Journal of Geophysical Research: Oceans

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ISSN / EISSN : 2169-9275 / 2169-9291
Published by: American Geophysical Union (AGU) (10.1029)
Total articles ≅ 4,602
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, C. Troy, H. Bootsma, , R. Maclellan‐Hurd
Journal of Geophysical Research: Oceans; https://doi.org/10.1029/2021jc017533

Abstract:
In this study, we report on turbulent mixing observed during the annual stratification cycle in the hypolimnetic waters of Lake Michigan (USA), highlighting stratified, convective, and transitional mixing periods. Measurements were collected using a combination of moored instruments and microstructure profiles. Observations during the stratified summer showed a shallow, wind-driven surface mixed layer with locally elevated dissipation rates in the thermocline () potentially associated with internal wave shear. Below the thermocline, turbulence was weak () and buoyancy-suppressed ( < 8.5), with low hypolimnetic mixing rates () limiting benthic particle delivery. During the convective winter period, a diurnal cycle of radiative convection was observed over each day of measurement, where temperature overturns were directly correlated with elevated turbulence levels throughout the water column (; ). A transitional mixing period was observed for spring conditions when surface temperatures were near the temperature of maximum density (TMD3.98) and the water column began to stably stratify. While small temperature gradients allowed strong mixing over the transitional period (), hypolimnetic velocity shear was overwhelmed by weakly stable stratification (; ), limiting the development of the surface mixed layer. These results highlight the importance of radiative convection for breaking down weak hypolimnetic stratification and driving energetic, full water column mixing during a substantial portion of the year (>100 days at our sample site). Ongoing surface water warming in the Laurentian Great Lakes is significantly reducing the annual impact of convective mixing, with important consequences for nutrient cycling, primary production, and benthic-pelagic coupling.
Journal of Geophysical Research: Oceans, Volume 126; https://doi.org/10.1029/2020jc017043

Abstract:
Current down-scaling numerical modelling system around Hong Kong achieved a considerable prediction skill for the estuarine–shelf circulation off the Pearl River Estuary (PRE) without data assimilation (DA). In order to further improve the reliability of this modelling system, the cost-effective Ensemble optimal interpolation approach is implemented to test the potential benefits from assimilating the cruise temperature and salinity (T/S) profiles to reproduce the variable coast waters. Regarding assimilation parameters (e.g. assimilation window, observation spatiotemporal scales, and ensemble composition), four parallel experiments are conducted in summer 2015. Against the assimilated T/S profiles, the vertical structures of the analyzed T/S are improved by the DA, although the waters experience strong mixing on the shelf. Compared with the run without DA, the root mean square errors of the predicted T/S are generally reduced by 9.8%–23.5% and 4.2%–14.0% in the assimilation runs. The results also show the salinity stratification is improved in the shelf by the assimilation of T/S profiles, although the improvement is sensitive to the selected ensemble and the assimilation window. Further, we investigate the impact of the temporal scales of the river-estuary-shelf waters on the assimilation results by the sampling of the model-state ensemble. The water exchanges between the estuary and the shelf are also better captured through this assimilation. The assimilation impact analysis shows that DA has advantages in reproducing the distribution of water masses of the river-estuary-shelf waters, although the quality of the reproduced water mass distribution is related to the adopted sample ensemble in DA.
Journal of Geophysical Research: Oceans; https://doi.org/10.1029/2020jc017075

Abstract:
El Niño related sea surface temperature (SST) anomalies over the tropical Pacific Ocean impact global climates, but these impacts differ substantially for conventional cold tongue El Niño (CT El Niño) and the central Pacific El Niño (CP El Niño) events. This study is motivated by the need for a better understanding of the recharge/discharge processes associated with these two different flavors of El Niño. Composite analysis based on improved CT and CP El Niño identification methods applied to the Simple Ocean Data Assimilation demonstrates that the recharge/discharge processes are active during CT El Niño events. In contrast, for CP El Niño events, the recharge/discharge processes do not play a significant role. Prior to a CT El Niño, warm water accumulates over the western Pacific due to off-equatorial anticyclonic wind stress curl. The onset of a CT El Niño is closely associated with the formation of a cyclonic atmospheric circulation over the northwest Pacific in the winter and spring, which induces westerly wind anomalies in the equatorial western Pacific and initiates eastward warm water transport. This leads to peak warming in the eastern equatorial Pacific the following winter, followed by the poleward discharge of warm water. This quasi-cyclical behavior provides a measure of predictability. In contrast, the CP El Niño events do not show a precursor subsurface warming signal along the tropical Pacific thermocline. Instead, modest warm SST anomalies appear in boreal summer and peak in the fall, with weak subsurface warming mainly in the fall during CP El Niños. Hence, CP El Niños are less predictable in terms of an equatorial thermocline precursor than CT El Niño events.
, Orlando Cordero, , Christine Angelini
Journal of Geophysical Research: Oceans; https://doi.org/10.1029/2021jc017357

