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Ocean deoxygenation due to anthropogenic warming represents a major threat to marine ecosystems and fisheries. Challenges remain in simulating the modern observed changes in the dissolved oxygen (O2). Here, we present an analysis of upper ocean (0-700m) deoxygenation in recent decades from a suite of the Coupled Model Intercomparison Project phase 6 (CMIP6) ocean biogeochemical simulations. The physics and biogeochemical simulations include both ocean-only (the Ocean Model Intercomparison Project Phase 1 and 2, OMIP1 and OMIP2) and coupled Earth system (CMIP6 Historical) configurations. We examine simulated changes in the O2inventory and ocean heat content (OHC) over the past 5 decades across models. The models simulate spatially divergent evolution of O2trends over the past 5 decades. The trend (multi-model mean and spread) for upper ocean global O2inventory for each of the MIP simulations over the past 5 decades is 0.03 ± 0.39×1014 [mol/decade] for OMIP1, −0.37 ± 0.15×1014[mol/decade] for OMIP2, and −1.06 ± 0.68×1014[mol/decade] for CMIP6 Historical, respectively. The trend in the upper ocean global O2inventory for the latest observations based on the World Ocean Database 2018 is −0.98×1014[mol/decade], in line with the CMIP6 Historical multi-model mean, though this recent observations-based trend estimate is weaker than previously reported trends. A comparison across ocean-only simulations from OMIP1 and OMIP2 suggests that differences in atmospheric forcing such as surface wind explain the simulated divergence across configurations in O2inventory changes. Additionally, a comparison of coupled model simulations from the CMIP6 Historical configuration indicates that differences in background mean states due to differences in spin-up duration and equilibrium states result in substantial differences in the climate change response of O2. Finally, we discuss gaps and uncertainties in both ocean biogeochemical simulations and observations and explore possible future coordinated ocean biogeochemistry simulations to fill in gaps and unravel the mechanisms controlling the O2changes.
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This content will become publicly available on April 1, 2026
Annual Cycle Changes in the Vertical Structure of Ocean Temperature: A Fingerprint of Human Influence on Climate
Abstract We investigate changes in the vertical structure of the ocean temperature annual cycle amplitude (TEMPAC) down to a depth of 300 m, providing important insights into the relative contributions of anthropogenic and natural influences. Using observations and phase 6 of the Coupled Model Intercomparison Project (CMIP6) simulations, we perform a detection and attribution analysis by applying a standard pattern-based “fingerprint” method to zonal-mean TEMPACanomalies for three major ocean basins. In all model historical simulations and observational datasets, TEMPACincreases significantly in the surface layer, except in the Southern Ocean, and weakens within the subsurface ocean. There is a decrease in TEMPACbelow the annual-mean mixed layer depth, mainly due to a deep-reaching winter warming signal. The temporal evolution of signal-to-noise (S/N) ratios in observations indicates an identifiable anthropogenic fingerprint in both surface and interior ocean annual temperature cycles. These findings are consistent across three different observational datasets, with variations in fingerprint detection time likely related to differences in dataset coverage, interpolation method, and accuracy. Analysis of CMIP6 single-forcing simulations reveals the dominant influence of greenhouse gases and anthropogenic aerosols on TEMPACchanges. Our identification of an anthropogenic TEMPACfingerprint is robust to the selection of different analysis periods. S/N ratios derived with model data only are consistently larger than ratios calculated with observational signals, primarily due to model versus observed TEMPACdifferences in the Atlantic. Human influence on the seasonality of surface and subsurface ocean temperature may have profound consequences for fisheries, marine ecosystems, and ocean chemistry. Significance StatementThe seasonal cycle is a fundamental aspect of our climate, and gaining insight into how anthropogenic forcing has impacted seasonality is of scientific, economic, and societal importance. Using observations and CMIP6 model simulations, this research applies a pattern-based detection and attribution method to ocean temperature annual cycle amplitude (TEMPAC) down to 300 m across three major ocean basins. Key findings reveal significant increases in surface layer TEMPACexcept in the Southern Ocean and a weakening of TEMPACwithin the subsurface ocean. Importantly, the analysis confirms human influence on TEMPAC. These findings underscore the profound influence of human-caused climate change on the world’s oceans and have important implications for marine ecosystems, fisheries, and ocean chemistry.
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- Award ID(s):
- 2048336
- PAR ID:
- 10577988
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 38
- Issue:
- 7
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- 1595 to 1610
- Subject(s) / Keyword(s):
- Ocean Ocean dynamics Climate Climate change Pattern detection
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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