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Understanding deep ocean state changes with an ocean state estimate and observations
Date
2025
Authors
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Publisher
University of Delaware
Abstract
The deep ocean (>2000 m), which accounts for about half of the total ocean volume, plays a crucial role in global heat uptake and carbon sequestration, yet its state changes under a warming climate remain poorly understood due to limited observational coverage in both space and time. My dissertation aims to enhance our understanding of deep-ocean state changes through three independent but interconnected studies. ☐ Firstly, while repeat hydrographic measurements suggest a global deep-ocean warming trend, a widely used ocean state estimate, ECCO Version 4 Release 4 (ECCO V4r4), shows a contrasting cooling trend in the deep Pacific and Indian Oceans. To investigate whether the sparseness of hydrographic observations leads to these discrepancies, we conducted a sampling experiment using ECCO, mimicking the spatial and temporal coverage of historical hydrographic sections. Our results show that deep OHC trends from sampled data are broadly consistent with full-resolution ECCO estimates, confirming that the existing observational framework effectively captures large-scale trends. However, uncertainties remain high in regions such as the Northwest Atlantic and the Southern Ocean, where filled with newly formed deep water masses and data coverage is particularly sparse. These findings suggest that the discrepancies between ECCO and observationally derived deep-OHC trends are unlikely due to sampling limitations. ☐ Another approach to indirectly estimating deep-ocean steric changes is the residual method, which derives deep steric sea level as the difference between total sea level (satellite altimetry), ocean mass (satellite gravimetry), and upper-ocean steric height (Argo). While previous studies found this approach ineffective at global and basin scales over short periods, this study extends the analysis to ~20 years to assess its detectability at regional scales, particularly in seven locations with long-term full depth or deep hydrographic records. Our results indicate that detecting deep steric sea level signals using the residual method remains challenging due to small magnitude of deep-ocean variations and large regional uncertainties. Additionally, we identify a significant imbalance in the Northeast Atlantic residual sea level budget, suggesting potential biases in satellite gravimetry ocean mass estimates. ☐ Lastly, while analyzing deep steric sea level variations to validate the residual sea level time series, we observed a consistent freshening trend at six locations in the tropical and subtropical North Atlantic. This robust signal reveals long-term freshening, but with a noticeable cessation in the recent decade. By linking the long-term time-series in these regions and the subpolar North Atlantic, we propose that the freshening reflects a multi-decadal signal advected from the subpolar North Atlantic. Based on the observed salinity patterns in subpolar time-series, we predict a reversal of deep-ocean salinity in the subtropical and tropical North Atlantic along the Deep Western Boundary Current (DWBC) within the next decade. ☐ My thesis demonstrates the complexity of deep-ocean state changes, where climate signals can penetrate into the deep ocean, particularly in deep-water formation regions and spread along pathways such as the DWBC. Our ability to observe and understand deep-ocean variability remains limited, emphasizing the importance of expanded and sustained observations. In addition, long-term, systematic deep-ocean time-series are essential for unraveling climate variability across different timescales and improving our understanding of deep-ocean responses to global climate change.
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Keywords
Deep ocean state changes, Ocean state estimate, Hydrographic observations, Satellite gravimetry, Deep Western Boundary Current