The role of polar gyres in the Southern Ocean overturning circulation

Abstract:

The polar gyres of the Southern Ocean form an important dynamical bridge between the Antarctic Circumpolar Current (ACC) and the major sites of bottom water formation around Antarctica. Among their many functions, these gyres advect upwelled Circumpolar Deep Water (CDW) toward the coastline and create modified water masses that serve as key end-members in the formation of Antarctic Bottom Water (AABW). Despite their uniquely important role, these gyres are rarely acknowledged in conceptual models of the Southern Ocean, which tend to focus on the overturning that occurs across the ACC. Here, we challenge this paradigm by presenting idealized simulations of the Southern Ocean that isolate the effect of a polar gyre on the residual-mean overturning. We find that the formation of a polar gyre coincides with a northward shift in the surface outcrop positions of mid-depth isopycnals. This northward shift is due to the steepening of isopycnals over the zonal ridge that exists along the northern periphery of the gyre. The deeper isopycnals that do not outcrop above the zonal ridge continue across the gyre, with relatively shallow slopes, before eventually outcropping near the Antarctic continental shelf. Thus, by preferentially steering denser isopycnals to the Antarctic coastline, polar gyres restrict the abyssal overturning to the higher density classes of the circumpolar flow. More generally, these results suggest polar gyre dynamics play a critical role in the bifurcation of CDW into AABW and the less dense Antarctic Intermediate Water.