Harden Benjamin E.

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Benjamin E.

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Now showing 1 - 6 of 6
  • Article
    Offshore transport of dense water from the East Greenland Shelf
    (American Meteorological Society, 2014-01) Harden, Benjamin E. ; Pickart, Robert S. ; Renfrew, Ian A.
    Data from a mooring deployed at the edge of the East Greenland shelf south of Denmark Strait from September 2007 to October 2008 are analyzed to investigate the processes by which dense water is transferred off the shelf. It is found that water denser than 27.7 kg m−3—as dense as water previously attributed to the adjacent East Greenland Spill Jet—resides near the bottom of the shelf for most of the year with no discernible seasonality. The mean velocity in the central part of the water column is directed along the isobaths, while the deep flow is bottom intensified and veers offshore. Two mechanisms for driving dense spilling events are investigated, one due to offshore forcing and the other associated with wind forcing. Denmark Strait cyclones propagating southward along the continental slope are shown to drive off-shelf flow at their leading edges and are responsible for much of the triggering of individual spilling events. Northerly barrier winds also force spilling. Local winds generate an Ekman downwelling cell. Nonlocal winds also excite spilling, which is hypothesized to be the result of southward-propagating coastally trapped waves, although definitive confirmation is still required. The combined effect of the eddies and barrier winds results in the strongest spilling events, while in the absence of winds a train of eddies causes enhanced spilling.
  • Article
    Liquid freshwater transport estimates from the East Greenland Current based on continuous measurements north of Denmark Strait
    (John Wiley & Sons, 2017-01-10) de Steur, Laura ; Pickart, Robert S. ; Macrander, Andreas ; Våge, Kjetil ; Harden, Benjamin E. ; Jónsson, Steingrímur ; Østerhus, Svein ; Valdimarsson, Héðinn
    Liquid freshwater transports of the shelfbreak East Greenland Current (EGC) and the separated EGC are determined from mooring records from the Kögur section north of Denmark Strait between August 2011 and July 2012. The 11 month mean freshwater transport (FWT), relative to a salinity of 34.8, was 65 ± 11 mSv to the south. Approximately 70% of this was associated with the shelfbreak EGC and the remaining 30% with the separated EGC. Very large southward FWT ranging from 160 mSv to 120 mSv was observed from September to mid-October 2011 and was foremost due to anomalously low upper-layer salinities. The FWT may, however, be underestimated by approximately 5 mSv due to sampling biases in the upper ocean. The FWT on the Greenland shelf was estimated using additional inshore moorings deployed from 2012 to 2014. While the annual mean ranged from nearly zero during the first year to 18 mSv to the south during the second year, synoptically the FWT on the shelf can be significant. Furthermore, an anomalous event in autumn 2011 caused the shelfbreak EGC to reverse, leading to a large reduction in FWT. This reversed circulation was due to the passage of a large, 100 km wide anticyclone originating upstream from the shelfbreak. The late summer FWT of −131 mSv is 150% larger than earlier estimates based on sections in the late-1990s and early-2000s. This increase is likely the result of enhanced freshwater flux from the Arctic Ocean to the Nordic Seas during the early 2010s.
  • Article
    The annual salinity cycle of the Denmark Strait Overflow
    (American Geophysical Union, 2022-03-22) Opher, Jacob G. ; Brearley, J. Alexander ; Dye, Stephen R. ; Pickart, Robert S. ; Renfrew, Ian A. ; Harden, Benjamin E. ; Meredith, Michael P.
    The Denmark Strait Overflow (DSO) is an important source of dense water input to the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). It is fed by separate currents from the north that advect dense water masses formed in the Nordic Seas and Arctic Ocean which then converge at Denmark Strait. Here we identify an annual salinity cycle of the DSO, characterized by freshening in winter and spring. The freshening is linked to freshening of the Shelfbreak East Greenland Current in the Blosseville Basin north of the Denmark Strait. We demonstrate that the East Greenland Current advects fresh pycnocline water above the recirculating Atlantic Water, which forms a low salinity lid for the overflow in Denmark Strait and in the Irminger Basin. This concept is supported by intensified freshening of the DSO in lighter density classes on the Greenland side of the overflow. The salinity of the DSO in the Irminger Basin is significantly correlated with northerly/northeasterly winds in the Blosseville Basin at a lag of 3–4 months, consistent with estimated transit times. This suggests that wind driven variability of DSO source water exerts an important influence on the salinity variability of the downstream DSO, and hence the composition of the deep limb of the AMOC.
  • Preprint
    Upstream sources of the Denmark Strait Overflow : observations from a high-resolution mooring array
    ( 2016-02-19) Harden, Benjamin E. ; Pickart, Robert S. ; Valdimarsson, Héðinn ; Våge, Kjetil ; de Steur, Laura ; Richards, Clark G. ; Bahr, Frank B. ; Torres, Daniel J. ; Børve, Eli ; Jonsson, Steingrimur ; Macrander, Andreas ; Østerhus, Svein ; Håvik, Lisbeth ; Hattermann, Tore
    We present the first results from a densely instrumented mooring array upstream of the Denmark Strait sill, extending from the Iceland shelfbreak to the Greenland shelf. The array was deployed from September 2011 to July 2012, and captured the vast majority of overflow water denser than 27.8 kgm-3 approaching the sill. The mean transport of overflow water over the length of the deployment was 3.54 ± 0.16 Sv. Of this, 0.58 Sv originated from below sill depth, revealing that aspiration takes place in Denmark Strait. We confirm the presence of two main sources of overflow water: one approaching the sill in the East Greenland Current and the other via the North Icelandic Jet. Using an objective technique based on the hydrographic properties of the water, the transports of these two sources are found to be 2.54 ± 0.17 Sv and 1.00 ± 0.17 Sv, respectively. We further partition the East Greenland Current source into that carried by the shelfbreak jet (1.50 ± 0.16 Sv) versus that transported by a separated branch of the current on the Iceland slope (1.04 ± 0.15 Sv). Over the course of the year the total overflow transport is more consistent than the transport in either branch; compensation takes place among the pathways that maintains a stable total overflow transport. This is especially true for the two East Greenland Current branches whose transports vary out of phase with each other on weekly and longer time scales. We argue that wind forcing plays a role in this partitioning.
  • 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
    High-frequency variability in the North Icelandic Jet
    (Sears Foundation for Marine Research, 2018-03) Harden, Benjamin E. ; Pickart, Robert S.
    We describe the high-frequency variability in the North Icelandic Jet (NIJ) on the Iceland Slope using data from the densely instrumented Kögur mooring array deployed upstream of the Denmark Strait sill from September 2011 to July 2012. Significant sub-8-day variability is ubiquitous in all moorings from the Iceland slope with a dominant period of 3.6 days. We attribute this variability to topographic Rossby waves on the Iceland slope with a wavelength of 62 ± 3 km and a phase velocity of 17.3 ± 0.8 km/day−1 directed downslope (−9◦ relative to true-north). We test the theoretical dispersion relation for these waves against our observations and find good agreement between the predicted and measured direction of phase propagation.We additionally calculate a theoretical group velocity of 36 km day−1 directed almost directly up-slope (106◦ relative to true-north) that agrees well with the propagation speed and direction of observed energy pulses. We use an inverse wave tracing model to show that this wave energy is generated locally, offshore of the array, and does not emanate from the upstream or downstream directions along the Iceland slope. It is hypothesized that either the meandering Separated East Greenland Current located seaward of the NIJ or intermittent aspiration of dense water into the Denmark Strait Overflow are the drivers of the topographic waves.