Abstract:
Intertidal creeks (channel width <5 m) weave through salt marshes, delivering water, nutrients, and sediments into the marsh interior and affecting spatial heterogeneity in plant and animal distributions. Despite their global prevalence, creek connectivity, and the mechanisms controlling cross-marsh hydrodynamics, remain poorly resolved. In this study, we measured flow and total suspended solids (TSS) transport in three intertidal creeks within a confined drainage basin in a Georgia, USA salt marsh. We discovered that the effective drag is 3 to 12 times greater than bed drag, reaching levels similar to those observed in coral reefs. Furthermore, the drag between tidal flood and ebb phases differs, indicating an asymmetric drag. Analyses of along-channel momentum reveal that pressure gradient m/s2 and friction m/s2 dominate creek momentum balance. Divergence in tidal and suspended solids (TSS) transport between adjacent creeks revealed contrasting tidal transport asymmetries (i.e., flood or ebb dominated) within this confined basin. We discuss how these differences may alter the eco-geospatial evolution of salt marshes and their response to sea-level rise.
Journal of Geophysical Research: Oceans; https://doi.org/10.1029/2020jc016913

Abstract:
We conducted a current velocity and hydrographic time series observation in the upper ocean by deploying a mooring buoy at station K2 (N, E) from July 2015 to June 2019 to understand the physical conditions that affect the acidification of the winter mixed layer (ML) water in the western subarctic North Pacific. Disturbances with a substantial current velocity shear were excited by the wind changes remotely related to the 2015/2016 and 2018/2019 El Niño events. The density stratification beneath the ML was also weakened in winter, particularly the El Niño years. Based on the bulk Richardson number mixing scheme, we found that the El Niño-related strengthening of the current velocity shear enhanced the vertical diffusivity beneath the winter ML driven by the near-inertial current velocity variation. The weakening of the density stratification is ineffective in enhancing the vertical diffusivity. It is suggested that the El Niño-related large vertical mixing accelerates the acidification of the winter ML water through the enhancement of the entrainment of the deep water that is rich in dissolved inorganic carbon.
, Zizhou Liu, , , Luoyu Hu, Liyuan Sun, Baoqing Yang
Journal of Geophysical Research: Oceans; https://doi.org/10.1029/2021jc017562

Abstract:
Traditionally, the Yellow Sea (YS) is believed to be well mixed throughout the water column or to experience significant temperature inversions in winter. Recently, we found significant positive surface-bottom temperature difference (SBTD) as large as 3.7 °C in northeastern coastal waters of the Shandong Peninsula in the YS in winter. By analyzing four years (June 2016-September 2020) of online in situ observations, this study examined monthly and interannual variations in winter positive SBTD and the underlying dynamics. The winter positive SBTD generally formed in November, peaked in December/January and disappeared in March. At the interannual time scale, this SBTD was strongest in 2018 and weakest in 2020. The underlying dynamics involved the combined effects of prevailing winter monsoon winds and resultant surface heat loss and current heat advection. When surface northerly wind was strong, strengthened southward advection of cold air over the ocean cooled the water column intensely, whereas wind-enhanced southward advection of warm surface water partly offset the sea surface cooling and resulted in surface temperature cooling that was considerably slower than bottom temperature or even surface temperature warming. These processes together generated strong positive SBTDs. When the surface northerly wind was weak, all of the above processes became weak and resulted in weak positive SBTDs. The relationship with Yellow River runoff was also discussed. This study enriches our knowledge of winter thermal structure in the YS. More observations and coupled physical-biogeochemical simulations are needed to further clarify the impacts on climate and marine ecology.
Journal of Geophysical Research: Oceans, Volume 126; https://doi.org/10.1029/2021jc017537

Abstract:
Mixed-layer dynamics exert a first order control on nutrient and light availability for phytoplankton. In this study, we examine the influence of mixed-layer dynamics on net community production (NCP) in the Southern Ocean on intra-seasonal, seasonal, interannual, and decadal timescales, using biogeochemical Argo floats and satellite-derived NCP estimates during the period from 1997 to 2020. On intra-seasonal timescales, the shoaling of the mixed layer is more likely to enhance NCP in austral spring and winter, suggesting an alleviation of light limitation. As expected, NCP generally increases with light availability on seasonal timescales. On interannual timescales, NCP is correlated with mixed layer depth (MLD) and mixed-layer-averaged photosynthetically active radiation (PAR) in austral spring and winter, especially in regions with deeper mixed layers. Though recent studies have argued that winter MLD controls the subsequent growing season’s iron and light availability, the limited number of Argo float observations contemporaneous with our satellite observations do not show a significant correlation between NCP and the previous-winter’s MLD on interannual timescales. Over the 1997-2020 period, we observe regional trends in NCP (e.g., increasing around S. America), but no trend for the entire Southern Ocean. Overall, our results show that the dependence of NCP on MLD is a complex function of timescales.
, Kai Wu, Shuai Gu, Peng Jiang, Huifang Li, ,
Journal of Geophysical Research: Oceans; https://doi.org/10.1029/2021jc017585

