Variability of internal tide energy, mixing and nitrate fluxes in response to changes in stratification on the northeast New Zealand continental shelf

Sharples, Jonathan ORCID: 0000-0002-7031-3593 and Zeldis, John R (2021) Variability of internal tide energy, mixing and nitrate fluxes in response to changes in stratification on the northeast New Zealand continental shelf. NEW ZEALAND JOURNAL OF MARINE AND FRESHWATER RESEARCH, 55 (1). pp. 94-111.

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Official URL: http://dx.doi.org/10.1080/00288330.2019.1705357

Abstract

A 4-month time series of water column temperature structure on the shelf of North Island New Zealand is used to calculate the energy in the internal tide as stratification evolved between spring and summer. Average total energy in the internal tidal wave was 200 J m−2, with peaks reaching 600 J m−2. Wave energy was weakly correlated with stratification (r = 0.2) and with the spring-neap cycle of tidal currents (r = 0.17). Overall there was little predictability in internal tide behaviour in response to the physical environment. Reduction in wave energy was associated with a downwelling-favourable wind event which reduced stratification by mixing and by removing deeper water off the shelf. Vertical eddy diffusivity driven by internal wave dissipation ranged between 5.6 × 10−5 and 3.2 × 10−4 m2 s−1. Combined with nitrate data this diffusivity resulted in diapycnal nitrate fluxes towards the sea surface of 1.6–2.2 mmol m−2 day−1. Strong stratification in summer reduced the eddy diffusivity but had little effect on nitrate flux as a strengthened vertical nitrate gradient compensated for the reduced diffusivity. This compensation will be important when predicting how stronger stratification in a warmer climate might alter diapycnal nutrient supplies to the upper ocean.

Item Type:	Article
Uncontrolled Keywords:	Internal tide, energy dissipation, stratification, vertical diffusivity, nitrate flux
Depositing User:	Symplectic Admin
Date Deposited:	13 Jan 2020 11:13
Last Modified:	19 Jan 2023 00:10
DOI:	10.1080/00288330.2019.1705357
Open Access URL:	https://doi.org/10.1080/00288330.2019.1705357
Related URLs:	Author Publisher
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3070458