A Deep Eastern Boundary Current Carrying Indian Deep Water South of Australia

Abstract

In the Southern Hemisphere, the ocean's deep waters are predominantly transported from low to high latitudes via boundary currents. In addition to the Deep Western Boundary Currents, pathways along the eastern boundaries of the southern Atlantic, Indian, and Pacific transport deep water poleward into the Southern Ocean where these waters upwell to the sea surface. These deep eastern boundary currents and their physical drivers are not well characterized, particularly those carrying carbon and nutrient‐rich deep waters from the Indian and Pacific basins. Here we describe the poleward deep eastern boundary current that carries Indian Deep Water along the southern boundary of Australia to the Southern Ocean using a combination of hydrographic observations and Lagrangian experiments in an eddy‐permitting ocean state estimate. We find strong evidence for a deep boundary current carrying the low‐oxygen, carbon‐rich signature of Indian Deep Water extending between 1,500 and 3,000 m along the Australian continental slope, from 30°S to the Antarctic Circumpolar Current southwest of Tasmania. From the Lagrangian particles it is estimated that this pathway transports approximately 5.8 ± 1.3 Sv southward from 30°S to the northern boundary of the Antarctic Circumpolar Current. The volume transport of this pathway is highly variable and is closely correlated with the overlying westward volume transport of the Flinders Current.

The Deep Ocean's Carbon Exhaust

“…The leakage of old, pre-industrial CO 2 from the deepest ocean layers is unique to the high-latitude Southern Ocean, because this is the only region where deep waters ascend to the surface along rising, steep isopycnals (Morrison et al, 2015;Speer et al, 2000;Talley, 2013;Toggweiler & Samuels, 1995). Deep waters exit the Indian, Pacific and Atlantic basins in both western and eastern boundary currents, and spiral southeastward and upward until reaching the base of the mixed layer in the southern Antarctic Circumpolar Current (ACC; Tamsitt et al, 2017Tamsitt et al, , 2019. This upwelling process is thought to be the major return pathway for remineralized carbon from the ocean's interior to the surface (Kwon et al, 2009;Sarmiento et al, 1988;Skinner et al, 2010).…”

Evolution of the Oceanic 13C Suess Effect in the Southeastern Indian Ocean Between 1994 and 2018

“…This corresponds with the core of Upper Circumpolar Deep Water (UCDW), characterized by isoneutral contours ϒ N = 27.8–28.04 kg/m 3 and an oxygen minimum (O 2 = <180 umol/kg −1 ; Talley, 2013). This O 2 minimum is imparted by return flow of Indian Deep Water (IDW), which flows southward at depths of 2,000–2,500 m along the eastern flank of the Indian Ocean (Tamsitt et al., 2019). IDW is rich in remineralized carbon and correspondingly less enriched in 13 C (Talley et al., 2013), leading to the δ 13 C DIC minimum we observed in the east of our study region, particularly within the Perth Basin.…”

Internal Wave Imprints on Temperature Fluctuations as Revealed by Rapid‐Sampling Deep Profiling Floats

“…The original purpose of maintaining over 1,000 Deep Argo floats is to close the heat, freshwater, and sea-level budgets, characterize decadal variability in deep ocean water masses and estimate the mean and decadal variability in deep ocean circulation including the meridional overturning circulations (Johnson et al, 2015). Because of the nearly 250 Deep Argo floats deployed in deep areas of the South Pacific, South Indian, and Atlantic Ocean, we are currently able to estimate the warming rate of deep water in some basins (Johnson et al, 2019(Johnson et al, , 2020 and reexamine the deep currents in some basins (Racapé et al, 2019;Tamsitt et al, 2019;Zilberman et al, 2020). Likewise, it is possible to have assessments on internal (Johnson et al, 2019).…”