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1. A Comparison of the Variability and Changes in Global Ocean Heat Content from Multiple Objective Analysis Products During the Argo Period

   https://doi.org/10.1175/JCLI-D-20-0794.1  

   Liang, Xinfeng; Liu, Chao; Ponte, Rui M.; Chambers, Don P. (July 2021, Journal of Climate)

   null (Ed.)

   Abstract Ocean heat content (OHC) is key to estimating the energy imbalance of the earth system. Over the past two decades, an increasing number of OHC studies were conducted using oceanic objective analysis (OA) products. Here we perform an intercomparison of OHC from eight OA products with a focus on their robust features and significant differences over the Argo period (2005-2019), when the most reliable global scale oceanic measurements are available. For the global ocean, robust warming in the upper 2000 m is confirmed. The 0-300 m layer shows the highest warming rate but is heavily modulated by interannual variability, particularly the El Niño–Southern Oscillation. The 300-700 m and 700-2000 m layers, on the other hand, show unabated warming. Regionally, the Southern Ocean and mid-latitude North Atlantic show a substantial OHC increase, and the subpolar North Atlantic displays an OHC decrease. A few apparent differences in OHC among the examined OA products were identified. In particular, temporal means of a few OA products that incorporated other ocean measurements besides Argo show a global-scale cooling difference, which is likely related to the baseline climatology fields used to generate those products. Large differences also appear in the interannual variability in the Southern Ocean and in the long-term trends in the subpolar North Atlantic. These differences remind us of the possibility of product-dependent conclusions on OHC variations. Caution is therefore warranted when using merely one OA product to conduct OHC studies, particularly in regions and on timescales that display significant differences. 

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2. Refined Estimates of Global Ocean Deep and Abyssal Decadal Warming Trends

   https://doi.org/10.1029/2024GL111229  

   Johnson, Gregory C; Purkey, Sarah G (September 2024, Geophysical Research Letters)

   Abstract Deep and abyssal layer decadal temperature trends from the mid‐1980s to the mid‐2010s are mapped globally using Deep Argo and historical ship‐based Conductivity‐Temperature‐Depth (CTD) instrument data. Abyssal warming trends are widespread, with the strongest warming observed around Antarctic Bottom Water (AABW) formation regions. The warming strength follows deep western boundary currents transporting abyssal waters north and decreases with distance from Antarctica. Abyssal cooling trends are found in the North Atlantic and eastern South Atlantic, regions primarily ventilated by North Atlantic Deep Water (NADW). Deep warming trends are prominent in the Southern Ocean south of about 50°S, the Greenland‐Iceland‐Norwegian (GIN) Seas and the western subpolar North Atlantic, with cooling in the eastern subpolar North Atlantic and the subtropical and tropical western North Atlantic. Globally integrated decadal heat content trends of 21.6 (±6.5) TW in the deep and 12.9 (±1.8) TW in the abyssal layer are more certain than previous estimates. 

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3. Enhanced Upper Ocean Warming Projected by the Eddy‐Resolving Community Earth System Model

   https://doi.org/10.1029/2023GL106100  

   Xu, Gaopeng; Chang, Ping; Small, Justin; Danabasoglu, Gokhan; Yeager, Stephen; Ramachandran, Sanjiv; Zhang, Qiuying (November 2023, Geophysical Research Letters)

   Abstract Ocean warming is a key factor impacting future changes in climate. Here we investigate vertical structure changes in globally averaged ocean heat content (OHC) in high‐ (HR) and low‐resolution (LR) future climate simulations with the Community Earth System Model (CESM). Compared with observation‐based estimates, the simulated OHC anomalies in the upper 700 and 2,000 m during 1960–2020 are more realistic in CESM‐HR than ‐LR. Under RCP8.5 scenario, the net surface heat into the ocean is very similar in CESM‐HR and ‐LR. However, CESM‐HR has a larger increase in OHC in the upper 250 m compared to CESM‐LR, but a smaller increase below 250 m. This difference can be traced to differences in eddy‐induced vertical heat transport between CESM‐HR and ‐LR in the historical period. Moreover, our results suggest that with the same heat input, upper‐ocean warming is likely to be underestimated by most non‐eddy‐resolving climate models. 

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4. “Young Get Older, But Old Get Younger”: Warming‐Induced Circulation Change Explains Contrasting Pattern of Projected Pacific Deoxygenation

   https://doi.org/10.1029/2025AV002278  

   Taylor, Benjamin A; Eddebbar, Yassir A; Takano, Yohei; Talley, Lynne D; Xie, Shang‐Ping (September 2026, AGU Advances)

   Abstract Future projections of oxygen and water mass age change in the upper Pacific Ocean show a consistent pattern: strong deoxygenation and increased age in the well‐ventilated subtropical gyres, contrasted by weak deoxygenation and younger age in the poorly ventilated tropical oxygen minimum zones. We investigate the underlying dynamics of this pattern by forcing an intermediate‐complexity ocean circulation and biogeochemistry model with globally uniform and regionally‐distinct idealized warming experiments. We find that this meridional contrast in oxygen and age change arises from a slowdown of vertical velocities driven by a combination of Pacific basin and remote Southern Ocean heat forcing. Warming and stratifying the upper Pacific basin compresses the subtropical gyre circulation into a shallower, flatter layer, thereby reducing ventilation along the base of the North Pacific ventilated thermocline. Warming in the mid‐latitude Southern Ocean reduces downwelling locally, but remotely reduces upwelling of old, low‐oxygen water in the Tropical and North Pacific via a Kelvin wave‐mediated adjustment of the circulation. The combination of these two effects drives the “young get older, but old get younger” response of the Pacific thermocline. Continued monitoring of upper ocean oxygen through the Biogeochemical‐Argo float program and improved model representation of mid‐ and high‐latitude ventilation processes are critical to inform how marine ecosystems will adapt to a warmer and more stratified ocean. 

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5. Oceanographic Variability in the Rapidly Warming Coastal Mid-Atlantic (USA) over the last 200 years: Insights from Shell-Based Geochemistry and Growth Histories

   Sun, H.; Wanamaker, A.; Thatcher, D.; Whitney, N.; Jarosinski, L.; Stewart, J.; Williams, B.; LaVigne, M. (December 2022, American Geophysical Union Fall Meeting)

   Warming in recent decades in the North Atlantic Ocean has been heterogeneous, with locations along the northwestern Atlantic experiencing some of the largest and fastest warming in the last 100 years. This region is important for fisheries but has limited spatial and temporal hydrographic instrumental series extending beyond the past decades, especially along the coastal United States portion of the northwestern Atlantic, thus impacting our understanding of past climatic variability. To provide a longer temporal context for these changes, we constructed a continuous master shell growth chronology spanning the last two centuries and provided geochemical records from the Mid-Atlantic region using the long-lived marine bivalve Arctica islandica. Shells were collected on the outer shelf region off Ocean City, Maryland, in ~ 60 m water depth. This region is sensitive to large-scale North Atlantic Ocean dynamics, including the Atlantic Meridional Overturning Circulation (AMOC) and Gulf Stream eddies. Based on growth histories and shell oxygen isotopes, we provide evidence of hydrographic variability beyond the relatively short instrumental period and evaluate the likely causes for these changes. These data allow us to better characterize recent and past oceanographic changes in the Mid-Atlantic region, synthesize the new results with previously developed paleo-records in the northwestern Atlantic, and provide guidance for the management of fisheries in this region. 

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