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(searched for: doi:10.1029/2012jc008369)
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, Franziska U. Schwarzkopf, , , , Markus Scheinert, Tobias Schulzki, , ,
Published: 2 September 2021
Ocean Science, Volume 17, pp 1177-1211; https://doi.org/10.5194/os-17-1177-2021

Abstract:
A hierarchy of global 1/4∘ (ORCA025) and Atlantic Ocean 1/20∘ nested (VIKING20X) ocean–sea-ice models is described. It is shown that the eddy-rich configurations performed in hindcasts of the past 50–60 years under CORE and JRA55-do atmospheric forcings realistically simulate the large-scale horizontal circulation, the distribution of the mesoscale, overflow and convective processes, and the representation of regional current systems in the North and South Atlantic. The representation of the Atlantic Meridional Overturning Circulation (AMOC), and in particular the long-term temporal evolution, strongly depends on numerical choices for the application of freshwater fluxes. The interannual variability of the AMOC instead is highly correlated among the model experiments and also with observations, including the 2010 minimum observed by RAPID at 26.5∘ N. This points to a dominant role of the wind forcing. The ability of the model to represent regional observations in western boundary current (WBC) systems at 53∘ N, 26.5∘ N and 11∘ S is explored. The question is investigated of whether WBC systems are able to represent the AMOC, and in particular whether these WBC systems exhibit similar temporal evolution to that of the zonally integrated AMOC. Apart from the basin-scale measurements at 26.5∘ N, it is shown that in particular the outflow of North Atlantic Deepwater at 53∘ N is a good indicator of the subpolar AMOC trend during the recent decades, once provided in density coordinates. The good reproduction of observed AMOC and WBC trends in the most reasonable simulations indicate that the eddy-rich VIKING20X is capable of representing realistic forcing-related and ocean-intrinsic trends.
Muping Zhou, Guihua Wang, Wenhu Liu, Changlin Chen
Journal of Physical Oceanography, Volume 50, pp 2953-2963; https://doi.org/10.1175/jpo-d-20-0013.1

Abstract:
The existence of a deep western boundary current (DWBC) in the South China Sea (SCS) was verified by direct observations from three current moorings deployed from September 2015 to September 2018. The average current speeds observed in the DWBC were around 1 cm s−1 along the northern boundary and 3 cm s−1 along the western boundary. The DWBC demonstrates significant intraseasonal variability in the 30–120-day-period band, which may come from the variability in the Luzon overflow or the eddies in the deep SCS forced by a stable Luzon overflow. In addition, observations found that this DWBC along the northern boundary can reverse its direction meridionally in the spring. Model results suggest that if the Luzon overflow decreases one-third of its typical transport, this current reversal can occur. This behavior can be explained through “relaxation” theory.
, , D. Rayner, , I. Le Bras, , , S. Bacon
Journal of Geophysical Research: Oceans, Volume 124, pp 3246-3278; https://doi.org/10.1029/2018jc014730

Abstract:
The Deep Western Boundary Current (DWBC) in the subpolar North Atlantic is the lower limb of the Atlantic Meridional Overturning Circulation and a key component of the global climate system. Here, a mooring array deployed at 60°N in the Irminger Sea, between 2014 and 2016 provides the longest continuous record of total DWBC volume transport at this latitude. The 1.8 year averaged transport of water denser than σθ = 27.8 kg m‐3 was ‐10.8 ± 4.9 Sv (mean ± 1 std; 1 Sv = 106 m3 s‐1). Of this total, we find ‐4.1 ± 1.4 Sv within the densest layer (σθ > 27.88 kg m‐3) that originated from the Denmark Strait Overflow. The lighter North East Atlantic Deep Water layer (σθ = 27.8‐27.88 kg m‐3) carries ‐6.5 ± 7.7 Sv. The variability in transport ranges between 2 and 65 days. There is a distinct shift from high to low frequency with distance from the East Greenland slope. High frequency fluctuations (2‐8 days) close to the continental slope are likely associated with topographic Rossby waves and/or cyclonic eddies. Here, perturbations in layer thickness make a significant (20‐60%) contribution to transport variability. In deeper water, toward the centre of the Irminger Basin, transport variance at 55 days dominates. Our results suggest that there has been a 1.8 Sv increase in total transport since 2005‐2006, but this difference can be accounted for by a range of methodological and data limitation biases.
, , Isabela A. Le Bras, , Michael S. McCartney
Journal of Geophysical Research: Oceans, Volume 122, pp 7488-7505; https://doi.org/10.1002/2017jc012984

