A modelling synthesis of the volume flux through Cook Strait

Abstract

We present hindcasts of currents in Cook Strait from a baroclinic, tide-resolving ocean model. Three hindcasts are run, forced respectively at the lateral boundaries with data from the Bluelink Reanalysis (BRAN), the global Hybrid Coordinate Ocean Model (HYCOM) and a New Zealand Shelf Seas baroclinic hindcast (NZROMS). The hindcast forced by BRAN is evaluated against measured currents at locations in the Cook Strait Narrows and South Taranaki Bight. The model reproduces the observed tidal, subtidal and mean currents well. The three hindcasts produce a 3year-mean volume flux southward through the Narrows of 0.360, 0.734 and 0.230 Sv, respectively. When corrected for the observed discrepancies between model and measurements in the Narrows, the estimates are 0.430, 0.498 and 0.337 Sv. We conclude that true 3-year mean volume flux through Cook Strait is 0.42 ± 0.08 Sv. The tidal volume flux amplitude is 4.68 Sv (M2). The subtidal fluctuations in volume flux have a standard deviation of 0.62 Sv. The effect of wind is investigated with a wind-forced barotropic model, which produces volume flux fluctuations that are highly correlated (r = 0.93) with the subtidal fluctuations from the baroclinic model, implying that the latter are largely wind-generated.

Evolution of marine gravel dunes on the open shelf under multi-directional currents conditions

“…In Cook Strait, the tidal flow is the dominant transport mechanism, with tidal amplitudes reaching just under 6 Sv with a mean south-eastward flow of 0.42 ± 0.08 Sv (Hadfield & Stevens, 2020). This residual flow is attributed to winddriven currents, the pressure gradient across Cook Strait, and tidal rectification (Walters et al, 2010;Stevens, 2014;Hadfield & Stevens, 2020).…”

“…The model initial state and boundary conditions for temperature, salinity, and non-tidal currents came from a larger 2 km-scale Cook Strait model (Chiswell et al, 2019;Hadfield & Stevens, 2020) and tidal currents for 13 tidal constituents (M2, S2, N2, K2, Kl, 01, PI, Ql, 2N2, MU2, NU2, Frontiers in Earth Science frontiersin.org L2, T2) came from the NIWA New Zealand region tidal model (Walters et al, 2001). Surface wind stress came from the New Zealand Limited Area Model (NZLAM) (Webster et al, 2008) and surface heat fluxes from National Centre for Environmental Prediction (NCEP) models (Kalnay et al, 1996).…”

“…To further investigate these tidal effects, a higher resolution hydrodynamic model is more appropriate than a long-term simulation that captures seasonal variability with lower resolution outputs. While there are other oceanographic processes, such as wind-driven currents (Walters et al, 2010;Hadfield & Stevens, 2020), internal tides or the mean flow due to differences in sea level height between the north-eastern and the south-western side of Cook Strait (Walters et al, 2010); the short period modelled here will not capture the seasonal variability in these processes. Although seasonal variability is not expected to alter the main findings here, it is expected that it will create slight differences in the currents for each tidal cycle.…”

“…In Cook Strait, the tidal flow is the dominant transport mechanism, with tidal amplitudes reaching just under 6 Sv with a mean south-eastward flow of 0.42 ± 0.08 Sv (Hadfield & Stevens, 2020). This residual flow is attributed to winddriven currents, the pressure gradient across Cook Strait, and tidal rectification (Walters et al, 2010;Stevens, 2014;Hadfield & Stevens, 2020). The modelling study performed Frontiers in Earth Science frontiersin.org here is too short to investigate seasonal variability and eventscale changes in this residual flow, but Stevens (2014) has studied these processes in moorings situated in Cook Strait.…”

Characterizing the Variability of Boundary Currents and Ocean Heat Content Around New Zealand Using a Multi‐Decadal High‐Resolution Regional Ocean Model

“…Flows in the Cook Strait are largely dominated by the semidiurnal tide with a much smaller residual flow from west to east (Stevens, 2014; Walters et al., 2010). The mean residual volume flux through Cook Strait was estimated at 0.42 ± 0.08 Sv compared to the M 2 tidal volume flux amplitude of 4.68 Sv, and the subtidal volume flux fluctuations are predominantly wind‐driven with a standard deviation of 0.62 Sv (Hadfield & Stevens, 2020). The flow to the east of central NZ is complex; the outflow on the eastern side of central NZ is a combination of water masses from the SC passing northward along the east coast of the South Island, the D’Urville Current exiting the Greater Cook Strait and meanders from the ECC (Bowman et al., 1983; Chiswell, 2000; Heath, 1971).…”