Journal of Geophysical Research-Oceans最新文献

Increasing our knowledge about the structure and metabolism of the microbial plankton community in coastal upwelling systems is essential for efficiently managing the resources of these highly productive ecosystems. Here we explore seasonal variations of this community in the NW Iberian coastal upwelling system by measuring variables informing on community structure and metabolism. The microbial community remained mostly autotrophic year-round, with variations imposed by hydrographic conditions. Changes in plankton biomass were driven by the rapid response of larger cells to water column shifts. During upwelling, large diatoms (>20 μm) dominated. Conversely, when downwelling prevailed, autotrophic nanoflagellates (2–20 μm) and picoflagellates (<2 μm) were the main autotrophs. However, peak values of biomass and metabolism were recorded in summer stratified periods during upwelling season, with elongated and the medium sized diatom species Leptocylindrus danicus dominating. The autumn period characterized by the presence of the Iberian Poleward Current featured warm, nutrient-poor waters, enhanced heterotrophy, occurrence of red-tide forming organisms, and presence of mixotrophic dinoflagellates, suggesting metabolic adaptability as a response to nutrient limitation. Seasonal shifts in size structure and metabolism reveals a dynamic microbial community tightly linked to physical forcing. Additionally, by comparing our shelf station with previous studies in the inner bays (Rías) and the shelf break, we define a land-sea gradient in microbial structure and metabolic regime: from consistently autotrophic, microplankton-dominated communities inside the Rías to smaller, more heterotrophic assemblages offshore. These findings provide a conceptual framework for understanding how microbial productivity may shift under future changes in upwelling regimes.

The Beaufort Shelf has historically been reported to exhibit limited polynya activity in winter. Yet, recent satellite observations show episodic recurrence of a large polynya west of Mackenzie Canyon, a site of significant shelf-basin exchange. Here, we investigate satellite-detected occurrences of this polynya over winters 2003–2025, including their relation to regional winds, ice drift, and ocean conditions. The polynya is observed to open when easterly winds drive rapid ice drift over the shelf, mechanically opening the ice near Qikiqtaruk (Herschel Island). Under strong and persistent forcing, open water extends northwestward, sometimes occupying large portions of the shelf. Its comparison to a 1-D coastal polynya model suggests that this observed polynya growth could reflect contributions from ocean heating. Fluxes of interior ocean heat to the shelf are confirmed across two winters of mooring observations, which revealed coincident upwelling along the western flank of Mackenzie Canyon as polynyas formed. Warm upwelled waters were advected by a strong shelf current directed along the axis of polynya extension. Transported heat could suppress an estimated $��10�\pm �8��c�m�$ of daily ice growth over the shelf, comparable to that otherwise expected from the estimated surface heat losses. Recent years have featured several extreme polynyas, some exceeding 400 km in length. These events are rare and occur under exceptional wind forcing. However, increased ice drift speeds in the last decade coincide with more frequent and extensive openings, suggesting that large polynyas may be becoming a more prominent feature over the shelf as the mobility of the winter ice cover increases.

The West Greenland boundary current system plays a central role in modulating deep convection in the Labrador Sea. Here, we use 6 years of mooring data, 2014–2020, together with atmospheric and oceanic reanalysis fields, to investigate wind-driven downwelling along the southwest Greenland shelf/slope. A total of 49 downwelling events were identified using timeseries of alongcoast wind and bottom density anomaly. On average, the events last 4.5 days and are characterized by increased southeasterly winds followed by an increase in alongstream velocity by ∼0.12 m/s and decrease in bottom density by ∼0.18 kg/m³. A cross-stream Ekman cell develops, although the response is weaker offshore. The events are driven by low-pressure systems originating from the southwest/south that progress into the Labrador Sea and generate strong southeasterly winds along southwest Greenland. More downwelling events occur in summer, but the ocean response is weaker due to less intense winds during this season. The largest number of events occurred in 2017–2018, coinciding with the period when the deepest convection occurred in the interior Labrador Sea over the past 30 years. The ocean reanalysis fields reveal significant positive anomalies of upper-layer salinity and density in the interior Labrador Sea during the fall to early winter of 2017–2018. These anomalies likely reflect reduced seaward spreading of freshwater from the shelf due to the more frequent downwelling winds. Our results highlight the important role of wind-driven downwelling along the west Greenland coast in preconditioning Labrador Sea deep convection, thereby influencing the large-scale ocean circulation and climate system.

