Evaluating Recent Trends in Mixed Layer Depth Around the Kerguelen Plateau

Abstract

The Kerguelen Plateau in the Southern Indian Ocean sustains intense phytoplankton blooms through local iron enrichment. Changes in mixed‐layer depth (MLD) can therefore affect regional ecosystem functioning and air–sea carbon exchange. Using 17 years (2007–2023) of observations from animal‐borne sensors and Argo floats, we reconstruct monthly MLD from sparse in situ data using principal components analysis through conditional expectation (PACE). The method is evaluated using GLORYS and a masked version reproducing the observational sampling pattern. The reconstruction captures the dominant seasonal cycle and leading spatial pattern of MLD variability. The masked‐reanalysis experiment shows that broad interannual fluctuations are more stable than fitted trends, which are particularly sampling‐sensitive. We examine these trends to determine whether the observational record supports sustained MLD change or instead reflects sampling‐induced variability. We find a deeper‐MLD region over and west of the Kerguelen Plateau, surrounded by a shallower‐MLD region and bounded to the north by the deep mixing band. Observation‐based trend estimates for 2007–2023 indicate a weak summer deepening tendency of about , consistent with GLORYS but not statistically significant. A winter shallowing tendency is obtained from the sparse reconstructions; however, it is mostly a result of poor data sampling. In the early 1990s, bottle profiles from the KERFIX station give deeper MLD than at the nearest grid point in 2007–2023, in winter and summer, but this contrast is difficult to interpret as a long‐term trend. Overall, the results suggest that domain‐wide MLD has not changed significantly around the Kerguelen region in the last two decades.