Study of the Antarctic Circumpolar Current via the Shallow Water Large Scale Modelling

This paper proposes a modelling of the Antarctic Circumpolar Current (ACC) by means of a two-point boundary value problem. As the major means of exchange of water between the great ocean basins (Atlantic, Pacific and Indian), the ACC plays a highly important role in the global climate. Despite its importance, it remains one of the most poorly understood components of global ocean circulation. We present some recent results on the existence and uniqueness of solutions of a two-point nonlinear boundary value problem that arises in the modeling of the flow of the (ACC) (see discussions in [4-9]).

Bathymetric control of subpolar gyres and the overturning circulation in the Southern Ocean

The subpolar gyres of the Southern Ocean form an important dynamical link between the Antarctic Circumpolar Current (ACC) and the coastline of Antarctica. Despite their key involvement in the production and export of bottom water and the poleward transport of oceanic heat, these gyres are rarely acknowledged in conceptual models of the Southern Ocean circulation, which tend to focus on the zonally-averaged overturning across the ACC. To isolate the effect of these gyres on the regional circulation, we carried out a set of numerical simulations with idealized representations of the Weddell Sea sector in the Southern Ocean. A key result is that the zonally-oriented submarine ridge along the northern periphery of the subpolar gyre plays a fundamental role in setting the stratification and circulation across the entire region. In addition to sharpening and strengthening the horizontal circulation of the gyre, the zonal ridge establishes a strong meridional density front that separates the weakly stratified subpolar gyre from the more stratified circumpolar flow. Critically, the formation of this front shifts the latitudinal outcrop position of certain deep isopycnals such that they experience different buoyancy forcing at the surface. Additionally, the zonal ridge modifies the mechanisms by which heat is transported poleward by the ocean, favoring heat transport by transient eddies while suppressing that by stationary eddies. This study highlights the need to characterize how bathymetry at the subpolar gyre-ACC boundary may constrain the transient response of the regional circulation to changes in surface forcing.

Cenozoic evolution of the Campbell Plateau, Subantarctic New Zealand: Insights from sub-bottom profile data

<p>The Campbell Plateau represents ~30% of the submerged continent of Zealandia and represents part of the Gondwana super-continent that began to break-up ~98Ma. The focus of this MSc thesis is to use sub-bottom, profile data collected in 2017 and 2018 from Campbell Plateau to improve our understanding of the Cenozoic evolution of the region. The sub-bottom profiles show a rugged basement overlain by a variety of sedimentary sequences and subsurface features such as volcanoes, onlap, and downlap surfaces as well as multiple unconformities that can be traced throughout the Cenozoic (65Ma). The sub-bottom profiles are compared to 2 drill cores; Ocean Drilling Program (ODP) site 1120 on the eastern side of the plateau and Deep Sea Drilling Program (DSDP) site 277 in the south. These drill cores indicate that the lithology from the Cretaceous onwards is predominantly biogenic calcareous sandstone and mudstone, which changes to nannofossil-rich oozes in the Miocene and foraminiferal oozes and nannofossil oozes dated early to late Pleistocene. The northern plateau appears to be relatively quiescent with thin, relatively uniform strata, only influenced by small reverse faults. Sedimentary deposits such as wedges and contourites are also evident in the central and north-western part of the study area. The southern plateau appears to be have been highly dynamic with onlap/downlap surfaces, interpreted as current scours, and erosional surfaces. There is a plateau-wide unconformity during the Pliocene, as derived from the nannofossils of the ODP1120 drill core, which appears to have been a large-scale erosional event. The Southern Ocean circulation, dominated by Antarctic Circumpolar Current, the Subtropical Front, and local wind-driven currents, are the main drivers of these lithological changes and plateau-wide sedimentological structures. Previous interpretations of the sub-surface structure of the plateau are seen to be invalid in relation to this study, with the sub-surface seen to be relatively undeformed with only minor reverse faulting present. Areas of possible uplifted basement seen near Campbell Island also indicate that the Campbell Plateau has been through substantial erosion and deformation since its’ separation from Gondwana ~98Ma and movement to its modern-day position.</p>

