Ocean Processes Research Articles

Reactivation of a Subglacial Channel Around the Grounding Zone of Roi Baudouin Ice Shelf, Antarctica

AbstractSubglacial water beneath the Antarctic Ice Sheet is often funneled via subglacial channels, which inject freshwater into ice‐shelf cavities where it interacts with ocean water. The temporal variability of this system has been poorly observed, but its importance for ice dynamics is well recognized. Airborne radar data show a subglacial channel evolving within a decade near of the grounding zone of the Roi Baudouin Ice Shelf (East Antarctica), while topographic signatures on the ice shelf indicate prior inactivity for 60 years. Combining our observations with subglacial hydrological modeling, we suggest that the interplay between episodic subglacial water pulses and ocean water intrusion drive the opening and closing of the channels. Our findings illuminate the short‐term transient nature of subglacial channel activity. This impacts ice‐shelf–ocean processes, which are important for constraining increasing ocean warming onto ice‐shelf basal mass balance, but pose significant challenges for subglacial hydrological modeling at the grounding zone.

A new vision of the Adriatic Dense Water future under extreme warming

Abstract. We use the Adriatic Sea and Coast (AdriSC) kilometer-scale atmosphere–ocean model to assess the impact of a far-future extreme-warming scenario on the formation, spreading, and accumulation of North Adriatic Dense Water (NAddW) over the entire basin, including the Jabuka Pit accumulation site, and Adriatic Deep Water (AdDW) over the Southern Adriatic Pit (SAP). Our key findings differ from previous studies that used coarser Mediterranean climate models and did not update the thresholds for dense-water and deep-water definitions to account for the far-future background density changes caused by warmer sea surface temperatures. We show that surface buoyancy losses at NAddW generation sites, driven by evaporation, are expected to increase by 15 % under extreme warming, despite a 25 % reduction in the intensity and spatial extent of Bora winds. As a result, future NAddW formation will remain similar to present conditions. However, the volume of dense water in the Jabuka Pit will decrease due to the increased far-future stratification. Additionally, dense-water transport between the Jabuka Pit and the deepest part of the SAP will stop, as future NAddW will be lighter than the AdDW. Regarding Ionian–Adriatic exchanges, extreme warming will not affect the impact of the bimodal oscillation system on the Adriatic salinity variability, but future AdDW dynamics will be determined by density changes in the northern Ionian Sea. Our findings highlight the complexity of climate change impacts on Adriatic atmosphere–ocean processes and the importance of high-resolution models for more accurate far-future projections in the Adriatic Sea.

Bomb-produced radiocarbon dynamics and salinity-water isotopes relationship at two stations in the tropical South Pacific Ocean and implications for upper ocean processes

Bomb-produced radiocarbon dynamics and salinity-water isotopes relationship at two stations in the tropical South Pacific Ocean and implications for upper ocean processes

On the intraseasonal oceanic processes constrained by data assimilation: a case study of the Tropical Pacific

On the intraseasonal oceanic processes constrained by data assimilation: a case study of the Tropical Pacific

Marine phytoplankton impose strong selective pressures on in vitro microbiome assembly, but drift is the dominant process

Abstract Phytoplankton are known ecosystem engineers that modulate ocean community assembly processes, but the universality and extent of their microbiome control remains unclear. We used in vitro incubations and 16S rRNA amplicon sequencing to test the influence of Southern and South Pacific Oceans dominant phytoplankton on assembly processes and community successions in response to phytoplankton blooms. Presence of phytoplankton grown with reduced diversity cultures or supplemented with exogenously added microbiomes showed reduced diversity, suggesting environmental filtering. Community profiles were distinct under all culture conditions, further confirming strong selection for specific microbiomes based on phytoplankton. Analysis of core, abundant and rare organisms in each culture condition showed a conserved response where core organisms were enriched under exogenously added phytoplankton. Progression through phytoplankton growth phases selected first for rare and abundant organisms, with increased selection for core members during exponential phase and relaxing of selection during death phase, as seen throughout incubations for microbiome only controls. Surprisingly, selection process quantification identified drift as the dominant process across all conditions and growth phases, with homogenous selection and dispersal limitation accounting for the remainder. Altogether, we confirm the role phytoplankton play in community assembly using Southern Ocean derived model organisms but demonstrate that stochastic processes still predominately drive community selection.

