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

Abstract

The origins of the upper branch of the Atlantic meridional overturning circulation (AMOC) are traced with backward-in-time Lagrangian trajectories, quantifying the partition of volume transport between different routes of entry from the Indo-Pacific into the Atlantic. Particles are advected by the velocity field from a recent release of "Estimating the Circulation and Climate of the Ocean" (ECCOv4). This global time-variable velocity field is a dynamically consistent interpolation of over 1 billion oceanographic observations collected between 1992 and 2015. Of the 13.6 Sverdrups (1 Sv = 10 6 m 3 /s) flowing northward across 6 • S, 15% enters the Atlantic from Drake Passage, 35% enters from the straits between Asia and Australia (Indonesian Throughflow), and 49% comes from the region south of Australia (Tasman Leakage). Because of blending in the Agulhas region, water mass properties in the South Atlantic are not a good indicator of origin. Plain Language Summary Particle trajectories in the upper limb of the Atlantic meridional overturning circulation (AMOC) are traced from the equatorial Atlantic to different sections of origin in the Southern Ocean. The velocity moving the particles is an estimate combining over 1 billion observations with a global ocean model that conserves mass, momentum, temperature, salinity, and sea-ice over a regular grid. Ninety-seven percent of the particles enters the Atlantic from the tip of South Africa, as a relatively warm and salty water mass, while the remaining 3% enters from the tip of South America cold and fresh. The thermodynamical properties of waters of different origin are blended at the tip of South Africa: Water mass analysis cannot serve as a substitute of particle trajectories to track water routes. The MOC includes northward flow of intermediate and upper waters from the Southern Ocean into the Atlantic, which are eventually transformed into North Atlantic Deep Water (NADW) in the Labrador and Nordic Seas. NADW then flows southward at depth upwelling in the Southern Ocean to close the mid-depth cell (red contours in Figure 1). An equivalent mid-depth cell is absent in the Indo-Pacific sector (Cessi, 2019). Water that has upwelled from the lower, southward branch of the mid-depth cell in the Indo-Pacific sector (south of 30 • S) can return to the North Atlantic through two pathways: the warm route, that is, westward and northward around the tip of South Africa (Gordon, 1986), or the cold route, that is, eastward and northward around Drake Passage (Rintoul, 1991). The quantitative contributions of these two routes differ among estimates, but this partition is important for the transport of heat and freshwater into the Atlantic. Water that enters the South Atlantic through the warm route is warm and salty, while that entering through the cold route is fresh and cold. Many model simulations have shown that if the cold route prevails, then the MOC is robust to freshwater perturbation in the high latitudes of the North Atlantic and Arctic. Vice versa, if the exchange is mo...

Late Miocene Onset of Tasman Leakage and Southern Hemisphere Supergyre Ushers in Near‐Modern Circulation

“…The TL can be readily identified in salinity profiles around Australia (Figures 1 and S2; Table S1). TL flow is ∼4 Sv in both modeling (van Sebille et al, 2012) and float (Rosell-Fieschi et al, 2013) studies, compared to 14.3 Sv through the ITF (van Sebille et al, 2014), whereas more recent modeling suggests a somewhat greater role for TL (Rousselet et al, 2020). TL flow varies seasonally (Rosell-Fieschi et al, 2013;van Sebille et al, 2012) and largely depends on large-scale wind patterns over the Pacific (van Sebille et al, 2014).…”

“…The reality, however, is more complex (e.g., , with the Indian Ocean increasingly seen as playing a more critical role in thermohaline circulation (Schott et al, 2009). It is now clear that both the ITF and TL are important inter-basinal connections that ultimately provide return waters for the North Atlantic through the Agulhas Current (Durgadoo et al, 2017;Rousselet et al, 2020). Flow through both the Tasman and Agulhas Leakages has been increasing over the past decades (Qu et al, 2019; Figure 1).…”

IndividualDisplacements.jl: a Julia package to simulate and study particle displacements within the climate system

“…IndividualDisplacements.jl is most directly related to the MITgcm/flt Fortran package which the author also recently extended (Campin et al, 2020). This new Julia package was in fact partly motivated by a need to provide an easier and simpler alternative to using MITgcm/flt in its offline mode in order to handle global grids not readily supported by other packages (G. Forget et al, 2015;Gaël Forget, 2020;Rousselet et al, 2020).…”

“…At this stage, IndividualDisplacements.jl is considered production-ready, such that we could immediately start transferring several ongoing research collaborations that use MITgcm and ECCO (Gaël Forget & Ferreira, 2019;Rousselet et al, 2020) from Fortran to Julia. Extension to other models may start with MERRA2 / MITgcm coupled model runs (Strobach et al, 2020) and near-term applications could also include data assimilation to better simulate plastic garbage patches (Peytavin et al, 2021).…”

Centennial Changes in the Indonesian Throughflow Connected to the Atlantic Meridional Overturning Circulation: The Ocean's Transient Conveyor Belt

“…However, later observational studies suggested that deep waters, including PDW, mainly return to the surface via along‐isopycnal pathways in the Southern Ocean (e.g., Marshall & Speer, 2012). In this paradigm, the ITF is relegated to a component of the circum‐Australia circulation with a small role in the global overturning circulation (Rousselet et al, 2020; Sloyan & Rintoul, 2001). Thus, most studies of ITF transport variability have focused on local processes in the Indo‐Pacific Basin (e.g., Feng et al, 2018; Godfrey, 1996; Sprintall et al, 2019).…”