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Journal of Geophysical Research
American Geophysical Union
Journal of Geophysical Research

American Geophysical Union

2169-9275

Journal of Geophysical Research/Journal Journal of Geophysical Research
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    Pathways, Form Drag, and Turbulence in Simulations of an Ocean Flowing Through an Ice Melange

    Hughes, Kenneth G.
    17页
    查看更多>>摘要：A collection of calved icebergs frozen in place by surrounding sea ice (a "melange") is common in many Greenlandic fjords. Being an obstacle to ocean currents, a melange alters ocean dynamics and thermodynamics over the many kilometers that it extends out in front of the glacier. Here we focus on a set of dynamics-flow pathways, form drag, and turbulence-using many simulations that each resolve the flow around hundreds of volume-occupying, but otherwise passive, cuboid "icebergs." Some streaks of fast flow do arise where currents are squeezed through gaps between icebergs but, on average, the near-surface current is slowed. Maximum average currents speeds are instead found at or below the drafts of the deepest icebergs. Form drag scales with U(2)h when few icebergs exist and smoothly transitions to scaling with NUh(2) when many icebergs exist, where U is the far-field flow velocity, N is the buoyancy frequency, and h is a typical iceberg draft. Turbulence predominately occurs in wakes behind the icebergs (as opposed to below icebergs due to internal wave breaking). Consequently, turbulent dissipation is elevated at depths less than or near the drafts of larger icebergs (similar to 100 m).
      原文链接:
    • NSTL
    • Amer Geophysical Union

    On the Variability of Floc Characteristics in a Shallow Estuary

    Manning, Andrew J.Monismith, StephenFringer, OliverEgan, Galen...
    15页
    查看更多>>摘要：We conducted field work in South San Francisco Bay to examine cohesive sediment flocculation dynamics in a shallow, wave- and current-driven estuarine environment. Drawing on data collected using a suite of acoustic and optical instrumentation over three distinct seasons, we found that the factors driving floc size variability differed substantially when comparing locally sourced sediment (i.e., through wave-driven resuspension) to suspended sediment advected from upstream. Statistical analysis of our extensive field data revealed additional seasonal variability in these trends, with wave stress promoting floc breakup during the summer and winter months, and biological processes encouraging floc growth during the spring productive period. Combining these data with fractal dimension estimates, we found that seasonally varying floc composition can lead to differences in floc settling velocity by a factor of approximately two to five for a given floc size. Finally, by analyzing co-located turbulence and sediment flux measurements from the bottom boundary layer, we present evidence that the relationship between floc size and the inverse turbulent Schmidt number varies with floc structure. These results can be used to inform sediment transport modeling parameterizations in estuarine environments.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Tides, Stratification, and Counter Rotation: The German Bight ROFI in Comparison to Other Regions of Freshwater Influence

    Kopte, RobertBecker, MariusHoltermann, PeterWinter, Christian...
    16页
    查看更多>>摘要：The characteristics of tidal velocity profiles and their relation to stratification are investigated based on high-resolution field data collected at four locations in the German Bight Region of Freshwater Influence (ROFI) in the North Sea. The deployments each include two to three tidal cycles and were conducted during field campaigns in August 2016 and May 2018. The depth-averaged semidiurnal tidal motion is dominated by a standing wave directed toward the coast, but modified by a smaller, coast-parallel progressive wave contribution. The time series of the tidal velocity profiles consistently show tidal asymmetries with higher flood than ebb velocities near the surface and counter-clockwise rotation of the velocity trajectories at depth. Near the surface, phase-locked periodic changes in the sense of rotation within the tidal cycle are evident for three deployments, resulting in periodic counter-rotation of the upper and lower layer. During these episodes, stratification of the water column is observed. Counter-rotation is initiated after a sudden decoupling developing from the surface downward, with subsequent rapid development of stratification and velocity shear. The observed decoupling is most likely triggered by advection of the plume-induced lateral surface density gradient by weakly sheared ebb currents toward the study site. Due to the dominance of the standing wave in the German Bight ROFI, the observed intra-tidal variations of stratification are more similar to the Liverpool Bay and differ significantly from the Rhine ROFI, where the tidal dynamics are controlled by a progressive Kelvin wave.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Monthly to Decadal Variability of Mesoscale Stirring in the California Current System: Links to Upwelling, Climate Forcing, and Chlorophyll Transport

