von Appen Wilken-Jon

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  • Preprint
    Water column structure and statistics of denmark strait overflow water cyclones
    ( 2013-10-30) von Appen, Wilken-Jon ; Pickart, Robert S. ; Brink, Kenneth H. ; Haine, Thomas W. N.
    Data from seven moorings deployed across the East Greenland shelfbreak and slope 280 km downstream of Denmark Strait are used to investigate the characteristics and dynamics of Denmark Strait Overflow Water (DSOW) cyclones. On average, a cyclone passes the mooring array every other day near the 900 m isobath, dominating the variability of the boundary current system. There is considerable variation in both the frequency and location of the cyclones on the slope, but no apparent seasonality. Using the year-long data set from September 2007 to October 2008, we construct a composite DSOW cyclone that reveals the average scales of the features. The composite cyclone consists of a lens of dense overflow water on the bottom, up to 300 m thick, with cyclonic flow above the lens. The azimuthal flow is intensified in the middle and upper part of the water column and has the shape of a Gaussian eddy with a peak depth-mean speed of 0.22 m/s at a radius of 7.8 km. The lens is advected by the mean flow of 0.27 m/s and self propagates at 0.45 m/s, consistent with the topographic Rossby wave speed and the Nof speed. The total translation velocity along the East Greenland slope is 0.72 m/s. The self-propagation speed exceeds the cyclonic swirl speed, indicating that the azimuthal flow cannot kinematically trap fluid in the water column above the lens. This implies that the dense water anomaly and the cyclonic swirl velocity are dynamically linked, in line with previous theory. Satellite sea surface temperature (SST) data are investigated to study the surface expression of the cyclones. Disturbances to the SST field are found to propagate less quickly than the in-situ DSOW cyclones, raising the possibility that the propagation of the SST signatures is not directly associated with the cyclones.
  • Article
    Eddies and the distribution of eddy kinetic energy in the Arctic Ocean
    (Oceanography Society, 2022-04-27) von Appen, Wilken-Jon ; Baumann, Till M. ; Janout, Markus A. ; Koldunov, Nikolay ; Lenn, Yueng-Djern ; Pickart, Robert S. ; Scott, Robert B. ; Wang, Qiang
    Mesoscale eddies are important to many aspects of the dynamics of the Arctic Ocean. Among others, they maintain the halocline and interact with the Atlantic Water circumpolar boundary current through lateral eddy fluxes and shelf-basin exchanges. Mesoscale eddies are also important for transporting biological material and for modifying sea ice distribution. Here, we review what is known about eddies and their impacts in the Arctic Ocean in the context of rapid climate change. Eddy kinetic energy (EKE) is a proxy for mesoscale variability in the ocean due to eddies. We present the first quantification of EKE from moored observations across the entire Arctic Ocean and compare those results to output from an eddy resolving numerical model. We show that EKE is largest in the northern Nordic Seas/Fram Strait and it is also elevated along the shelf break of the Arctic Circumpolar Boundary Current, especially in the Beaufort Sea. In the central basins, EKE is 100–1,000 times lower. Generally, EKE is stronger when sea ice concentration is low versus times of dense ice cover. As sea ice declines, we anticipate that areas in the Arctic Ocean where conditions typical of the North Atlantic and North Pacific prevail will increase. We conclude that the future Arctic Ocean will feature more energetic mesoscale variability.
  • Article
    Frontogenesis and variability in Denmark Strait and its influence on overflow water
    (American Meteorological Society, 2019-07-01) Spall, Michael A. ; Pickart, Robert S. ; Lin, Peigen ; von Appen, Wilken-Jon ; Mastropole, Dana M. ; Valdimarsson, Héðinn ; Haine, Thomas W. N. ; Almansi, Mattia
    A high-resolution numerical model, together with in situ and satellite observations, is used to explore the nature and dynamics of the dominant high-frequency (from one day to one week) variability in Denmark Strait. Mooring measurements in the center of the strait reveal that warm water “flooding events” occur, whereby the North Icelandic Irminger Current (NIIC) propagates offshore and advects subtropical-origin water northward through the deepest part of the sill. Two other types of mesoscale processes in Denmark Strait have been described previously in the literature, known as “boluses” and “pulses,” associated with a raising and lowering of the overflow water interface. Our measurements reveal that flooding events occur in conjunction with especially pronounced pulses. The model indicates that the NIIC hydrographic front is maintained by a balance between frontogenesis by the large-scale flow and frontolysis by baroclinic instability. Specifically, the temperature and salinity tendency equations demonstrate that the eddies act to relax the front, while the mean flow acts to sharpen it. Furthermore, the model reveals that the two dense water processes—boluses and pulses (and hence flooding events)—are dynamically related to each other and tied to the meandering of the hydrographic front in the strait. Our study thus provides a general framework for interpreting the short-time-scale variability of Denmark Strait Overflow Water entering the Irminger Sea.
