Agulhas Meanders vs. Subtropical Front: Influence on Retroflection path over 28 years

Agulhas Current meanders, also known as Natal Pulses, are the dominant modes of variability within the Agulhas Current. These meanders significantly impact local hydrological dynamics and ecosystems. Previous observations and model outputs suggest that the influence of these meanders on Agulhas ring shedding is limited, while their effect on the Agulhas retroflection remains unclear. Models also imply that the Southern Ocean supergyre has more influence on Agulhas leakage variability than upstream drivers. Here, we developed an algorithm based on 28 years of daily-averaged Global Ocean Physics Reanalysis data with a spatial resolution of 0.083° × 0.083°. Using current velocities and sea surface height anomalies (SSHA), we tracked Agulhas Current meanders from the Agulhas Current Time-series Experiment (ACT) region to the Agulhas Return Current. Specifically, we examine the influence of the meanders on the westmost extent, i.e. minimum longitude, of the Agulhas retroflection on the basis that westwards excursions of the Agulhas current increase the probability of leakage over a longer path through this region. This is compared with the potential influence of the Subtropical Front (STF) which is the southmost front of the Southern Ocean supergyre. Our algorithm detects between 1-6 meanders per year in the ACT region, with an increasing trend over the 28-year period. These are more than reported by recent studies. More work is required to understand why we identify more meanders in the reanalysis than was found from in situ and satellite observations, however the increasing trend in the number of meanders is similar. The meanders correspond to negative SSHA propagating along the mean Agulhas Current path that is well defined down to Agulhas Bank at 22O E. The maximum lagged correlation coefficient between the minimum longitude of the Agulhas retroflection and 22O E SSHA is –0.23, which is three times higher than that between the minimum longitude of the retroflection and the subtropical front. This suggests that meanders exert a stronger influence on the zonal movement of the Agulhas retroflection compared to the STF. Additionally, the 10-day lagged correlation coefficient of SSHA between the Agulhas retroflection and the return current is 0.18, indicating that Agulhas meanders also have significant impact on the return current. These findings support the hypothesis that Agulhas meanders play an important role in shifting the position of the Agulhas retroflection. Finally, we quantify heat loss along the lagrangain trajectories in the Agulhas retroflection region to evaluate the likelihood of greater leakage for longer trajectories.