    Giddings, A.Franks, P. J. S.Baumann-Pickering, S.
    20页
    查看更多>>摘要：The California Current System (CCS) is a highly productive eastern boundary upwelling system. Cross-shore transport is driven by horizontal stirring from mesoscale (and submesoscale) eddies, fronts, and filaments, which shift surface productivity away from the narrow upwelling zone inshore. Using an unprecedented 25-year daily record of finite size Lyapunov exponents (FSLEs), we characterize the spatial and temporal patterns of mesoscale stirring in the entire CCS and quantify correlations of FSLEs with satellite-measured chlorophyll a and climate indices. The annual cycle of mesoscale stirring is highly correlated with seasonal upwelling in the CCS, with bands of intense FSLE propagating to the west during the upwelling season. Annual cycles and interannual variability in FSLE intensity were most prominent to the north and inshore of the CCS, and weaker to the south and offshore. Interannual fluctuations in mesoscale stirring showed 3-12 month lagged responses to climate indices, with the CCS oscillating out of phase with waters west and north. Chlorophyll a is positively correlated with FSLE intensity in a meridional band 200-600 km offshore, consistent with the hypothesis that increased horizontal stirring increases the export of chlorophyll a to offshore waters. When mesoscale stirring intensifies during negative phases of the Pacific Decadal Oscillation and Ocean Nino index, we predict greater offshore transport of organic carbon. Our analyses underscore the predictability of annual and interannual cycles of mesoscale stirring and chlorophyll a fluxes in the CCS and their links with climate indices.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Physical Drivers of Biogeochemical Variability in the Polar Front Meander

    Yang, XiangStrutton, Peter G.Phillips, Helen E.Pittman, Nicholas A....
    19页
    查看更多>>摘要：The Southern Ocean plays a vital role in global ocean circulation, and the Polar Front (PF) is one of its most important physical features. The PF meander south of Tasmania, around 153 degrees E, 55 degrees S, is a very dynamic region which spawns mesoscale eddies, and influences local biogeochemistry and sea-air interaction. By using voyage and ancillary data, we investigated the unusually strong spring bloom in the vicinity of the PF meander in 2018. We infer that the upwelling of deep water at the front and in eddies, brings macronutrients and dissolved iron (dFe) to the surface. Chlorophyll concentration peaked at over 0.6 mg m(-3), which is anomalously high for this area. With reduced iron limitation, the physiological characteristics of phytoplankton in the northern, downstream part of the study area also changed. The photochemical efficiency was improved and released this area from its usual high-nutrient low-chlorophyll (HNLC) status. This was mainly indicated by the increase in the dawn Fv/Fm maximum (indictor of photochemical efficiency) from 0.2 to over 0.5. With the biomass increase and healthier community status, we observed consumption of surface dissolved inorganic carbon and increased particulate organic carbon production to about 40 mu mol L-1, forming a weak local carbon sink. Through the investigation of multiple years, a weak positive correlation between mixed layer depth shoaling and phytoplankton growth was found, but there was significant interannual variability in this relationship, likely caused by variable eddy conditions and dFe delivery.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Optimal Precursor Triggering Kuroshio Large Meander Decay Obtained in a Regional Ocean Model

    Liu, XiaWang, QiangZhang, Hui
    18页
    查看更多>>摘要：This study investigates the optimal precursor (OPR) triggering Kuroshio large meander (LM) decay using a conditional nonlinear optimal perturbation (CNOP) approach and a regional ocean modeling system (ROMS). The results show that the large amplitudes of the OPR are mainly concentrated at 1,000-2,500 m in the LM upstream region (west of 137 degrees E), and their propagation downstream along the Kuroshio axis contributes to triggering the transition from the LM path to the nonlarge meander (NLM) path. Using the growth of the OPR, it is found that baroclinic instability contributes to the fast vertical development of the perturbations in the early stage of the LM decay, while barotropic processes play a leading role during the cyclonic ring shedding of the LM decay. In particular, the LM decay process is evaluated by the depth-integrated vorticity equation, implying that the advection term dominates over the beta effect in the LM decay. Furthermore, the vorticity balance associated with the cyclonic eddy indicates that the advection and divergence terms represent a main balance in the spin-down process of the cyclonic eddy. Under the effects of both, high potential vorticity water flows out from the LM meander tip and then the amplitude of the LM experiences a remarkable decrease. With the detachment of a cyclonic ring, a transition from the LM path to the NLM path occurs. This study further deepens our understanding of the LM decay and provides us some useful hints about designing the observation system of the LM.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    PAHs in the North Atlantic Ocean and the Arctic Ocean: Spatial Distribution and Water Mass Transport

