The circulation of the Southern Ocean and the adjacent ocean basins determined by inverse methods
Creators: name: Sloyan, Bernadette M
Publisher: name: University Of Tasmania
Year: 2023

description: Inverse methods which conserve mass, heat and salt are applied to high resolution hydrographic data in the Southern Ocean and adjacent ocean basins. Dianeutral fluxes are incorporated into the inverse model by including a separate dianeutral flux unknown for each property. Further model development includes the addition of air-sea forcing (wind stress, air-sea heat flux and freshwater flux). The inverse model is then used to test different circulation hypotheses, including the estimated size of the heat loss over the Weddell Sea and the strength of the Malvinas Current. The accuracy of inverse methods are tested using the Fine Resolution Antarctic Model. This shows that the lateral and dianeutral fluxes are well determined by inverse methods and that the \true\" effective diffusion across neutral surfaces in the inverse model can be much larger and in some cases of opposite sign to the explicit diffusion in the numerical model when layers outcrop or undercrop. The latter results has implications for how diffusivity estimates obtained from inverse models should be interpreted. In the initial inverse model the Antarctic Circumpolar Current transport varies from 133¬¨¬±2x106m3s-1 at Drake Passage to 148¬¨¬±4.5 south of Australia. There is a -15¬¨¬±8x106m3s-1 southward transport through the Indian Ocean and a similar sized northward transport into the Pacific Ocean at 32¬¨‚àûS. The addition of air-sea fluxes results in a 5x106m3s-1 decrease in the southward eastward and northward transport in the Indian ocean south of Australia and across 32¬¨‚àûS in the Pacific. The imposition of a large northward Malvinas Current (60x 106m3 s-1 ) adjacent to the South American coast maintains a similar eastward transport at Drake Passage south of Africa and south of Australia. The southward and northward transport through the Indian Ocean and across 32¬¨‚àûS in the Pacific is decreased further to -8.3¬¨¬±5.lx106m3s-1 and 8.8¬¨¬±14.2x 106m3 s-1 respectively. By including air-sea forcing and interior dianeutral fluxes the water mass production and modification that occurs in the regions defined by the hydrographic sections can be quantified. In the Indian Ocean sector the 1 Agulhas Current transports warm Indonesian throughflow thermocline water into the Southern Ocean. The thermocline water (10x106m3 s-1 ) is transferred to SAMW by air-sea (cooling and evaporation) processes and interior dianeutral fluxes. As the SAMW moves eastward with the ACC Antarctic surfaces water (16x106m3 s-1 ) is converted to SAMW across the Polar Front Zone resulting in progressive cooling and freshening of SAMW. 10x106m3s-1 of the locally produced SAMW in the Southern Ocean Indian exits the region northward across 32¬¨‚àûS ventilating the subtropical gyre while the remaining 15x106m3s-1 moves eastward with the ACC into the Southern Ocean Pacific sector. In the Southern Ocean Pacific sector 11x106m3 s-1 of SAMW is transferred into thermocline water by air-sea fluxes and exported northward in the Pacific Ocean across 32¬¨‚àûS. This circulation path of thermocline water between the Indian and Pacific Oceans describes the Indonesian throughflow as a circum-Australia feature. North Atlantic Deep Water (20x106m3 s-1 ) moves southward across 15¬¨‚àûS in the Atlantic and is the major source of Lower Circumpolar Deep Water. Lower Circumpolar Deep Water salinity and oxygen maxima are eroded during the eastward progression of the Antarctic Circumpolar Current. Strong deep overturning circulation are found in the Indian (26x106m3 s-1 ) and Pacific (30x106m3s-1 ) Oceans. The northward transport of Antarctic Bottom Water and Circumpolar Deep Water into the Indian and Pacific Oceans is essentially balanced by the return of slightly less dense low oxygen Indian and Pacific Deep water. The southward inflow of Indian and Pacific deep water to the Southern Ocean results in the formation of Upper Circumpolar Deep Water. The eastward flow of Antarctic Bottom Water by the Antarctic Circumpolar Current into the Southern Ocean Pacific sector is not sufficient to balance the northward export of Antarctic Bottom Water into the Pacific Ocean across 32¬¨‚àûS. This implies there are sources of Antarctic Bottom Water within the Southern Ocean Pacific sector. The circulation results in a meridional heat flux at 30¬¨‚àûS of -0.51 PW. The southward heat flux is dominated by a large southward heat flux in the Indian Ocean (-0.94¬¨¬±0.18 PW) a northward heat flux in the Atlantic (0.34¬¨¬±0.13 PW) and a small northward heat flux in the Pacific (0.09¬¨¬±0.26 PW)."