Warming trends of the Agulhas Current from 1982 to 2019

Abstract

The Agulhas Current system has been reported to be warming significantly since the 1980s, with warming rates as high as 0.7 oC/decade for the highest warming regions, such as the Agulhas Retroflection and the Agulhas Return Current. Due to the different temporal scale and spatial resolution of datasets used to investigate the warming trends of the Agulhas Current system in previous studies, different warming rates have been reported. The poleward shift and intensification of the subtropical Western Boundary Currents (WBCs), including the Agulhas Current, have been linked to their accelerated warming. While prior research has demonstrated distinct and significant warming trends in the Agulhas Current region, more recent datasets with improved spatial resolution and longer time scales have been made available and more easily accessible. As a result, the general purpose of this study was to update earlier studies with more recent Sea Surface Temperature (SST) timeseries and compare the latest trends with previously reported trends. The main aims of this study were fourfold: (1) to investigate the warming trends in the Agulhas Current and determine the strongest warming regions using the National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation SST (OISST, v.2) product with a higher spatial resolution (1/4o) and longer temporal coverage (1982–2019) compared to datasets used in previous studies. (2) to calculate the linear trends for the satellite-derived geostrophic currents to determine if the Agulhas Current has intensified from 1993 to 2019. (3) To assess the relationship between SST and the geostrophic current speed by calculating Pearson’s correlation. (4) Finally, to assess the influence of the Southern Annular Mode (SAM) and El Niño-Southern Oscillation (ENSO) on SST and geostrophic current speed anomalies over the different regions in the Agulhas Current Results from the linear least squares regression revealed significant (p< 0.05) linear warming trends that were consistent with other studies; however, the maximum warming rate of up to 0.57 oC/decade was slightly lower than anticipated warming rates that were reported previously. The present study also found that the Agulhas Retroflection and the Agulhas Return Current are still the strongest warming regions in the Agulhas Current system, consistent with previous research findings. In terms of the overall geostrophic current flow, no significant trend of strengthening or weakening was observed between 1993 and 2019, though localised trends were noted both annually and seasonally. However, based on the overall results, while the correlation is statistically significant, SST may not be directly affecting geostrophic current, and other variables may also play important roles in influencing geostrophic current anomalies. Furthermore, SST was significantly correlated to the geostrophic current speed over the Agulhas Current. The correlation was moderate annually and strong seasonally; a significant and positive moderate annual Pearson’s correlation coefficient of around 0.63 was found in most regions. Seasonally, the correlation between SST and geostrophic current anomalies was greater than 0.8 in some regions. The relationship between climate indices (SAM and ONI), SST, and geostrophic anomalies demonstrated a statistically significant influence of the climate indices on environmental parameters in the Agulhas Current, especially SST. However, while statistically significant in some cases, these correlations are negligible, implying that the influence is weak and other local or mesoscale factors may also be at work.