    Liu, MengyangCai, MinggangDuan, MengshanChen, Meng...
    14页
    查看更多>>摘要：In the Arctic Ocean, it is still unclear what role oceanic transport plays in the fate of semivolatile organic compounds. The strong-stratified Arctic Ocean undergoes complex inputs and outputs of polycyclic aromatic hydrocarbons (PAHs) from the neighboring oceans and continents. To better understand PAHs' transport processes and their contribution to high-latitude oceans, surface seawater, and water column, samples were collected from the North Atlantic Ocean and the Arctic Ocean in 2012. The spatial distribution of dissolved PAHs ( n-ary sumation (9)PAH) in surface seawater showed an "Arctic Shelf > Atlantic Ocean > Arctic Basin" pattern, with a range of 0.3-10.2 ng L-1. Positive matrix factorization modeling results suggested that vehicle emissions and biomass combustion were the major PAHs sources in the surface seawater. According to principal component analysis, PAHs in different water masses showed unique profiles indicating their different origins. Carried by the Norwegian Atlantic Current (0-800 m) and East Greenland Current (0-300 m), PAH individuals' net transport mass fluxes ranged from -4.4 +/- 1.7 to 53 +/- 39 tons year(-1) to the Arctic Ocean. We suggested the limited contribution of ocean currents on PAHs' delivery to the Arctic Ocean, but their role in modulating PAHs' air-sea interactions and other biogeochemical processes needs further studies.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Distinct Oceanic Responses at Rapidly Intensified and Weakened Regimes of Tropical Cyclone Ockhi (2017)

    Jyothi, LingalaJoseph, SudheerSuneetha, P.Huber, Matthew...
    22页
    查看更多>>摘要：This study analyzed the upper ocean responses to tropical cyclone (TC) Ockhi (2017) from two regions where it underwent (a) rapid intensification (RI) and (b) rapid weakening (RW) using simulations from the high-resolution HYbrid Coordinate Ocean Model. Pre-existed oceanic conditions in the RI region were warmer and fresher than in the RW region. Though the translation speed was slower over the RI region, the storm-induced sea surface temperature cooling was weaker (-0.8 degrees C) while a significant cooling (-1.9 degrees C) was observed at RW. The mixed layer heat budget analysis revealed that the dominant process driving the temperature tendency in the RI and RW regions were surface thermal forcing (-0.01 degrees C h(-1)) and entrainment (-0.015 degrees C h(-1)), respectively. The role of entrainment was restrained by a thick barrier layer in the RI region. The TC-induced mixing over a weakly stratified ocean at RW resulted in a significant drop in dynamic temperature (T-dy) compared to the RI. The cooling differences were attributed to the contrasting salinity stratification at RI and RW regimes which modulated the ocean's negative feedback to TC intensity changes. Further, the difference between the depths of mixing length and 26 degrees C isotherm at RI and RW regimes indicate that 26 degrees C isotherm does not necessarily represent the mixing depth under the influence of TC. Hence, this study suggests T-dy may be a better metric for cyclone intensity changes than heat content above a fixed isotherm, particularly in the salinity stratified regions.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Linking Phytoplankton Variability to Atmospheric Blocking in an Eastern Boundary Upwelling System

    Le, ChengfengWu, MingSun, HongweiLong, Shang-min...
    12页
    查看更多>>摘要：The oceanic ecosystems in the eastern boundary upwelling systems (EBUS) are expected to be influenced by climate change, but specific mechanisms describing their connections are not well understood. Using a 20-year time-series record of satellite and climate data, we identified a strong linkage between phytoplankton variability in a EBUS and a high latitude, large-scale atmospheric process. The winter-spring phytoplankton variability in the Canary Current Upwelling System was associated to the atmospheric blocking events over Greenland. These Greenland blocking events were primarily caused by the persistence of quasi-stationary, large-scale high atmosphere pressure fields. These fields forced the eddy-driven jet stream to shift south, which caused a weakening of the equatorward alongshore winds over the upwelling system. As a result, the wind-driven upwelling was suppressed, and phytoplankton growth was inhibited. These results identified a previously unknown source driving interannual phytoplankton variability in the EBUS, underscoring the importance of contributions of atmospheric connections on the coastal upwelling ecosystems.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Bottom-Reached Near-Inertial Waves Induced by the Tropical Cyclones, Conson and Mindulle, in the South China Sea

    Ma, YongguiWang, DongxiaoShu, YeqiangChen, Ju...
    13页
    查看更多>>摘要：Moored current observations in the northwestern South China Sea (SCS) captured two events of near-inertial waves (NIWs) induced by the tropical cyclones Conson and Mindulle, which propagated to the seafloor from the base of the mixed layer. The time taken by the Conson-induced near-inertial kinetic energy (NIKE) to reach the bottom of the ocean was approximately 12 days. This NIKE was twice as fast as the Mindulle-induced NIKE to reach the bottom of the ocean, which was mainly dependent on the properties of the NIWs deeply affected by the background vorticity. The NIWs in the upper layer induced by Conson/Mindulle showed a significant blue/red shift (1.07/0.85 f $f$) as the observation site was located outside/within an exceptional anticyclonic eddy. The bottom NIWs showed an obvious blue shift, aligning with the result of ray tracing, which revealed that the bottom wave packets originated from higher latitudes. We estimated an NIW vertical energy flux of up to 0.22 m W m(-2) in the abyssal ocean, which represents about 29% of the upper layer NIKE. The reached seabed NIWs had high vertical shear and might dissipate locally and play an important role in the maintenance of deep mixing of the SCS.
      原文链接:
    • NSTL
    • Amer Geophysical Union