Progress in Oceanography最新文献

The Seychelles-Chagos Thermocline Ridge (SCTR) is a biologically important region of open ocean upwelling within the south west Indian Ocean (5–10°S and 45–90°E), driven by the tropical gyre. The SCTR refers to an elongated feature that joins two local minima in thermocline depth; the Seychelles Dome (SD) and Chagos Dome (CD). Entering the ocean basin from the east, the Indonesian Throughflow (ITF) has been shown to interact with the upwelling region, although the relationship between the phytoplankton bloom associated with the SCTR and the ITF are so far unexplored. Using in situ observations and remotely sensed data, the buoyancy fluxes from the ITF are shown to strongly condition surface chlorophyll-a (chl-a) concentrations over the Chagos Dome, the eastern extreme of the SCTR, at seasonal and interannual scales. Accordingly, we find a significant inverse correlation (r = −0.43) between the altimeter-derived volume transport of the ITF and the surface chl-a concentrations. This inverse correlation increases (r = −0.61) when only the 10th and 90th percentile of the ITF volume transport anomalies are considered, indicating the influence of the ITF may be overcoming other physical drivers, especially under extreme ITF events. We hypothesise that the buoyancy flux of a strong ITF input ‘caps’ the Chagos Dome with warm, less saline waters, suppressing surface phytoplankton and reducing the surface chl-a concentrations. This hypothesis is supported by a strong, significant correlation (r = 0.66) between remotely sensed surface salinity and surface chl-a over the region. This relationship is not found over the Seychelles Dome, where the ITF has a weaker direct impact over the bloom. These results suggest that the westward travel of ITF waters may condition the eastward expansion of the SCTR and, therefore, the zonal extent of the associated chl-a bloom. This happens at seasonal and interannual time-scales concomitantly with the propagation of downwelling Rossby waves, deepening the thermocline and facilitating the westward advance of ITF waters. This is visible through a combination of remotely sensed and in situ observations at depth from the RAMA mooring array at the eastern domain of the SCTR, where intrusions of warm, less saline waters, typical of ITF waters, coincide with downwelling Rossby waves deepening the thermocline. Thus, both the westward travel of ITF waters and the propagation of downwelling Rossby waves shape the eastward expansion of the SCTR and, therefore, the zonal extent of the associated surface chl-a bloom on a year-to-year basis.

Sedimentary properties and accumulation rates on the continental shelf and in the deep sea reflect temporal oceanographic, biological and chemical processes occurring in the water column and the sediment surface. We used the radionuclides ²¹⁰Pb, ²²⁶Ra, and ¹³⁷Cs activities to estimate sedimentation rates during the last century at nine stations in the northern Barents Sea region. Elemental (C, N) and stable isotopic composition (δ¹³C, δ¹⁵N) were also analysed from the nine stations sampled in August 2018, and, for five other stations sampled in August and December 2019, and in March and May 2021. Sediment accumulation rates varied between 130 and 1 410 g m⁻² y⁻¹. The < 63 μm normalized total organic carbon (TOC₆₃) and the total nitrogen from the sediment surface varied between 0.90–2.56 % and 0.13–0.33 %, respectively. Ice-free shelf stations had higher TOC₆₃ and possibly fresher organic matter (high δ¹³C, low δ¹⁵N) than ice-covered more northern stations. The opposite trend was observed for total inorganic carbon. We found that these trends in biogeochemical parameters were spatially structured by the winter sea ice concentration and biological production differences, and exhibited a south-north separation of the Polar Front region. The low and stable organic carbon accumulation rate (1.7–13.4 g C_org m^-2 y⁻¹; AR_toc) is a function of slow sedimentation rates, and high degradation and residence time in the water column and at the sediment–water interface. Overall, the AR_toc has been stable for the past 100 years, with a slight increase from the early 1970s to the present at the shelf and slope stations. Our results highlight that spatial scales of variability of the studied sedimentary parameters are linked to spatial patterns of important environmental variables (e.g., chlorophyll-a, sea ice concentration) in the region. In contrast, no seasonal differences were observed in the sediment parameters of revisited stations, and the dated sediment geochemical profiles did not exhibit substantial longer-term variation. This means that climate-induced changes in variables that modify the sedimentary geochemistry of the environment may affect benthic community activity and structure before leaving a record in AR_toc.

