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A clustering-based approach to ocean model–data comparison around Antarctica
Ist Teil von
Ocean science, 2021-01, Vol.17 (1), p.131-145
Ort / Verlag
Katlenburg-Lindau: Copernicus GmbH
Erscheinungsjahr
2021
Quelle
EZB Electronic Journals Library
Beschreibungen/Notizen
The Antarctic Continental Shelf seas (ACSS) are a critical, rapidly changing
element of the Earth system. Analyses of global-scale general circulation
model (GCM) simulations, including those available through the Coupled Model
Intercomparison Project, Phase 6 (CMIP6), can help reveal the origins of
observed changes and predict the future evolution of the ACSS. However, an
evaluation of ACSS hydrography in GCMs is vital: previous CMIP ensembles
exhibit substantial mean-state biases (reflecting, for example, misplaced
water masses) with a wide inter-model spread. Because the ACSS are also a
sparely sampled region, grid-point-based model assessments are of limited
value. Our goal is to demonstrate the utility of clustering tools for
identifying hydrographic regimes that are common to different source fields
(model or data), while allowing for biases in other metrics (e.g., water mass
core properties) and shifts in region boundaries. We apply K-means
clustering to hydrographic metrics based on the stratification from one GCM
(Community Earth System Model version 2; CESM2) and one observation-based
product (World Ocean Atlas 2018; WOA), focusing on the Amundsen,
Bellingshausen and Ross seas. When applied to WOA temperature and salinity
profiles, clustering identifies “primary” and “mixed” regimes that have
physically interpretable bases. For example, meltwater-freshened coastal
currents in the Amundsen Sea and a region of high-salinity shelf water
formation in the southwestern Ross Sea emerge naturally from the algorithm.
Both regions also exhibit clearly differentiated inner- and outer-shelf
regimes. The same analysis applied to CESM2 demonstrates that, although
mean-state model biases in water mass T–S characteristics can be substantial,
using a clustering approach highlights that the relative differences between
regimes and the locations where each regime dominates are well represented
in the model. CESM2 is generally fresher and warmer than WOA and has a limited
fresh-water-enriched coastal regimes. Given the sparsity of observations of
the ACSS, this technique is a promising tool for the evaluation of a larger
model ensemble (e.g., CMIP6) on a circum-Antarctic basis.