Abstract:
A moored array spanning the continental slope southeast of Cape Cod sampled the equatorward-flowing Deep Western Boundary Current (DWBC) for a 10 year period: May 2004 to May 2014. Daily profiles of subinertial velocity, temperature, salinity, and neutral density are constructed for each mooring site and cross-line DWBC transport time series are derived for specified water mass layers. Time-averaged transports based on daily estimates of the flow and density fields in Stream coordinates are contrasted with those derived from the Eulerian-mean flow field, modes of DWBC transport variability are investigated through compositing, and comparisons are made to transport estimates for other latitudes. Integrating the daily velocity estimates over the neutral density range of 27.8–28.125 kg/m3 (encompassing Labrador Sea and Overflow Water layers), a mean equatorward DWBC transport of 22.8 × 106 ± 1.9 × 106 m3/s is obtained. Notably, a statistically significant trend of decreasing equatorward transport is observed in several of the DWBC components as well as the current as a whole. The largest linear change (a 4% decrease per year) is seen in the layer of Labrador Sea Water that was renewed by deep convection in the early 1990s whose transport fell from 9.0 × 106 m3/s at the beginning of the field program to 5.8 × 106 m3/s at its end. The corresponding linear fit to the combined Labrador Sea and Overflow Water DWBC transport decreases from 26.4 × 106 to 19.1 × 106 m3/s. In contrast, no long-term trend is observed in upper ocean Slope Water transport. These trends are discussed in the context of decadal observations of the North Atlantic circulation, and subpolar air-sea interaction/water mass transformation.
Chun Zhou, , Jiwei Tian, Xiaolong Zhao, Yuchao Zhu, Qingxuan Yang, Tangdong Qu
Published: 24 August 2017
Scientific Reports, Volume 7, pp 1-7; https://doi.org/10.1038/s41598-017-09436-2

Abstract:
Deep western boundary current (DWBC) was observed for the first time by an array of 6 current meter moorings southeast of the Zhongsha Islands in the South China Sea (SCS) deep basin during the period from August 2012 to January 2014. In the mean, the DWBC in the SCS flows southwestward with core velocity of 2.0 cm/s and a volume transport of 1.65 Sv (1 Sv = 1 × 106 m3/s). Its temporal variability is dominated by intraseasonal fluctuations with period around 90 days. The main axis of the DWBC, characterized by a low temperature core, tends not to shift with the 90-day fluctuation.
Isabela Astiz Le Bras, ,
Journal of Geophysical Research: Oceans, Volume 122, pp 5348-5366; https://doi.org/10.1002/2017jc012921

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, Steven J. Manganini, Jongjin Park, Daniel B. Montluçon, ,
Journal of Geophysical Research: Oceans, Volume 122, pp 4539-4553; https://doi.org/10.1002/2016jc012549

Abstract:
Biogenic matter characteristics and radiocarbon contents of organic carbon (OC) were examined on sinking particle samples intercepted at three nominal depths of 1000 m, 2000 m, and 3000 m (∼50 m above the seafloor) during a 3 year sediment trap program on the New England slope in the Northwest Atlantic. We have sought to characterize the sources of sinking particles in the context of vertical export of biogenic particles from the overlying water column and lateral supply of resuspended sediment particles from adjacent margin sediments. High aluminum (Al) abundances and low OC radiocarbon contents indicated contributions from resuspended sediment which was greatest at 3000 m but also significant at shallower depths. The benthic source (i.e., laterally supplied resuspended sediment) of opal appears negligible based on the absence of a correlation with Al fluxes. In comparison, CaCO3 fluxes at 3000 m showed a positive correlation with Al fluxes. Benthic sources accounted for 42 ∼ 63% of the sinking particle flux based on radiocarbon mass balance and the relationship between Al flux and CaCO3 flux. Episodic pulses of Al at 3000 m were significantly correlated with the near-bottom current at a nearby hydrographic mooring site, implying the importance of current variability in lateral particle transport. However, Al fluxes at 1000 m and 2000 m were coherent but differed from those at 3000 m, implying more than one mode of lateral supply of particles in the water column.
Geophysical & Astrophysical Fluid Dynamics, Volume 108, pp 363-386; https://doi.org/10.1080/03091929.2014.891023