Full-depth hydrographic sections of the BROKE experiment in 1996 (across the Antarctic margin from 80 to 150°E; Bindoff et al., 2000) were revisited for the first time during the 2018/2019 austral summer. We describe the subsurface physical oceanography in 2019 and the hydrographic changes between 1996 and 2019 not documented in earlier studies. The survey captured decadal changes in ocean structure from the southern flank of the Antarctic Circumpolar Current (ACC) to the continental shelves. In five cross-slope meridional sections, where 1996 and 2019 measurements are comparable (112, 120, 128, 140, and 150°E), the poleward shift of the southern boundary of the ACC (50–120 km) prevailed near the continental rise. The simultaneous displacement of barotropic ACC fronts and poleward migration of deep water contributed to full-depth warming (0.1–1.6 °C) and potentially to a reduction in the bottom water volume. Freshening was widely observed from the deep to bottom layers (∼0.02 g/kg), with the signal extending from the upper continental slope. Bottom-intensified freshening was accompanied by an oxygenation of 10–20 μmol/kg, indicating that freshening-driven oxygenation of bottom layers counteracted the deoxygenation effect of the poleward barotropic frontal shift. Westward transport of the Antarctic Slope Current decreased by more than 10 Sv from 1996 to 2019 in the five cross-slope sections; its frontal features and current axis shifted offshore by more than 20 km in 112–140°E. Additionally, subsurface warming along modified Circumpolar Deep Water by up to 0.4 °C was commonly detected across the upper continental slope. For the 2019 hydrography, shelf water sufficiently dense to form bottom water (>28.35 kg/m³) was found to the east of Mertz Polynya (142–148°E), implying a pathway for dense shelf water export from the eastern margin of Mertz Polynya. Our findings underscore the importance of sustained efforts for in-situ observations that widely cover the East Antarctic margin.

Our recent study reported the existence of internal solitary waves with the diurnal tidal cycle (ISW-D) in the Sulu Sea, however, the three-dimensional characteristics and generation mechanism of ISW-D are still unclear (Huang et al., 2023). In this work, the spatial–temporal characteristics, generation mechanism and propagating process of ISW-D in the Sulu Sea are first preliminary investigated based on high-temporal-resolution Geostationary Orbiting Satellite (GOS) images and high-spatial-resolution two-dimensional numerical model (MITgcm). GOS images from 2018-2022 are retrieved and a total of 13 pairs of ISW-D packets are found. ISW-D occur at spring tide during May–August, with an average interpacket distance of 198 km and a phase speed of 2.30 m/s. To further knowledge of the generation mechanism and propagation process of ISW-D, the non-linear and non-hydrostatic numerical simulations are conducted. The comparison of ISW-D parameters with GOS images proves the validity of numerical simulations. The results of numerical simulations and theoretical parameters indicate that ISW-D are generated at the sill near Pearl Bank by internal tide release mechanism. Moreover, three sensitivity experiments are designed to investigate the effects of tidal force and seawater stratification on the generation and propagation of ISW-D. The results reveal that the ISW-D is generated by diurnal tides with stronger intensity than semidiurnal tides. Seawater stratification does not influence the generation of ISW-D but modulates the propagation process. Phase speeds from GOS images and theoretical model show a positive correlation between the phase speeds of ISW-D and the intensity of seawater stratification.

Stratification and its thermal and haline contributions are important ocean properties of fundamental climatic influence. Upper-ocean stratification shapes marine ecosystems by regulating nutrient availability and deep-ocean stratification is important for carbon sequestration and ventilating the ocean interior. Here, we first assess the applicability of an ocean reanalysis product in representing stratification in the Nordic Seas and East Greenland Shelf. While the reanalysis performs well in most interior basins, it exhibits significant shortcomings on the East Greenland shelf, raising concerns about the reanalysis product in these areas. We then examine the development in the thermal and haline contributions to summer upper- (100 m) and winter intermediate- (1000 m) ocean stratification in the Greenland Sea from 1980 to 2020. We find that there has been a transition in the controls of winter stratification in the upper 1000 m of the Greenland Sea. The transition was associated with a westward migration of the boundary between salinity- and temperature-stratified waters and eventual switch from haline to thermal control of winter stratification. With that follows a change in the type of forcing that can lead to convection: The Greenland Sea is now less dependent on eroding salinity gradients but rather depends on cooling to overcome stratification. There has been a similar switch in summer stratification in the upper-ocean of the Greenland Sea where surface waters shifted from variable stratification, alternating between salinity and temperature dominance, to a stable temperature-stratified regime. This switch coincided with declining sea-ice concentrations related to the disappearance of the Odden ice tongue after 1997. The high sea-ice conditions previously characteristic of the Greenland Sea are now rare suggesting the transition will persist with potential implications for marine ecology and local sea-ice formation. Our findings reveal differences in how thermal and haline stratification has developed over the last 40 years, which may help explain or predict plankton production and carbon uptake and export.