Cenozoic evolution of the Campbell Plateau, Subantarctic New Zealand: Insights from sub-bottom profile data

<p>The Campbell Plateau represents ~30% of the submerged continent of Zealandia and represents part of the Gondwana super-continent that began to break-up ~98Ma. The focus of this MSc thesis is to use sub-bottom, profile data collected in 2017 and 2018 from Campbell Plateau to improve our understanding of the Cenozoic evolution of the region. The sub-bottom profiles show a rugged basement overlain by a variety of sedimentary sequences and subsurface features such as volcanoes, onlap, and downlap surfaces as well as multiple unconformities that can be traced throughout the Cenozoic (65Ma). The sub-bottom profiles are compared to 2 drill cores; Ocean Drilling Program (ODP) site 1120 on the eastern side of the plateau and Deep Sea Drilling Program (DSDP) site 277 in the south. These drill cores indicate that the lithology from the Cretaceous onwards is predominantly biogenic calcareous sandstone and mudstone, which changes to nannofossil-rich oozes in the Miocene and foraminiferal oozes and nannofossil oozes dated early to late Pleistocene. The northern plateau appears to be relatively quiescent with thin, relatively uniform strata, only influenced by small reverse faults. Sedimentary deposits such as wedges and contourites are also evident in the central and north-western part of the study area. The southern plateau appears to be have been highly dynamic with onlap/downlap surfaces, interpreted as current scours, and erosional surfaces. There is a plateau-wide unconformity during the Pliocene, as derived from the nannofossils of the ODP1120 drill core, which appears to have been a large-scale erosional event. The Southern Ocean circulation, dominated by Antarctic Circumpolar Current, the Subtropical Front, and local wind-driven currents, are the main drivers of these lithological changes and plateau-wide sedimentological structures. Previous interpretations of the sub-surface structure of the plateau are seen to be invalid in relation to this study, with the sub-surface seen to be relatively undeformed with only minor reverse faulting present. Areas of possible uplifted basement seen near Campbell Island also indicate that the Campbell Plateau has been through substantial erosion and deformation since its’ separation from Gondwana ~98Ma and movement to its modern-day position.</p>

Extremely Southward Migration of the Antarctic Circumpolar Current During the Last Interglacial

The Antarctic Circumpolar Current (ACC) acts as a critical component to regulate the global thermohaline circulation and climate. However, active debate remains about the relative strength of ACC during current/past warm periods and underlying driving mechanisms. Here, we present sortable silt mean grain size records from the Scotia Sea to infer the ACC strength over the past 160 ka. The 22-ka cycles of sortable silt mean grain size suggest that the precession-driven contraction/expansion of Subtropical Jet dominates the migration of ACC fronts, and thus ACC speed and potential Atlantic Meridional Overturning Circulation stability. We find that the bottom flow speed during MIS 5e was over three times faster than the Holocene, with no apparent difference in ACC speed between the Holocene and the Last Glacial Maximum. We suggest that a southward shift of oceanic fronts of ~5° could cause the additional speed-up of ACC during MIS 5e. This could induce warmer water flowing in the ACC to approach and melt the Antarctica continental ice shelves, with corresponding effects on global sea level and the global climate.

Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling

Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. Here we use high-resolution ocean simulations with detailed paleobathymetry to demonstrate that tectonics did play a fundamental role in reorganising Southern Ocean circulation patterns and heat transport, consistent with available proxy data. When at least one gateway (Tasmanian or Drake) is shallow (300 m), gyres transport warm waters towards Antarctica. When the second gateway subsides below 300 m, these gyres weaken and cause a dramatic cooling (average of 2–4 °C, up to 5 °C) of Antarctic surface waters whilst the ACC remains weak. Our results demonstrate that tectonic changes are crucial for Southern Ocean climate change and should be carefully considered in constraining long-term climate sensitivity to CO2.