Transport mechanisms between the deep ocean and adjacent continental shelf seas play an important role in the spatial distribution of nutrient delivery to the coastal ocean and in the temporal variability of shelf biogeochemical processes. Along the North West European Shelf (NWES) edge, nutrient-rich waters of oceanic origin are found below the mixed layer, representing a potential nutrient source for fueling new production on the shelf. We find persistent cross-isobath geostrophic transport integrated over along-isobath segments of the NWES edge in hydrographic climatologies and altimetric sea surface height gradients. This transport is O(1 cm/s), has little vertical structure, and is onshore along the entire extent of the 200 m isobath, except along the southern rim of the Norwegian Trench. Despite strong temporal variability in the shelf-edge hydrography on seasonal to decadal timescales, changes in the ocean-shelf geostrophic transport are subtle. This is due to a persistent large-scale steric sea surface slope along the shelf edge. The geostrophic flow induces local depth-integrated cross-isobath nitrate fluxes of O(1–10 mmol/m/s). This is similar in magnitude to the winter wind-driven nitrate transport, but is much less variable at seasonal and inter-annual time scales. Variability in the geostrophic advection of nitrate is thus determined by the ocean-shelf nitrate gradient's variability, rather than by the cross-isobath flow's variability. Geostrophic transport may therefore be an important baseline component of the nutrient and carbon budgets on the NWES and other continental shelves, and should be considered in their long-term response to climate-scale forcing.

Sea-surface topography observations from the Surface Water & Ocean Topography (SWOT) satellite altimeter mission are used to map the baroclinic internal tides in a region off California during the 1-day calibration/validation orbit phase of the mission. The observations, which cover the period from 30 March through 10 July 2023, reveal a large-scale internal wave field dominated by waves generated at the Mendocino Fracture zone approximately 500 km north of the region studied. Tidal harmonic constants of baroclinic sea level are derived from mooring observations at four sites within the KaRIn swath and compared with the SWOT data. The harmonic constants from the moorings and altimeter agree qualitatively; however, the SWOT results are significantly uncertain due to long-wavelength platform errors and non-astronomical tidal modulations. Spectral analysis of the time series reveal that the phase-locked tide explains about 75% of the baroclinic sea level variance in the semidiurnal band.

Phytoplankton play a crucial role in regulating marine biogeochemical pathways but studying their spatial and temporal dynamics often requires extensive and laborious oceanographic expeditions. Here, we report a novel use of Biogeochemical-Argo data to delineate planktonic habitats in the Tropical North Atlantic Ocean (TNA). We then investigated the phytoplankton biomass via in situ measurement of the BGC-Argo floats and phytoplankton community structure of each habitat using Phytoplankton Functional Types derived from satellite observations. Our habitat delineation approach provided a finer scale and dynamic overview of phytoplankton assemblages and their surrounding environment, complementary to the widely used and static biogeochemical provinces of Longhurst. While picophytoplankton remained dominant for most communities, we found elevated abundance of microphytoplankton, including diatoms and dinophytes, and nanophytoplankton, including haptophyte and green algae, following the seasonal displacement of the Intertropical Convergence Zone, and the Amazon and Orinoco River discharges into the Tropical Atlantic and Eastern Caribbean Sea. At midlatitude, our method was able to capture both the seasonal and spatial variations of the oligotrophic region of the North Atlantic subtropical gyre. These findings shed light on the seasonality of phytoplankton distribution driven by meteorological and oceanic forcings and demonstrated the potential of the BGC-Argo in studying and monitoring marine ecosystem.

The slope current in the northwest South China Sea (SCS) plays a vital role in regulating mass and nutrient exchange between the coastal region and the open ocean. Over the past three decades, this current has exhibited a significant increasing trend, indicating a much stronger marginal sea-open ocean interaction. Using observational data, we show that the intensified anticyclonic warm eddy shedding from the Luzon Strait propagates along the continental slope, leading to a stronger slope current. During 1993–2022, a reduction in Subtropical Mode Water was accompanied by decreases in sea surface height (SSH) and Kuroshio transport in the Luzon Strait, as well as an enhanced looping pathway of the Kuroshio in the northern SCS. These conditions favor anticyclonic warm eddies (positive SSH anomaly) shedding from the northwestern Luzon Strait and propagating along the continental slope southwestward. At the same time, the accompanying cold eddies (negative SSH anomaly) propagate westward towards western SCS. The combined propagation of these dipole-like eddies further strengthens the SSH gradient between the shelf and inner ocean in the SCS, thereby intensifying the slope current. Importantly, it is the increased mean strength of the anticyclonic eddies, rather than their number, that drives this long-term current intensification. In contrast, neither local surface wind nor buoyancy forcing can account for the observed enhancement of the slope current, although wind forcing likely contributes to strengthening its southern segment. These findings highlight the crucial role of Kuroshio intrusion and eddy-current interaction in regulating the long-term variability of circulation in the SCS.