  • Preprint
    The East Greenland Spill Jet as an important component of the Atlantic Meridional Overturning Circulation
    ( 2014-06-03) von Appen, Wilken-Jon ; Koszalka, Inga M. ; Pickart, Robert S. ; Haine, Thomas W. N. ; Mastropole, Dana M. ; Magaldi, Marcello G. ; Valdimarsson, Héðinn ; Girton, James B. ; Jochumsen, Kerstin ; Krahmann, Gerd
    The recently discovered East Greenland Spill Jet is a bottom-intensified current on the upper continental slope south of Denmark Strait, transporting intermediate density water equatorward. Until now the Spill Jet has only been observed with limited summertime measurements from ships. Here we present the first year-round mooring observations demonstrating that the current is a ubiquitous feature with a volume transport similar to the well-known plume of Denmark Strait overflow water farther downslope. Using reverse particle tracking in a high-resolution numerical model, we investigate the upstream sources feeding the Spill Jet. Three main pathways are identified: particles flowing directly into the Spill Jet from the Denmark Strait sill; particles progressing southward on the East Greenland shelf that subsequently spill over the shelfbreak into the current; and ambient water from the Irminger Sea that gets entrained into the flow. The two Spill Jet pathways emanating from Denmark Strait are newly resolved, and long-term hydrographic data from the strait verifies that dense water is present far onto the Greenland shelf. Additional measurements near the southern tip of Greenland suggest that the Spill Jet ultimately merges with the deep portion of the shelfbreak current, originally thought to be a lateral circulation associated with the sub-polar gyre. Our study thus reveals a previously unrecognized significant component of the Atlantic Meridional Overturning Circulation that needs to be considered to understand fully the ocean’s role in climate.
  • Article
    Two configurations of the Western Arctic Shelfbreak Current in summer
    (American Meteorological Society, 2012-03) von Appen, Wilken-Jon ; Pickart, Robert S.
    Data from a closely spaced array of moorings situated across the Beaufort Sea shelfbreak at 152°W are used to study the Western Arctic Shelfbreak Current, with emphasis on its configuration during the summer season. Two dynamically distinct states of the current are revealed in the absence of wind, with each lasting approximately one month. The first is a surface-intensified shelfbreak jet transporting warm and buoyant Alaskan Coastal Water in late summer. This is the eastward continuation of the Alaskan Coastal Current. It is both baroclinically and barotropically unstable and hence capable of forming the surface-intensified warm-core eddies observed in the southern Beaufort Sea. The second configuration, present during early summer, is a bottom-intensified shelfbreak current advecting weakly stratified Chukchi Summer Water. It is baroclinically unstable and likely forms the middepth warm-core eddies present in the interior basin. The mesoscale instabilities extract energy from the mean flow such that the surface-intensified jet should spin down over an e-folding distance of 300 km beyond the array site, whereas the bottom-intensified configuration should decay within 150 km. This implies that Pacific Summer Water does not extend far into the Canadian Beaufort Sea as a well-defined shelfbreak current. In contrast, the Pacific Winter Water configuration of the shelfbreak jet is estimated to decay over a much greater distance of approximately 1400 km, implying that it should reach the first entrance to the Canadian Arctic Archipelago.
