A. M. Thurnherr

Lamont - Doherty Earth Observatory Columbia University, New York City, New York, United States

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Publications (49)150.77 Total impact

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    ABSTRACT: The Southern Ocean plays a pivotal role in the global ocean circulation and climate1–3. There, the deep water masses of the world ocean upwell to the surface and subsequently sink to intermediate and abyssal depths, forming two overturning cells that exchange substantial quantities of heat and carbon with the atmosphere4,5. The sensitivity of the upper cell to climatic changes in forcing is relatively well established6. However, little is known about how the lower cell responds, and in particular whether small-scale mixing in the abyssal Southern Ocean, an important controlling process of the lower cell7,8, is influenced by atmospheric forcing. Here, we present observational evidence that relates changes in abyssal mixing to oceanic eddy variability on timescales of months to decades. Observational estimates of mixing rates, obtained along a repeat hydrographic transect across Drake Passage, are shown to be dependent on local oceanic eddy energy, derived from moored current meter and altimetric measurements. As the intensity of the regional eddy field is regulated by the Southern Hemisphere westerly winds9,10, our findings suggest that Southern Ocean abyssal mixing and overturning are sensitive to climatic perturbations in wind forcing.
    Nature Geoscience 08/2014; 7:577-582. · 11.67 Impact Factor
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    ABSTRACT: sectors of the Antarctic ice sheet are vulnerable to increases in melting at the bases of fringing ice shelves, with melt rates depending on ocean temperatures and circulations in the sub-ice cavities. Here we analyze an oceanographic data set obtained in austral summer 2009 in Pine Island Bay, which is bounded in the east by the calving front of the Amundsen Sea's fast-moving Pine Island Ice Shelf. The upper-ocean velocity field in the ice-free bay was dominated by a 700 m deep and 50 km wide gyre circulating 1.5 Sv of water clockwise around the bay. Ice cavity water was observed in a surface-intensified and southward-intensified boundary current along the ice front, and in a small ice cove at the end of the southern shear margin of the ice shelf. Repeat measurements in the cove reveal persistent cavity water export of ≈ 0.25 Sv during 10 days of sampling. Vertical velocities in the cove above the ice draft were dominated by buoyancy-frequency oscillations with amplitudes of several cm/s but without significant net upwelling. In combination with the seawater properties, this observation indicates that much of the upwelling occurs within fractured ice near the cove, potentially contributing to weakening the ice shelf shear margin.
    Journal of Geophysical Research: Oceans 02/2014; 119(3). · 3.44 Impact Factor
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    ABSTRACT: The fate of a deep boundary current that originates in the Southeast Pacific and flows southward along the continental slope of South America is elucidated. The current transports poorly ventilated water of low salinity (a type of Pacific Deep Water, PDW), into Drake Passage. East of Drake Passage, the boundary current breaks into fresh anticyclonic eddies, nine examples of which were observed in mooring data from December 2009 to March 2012. The observed eddies appear to originate mainly from a topographic separation point close to 60°W, have typical diameters of 20–60 km and accompanying Rossby numbers of 0.1–0.3. These features are likely to be responsible for transporting PDW meridionally across the Antarctic Circumpolar Current, explaining the near homogenization of Circumpolar Deep Water properties downstream of Drake Passage. This mechanism of boundary current breakdown may constitute an important process in the Southern Ocean overturning circulation. Mid‐depth eddies observed east of Drake PassageEddies contain fresh Pacific Deep WaterEddies induce transport of water properties across the ACC
    Geophysical Research Letters 01/2014; 41(1). · 3.98 Impact Factor
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    ABSTRACT: The vertical position of larvae of vent species above a mid-ocean ridge potentially has a strong effect on their dispersal. Larvae may be advected upward in the buoyant vent plume, or move as a consequence of their buoyancy or by active swimming. Alternatively, they may be retained near the bottom by the topography of the axial trough, or by downward swimming. At vents near 9°50′N on the axis of the East Pacific Rise, evidence for active larval positioning was detected in a comparison between field observations of larvae in the plankton in 2006 and 2007 and distributions of non-swimming larvae in a two-dimensional bio-physical model. In the field, few vent larvae were collected at the level of the neutrally buoyant plume (~75 m above the bottom); their relative abundances at that height were much lower than those of simulated larvae from a near-bottom release in the model. This discrepancy was observed for many vent species, particularly gastropods, suggesting that they may actively remain near the bottom by sinking or swimming downward. Near the seafloor, larval abundance decreased from the ridge axis to 1000 m off axis much more strongly in the observations than in the simulations, again pointing to behavior as a potential regulator of larval transport. We suspect that transport off axis was reduced by downward-moving behavior, which positioned larvae into locations where they were isolated from cross-ridge currents by seafloor topography, such as the walls of the axial valley—which are not resolved in the model. Cross-ridge gradients in larval abundance varied between gastropods and polychaetes, indicating that behavior may vary between taxonomic groups, and possibly between species. These results suggest that behaviorally mediated retention of vent larvae may be common, even for species that have a long planktonic larval duration and are capable of long-distance dispersal.