The world-renowned tidal bore forms in the Qiantang River Estuary (QRE) and leads to distinctive hydro-sediment dynamics and the reshaping of coastal geomorphology. This study presents a time series of in situ data of wave, current, and suspended sediment concentration (SSC) in the tidal bore, covering the spring-neap tidal cycle in 2020. Field data analysis reveals the spring-neap and flood-ebb asymmetries in hydro-sediment dynamics. Current-induced bed shear stress is mostly larger than that induced by waves. The interactions of semi-diurnal tide and shallow water tide play a leading role in the tidal asymmetry. Turbulence, particularly ejection and sweep, contributes to the sediment inception and increased turbidity. Peak turbulent kinetic energy (TKE) and strong high-frequency water level oscillations occur during both flood and ebb tides, driven by different mechanisms. During the flood tides, they are initiated by the breaking of the tidal bore and its secondary waves. During ebb tides, the wave-current interactions enhance TKE and generate intense high-frequency oscillations, which is a process previously under-documented. The findings reveal the dynamic mechanism of turbulence asymmetry and high-frequency oscillation in water level due to current-wave interactions and shed light on the evolution of dynamic geomorphology in macro-tidal turbid estuaries.

The northward propagation of the Monsoon Intraseasonal Oscillations (MISOs) in the Bay of Bengal (BoB) is an intrinsic characteristic of the Indian summer monsoon (ISM). Previous studies have demonstrated the critical role of air-sea interactions in modulating MISO propagation. This study elucidates the intraseasonal variability of ocean heat content (OHC) in the upper 200 m of the BoB and its dynamic relationship with MISO. Similar to sea surface temperature (SST), the positive OHC anomalies lead MISO's northward propagation, showing two prominent maxima located east of Sri Lanka and in the northwestern BoB. The OHC anomalies are stronger east of Sri Lanka, penetrating the thick barrier layer during MISO events, whereas temperature anomalies in the northwestern BoB remain confined to the mixed layer. Diagnostic analyses reveal that the intraseasonal OHC variability, unlike that of SST, stems from intensified downward vertical advection driven by intraseasonal vertical velocity. In contrast to the wind-dominated intraseasonal vertical velocity in NBOX, the pronounced intraseasonal OHC variability east of Sri Lanka stems from sea level anomaly generated by both westward-propagating Rossby waves and MISO-related winds. Subsequently, with the arrival of MISO, upward vertical advection and thick barrier layer prolong warm SST anomalies east of Sri Lanka, providing additional heat and moisture to enhance MISO convection and rainfall intensity. These results highlight the essential role of the memory effect of upper ocean heat exchange and redistribution processes in sustaining MISO propagation.

In the South Atlantic Bight (SAB), changes in the Gulf Stream (GS), particularly its strength and proximity to the coast, are thought to be primary factors determining the shelf-break upwelling rate. However, it is still not entirely clear if and to what extent those factors influence cross-shelf nutrient fluxes and shape the ocean biogeochemistry at interannual and longer timescales. Here, we use a high-resolution regional ocean-biogeochemical model and an ocean reanalysis product (1993–2022), along with a satellite-derived chlorophyll data set (1997–2022), to investigate the interannual ocean-biogeochemical variability in the SAB. Regional model outputs suggest that year-to-year changes in phytoplankton production are indeed largely driven by upwelling of cold and nutrient-rich water to the shelf-break. The upwelling variability, reflected in bottom temperature and vertically integrated production patterns, is strongly linked to surface velocity changes in the GS near the shelf break, but weakly related to the depth-integrated GS transport. The GS's velocity changes, and the temperature and production anomalies, are well correlated to the alongshore wind stress, suggesting that local wind is the leading driver of the shelf-break upwelling variability at interannual timescales. Those relationships are also supported by circulation patterns from ocean reanalysis and satellite chlorophyll anomalies. Finally, examining the simulated shelf-slope interchanges in the carbonate system, we find that shelf-break upwelling significantly increases bottom acidification, a pattern linked to the low carbonate concentration in the slope waters. This study thus provides new insight for understanding and predicting GS and winds impacts on biogeochemical patterns from the SAB.