Importance of mesoscale eddies and mean circulation in ventilation of the Southern Ocean

Abstract

This study examines the relative importance of the mean advection and mesoscale currents in the property exchange between the Southern Ocean mixed layer and downstream in the upper 2000 m; this exchange is referred to as ventilation. A new, highly efficient off‐line tracer model employed here uses precalculated velocities to advect dynamically passive tracers. Two idealized tracers are considered: the Boundary Impulse Response (BIR) tracer, which helps to determine the ventilation pathways and time scales, and the Transient Surface Tracer (TST), which is relevant to transient atmospheric tracers. The importance of eddies is isolated by contrasting the control simulation with a simulation without mesoscale currents. The analysis reveals complex three‐dimensional ventilation pathways, controlled by the interplay between the mean advection and eddy‐induced spreading. The mean currents carry the tracers eastward within ACC and contribute to the formation of the Antarctic Intermediate Water (AAIW) in the South Pacific and South Atlantic. The main effect of eddies is to disperse tracers away from the mean pathways, and this dispersion acts to retain the BIR tracer in the Atlantic and Indian sectors and reduce the upstream influence of these regions on the South Pacific. In addition, the eddy‐induced along‐isopycnal spreading within ACC increases the ventilated depth and the inventory of TST. The results can be used to interpret distribution of tracers in the ocean in numerical simulations and observations.

Complexity of Mesoscale Eddy Diffusivity in the Ocean

“…For example, the QG analysis in this study employs the low‐pass spatial filtering <…> intended to remove scales shorter than 112.5 km (Rossby deformation radius is 40 km), while the GCM analysis uses a square filter width of approximately 2° longitude. Large‐scale GCM velocities are also averaged over 5 years for the most efficient separation of the large‐scale circulation (Kamenkovich et al., 2017).…”

Complexity of Mesoscale Eddy Diffusivity in the Ocean

“…The second model is a comprehensive, general circulation model (GCM) of the entire Atlantic, used in the "offline" regime, which means that tracers are simulated using previously computed daily physical fields, thus, making the model computationally very efficient (Kamenkovich et al, 2017). The physical variables used in offline models are calculated in a separate "online" simulation with the hybrid coordinate ocean model (HYCOM) (Bleck, 2002;Chassignet et al, 2003), which uses isopycnal coordinates in the open ocean and below the mixed layer.…”

“…For example, the QG analysis in this study employs the low-pass spatial filtering <…> intended to remove scales shorter than 112.5 km (Rossby deformation radius is 40 km), while the GCM analysis uses a square filter width of approximately 2° longitude. Large-scale GCM velocities are also averaged over 5 years for the most efficient separation of the large-scale circulation (Kamenkovich et al, 2017).…”

Characterizing the chaotic nature of ocean ventilation

“…Several studies indicate that the turbulent nature of ocean circulation is of central importance to thermocline ventilation [ Robbins et al ., ; Mecking et al ., ; Booth and Kamenkovich , ; Koch‐Larrouy et al ., ; Abernathey and Ferreira , ; Kamenkovich et al ., ]. Quantifying its impact, however, remains challenging due to the difficulty in separating out the effects of different spatial and temporal scales [ Booth and Kamenkovich , ].…”

“…Quantifying its impact, however, remains challenging due to the difficulty in separating out the effects of different spatial and temporal scales [ Booth and Kamenkovich , ]. Consequently, analyses are often largely qualitative by necessity [e.g., Mecking et al ., ; Kamenkovich et al ., ]. Notable exceptions to this are attempts to quantify, through a variety of means, a nondimensional Peclet number, which characterizes the relative importance of advection and mixing for tracer transport [e.g., Rhines and Schopp , ; Waugh et al ., ; Shao et al ., ].…”