximum near-surface wind speeds. Simulations with other geometries, mimicking key features of North America and Eurasia, investigate the impacts of continental length and width, sloping coastlines, and inland bodies of water on regional temperature variance. The largest effect comes from tapering the northwest corner of the continent, similar to Eurasia, which substantially reduces the maximum temperature variance. Narrower continents have smaller temperature variance in isolation, implying that the high variances over North America must be due to the nonlocal influence of stationary waves. Support for this hypothesis is provided by simulations with two midlatitude continents, which show how continental geometry and stationary waves can combine to shape regional temperature variance.more » « less
-
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.more » « less
-
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.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1175/JPO-D-21-0036.1