Seasonal Mixed Layer Temperature in the Congolese Upwelling System

AbstractThe Congolese upwelling system (CoUS), located along the West African coast north of the Congo River, is one of the most productive and least studied systems in the Gulf of Guinea. The minimum sea surface temperature in the CoUS occurs in austral winter, when the winds are weak and not particularly favorable to coastal upwelling. Here, for the first time, we use a high‐resolution regional ocean model to identify the key atmospheric and oceanic processes that control the seasonal evolution of the mixed layer temperature in a 1°‐wide coastal band from 6°S to 4°S. The model is in good agreement with observations on seasonal timescales, and in particular, it realistically reproduces the signature of the surface upwelling during the austral winter, the shallow mixed layer due to salinity stratification, and the signature of coastal wave propagation. The analysis of the mixed layer heat budget for the year 2016 reveals a competition between warming by air‐sea fluxes, dominated by the incoming shortwave radiation throughout the year, and cooling by vertical mixing at the base of the mixed layer, as other tendency terms remain weak. The seasonal cooling is induced by vertical mixing, where local wind‐driven dynamics play a secondary role compared to subsurface processes. A subsurface analysis shows that remotely forced coastal‐trapped waves raise the thermocline from April to August, which strengthens the vertical temperature gradient at the base of the mixed layer and leads to the mixing‐induced seasonal cooling in the Congolese upwelling system.

Weakening of subsurface ocean temperature seasonality over the past four decades

The seasonal cycle, responsible for much of the temperature variability in the upper ocean, exerts profound climatic and ecological influence. While surface intensification of temperature seasonality has been widely examined, changes beneath the ocean surface remain unknown. Here we analyze multiple ocean temperature datasets, revealing a robust, substantial weakening of subsurface seasonality by 5.7 ± 1.8% below the mixed layer in extratropical oceans since the 1980s. Using a hierarchy of climate models and an idealized diffusive model, we attribute this weakening to increased ocean heat uptake driven by rising greenhouse gases. This process strengthens upper ocean stratification, suppresses vertical mixing, and limits heat penetration into deeper ocean layers, resulting in a more quiescent subsurface ocean with reduced seasonal variability. Our findings highlight a new fingerprint of anthropogenic influence on subsurface ocean seasonality, with important implications for ocean biogeochemical processes and marine ecosystems.

Influence of mesoscale sea surface temperature anomaly on polar lows

Abstract Sea surface temperature anomaly (SSTA) associated with mesoscale oceanic processes, which are prevalent throughout the ocean, can significantly influence the atmospheric boundary layer and consequently atmospheric systems. While its influences on tropical and extratropical cyclones have been well-documented, the influence of mesoscale SSTA on polar lows (PLs) remains unexplored. To bridge this knowledge gap, we conducted a series of sensitivity numerical experiments with different SST configurations. The simulation results indicate that, over the lifespan of a PL, SSTA does not significantly influence PL intensity but does enhance latent heat release. On a longer time scale, based on simulations of five winter seasons over the Nordic Sea, we found that the accumulated impact of mesoscale SSTA creates favorable environments for PL intensification, characterized by higher moisture levels and lower static stability. These results highlight the importance of considering high-resolution SST boundary conditions, i.e. resolving mesoscale SST, in climate simulations of PLs.

Seasonality of feedback mechanisms involved in Pacific coastal Niño events

AbstractThe 2017 Pacific Coastal Niño Event was the strongest of its type. It caused torrential rainfall and devastating flooding in Peru and Ecuador and thus rapidly caught the attention of the scientific community. From reanalysis data, three similar events, occurring in 2008, 2012 and 2014, are identified which are however all weaker, peaked later during the year and led to very little socioeconomic impact. This study focuses on the role of seasonality for the evolution and impact of Coastal Niño events. Reanalysis products as well as historical simulations from a coupled climate model and targeted model sensitivity experiments are utilized to assess the seasonal varying contributions of surface heat fluxes, horizontal advection and subsurface processes to the modulation of sea surface temperatures off the Peruvian coast. As the atmospheric conditions underlay a strong seasonal cycle with convection only occurring between December and April, warm events in this season are shown to lead to stronger precipitation anomalies. Pacific coastal Niño events in general are shown to be primarily forced via oceanic processes, but in individual cases local atmospheric forcing plays an important role. However, there is a very high variability between the individual events, with especially the 2017 event standing out due to its forcing, timing, strength and associated precipitation response.

