Interaction of transient shelf currents with a buoyancy-driven coastal current
Observations on the inner New Jersey shelf (1996) showed that transient wind-driven currents of 3–4-day periods strongly interacted with a buoyancy-driven coastal current (or buoyant plume) originating from the Hudson estuary. In particular, the depth-averaged current fluctuations were amplified in the buoyant water. This phenomenon is studied here using a primitive equation numerical model (SPEM5). Transient (periodic) shelf currents are introduced in the form of incident barotropic shelf waves (BSW). The model domain is an idealized channel with open upstream/downstream boundaries and the depth exponentially increasing offshore, which allows the BSW propagation through the model domain. In most cases, the wave period is five days. The buoyancy-driven coastal current is forced by constant buoyant discharge through a coastal gap. Propagating BSWs reduce the growth of a buoyant anticyclonic bulge at the source region while a coastal current downstream from the source contains more buoyant water with a sharper density gradient in the frontal zone compared to the case without BSWs. The amplitude of vertically averaged transient currents increases in the buoyant layer (for instance, by 20–30% for the inflow density anomaly of -3 to -4 kg m-3). This amplification depends on the density anomaly of buoyant inflow. On the other hand, variations in the inflow velocity and/or the net transport do not affect the BSW amplification. The amplification of velocity amplitude is associated with the incident BSW scattering into higher wave modes. The total energy flux should remain approximately the same as BSWs propagate through the domain. The higher modes have lower group speeds and thus their amplitudes should be higher in order to maintain the same energy flux. Such interactions of transient currents with buoyant plumes are important for the mixing processes and the across-shelf exchange on the inner shelf.
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Document Type: Research Article
Publication date: 2004-07-01
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