  • Article
    Structure and variability of the shelfbreak East Greenland Current north of Denmark Strait
    (American Meteorological Society, 2017-10-31) Håvik, Lisbeth ; Våge, Kjetil ; Pickart, Robert S. ; Harden, Benjamin E. ; von Appen, Wilken-Jon ; Jónsson, Steingrímur ; Østerhus, Svein
    Data from a mooring array deployed north of Denmark Strait from September 2011 to August 2012 are used to investigate the structure and variability of the shelfbreak East Greenland Current (EGC). The shelfbreak EGC is a surface-intensified current situated just offshore of the east Greenland shelf break flowing southward through Denmark Strait. This study identified two dominant spatial modes of variability within the current: a pulsing mode and a meandering mode, both of which were most pronounced in fall and winter. A particularly energetic event in November 2011 was related to a reversal of the current for nearly a month. In addition to the seasonal signal, the current was associated with periods of enhanced eddy kinetic energy and increased variability on shorter time scales. The data indicate that the current is, for the most part, barotropically stable but subject to baroclinic instability from September to March. By contrast, in summer the current is mainly confined to the shelf break with decreased eddy kinetic energy and minimal baroclinic conversion. No other region of the Nordic Seas displays higher levels of eddy kinetic energy than the shelfbreak EGC north of Denmark Strait during fall. This appears to be due to the large velocity variability on mesoscale time scales generated by the instabilities. The mesoscale variability documented here may be a source of the variability observed at the Denmark Strait sill.
  • Article
    Structure, transport, and seasonality of the Atlantic Water boundary current north of Svalbard: Results from a yearlong mooring array
    (American Geophysical Union, 2019-02-15) Pérez-Hernández, M. Dolores ; Pickart, Robert S. ; Torres, Daniel J. ; Bahr, Frank B. ; Sundfjord, Arild ; Ingvaldsen, Randi B. ; Renner, Angelika H. H. ; Beszczynska-Möller, Agnieszka ; von Appen, Wilken‐Jon ; Pavlov, Vladimir
    The characteristics and seasonality of the Svalbard branch of the Atlantic Water (AW) boundary current in the Eurasian Basin are investigated using data from a six‐mooring array deployed near 30°E between September 2012 and September 2013. The instrument coverage extended to 1,200‐m depth and approximately 50 km offshore of the shelf break, which laterally bracketed the flow. Averaged over the year, the transport of the current over this depth range was 3.96 ± 0.32 Sv (1 Sv = 106 m3/s). The transport within the AW layer was 2.08 ± 0.24 Sv. The current was typically subsurface intensified, and its dominant variability was associated with pulsing rather than meandering. From late summer to early winter the AW was warmest and saltiest, and its eastward transport was strongest (2.44 ± 0.12 Sv), while from midspring to midsummer the AW was coldest and freshest and its transport was weakest (1.10 ± 0.06 Sv). Deep mixed layers developed through the winter, extending to 400‐ to 500‐m depth in early spring until the pack ice encroached the area from the north shutting off the air‐sea buoyancy forcing. This vertical mixing modified a significant portion of the AW layer, suggesting that, as the ice cover continues to decrease in the southern Eurasian Basin, the AW will be more extensively transformed via local ventilation.
  • Article
    Interannual variability in the pathways of the North Atlantic Current over the Mid-Atlantic Ridge and the impact of topography
    (American Meteorological Society, 2008-01) Bower, Amy S. ; von Appen, Wilken-Jon
    Recent studies have indicated that the North Atlantic Ocean subpolar gyre circulation undergoes significant interannual-to-decadal changes in response to variability in atmospheric forcing. There are also observations, however, suggesting that the southern limb of the subpolar gyre, namely, the eastward-flowing North Atlantic Current (NAC), may be quasi-locked to particular latitudes in the central North Atlantic by fracture zones (gaps) in the Mid-Atlantic Ridge. This could constrain the current’s ability to respond to variability in forcing. In the present study, subsurface float trajectories at 100–1000 m collected during 1997–99 and satellite-derived surface geostrophic velocities from 1992 to 2006 are used to provide an improved description of the detailed pathways of the NAC over the ridge and their relationship to bathymetry. Both the float and satellite observations indicate that in 1997–99, the northern branch of the NAC was split into two branches as it crossed the ridge, one quasi-locked to the Charlie–Gibbs Fracture Zone (CGFZ; 52°–53°N) and the other to the Faraday Fracture Zone (50°–51°N). The longer satellite time series shows, however, that this pattern did not persist outside the float sampling period and that other branching modes persisted for one or more years, including an approximately 12-month time period in 2002–03 when the strongest eastward flow over the ridge was at 49°N. Schott et al. showed how northward excursions of the NAC can temporarily block the westward flow of the Iceland–Scotland Overflow Water through the CGFZ. From the 13-yr time series of surface geostrophic velocity, it is estimated that such blocking may occur on average 6% of the time, although estimates for any given 12-month period range from 0% to 35%.