Coastal regions of the world’s oceans are critical to supporting the fishing sector, recreation, tourism, and the global blue economy. However, there is a paucity of subsurface, in situ ocean measurements in coastal and shelf regions worldwide that corresponds to the region where a majority of commercial fishing occurs. In Aotearoa New Zealand, the Moana Project and technology partner ZebraTech, Ltd. have co-designed a fully automatic system that measures, transmits, processes, and disseminates temperature observations in near real-time with a goal of providing broad-scale coverage of New Zealand’s coastal and shelf seas. In the first two years, more than 300 sensors were deployed by over 250 vessels with the cooperation and support of the commercial fishing sector, providing more than one million temperature measurements per month throughout New Zealand’s exclusive economic zone. Participation by the fishing sector is critical to program success with continuous improvement based on fishing sector feedback. Here we introduce the fishing-vessel-based temperature and pressure data collection on a national scale and present initial results showcasing a step change in research quality ocean temperature data collection. Next, we highlight the full-circle data pathway including improved ocean forecasts and near real-time return of the data to the vessels that obtained them. Finally, a discussion of key partnerships, use cases, and lessons learned in Aotearoa New Zealand provides a potential framework for deploying similar systems in data-poor regions worldwide with the support of the commercial fishing fleet and citizen scientists.

Benthic foraminifera are single-celled organisms inhabiting all marine environments. Despite their high tolerance to oxygen depletion, the prevailing hypothesis anticipates a reduction in their diversity in permanently oxygen-depleted environments, including oxygen minimum zones. Here we re-evaluate diversity and study the endemism of benthic foraminifera in the eastern Pacific, an oceanic area hosting the largest permanently oxygen-depleted waters of the world. We focus our analysis on the oxygen-depleted bottom waters and study how they compare with well-oxygenated waters. By utilizing extensive datasets of quantitative information on benthic foraminifera assemblages obtained from morphological traits, we present evidence that challenge traditional viewpoints. Contrary to prior inferences primarily derived from regional studies, our findings reveal that the median diversity (species richness and the Shannon index) calculated on both, living and dead assemblages does not decrease in the most oxygen-depleted bottom-waters. The analysis of unique (endemic) and shared species shows a divide between the neritic-bathyal oxygen-depleted bottom waters with low number of endemic species, and the well-oxygenated abyss hosting high number of unique species. These patterns could be explained by the long-term species exchange in the upper ocean and the isolation of the lower ocean.

Understanding the driving mechanism of phytoplankton dynamics is key to forecasting future changes in the ocean. Here, we report an apparent “trapezoidal” relationship between chlorophyll concentrations (Chl) and surface photosynthetically available radiation (PAR(0)) at the center of the South Pacific Gyre (cSPG) based on 18 years of MODIS Aqua measurements. A comparison of Chl with a photoacclimation model revealed that photoacclimation alone could not explain the temporal dynamics of Chl. Instead, the Chl dynamics were explained by a combination of photoacclimation, nutrient limitation, and the grazing pressure of zooplankton at different times throughout the year. An annual “trapezoidal” spiral relationship between Chl and PAR(0) suggested that the steady state of phytoplankton populations at the cSPG could be influenced by the alternation of co-regulation mechanisms during a year. Because this same pattern occurs in other subtropical gyres, this understanding of the underlying mechanisms not only facilitates simulating and forecasting phytoplankton dynamics but also provides a new perspective on how multiple stressors may impact phytoplankton communities in a warmer climate.

Disentangling microbial dynamics in the mesopelagic zone is crucial due to its role in processing sinking photic production, affecting carbon export to the deep ocean. The relative importance of photic zone processes versus local biogeochemical conditions in mesopelagic microbial dynamics, especially seasonal dynamics, is largely unknown. We employed rRNA gene transcript-based high-throughput sequencing on 189 samples collected from both the photic and mesopelagic zones, along with seasonal observations, to understand the South China Sea’s protistan-bacterial microbiota diversity, drivers, and mechanisms. Mesopelagic communities displayed unexpectedly greater seasonal but less vertical dynamics than photic counterparts. Temperature, dissolved oxygen, nutrients, and bacterial abundance drove mesopelagic communities vertically. Photic zone processes (using net community production and mixed layer depth as proxies) of past seasons, coinciding with strong monsoon periods, shaped seasonal fluctuations in mesopelagic communities, indicating a time-lag effect. Furthermore, certain microbes were identified as indicators for beta diversity by depth and season. This investigation deepens our understanding of how and why mesopelagic communities vary with season and depth. Recognizing the time-lagged effect of photic zone processes on mesopelagic communities is crucial for understanding the current and future configurations of the ocean microbiome, especially in the context of climate change and its effect on carbon export and ocean storage.