Irreversibility of winter precipitation over the Northeastern Pacific and Western North America against CO2 forcing

Comprehending the resilience of regional hydroclimate in response to CO2 removal is essential for guiding future mitigation and adaptation strategies. Using an ensemble of model simulations forced by idealized CO2 ramp-up followed by ramp-down, here we show that the winter precipitation over the Northeastern Pacific and Western North America (NPWNA) is irreversible even if global warming is reversed back to 2 °C level. This asymmetric change features a tripolar pattern and is tied to Aleutian Low intensification, which is driven by both zonal and meridional gradients of sea surface temperature (SST) anomalies in the tropical central-eastern Pacific. Distinct from the zonal SST gradient—explained by different timescales of surface and subsurface warming and ocean dynamical processes, amplified through the Bjerknes feedback—the meridional SST gradient originates from the southward shift of the intertropical convergence zone, maintained by the wind-evaporation-SST feedback. Our findings suggest that the regional hydrological risks over the NPWNA induced by CO2 ramp-up cannot be fully eliminated by CO2 removal even if the global warming level is restored back.

MASCS 1.0: synchronous atmospheric and oceanic data from a cross-shaped moored array in the northern South China Sea during 2014–2015

Abstract. This work presents a cross-shaped moored array dataset (MASCS 1.0) comprising five buoys and four moorings with synchronous atmospheric and oceanic data in the northern South China Sea during 2014–2015. The atmospheric data are observed by two meteorological instruments at the buoys. The oceanic data consist of sea surface waves measured using a wave recorder, temperature, and salinity from the surface to a depth of 400 m and at 10 and 50 m above the ocean bottom using conductivity, temperature, and depth recorders. They also include currents from the surface to a depth of 850 m measured using acoustic Doppler current profilers and measured at 10, 50, and 100 m above the floor using current meters. Additional measurements were taken for sea surface radiation, air visibility, chlorophyll, turbidity, and chromophoric dissolved organic matter at buoy 3 located at the center of the moored array. The data reveal air–sea interactions and oceanic processes in the upper and bottom ocean, especially the transition of the air–sea interface and ocean conditions from summer to winter monsoon and the effects of six tropical cyclones on the moored array. Multiscale processes were also recorded, such as air–sea fluxes, tides, internal waves, and low-frequency flows. The data are valuable and have many potential applications, including analyzing the phenomena and mechanisms of air–sea interactions and ocean dynamics and validating and improving numerical model simulations, data reanalysis, and assimilations. All the data described here are made publicly available at https://doi.org/10.5281/zenodo.14039870 (Zhang et al., 2024a).

HALOBATES: An Autonomous Surface Vehicle for High-Resolution Mapping of the Sea Surface Microlayer and Near-Surface Layer on Essential Climate Variables

Abstract Essential climate variables (ECVs) are physical, chemical, or biological parameters supporting the characterization of the climate system. The oceanic ECVs temperature and salinity have been defined for the sea surface (upper 1 mm) and bulk water (upper 2 m). Direct observation and sampling of the sea surface from ships and buoys are limited due to the potential loss of integrity of the sea surface and contamination by the research platform. The design, development, and operation of a research vehicle with autonomous capabilities based on commercially available components are described in this study. The autonomous operation and fully automated sampling enable high-resolution mapping of the sea surface microlayer (SML), i.e., the upper < 1-mm layer and the near-surface layer (<1.2 m). Sampling the SML is based on a rotating glass disk sampler. The autonomous surface vehicle HALOBATES has been equipped with multiple conductivity–temperature–depth (CTD) sensors in a flow-through system to measure both sea surface and bulk temperature and salinity. The performance of the autopilot under various scenarios is demonstrated. Typical anomalies of ECVs within the SML and near-surface layer are evident and forced by both atmospheric and oceanic processes. Data analyses reveal HALOBATES’ ability to resolve thermohaline structure on different spatial and temporal scales, for example, small-scale freshwater lenses and thermohaline changes across oceanic fronts. The integration of an acoustic Doppler current profiler supports the interpretation of thermohaline structures. HALOBATES is a platform with cutting-edge technology for understanding climate-related processes between the ocean and atmosphere and providing high-resolution sea surface data for validating satellite products of ECVs. Significance Statement An autonomous surface vehicle was developed to address the challenges in the in situ measurement of essential climate variables at the sea surface (upper 1 mm) and near-surface layer (upper 1 m) of the ocean. Autonomous operation for up to 12 h and fully automated sampling enable high-resolution mapping of the sea surface and near-surface layers. The ability of the vehicle to resolve thermohaline structures at various spatial and temporal scales is demonstrated, providing valuable data on freshwater fluxes (evaporation minus precipitation) between the ocean and the atmosphere. This cutting-edge technology enhances the understanding of climate-related processes, offering crucial “sea surface data” for validating satellite products of essential climate variables.

