Abstract
Abstract. The northward flow of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC) is fed by waters entering the South Atlantic from the Indian Ocean mainly via the Agulhas Current (AC) system and by waters entering from the Pacific through Drake Passage (DP), commonly referred to as the “warm” and “cold” water routes, respectively. However, there is no final consensus on the relative importance of these two routes for the upper limb's volume transport and thermohaline properties. In this study we revisited the AC and DP contributions by performing Lagrangian analyses between the two source regions and the North Brazil Current (NBC) at 6∘ S in a realistically forced high-resolution (1∕20∘) ocean model. Our results agree with the prevailing conception that the AC contribution is the major source for the upper limb transport of the AMOC in the tropical South Atlantic. However, they also suggest a non-negligible DP contribution of around 40 %, which is substantially higher than estimates from previous Lagrangian studies with coarser-resolution models but now better matches estimates from Lagrangian observations. Moreover, idealized analyses of decadal changes in the DP and AC contributions indicate that the ongoing increase in Agulhas leakage indeed may have induced an increase in the AC contribution to the upper limb of the AMOC in the tropics, while the DP contribution decreased. In terms of thermohaline properties, our study highlights the fact that the AC and DP contributions cannot be unambiguously distinguished by their temperature, as the commonly adopted terminology may imply, but rather by their salinity when entering the South Atlantic. During their transit towards the NBC the bulk of DP waters experiences a net density loss through a net warming, whereas the bulk of AC waters experiences a slight net density gain through a net increase in salinity. Notably, these density changes are nearly completely captured by Lagrangian particle trajectories that reach the surface mixed layer at least once during their transit, which amount to 66 % and 49 % for DP and AC waters, respectively. This implies that more than half of the water masses supplying the upper limb of the AMOC are actually formed within the South Atlantic and do not get their characteristic properties in the Pacific and Indian Oceans.
Highlights
Within the framework of the global overturning circulation, the South Atlantic is depicted as a conduit exporting cold, dense waters produced in the North Atlantic into the Indian and Pacific basins and enabling the return flow of warmer, lighter waters back into the North Atlantic, which in turn feed the deepwater formation (Broecker, 1991; Richardson, 2008)
Even though variability in the overall overturning strength and associated heat transport has been mostly related to variability in deepwater formation (e.g., Biastoch et al, 2008a; Yeager and Danabasoglu, 2014), the involved convection process is highly sensitive to the local stratification and is impacted by the thermohaline properties of the northward upper-layer return flow, that is, the upper limb of the Atlantic Meridional Overturning Circulation (AMOC)
Even though our results generally agree with the prevailing conception that the Agulhas Current (AC) contribution with around 50 % is the major source for the upper limb transport of the AMOC, they suggest a non-negligible Drake Passage (DP) or cold water route contribution www.ocean-sci.net/15/489/2019/
Summary
Within the framework of the global overturning circulation, the South Atlantic is depicted as a conduit exporting cold, dense waters produced in the North Atlantic into the Indian and Pacific basins and enabling the return flow of warmer, lighter waters back into the North Atlantic, which in turn feed the deepwater formation (Broecker, 1991; Richardson, 2008). It has been suggested that a net southward freshwater transport related to the northward advection of relatively high-saline waters along the AMOC’s upper limb in the South Atlantic introduces a positive feedback (e.g., Stommel, 1961; Rahmstorf, 1996; Drijfhout et al, 2011): a weakening (strengthening) of the AMOC results in reduced (enhanced) northward salt transport and corresponding freshening (salinification) of the North Atlantic, with further weakening (strengthening) of the overturning This salt–advection feedback constitutes the basis for the theory of rapid climate shifts associated with a bistability of the AMOC with either vigorous overturning (“on” state) or weak or reversed overturning (“off” state)
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