Abstract:
The cross-equatorial flow of grounded abyssal ocean currents in a differentially rotating meridional channel with parabolic bottom topography is examined. In particular, the dependence is determined of the cross-equatorial volume flux on the underlying flow parameters including the slope of the channel’s walls , the half-width of the channel , the half-width and height of the abyssal current and , respectively, the magnitude of the rotation vector , the Earth’s radius , and the reduced gravity . In addition, it is shown that the ratio between the width of the channel and the zonal wavelength of a narrow wave structure that is formed by the current in the equatorial region plays a crucial role in determining into which hemisphere the current flows after its interaction with the equator. It is found that some parameters (e.g. and ) do not have any significant effect on the zonal wavelength, while variations in other parameters (e.g. , , , and ) change the zonal wavelength and, consequently, can dramatically alter the qualitative trans-equatorial behavior of the abyssal current. After examining an auxiliary model of a particle in a rotating equatorial channel, it is shown that the zonal wavelength of the equatorial wave is linearly proportional to the equatorial length scale defined as , where is the equatorial value of the beta-parameter.
, , R. Zantopp, , , E. Behrens, C.W. Böning, D. Quadfasel, K. Jochumsen, H. Valdimarsson, et al.
Published: 13 April 2014
Progress in Oceanography, Volume 132, pp 233-249; https://doi.org/10.1016/j.pocean.2014.04.002

Abstract:
The Deep Western Boundary Current (DWBC) along the western margin of the subpolar North Atlantic is an important component of the deep limb of the Meridional Overturning near its northern origins. A network of moored arrays from Denmark Strait to the tail of the Grand Banks has been installed for almost two decades to observe the boundary currents and transports of North Atlantic Deep Water as part of an internationally coordinated observatory for the Atlantic Meridional Overturning Circulation. The dominant variability in all of the moored velocity time series is in the week-to-month period range. While the temporal characteristics of this variability change only gradually between Denmark Strait and Flemish Cap, a broad band of longer term variability is present farther along the path of the DWBC at the Grand Banks and in the interior basins (Labrador and Irminger Seas). The vigorous intra-seasonal variability may well mask possible interannual to decadal variability that is typically an order of magnitude smaller than the high-frequency fluctuations. Here, the intra-seasonal variability is quantified at key positions along the DWBC path using both, observations and high resolution model data. The results are used to evaluate the model circulation, and in turn the model is used to relate the discrete measurements to the overall pattern of the subpolar circulation. Topographic waves are found to be trapped by the steep topography all around the western basins, the Labrador and Irminger Seas. In the Labrador Sea, the high intra-seasonal variability of the boundary current regime is separated by a region of extremely low variability in narrow recirculation cells from the basin interior. There, the variability is also on intra-seasonal time scales, but at much longer periods around 50 days.
Journal of Geophysical Research: Oceans, Volume 118, pp 6461-6478; https://doi.org/10.1002/2013jc009228

Abstract:
[1] Data from two boundary arrays deployed along 34.5°S are combined to produce the first continuous in situ time series observations of the basin-wide meridional overturning circulation (MOC) in the South Atlantic. Daily estimates of the MOC between March 2009 and December 2010 range between 3 Sv and 39 Sv (1 Sv = 106 m3 s−1) after a 10 day low-pass filter is applied. Much of the variability in this ∼20 month record occurs at periods shorter than 100 days. Approximately two-thirds of the MOC variability is due to changes in the geostrophic (baroclinic plus barotropic) volume transport, with the remainder associated with the direct wind-forced Ekman transport. When low-pass filtered to match previously published analyses in the North Atlantic, the observed temporal standard deviation at 34.5°S matches or somewhat exceeds that observed by time series observations at 16°N, 26.5°N, and 41°N. For periods shorter than 20 days the basin-wide MOC variations are most strongly influenced by Ekman flows, while at periods between 20 and 90 days the geostrophic flows tend to exert slightly more control over the total transport variability of the MOC. The geostrophic shear variations are roughly equally controlled by density variations on the western and eastern boundaries at all time scales captured in the record. The observed time-mean MOC vertical structure and temporal variability agree well with the limited independent observations available for confirmation.
Journal of Physical Oceanography, Volume 43, pp 905-919; https://doi.org/10.1175/jpo-d-12-0150.1

Abstract:
Interactions between vortices and a shelfbreak current are investigated, with particular attention to the exchange of waters between the continental shelf and slope. The nonlinear, three-dimensional interaction between an anticyclonic vortex and the shelfbreak current is studied in the laboratory while varying the ratio ε of the maximum azimuthal velocity in the vortex to the maximum alongshelf velocity in the shelfbreak current. Strong interactions between the shelfbreak current and the vortex are observed when ε > 1; weak interactions are found when ε < 1. When the anticyclonic vortex comes in contact with the shelfbreak front during a strong interaction, a streamer of shelf water is drawn offshore and wraps anticyclonically around the vortex. Measurements of the offshore transport and identification of the particle trajectories in the shelfbreak current drawn offshore from the vortex allow quantification of the fraction of the shelfbreak current that is deflected onto the slope; this fraction increases for increasing values of ε. Experimental results in the laboratory are strikingly similar to results obtained from observations in the Middle Atlantic Bight (MAB); after proper scaling, measurements of offshore transport and offshore displacement of shelf water for vortices in the MAB that span a range of values of ε agree well with laboratory predictions.
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