Influence of water content on the partitioning of K between metal/sulfide and silicate phases in the core-mantle differentiation process of magma ocean

Influence of water content on the partitioning of K between metal/sulfide and silicate phases in the core-mantle differentiation process of magma ocean

The Late Jurassic high-Mg diorite and Early Cretaceous high-silica granite in central Tibet: Implication for the subduction process of the Meso-Tethys Ocean

The Late Jurassic high-Mg diorite and Early Cretaceous high-silica granite in central Tibet: Implication for the subduction process of the Meso-Tethys Ocean

Vertical Structures of Marine Heatwaves in the South China Sea: Characteristics, Drivers and Impacts on Chlorophyll Concentration

AbstractMarine heatwaves (MHWs) are characterized as extreme ocean warming events, which have multifaceted impacts on marine ecosystems. The warming of the ocean associated with MHWs can extend into the deep ocean. In the study, five main types of MHWs in the South China Sea (SCS) are identified with various vertical structures, including shallow, surface‐extension‐reversed, surface‐extension‐intensified, deep, and surface‐extension‐intensified‐reversed. Different types of MHWs have different spread patterns and depths. Shallow and surface‐extension‐reversed MHWs are common in coastal areas, and surface‐extension‐intensified MHW events often occur in deep areas. The vertical structures of MHWs are affected by ocean dynamical processes, particularly vertical processes and horizontal heat advection. These MHWs can alter ocean chlorophyll concentration to varying degrees and locations, contingent on their vertical thermal profiles. Furthermore, an analysis of the impacts of a long‐lived MHW event in the SCS in 2020 revealed that the MHWs could alter chlorophyll concentration. These results help to better understand the physical drivers of localized MHWs and their potential impacts in the context of climate change.

Scaling Simulations of Local Wind‐Waves Amid Sea Ice Floes

AbstractWave‐ice interactions are critical for correctly modeling air‐sea exchanges and ocean surface processes in polar regions. While the role of sea ice in damping open‐water swell waves has received considerable research interest, the impact of sea ice on locally generated wind‐waves in partial ice cover remains uncertain. The current approach in spectral wave models is to scale the wind input term by the open‐water fraction, , for the sea ice concentration (SIC), but this neglects the impact of subgrid‐scale patterns of sea ice coverage in limiting fetch for wind‐wave growth. Here, we use the spectral wave model SWAN to simulate local waves in realistic, synthetic fields of explicitly resolved sea ice floes over a range of SICs and floe size distributions (FSDs). We consider cases with floe sizes much larger than the wavelengths, and absent of interstitial frazil or pancake ice. Through geometric arguments, we show that the fetch available for wind‐wave growth, and thus the resulting wave statistics, depends on a combination of the SIC and the FSD. The combination of geometric scaling and empirical wave laws allows the prediction of bulk wave statistics as a function of SIC, a characteristic floe size, and wind speed. We show that due to the difference in spectral character from attenuated propagating open‐ocean swell, these waves may have an outsized impact on ocean mixing regimes.

Overlap of predator foraging and fishing over a cyclical annual biomass source in the South Caribbean

Oceanic processes influence the abundance and distribution of species and ecosystems, contributing to primary productivity and trophic webs. Caribbean ecosystems present cyclical upwelling where Sardinella aurita predominates among pelagic species facilitating the aggregation of predators. In the South Caribbean, western coast of Aragua, Venezuela, between 2004 and 2010, the existence of a biomass source was suggested by the overlapping areas of activity of Tursiops truncatus, Stenella frontalis and fishermen. Therefore (2019–2020), 30 bimonthly transects were conducted in the aforementioned study area to record sightings of feeding species: whale sharks, seabirds, bottlenose dolphins and fishermen. This allowed the calculation of the area of their records and the determination of the overlapping zone confirming this biomass source (Kernel fixed 50 %=23.96 km²). Then a spatial and quantitative analysis of [Chlorophyll-a], sea surface temperature (SST) and precipitation between 2018 and 2020 was performed finding historical means of 0.21±0.07 mg/m3, 27.37±1.22 °C and 86.39±60 mm, respectively. This led to a review of the total fishing mass by group showing an extraordinarily high and ephemeral peak of sardine fishing with a consequent growth of tuna, billfish and tuna-like fisheries. This mesotrophic-oligotrophic zone supports an annual biodiversity cycle and is located north of Henri Pittier National Park, contributing scientific evidence in support of the expansion of the park as a marine protected area, offering protection to fishes and human well-being, as under Venezuelan law artisanal fishing would be permitted.

