Role of the Agulhas Current in Indian Ocean circulation and associated heat and freshwater fluxes |
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| Institution: | 1. Key Laboratory of Biofuels, Shandong Provincial Key Laboratory of Energy Genetics, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China;2. College of Marine Life Sciences, Institute of Evolution and Marine Biodiversity, Ocean University of China, Qingdao 266003, China;3. Institute of Marine Microbes and Ecospheres, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361101, China;4. Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China;5. CAS Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;6. Key Laboratory of Testing and Evaluation for Aquatic Product Safety and Quality, Ministry of Agriculture and Rural Affairs, Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Qingdao 266071, China;1. CAS Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China;2. Department of Geophysics, Tohoku University, Sendai 980-8578, Japan;3. Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China;4. CNOOC Nanhai East Petroleum Bureau, Shenzhen, Guangdong 518054, China |
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| Abstract: | A reduced estimate of Agulhas Current transport provides the motivation to examine the sensitivity of Indian Ocean circulation and meridional heat transport to the strength of the western boundary current. The new transport estimate is 70 Sv, much smaller than the previous value of 85 Sv. Consideration of three case studies for a large, medium and small Agulhas Current transport demonstrate that the divergence of heat transport over the Indian Ocean north of 32°S has a sensitivity of 0.08 PW per 10 Sv of Agulhas transport, and freshwater convergence has a sensitivity of 0.03×109 kg s?1 per 10 Sv of transport. Moreover, a smaller Agulhas Current leads to a better silica balance and a smaller meridional overturning circulation for the Indian Ocean. The mean Agulhas Current transport estimated from time-series current meter measurements is used to constrain the geostrophic transport in the western boundary region in order to re-evaluate the circulation, heat and freshwater transports across 32°S. The Indonesian Throughflow is taken to be 12 Sv at an average temperature of 18°C. The constrained circulation exhibits a vertical–meridional circulation with a net northward flow below 2000 dbar of 10.1 Sv. The heat transport divergence is estimated to be 0.66 PW, the freshwater convergence to be 0.54×109 kg s?1, and the silica convergence to be 335 kmol s?1. Meridional transports are separated into barotropic, baroclinic and horizontal components, with each component conserving mass. The barotropic component is strongly dependent on the estimated size of the Indonesian Throughflow. Surprisingly, the baroclinic component depends principally on the large-scale density distribution and is nearly invariant to the size of the overturning circulation. The horizontal heat and freshwater flux components are strongly influenced by the size of the Agulhas Current because it is warmer and saltier than the mid-ocean. The horizontal fluxes of heat and salt penetrate down to 1500 m depth, suggesting that warm and salty Red Sea Water may be involved in converting the intermediate and upper deep waters which enter the Indian Ocean from the Southern Ocean into warmer and saltier waters before they exit in the Agulhas Current. |
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