Sources and Paths of the Lower Branch of the Abyssal Overturning Circulation

Abstract Lagrangian analysis using the velocity field of the Southern Ocean State Estimate (SOSE) at 1/6° resolution quantifies the origin of the lower branch of the abyssal circulation south of 30°S. Antarctic waters from 65°S contribute 62% of the northward bottom flow at 30°S. The remaining 38% comes from deep water recirculating from sources north of 30°S. Half of this deep water is North Atlantic Deep Water, with the remainder split between Indian and Pacific Deep Waters. Deep water densifies to bottom values following several circumpolar loops, which homogenize original properties to common characteristics of Lower Circumpolar Water. Transit times of deep water starting at 30°S, arriving as abyssal waters at 30°S, are about 150 years, twice as long as transit times of waters starting near Antarctica. Paths to 30°S originating from Antarctica avoid circumpolar loops and are steered by bottom topography.