Abstract:
The advective supply of allochthonous dissolved organic carbon (DOC) from open ocean to marginal seas through western boundary current intrusion influences the regional carbon inventory and microbial activities. However, there is limited observation about this process and its biogeochemical impacts on marginal seas. In this study, we investigated the biodegradation of allochthonous DOC carried by the intrusion of the Kuroshio Current into South China Sea (SCS). Using an isopycnal mixing model, the exchange and biodegradation processes of Kuroshio-intruded DOC were quantified. We estimated that approximate 10% of the surface DOC was remineralized due to the enhanced biodegradation in the SCS. This result was supported by the on-deck bioassay experiments were also conducted under different environmental contexts. The results of modeling and on-deck incubations indicate that DOC biodegradation was enhanced by the sharp gradient of environment factors, including nutrients supply, microbial species, and bio-lability of DOC in the frontal zone during the surface water mass mixing. The amount of carbon and nitrogen released from the enhanced DOC degradation by Kuroshio intrusion was estimated to be approximately equal to 8.6 Tg C yr-1. The amount of nitrogen could contribute 0.19 - 0.70 mmol N m-2 d-1 to the surface of SCS which is comparable to the total supply from deeper water and nitrogen fixation. This study suggests that the enhanced biodegradation of DOC during the western boundary currents intrusion could serve as an important sink of oceanic DOC, and thus provide an additional nutrient source to marginal seas.
, , , , , , H. Statscewich, K. Bernard, W. Fraser
Journal of Geophysical Research: Oceans; https://doi.org/10.1029/2021jc017304

Abstract:
Palmer Deep Canyon is one of the biological hotspots associated with deep bathymetric features along the West Antarctic Peninsula. The upwelling of nutrient-rich Upper Circumpolar Deep Water to the surface mixed layer in the submarine canyon has been hypothesized to drive increased phytoplankton biomass, attracting krill, penguins and other top predators to the area. However, observations in Palmer Deep Canyon lack a clear in-situ upwelling signal, laboratory experiments do not illustrate a physiological response by phytoplankton to Upper Circumpolar Deep Water, and surface residence times are too short for phytoplankton populations to reasonably respond to any locally upwelled nutrients. This suggests that local upwelling may not be the mechanism that links Palmer Deep Canyon to increased biological activity. Previous observations of isopycnal doming within the canyon suggested that a subsurface recirculating feature may be present. Here, using in-situ measurements and a circulation model, we demonstrate that the presence of a recirculating eddy may contribute to the maintenance of the biological hotspot by increasing residence times at depth and retaining a distinct layer of biological particles. Neutrally buoyant particle simulations showed that residence times increase to ∼175 days at 150 m within the canyon during the austral summer. In-situ particle scattering, flow cytometry, and water samples from within the subsurface eddy suggest that retained particles are detrital in nature. Our results suggest that this seasonal, retentive feature in Palmer Deep Canyon is important to the retention of biological material and may contribute to the maintenance of this hotspot.
, , Hugh F. J. Corr
Journal of Geophysical Research: Oceans, Volume 126; https://doi.org/10.1029/2021jc017290

Abstract:
In-situ phase-sensitive radar measurements from the Ronne Ice Shelf (RIS) reveal evidence of intermittent basal accretion periods at several sites that are melting in the long-term mean. Periods when ice is accreted at the ice-shelf base coincide with a decrease in the amplitude of the basal return of up to 4 dB. To quantify basal accretion we constrain simultaneously the dielectric constant, electrical conductivity, and thickness of the accreted ice. We do this by exploring the sensitivity of the received basal echo strength and phase to different transmit frequencies using the radar data in combination with a simple model. Along the western RIS we detect episodic basal accretion events leading to ice accumulation at a rate equivalent to 1-3 mm of meteoric ice per day. The inferred accumulation rates and electromagnetic properties of the accreted ice imply that these events are caused primarily by the deposition of frazil ice crystals. Our findings offer the possibility of monitoring and studying the evolution of boundaries between ice-shelf basal melting and accretion regimes using remote observations, collected from the ice-shelf surface.
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