Internal waves downstream of Norfolk Ridge, western Pacific, and their biophysical implications

Craig L. Stevens, Philip J. H. Sutton and Cliff S. Law

Limnol. Oceanogr., 57(4), 2012, 897-911 | DOI: 10.4319/lo.2012.57.4.0897

ABSTRACT: An 11-d quasi-Lagrangian surface layer experiment to the east of Norfolk Island tracked a 140-m-deep drifting vertical array (DVA) of instruments, including conductivity sensors, thermistors, and current meters. These are the first in situ data from the area to measure large-amplitude internal waves. The observations show isotherm peak-to-peak excursions reaching 50 m and were dominated by the semidiurnal forcing frequency. The DVA exhibited horizontal loops at the semidiurnal tidal frequency as it tracked water at depths of 80-140 m to within 10% of total horizontal displacement. The large vertical isotherm excursions generated substantial vertical shear. Temperature microstructure estimates of the vertical diffusivity of scalar properties at 70-m depth, around the center of the thermocline, were on the order of 10-4 m2 s-1 and around an order of magnitude larger at shallower depths. When combined with nitrate data, this implies vertical fluxes of nitrate in the pycnocline of ∼ 1 mmol m-2 d-1. The internal waves also potentially cause an interaction between the variability in tidal forcing and the diurnal radiation cycle that influences chlorophyll in the deep chlorophyll maximum. The average effect of the internal wave was to increase the light level for a particular isotherm over the static case. The internal wave-induced velocities were strong enough to dominate phytoplankton rise speeds and so potentially played a role in the formation and persistence of an observed Trichodesmium bloom.

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