Abstract
AbstractIn this article, we show that the class of low frequency (subinertial) waves known as coastal‐trapped waves (CTWs) are a significant agent of water volume exchange in a west Svalbard fjord, and by extension more widely along the west Svalbard and east Greenland margins where similar conditions prevail. We show that CTWs generated by weather systems passing across the sloping topography of the shelf break propagate into the fjord, steered by the topography of an across‐shelf trough. The CTWs have characteristic periods of ∼2 days, set by the passage time of weather systems. Phase speeds and wavelengths vary seasonally by a factor of two, according to stratification: winter (summer) values are Cp = 0.25 ms−1 (0.5 ms−1) and λ = 40 km (84 km). CTW‐induced flow velocities in excess of 0.2 ms−1 at 100 m water depth are recorded. Observationally scaled CTW model results allow their explicit role in volume exchange to be quantified. Of the estimated exchange terms, estuarine exchange is weakest ( m3s−1), followed by barotropic tidal pumping ( m3s−1), with intermediary exchange dominating ( m3s−1). Oscillatory flows display greatest activity in the 1–5 day period band, and CTW activity is identified as the likely source of variability in the 40–60 h period band. Within that band, intermediary exchange driven by CTWs is estimated as m3s−1; an exchange rate exceeding both barotropic and estuarine exchange estimates.
Highlights
The class of waves known as Coastal-Trapped Waves (CTWs) has long been recognized as an important source of geostrophic variability at subinertial frequencies along the world’s ocean margins [Allen, 1975; Huthnance, 1978; Mysak, 1980]
Four of five ‘‘events’’ occur between late June and early September during which time significant along-isobath current variability is apparent with a periods of several tens of hours. At this stage in the analysis, we have introduced the expectation, based on a discussion of the literature, that there is a strong likelihood of finding significant CTW activity in the troughs and fjords west of Svalbard
CTWs have been demonstrated to represent a major mode of flow variability within the fjord; we ask ‘‘what is effect of CTWs on fjord exchange?’’ In a linear, inviscid world the answer might be ‘‘none,’’ the introduction of mixing and finite amplitude effects raises the potential for significant CTW-induced exchange
Summary
The class of waves known as Coastal-Trapped Waves (CTWs) has long been recognized as an important source of geostrophic variability at subinertial frequencies along the world’s ocean margins [Allen, 1975; Huthnance, 1978; Mysak, 1980] Their explicit role as agents of water property exchange between ocean margins and adjacent coastal systems has received limited attention [e.g., Proehl and Rattray, 1984; Svendsen et al, 2002], and for broad fjords (i.e., wide with respect to the Rossby radius) CTWs are an implicit element of what has been termed ‘‘intermediary exchange’’ [see Stigebrandt, 2012 and references therein]. Freshwater input (via sub-glacial discharge) may influence intermediary baroclinic exchange (see below) Tides may drive both barotropic (tidal pumping), [e.g., Gillibrand, 2001], and baroclinic exchange (via enhanced diapycnal mixing increasing estuarine exchange). In broad fjords (relative to the Rossby deformation radius) a mean, geostrophically balanced lateral circulation can exist
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