Optimal Precursors Triggering Sudden Shifts in the Antarctic Circumpolar Current Transport Through Drake Passage

Due to the high nonlinearity, the accurate prediction of the Antarctic circumpolar current (ACC) transport is still challenging. Using the eddy-permitting regional ocean modeling system and the conditional nonlinear optimal perturbation approach, the sudden shift in the ACC transport through Drake Passage (DP) is investigated by exploring its optimal precursor (OPR). Here, the sudden shift in the ACC transport is defined as a fluctuation exceeding double STD (similar to 16 Sv; 1 Sv = 10(6) m(3) s(-1)) within 30 days. The results indicate that the OPRs for all three cases exhibit specific structures in the middle DP (58 degrees-62 degrees S, 72 degrees-64 degrees W) at the depth of 1,000-3,000 m, implying that the ACC transport is most sensitive to the initial perturbations there. The OPRs' evolutions show similar features: the OPR for each case triggers an eddy-like dipole perturbation with a northern cyclone and a southern anticyclone, leading to a sudden reduction in the ACC transport of similar to 40 Sv. It is verified that the density components in the OPRs determine such evolution processes. Furthermore, baroclinic instability is diagnosed to play a dominant role in the development of OPRs by analyzing the perturbed kinetic energy budget. This study suggests that the deep-layer density perturbations in the Southern Ocean should be carefully monitored when considering the short-range prediction of the ACC transport.

Automated summary

The accurate prediction of the Antarctic circumpolar current (ACC) transport remains challenging due to its high nonlinearity. This study investigates the sudden shifts in ACC transport through Drake Passage (DP) and finds that the optimal precursor (OPR) at specific depths and structures in the middle DP plays a crucial role in triggering sudden reductions in transport. Baroclinic instability is identified as a dominant factor in the development of OPRs, suggesting that careful monitoring of deep-layer density perturbations in the Southern Ocean is important for short-range ACC transport prediction.