Spatial and temporal variability of the Agulhas Retroflection: Observations from a new objective detection method

This study presents an improved method to identify the Location of the Agulhas Current's Core and Edges (LACCE), throughout the greater Agulhas system. In this study the method is applied to daily altimetric fields of absolute dynamic topography and geostrophic velocities. LACCE compares favourably with previous algorithms used to track the position of the Agulhas Current. The ability of the LACCE to correctly identify the core and edges of the Agulhas Current is also established through comparisons with along-track altimetry datasets and in situ observations of current speed and direction across both the Agulhas Current and the Agulhas Return Current. Our results show that LACCE can successfully be applied to a merged altimetry dataset to monitor changes in the Agulhas Current's position caused by mesoscale processes such as early retroflection events or transient features like Natal Pulses. To demonstrate the use of the LACCE, the Agulhas Current position outputs from the algorithm computed over a 26-year period (1993 to 2018) are then used to describe the spatial and temporal variability of the Retroflection. Our results show that the Agulhas Retroflection generally occurs in the region from 15.01-20.03 degrees E and 40.47-38.44 degrees S. The retroflection exhibits a trimodal distribution, indicating three preferential longitudes (15, 18 and 19.5 degrees E). Despite not being statistically significant, the results suggest some seasonal variation in the modal position of the Retroflection, with it frequenting western longitudes more during the austral summer, in agreement with previous studies. Early retroflections were found to occur more often during the austral spring and summer, with few taking place in winter and almost none in autumn. The observations from this study agree with previous studies that found the triggering mechanism for early retroflections (east of 22.54 degrees E) to be the interaction between large Agulhas Current meanders (Natal Pulses) with northward extensions of Agulhas Return Current meanders.

Automated summary

This study introduces an improved method (LACCE) to accurately identify the core and edges of the Agulhas Current, compared with previous algorithms. The results demonstrate the successful application of LACCE in monitoring changes in current position and reflection spatial-temporal variability. The study also confirms seasonal variations in the reflection position and identifies the triggering mechanism for early reflections.