[PAOCQ] Xiaoting Yang (WHOI)

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

The Meridional Overturning Circulation (MOC) carries a significant amount of mass, heat, carbon, and nutrients around the global oceans, therefore having an important and complicated influence on the global climate and ecosystem. These transports are accomplished by two overturning cells. The upper cell (the Atlantic Meridional Overturning Circulation, AMOC) is composed of northward near-surface flows and southward return flow of deep water at mid-depth, closed by upwelling in the Southern Ocean. The abyssal cell goes in the opposite direction, carrying northward a dense water mass originating from around Antarctica near the ocean bottom (Antarctic Bottom Watter, AABW). There has been debate on the composition of the AABW to origins from near Antarctica versus from the upper cell, since the two cells share some overlapping isopycnals and are therefore interconnected. In this work, we studied this composition from the Lagrangian perspective, using numerical parcel releasing and tracking techniques. 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.