    Deep Sea Research Part II Topical Studies in Oceanography 08/2013; 92:46–57. · 2.24 Impact Factor
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    ABSTRACT: [1] The spatial distribution of turbulent dissipation rates and internal wavefield characteristics is analyzed across two contrasting regimes of the Antarctic Circumpolar Current (ACC), using microstructure and finestructure data collected as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Mid-depth turbulent dissipation rates are found to increase from in the Southeast Pacific to in the Scotia Sea, typically reaching within a kilometer of the seabed. Enhanced levels of turbulent mixing are associated with strong near-bottom flows, rough topography, and regions where the internal wavefield is found to have enhanced energy, a less-inertial frequency content and a dominance of upward propagating energy. These results strongly suggest that bottom-generated internal waves play a major role in determining the spatial distribution of turbulent dissipation in the ACC. The energy flux associated with the bottom internal wave generation process is calculated using wave radiation theory, and found to vary between 0.8 mW m−2 in the Southeast Pacific and 14 mW m−2 in the Scotia Sea. Typically, 10%–30% of this energy is found to dissipate within 1 km of the seabed. Comparison between turbulent dissipation rates inferred from finestructure parameterizations and microstructure-derived estimates suggests a significant departure from wave-wave interaction physics in the near-field of wave generation sites.
    Journal of Geophysical Research: Oceans 06/2013; 118(6). · 3.44 Impact Factor
  • Journal of Physical Oceanography 08/2012; in press:120803124702009. · 3.18 Impact Factor
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    ABSTRACT: Deep-sea hydrothermal plume particles are known to sequester seawater trace elements and influence ocean-scale biogeochemical budgets. The relative importance of biotic versus abiotic oxidation–reduction and other particle-forming reaction, however, and the mechanisms of seawater trace element sequestration remain unknown. Suspended particulate material was collected from a non-buoyant hydrothermal plume by in situ filtration at 9°50′N East Pacific Rise during a 3-day, 24 sample, time-series. Twenty-three samples were digested for total elemental analysis. One representative sample was selected for particle-by-particle geochemical analyses including elemental composition by X-ray fluorescence, speciation of Fe, S, and C by 1s X-ray absorption near edge structure spectroscopy, and X-ray diffraction. Consistent with past studies, positive linear correlations were observed for P, V, As, and Cr with Fe in the bulk chemistry. Arsenic was associated with both Fe oxyhydroxides and sulfides but not uniformly distributed among either mineral type. Particle aggregation was common. Aggregates were composed of minerals embedded in an organic matrix; the minerals ranged from <20 nm to >10 μm in diameter. The speciation of major mineral forming elements (Fe, Mn, S) was complex. Over 20 different minerals were observed, nine of which were either unpredicted by thermodynamic modeling or had no close match in the thermodynamic database. Sulfur-bearing phases consisted of polysulfides (S6, S8), and metal sulfides (Fe, Cu, Zn, Mn). Four dominant species, Fe oxyhydroxide, Fe monosulfide, pyrrhotite, and pyrite, accounted for >80% of the Fe present. Particulate Mn was prevalent in both oxidized and reduced minerals. The organic matrix was: (1) always associated with minerals, (2) composed of biomolecules, and (3) rich in S. Possible sources of this S-rich organic matter include entrained near vent biomass and in situ production by S-oxidizing microorganisms. These results indicate that particle aggregation with organic material is prevalent in dispersing hydrothermal plume fluxes, as well as in sinking particulate matter at this site. Particle aggregation and organic coatings can influence the reactivity, transport, and residence time of hydrothermal particles in the water column. Thus a biogeochemical approach is critical to understanding the net effect of hydrothermal fluxes on ocean and sedimentary trace element budgets.