  • Thesis
    Moored observations of shelfbreak processes at the inflow to and outflow from the Arctic Ocean
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2013-02) von Appen, Wilken-Jon
    Two high-resolution mooring arrays extending from the outer shelf to the mid continental slope are used to elucidate shelf-basin exchange at the inflow to and the outflow from the Arctic Ocean. Pacific Water entering the Arctic Ocean forms the Western Arctic shelfbreak current along the Beaufort Sea slope. Data from the mooring array at 152°W—approximately 150 km east of Pt. Barrow, AK—reveals that this current has two distinct states in summer depending on the water mass it transports. When advecting Alaskan Coastal Water it is surface-intensified and both baroclinically and barotropically unstable. This configuration lasts about a month with an average transport of 0.7 Sv. When advecting Chukchi Summer Water the current is bottom-intensified and is only baroclinically unstable. This state also exists for approximately a month with an average transport of 0.6 Sv. The strong mean-to-eddy energy conversion causes both configurations of the current to spin down over a distance of a few hundred kilometers, suggesting that warm Pacific Water does not enter the Canadian Arctic Archipelago via this route. Dense water formed in the Nordic Seas overflows the Denmark Strait and undergoes vortex stretching, forming intense cyclones that propagate along the East Greenland slope. Data from the mooring array at 65°N—roughly 300 km downstream of Denmark Strait—was used to determine the full water column structure of the cyclones. On average a cyclone passes the array every other day in the vicinity of the 900 m isobath, although the depth range of individual cyclones ranges between the 500 m and 1600 m isobaths. The cyclones self-propagate at 0.45 m/s and are also advected by the mean flow of 0.27 m/s, resulting in a total propagation speed of 0.72 m/s. They have a peak azimuthal speed of 0.22 m/s at a radius of 7.8 km and contain overflow water in their core. In the absence of the cyclones, the background flow is dominated by the East Greenland Spill Jet. This is shown to be a year-round feature transporting 2–4 Sv of dense water equatorward along the upper continental slope.
  • Article
    Evolution of the East Greenland Current from Fram Strait to Denmark Strait : synoptic measurements from summer 2012
    (John Wiley & Sons, 2017-03-13) Håvik, Lisbeth ; Pickart, Robert S. ; Våge, Kjetil ; Torres, Daniel J. ; Thurnherr, Andreas M. ; Beszczynska-Möller, Agnieszka ; Walczowski, Waldemar ; von Appen, Wilken-Jon
    We present measurements from two shipboard surveys conducted in summer 2012 that sampled the rim current system around the Nordic Seas from Fram Strait to Denmark Strait. The data reveal that, along a portion of the western boundary of the Nordic Seas, the East Greenland Current (EGC) has three distinct components. In addition to the well-known shelfbreak branch, there is an inshore branch on the continental shelf as well as a separate branch offshore of the shelfbreak. The inner branch contributes significantly to the overall freshwater transport of the rim current system, and the outer branch transports a substantial amount of Atlantic-origin Water equatorward. Supplementing our measurements with historical hydrographic data, we argue that the offshore branch is a direct recirculation of the western branch of the West Spitsbergen Current in Fram Strait. The total transport of the shelfbreak EGC (the only branch sampled consistently in all of the sections) decreased toward Denmark Strait. The estimated average transport of dense overflow water (rh > 27.8 kg/m3 and h>08C) in the shelfbreak EGC was 2.860.7 Sv, consistent with previous moored measurements. For the three sections that crossed the entire EGC system the freshwater flux, relative to a salinity of 34.8, ranged from 127613 to 8168 mSv. The hydrographic data reveal that, between Fram Strait and Denmark Strait, the core of the Atlantic-origin Water in the shelfbreak EGC cools and freshens but changes very little in density.