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 0.2 m yr -1 , 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.
Plain Language SummaryThe Kerguelen region in the Southern Indian Ocean is a hotspot for marine life, partly because phytoplankton can grow there when iron is supplied to the upper ocean. The depth of the ocean mixed layer controls how strongly the surface ocean is connected to the waters below, and therefore affects light, nutrient supply, and biological productivity. In this study, we examine whether the mixed layer around the Kerguelen Plateau has changed over the last 17 years, a period during which many new observations became available from seals carrying oceanographic sensors as well as from Argo floats. Because the observations are sparse in space and time, we use a statistical reconstruction method to estimate monthly mixedlayer depth across the region. We identify two main regimes: deeper mixed layers over and west of the plateau, and shallower mixed layers around it. The regional reconstruction does not show a robust long-term change. Most importantly, even in this comparatively well-sampled Southern Ocean sector, the apparent long-term tendencies depend strongly on where and when observations were collected. The older KERFIX station record provides additional local context, but it is too short and localized to establish a regional trend.
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BibTeX
@article{doriot2026evaluating,
title = {{Evaluating Recent Trends in Mixed Layer Depth Around the Kerguelen Plateau}},
author = {Vincent Doriot and Fabien Roquet and David Nerini and Romain Caneill and Baptiste Picard and Christophe Guinet},
journal = {Journal of Geophysical Research. Oceans},
year = {2026},
month = oct,
doi = {10.1029/2026jc024633},
url = {https://hal.science/hal-05844190v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Evaluating Recent Trends in Mixed Layer Depth Around the Kerguelen Plateau
AU - Vincent Doriot
AU - Fabien Roquet
AU - David Nerini
AU - Romain Caneill
AU - Baptiste Picard
AU - Christophe Guinet
PY - 2026
DA - 2026/10/29
JO - Journal of Geophysical Research. Oceans
DO - 10.1029/2026jc024633
UR - https://hal.science/hal-05844190v1
AB - 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 0.2 m yr -1 , 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. Plain Language SummaryThe Kerguelen region in the Southern Indian Ocean is a hotspot for marine life, partly because phytoplankton can grow there when iron is supplied to the upper ocean. The depth of the ocean mixed layer controls how strongly the surface ocean is connected to the waters below, and therefore affects light, nutrient supply, and biological productivity. In this study, we examine whether the mixed layer around the Kerguelen Plateau has changed over the last 17 years, a period during which many new observations became available from seals carrying oceanographic sensors as well as from Argo floats. Because the observations are sparse in space and time, we use a statistical reconstruction method to estimate monthly mixedlayer depth across the region. We identify two main regimes: deeper mixed layers over and west of the plateau, and shallower mixed layers around it. The regional reconstruction does not show a robust long-term change. Most importantly, even in this comparatively well-sampled Southern Ocean sector, the apparent long-term tendencies depend strongly on where and when observations were collected. The older KERFIX station record provides additional local context, but it is too short and localized to establish a regional trend.
SN - 2169-9275
SN - 2169-9291
ER -