    Geochimica et Cosmochimica Acta 07/2012; 88:216–236. · 3.88 Impact Factor
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    Xinfeng Liang, Andreas M Thurnherr
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    ABSTRACT: AbstractMesoscale eddies are ubiquitous in the World Ocean and dominate the energy content on subinertial time scales. Recent theoretical and numerical studies suggest a connection between mesoscale eddies and diapycnal mixing in the deep ocean, especially near rough topography in regions of strong geostrophic flow. However, unambiguous observational evidence for such a connection has not yet been found, and it is still unclear what physical processes are responsible for transferring energy from mesoscale to small-scale processes. Here, the authors present observations demonstrating that finescale variability near the crest of the East Pacific Rise is strongly modulated by low-frequency geostrophic flows, including those due to mesoscale eddies. During times of strong subinertial flows, the authors observed elevated kinetic energy on vertical scales <50 m and in the near-inertial band, predominantly upward-propagating near-inertial waves, and increased incidence of layers with Richardson number . In contr...
    Journal of Physical Oceanography 07/2012; 42(7):1242-1248. · 3.18 Impact Factor
  • A. M. Thurnherr
    Journal of Atmospheric and Oceanic Technology 04/2012; 29(4):597-600. · 1.69 Impact Factor
  • Oceanography (Washington D.C.) 03/2012; 25(1):277-283. · 2.70 Impact Factor
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    ABSTRACT: From the momentof their discovery, chemosynthetic ecosystems in the deep sea have held intrinsic scientific value. At the same time that the scientific community is studying chemosynthetic ecosystems, other sectors are either engaged in, or planning for,activities that may adversely impact these ecosystems. There is an opportunity now to developc onservation strategies for networks of chemosynthetic ecosystem reserves in national and international waters throughcollaboration among concerned stakeholders.
    Marine Policy 03/2012; 36:378-381. · 1.87 Impact Factor
  • Article: Corrigendum
    A. M. Thurnherr
    Journal of Atmospheric and Oceanic Technology 06/2011; 28(6):852-852. · 1.69 Impact Factor
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    ABSTRACT: Atmospheric forcing, which is known to have a strong influence on surface ocean dynamics and production, is typically not considered in studies of the deep sea. Our observations and models demonstrate an unexpected influence of surface-generated mesoscale eddies in the transport of hydrothermal vent efflux and of vent larvae away from the northern East Pacific Rise. Transport by these deep-reaching eddies provides a mechanism for spreading the hydrothermal chemical and heat flux into the deep-ocean interior and for dispersing propagules hundreds of kilometers between isolated and ephemeral communities. Because the eddies interacting with the East Pacific Rise are formed seasonally and are sensitive to phenomena such as El Niño, they have the potential to introduce seasonal to interannual atmospheric variations into the deep sea.
    Science 04/2011; 332(6029):580-3. · 31.20 Impact Factor
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    A.M. Thurnherr
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    ABSTRACT: Vertical velocity is important for ocean dynamics on a vast range of scales, from isotropic turbulence to the global overturning circulation, and directly affects transport of biogeochemical tracers. In spite of this importance, vertical-velocity measurements in the ocean are scarce. In an effort to remedy this situation, a new method has been developed to obtain full-depth profiles of vertical velocity from data collected with standard Lowered Acoustic Doppler Current Profiler (LADCP) systems, such as the ones used during the CLIVAR repeat hydrography sections. Data from LADCP systems, which consist of CTDs and ADCPs lowered on hydrographic wires, are typically processed to obtain full-depth profiles of horizontal velocity. The fundamental difficulty underlying LADCP data processing is that the velocity measurements are relative to the moving instrument package. In order to obtain absolute ocean velocities, the instrument motion must be removed from each ADCP velocity profile. One method for achieving this consists in vertically integrating vertical shear of velocity, which can easily be obtained from LADCP velocity records and is independent of instrument motion, and to reference the resulting baroclinic velocity profiles with external constraints, such as package motion derived from bottom tracking. While this method can, in principle, be applied both to horizontal and to vertical velocity data the resulting uncertainties of \≈3-5 cm\·s<sup>-1</sup> are larger than the typical signal expected for vertical velocity in the ocean. In the new method presented here, vertical instrument motion is estimated from the temporal derivative of CTD pressure. While conceptually extremely simple, practical difficulties arise because vertical package motion (winch speed plus surface-wave induced heave) is usually associated with velocities on the order of 1 m\·s<sup>-1</sup>, whereas instantaneous vertical velocities in the ocean are typically 2 orders of ma- > - > gnitude smaller. Nevertheless, it is found that absolute vertical velocities accurate to \≈\θ.5 cm\·s<sup>-1</sup> can be obtained with available off-the-shelf instrumentation (Teledyne/RDI Workhorse ADCP, SeaBird 9plus CTD), as long as suitably lowpass-filtered high-frequency CTD pressure data are matched carefully to the corresponding ADCP time series. The new method can potentially be applied to available CTD/LADCP data from thousands of profiles collected all over the world's oceans. It is expected that the resulting vertical velocity data will provide novel insights into many dynamical processes, including internal waves, boundary currents, hydraulics, mesoscale and sub-mesoscale eddies, fronts, etc. It may furthermore be possible to use the vertical velocity data to improve "finestructure parameterization methods" that are increasingly being used to study turbulence and mixing from CTD/LADCP data.