“Other(ed)” Ocean Knowledges: Unlearning Integration in Ocean Governance for Recognitional Justice

There is an increasing call for the need to “integrate” Indigenous and local knowledge systems in ocean governance processes, on national and global scales. However, the knowledge systems, epistemes, and practices of different Indigenous and local coastal communities, whose stewardship of the planet sustains and protects marine ecosystems, pre‐date the institutionalised ocean sciences and governance with which they are meant to be integrated. The concept of integration often perpetuates othering and devaluation of various ocean knowledges that should not be subject to these problematic practices. Much of the current knowledge informing ocean governance is underpinned by colonial, military, and financial projects, in direct juxtaposition to epistemes and practices that are deeply interconnected with marine life. Writing from a marine social sciences perspective, we explore the inherent problems and limitations of integration approaches and propose reversing how we frame “knowledge” and its othering by suggesting that our scientific and governance practices are, in fact, “other” to longstanding ways of coexisting with the ocean. Without attempting to represent Indigenous knowledge systems or categorise these as unaware of scientific developments, we argue that researchers and scientists need to actively unlearn what is taught in prominent ocean sciences. By focusing on global governance through the International Seabed Authority and national ocean governance in South Africa, respectively, we explore knowledge othering and the process of unlearning what ocean governance teaches as knowledge integration to better critically consider how the ocean is, has been, and should be valued.

Southerly winds and rapid sea ice reductions along the Sea of Okhotsk coast of Hokkaido

The Sea of Okhotsk, between northern Japan and Siberia, is the southernmost sea region of the Northern Hemisphere with seasonal sea ice cover. During early spring, occasional rapid reduction in sea ice cover observed in the southern Sea of Okhotsk can lead to anomalously early disappearance of sea ice along the Hokkaido coast (northern Japan). This study investigated the atmospheric and oceanic processes leading to rapid reduction in sea ice in the southern Sea of Okhotsk during early spring. We detected six events of subseasonal continued reduction in sea ice cover in March between 1993 and 2019. Strong southerly winds with marked thermal advection and moisture transport over northern Japan were observed in five of the six events. The passage of extratropical cyclones brought warm moist air from the south, which led to drifting and melting of sea ice by changing the surface wind speed and surface heat budget in the southern Sea of Okhotsk. The perturbation in sensible heat flux attributable to warm air advection contributed substantially to sea ice reduction, whereas longwave radiation played a secondary role. In addition to the atmospheric processes, an enhanced Soya Current also transports heat from the Sea of Japan, possibly contributing to the sea ice melt. The initial sea ice reduction leads to further sea ice loss through ice–albedo feedback, resulting in prolonged sea ice reductions for 1–2 weeks.

Quantifying the Role of Ocean Dynamics in SST Variability across GCMs and Observations

Abstract Midlatitude SSTs forced by mesoscale oceanic processes can affect the large-scale atmosphere, pointing to the ocean’s crucial role outside the tropics. Previous studies have shown oceanic mesoscale processes’ effect on global and regional climate variability. This study quantifies the local contribution of ocean dynamics to mixed-layer temperature across the globe by directly estimating the ocean heat flux divergence resolved by state-of-the-art ocean reanalysis, eddy-resolving, and eddy-parameterized versions of two U.S. national climate models and indirectly from air–sea flux satellite-based estimates. Our results show that the eddy-resolving climate simulations resolve mixed-layer temperature variances that are larger and closer to those inferred from observations than both their eddy-parameterized counterparts and ECCO over much of the extratropics. The observations and the eddy-resolving models indicate a more significant role of ocean dynamics in the mixed-layer temperature variability than the surface fluxes over most extratropics compared to their eddy-parameterized versions. A frequency domain analysis shows that the better-resolved ocean mesoscale and thermal gradients enhance the variance over a time scale from 2 months to 30 years. Results show agreement in the ocean’s contribution among satellite-based estimates, ocean reanalysis products, and ocean eddy-resolving simulations. At the same time, differences emerge for ECCO and the eddy-parameterized models, suggesting that surface fluxes account for a larger fraction of the mixed-layer temperature variability in most of the extratropics.