Routes of the Upper Branch of the Atlantic Meridional Overturning Circulation according to an Ocean State Estimate

A deep Tasman outflow of Pacific waters during the last glacial period

The interoceanic exchange of water masses is modulated by flow through key oceanic choke points in the Drake Passage, the Indonesian Seas, south of Africa, and south of Tasmania. Here, we use the neodymium isotope signature (ε_Nd) of cold-water coral skeletons from intermediate depths (1460‒1689 m) to trace circulation changes south of Tasmania during the last glacial period. The key feature of our dataset is a long-term trend towards radiogenic ε_Nd values of ~−4.6 during the Last Glacial Maximum and Heinrich Stadial 1, which are clearly distinct from contemporaneous Southern Ocean ε_Nd of ~−7. When combined with previously published radiocarbon data from the same corals, our results indicate that a unique radiogenic and young water mass was present during this time. This scenario can be explained by a more vigorous Pacific overturning circulation that supported a deeper outflow of Pacific waters, including North Pacific Intermediate Water, through the Tasman Sea.

Using cold-water corals, this work identifies a deep outflow of Pacific waters via the Tasman Sea during the last ice age, thus highlighting the role of this area for the interoceanic exchange of water masses on climatic time scales.

Coupling of the mid-depth and abyssal components of the global overturning circulation according to a state estimate

Using velocities from a state estimate, Lagrangian analysis maps the global routes of North Atlantic Deep Water (NADW) exiting the Atlantic and reentering the upper branch of the Atlantic Meridional Overturning Circulation (AMOC). Virtual particle trajectories followed for 8100 years highlight an upper route (32%) and a lower route (68%). The latter samples σ₂ > 37.07 and is further divided into subpolar (20%) and abyssal cells (48%). Particles in the abyssal cell detour into the abyssal North Pacific before upwelling in the Southern Ocean. NADW preferentially upwells north of 33°S (67%). Total diapycnal transformations are largest in the lower route but of comparable magnitudes in the upper route, challenging its previous characterization as "adiabatic." Typical transit times are 300, 700, and 3600 years for the upper route, subpolar, and abyssal cells, respectively. The AMOC imports salinity into the Atlantic, indicating its potential instability to high-latitude freshwater perturbations.