A recount of Ross Sea waters

Oceanographic observations within the Ross Sea have grown dramatically in recent years, both in number and quality. This has prompted a parallel recount of the circulation and structure of all water masses in the southwestern continental margins of the Pacific Ocean. A high-resolution set of horizontal property distributions was combined into a new climatology, which in turn is the basis of a fine volumetric θ−S census of all Ross Sea water masses. Inshore of the shelf break (700 m isobath) the Ross Sea volume (25×104 km3) partitioning into layers of neutral density (γn) is: 25% in the top layer (γn<28.00 kg m−3) of Antarctic Surface Water (AASW), 22% in the middle layer (28.00 kg m−3<γn<28.27 kg m−3) of oceanic thermocline water (12%) and Modified Circumpolar Deep Water (MCDW; 10%), and 53% in the bottom layer (γn>28.27 kg m−3) of Shelf Water (SW, θ<−1.85 °C; 31%) and its modified form (MSW, θ>−1.85 °C; 22%), precursor of the Antarctic Bottom Water (AABW) offshore. AASW flows westward along the outer shelf and southward along the eastern coast past Cape Colbeck. Oceanic thermocline waters (28.00 kg m−3<γn<28.10 kg m−3) cross the shelf break west of 170°W and follow southward paths along banks, shoaling progressively to near the sea surface of the central and western inner shelves. Winter buoyancy loss converts AASW and oceanic thermocline water into denser types of MCDW and SW, which move cyclonically toward the sills of major troughs. The census shows a continuous mode of relatively dense MCDW (28.10 kg m−3<γn<28.27 kg m−3) directly linked to the Circumpolar Deep Water (CDW) offshore, effectively ventilating and freshening the deep ocean at levels below the salinity maximum and above AABW. MCDW outflows from the Ross Sea shelf are traced to the Antarctic Circumpolar Current: near 155°E, into the Australian-Antarctic Basin, along the northern flank of the Southwest Pacific Ridge, and to near 135°W, past the eastern end of the Ross Gyre. Two sources of salty SW are inferred near the coastal polynyas in the Terra Nova Bay and western Ross Ice Shelf areas. The latter contributes to a major outflow of Ice SW that reaches the shelf break along 180°. Vertical mixing of MCDW and SW produce MSW over the shelf, continuing over the sills as new AABW. Export of low salinity AABW (S<34.70) is common to all shelves showing SW (west of 170°W), but salty AABW only outflows the Joides and Drygalski troughs.