    Current, Waves and Turbulence Measurements (CWTM), 2011 IEEE/OES 10th; 04/2011
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    Xinfeng Liang, Andreas M. Thurnherr
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    ABSTRACT: Strong low-frequency currents are revealed in the deep ocean over the EPRSubinertial variability over the EPR is related to the mesoscale eddiesLocal topographic flows also contribute to the subinertial variability
    Geophysical Research Letters 03/2011; 38(6). · 3.98 Impact Factor
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    ABSTRACT: Topography has a strong effect on the physical oceanography over the flanks and crests of the global mid-ocean ridge system. Here, we present an analysis of the hydrography and circulation near the crest of the East Pacific Rise (EPR) between 9° and 10°N, which coincides with an integrated study site (ISS) of the RIDGE2000 program. The analysis is based primarily on survey and mooring data collected during the LADDER project, which aimed to investigate oceanographic and topographic influences on larval retention and dispersal in hydrothermal vent communities. Results indicate that the yearly averaged regional mean circulation is characterized by a westward drift of 0.5–1cms−1 across the EPR axis and by north- and southward flows along the western and eastern upper ridge flanks, respectively. The westward drift is part of a basin-scale zonal flow that extends across most of the Pacific ocean near 10°N, whereas the meridional currents near the ridge crest are a topographic effect. In spite of considerable mesoscale variability, which dominates the regional circulation and dispersal on weekly to monthly time scales, quasi-synoptic surveys carried out during the mooring deployment and recovery cruises indicate sub-inertial circulations that are qualitatively similar to the yearly averaged flow but associated with significantly stronger velocities. Weekly averaged mooring data indicate that the anticyclonically sheared along-flank flows are associated with core speeds as high as 10cms−1 and extend ≈10km off axis and 200m above the ridge-crest topography. Near the northern limit of the study region, the Lamont Seamount Chain rises from the western ridge flank and restricts along-EPR flow to five narrow passages, where peak velocities in excess of 20cms−1 were observed. Outside the region of the ridge-crest boundary currents the density field over the EPR near 10°N is characterized by isopycnals dipping into the ridge flanks. Directly above the EPR axis the ridge-crest boundary currents give rise to an isopycnal dome. During times of strong westward cross-EPR flow isopycnal uplift over the eastern flank causes the cross-ridge density field below the doming isopycnals to be asymmetric, with higher densities over the eastern than over the western flank. The data collected during the LADDER project indicate that dispersal of hydrothermal products from the EPR ISS on long time scales is predominantly to the west, whereas mesoscale variability dominates dispersal on weekly to monthly time scales, which are particularly important in the context of larval dispersal.
    Deep-sea Research Part I-oceanographic Research Papers - DEEP-SEA RES PT I-OCEANOG RES. 01/2011; 58(4):365-376.
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    A. M. Thurnherr, L. C. St. Laurent
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    ABSTRACT: While turbulence levels in the study region are generally low, mixing is intenseAway from seamounts, turbulence/mixing are dominated by internal tideTopographic organization related to seamounts dominate spatial mixing patterns
    Geophysical Research Letters 01/2011; 38(15). · 3.98 Impact Factor
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    Marine Policy 01/2011; 36(2012):378-381. · 1.87 Impact Factor
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    A. M. Thurnherr
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    ABSTRACT: 5 Processing data from Lowered Acoustic Doppler Current Profilers (LADCPs) is not trivial. There are currently (at least) two publicly available software packages that can be used for this purpose: one primarily written by Eric Firing at University of Hawaii and the other one by Martin Visbeck, who wrote it while working at LDEO (Columbia University). The purpose of this "how-to" document is to provide a cookbook-like introduction to using the LDEO software — it is assumed that the reader 10 knows what LADCP processing is all about and knows how to acquire LADCP data. Processing parameters will not be discussed in detail. Over the years many different versions of the LDEO LADCP processing software have been made available. This document was first written after I essentially re-implemented the "user interface" of the then-current LDEO software version (8b) during the CLIVAR P02 cruise in summer 2004. In 15 order to avoid confusion this version with the new interface is called "Version IX". I am striving for ease of maintenance in the face of expected future changes. Therefore, there will be as few specifics (e.g. as to file and directory names) as possible. In particular, you will not find any information on where to get the software — search the net or ask a colleague in the know. It is assumed that the reader of this document is familiar with Matlab and the underlying operating 20 system, whatever it is.
    01/2011;