Editing Cook Strait (section)

==Oceanography==
{{External media
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|image1=[https://www.niwa.co.nz/our-science/coasts/research-projects/all/physical-hazards-affecting-coastal-margins-and-the-continental-shelf/news/cookmov Animation of the tides in Cook Strait] – [[National Institute of Water and Atmospheric Research|NIWA]]
|image2=[https://www.niwa.co.nz/our-science/coasts/research-projects/all/physical-hazards-affecting-coastal-margins-and-the-continental-shelf/news/m2mov Animation of the lunar (M2) tide component around NZ]
|image3=[http://www.teara.govt.nz/en/engineering-on-the-sea-floor/1/2 Cook Strait currents before and after high tide at Wellington] – [[Te Ara: The Encyclopedia of New Zealand]]
|image4=[https://natlib.govt.nz/visiting/wellington/unfolding-the-map/coastal-charting Underwater topography of Cook Strait] – [[National Library of New Zealand]]
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{{see also|Cook Strait tides}}

The waters of Cook Strait are dominated by strong [[Ocean power in New Zealand#Tidal power|tidal flows]]. The tidal flow through Cook Strait is unusual in that the tidal elevation at the ends of the strait are almost exactly out of phase with one another, so high water on one side meets low water on the other.<ref>[https://niwa.co.nz/our-science/coasts/research-projects/all/physical-hazards-affecting-coastal-margins-and-the-continental-shelf/news/m2mov Lunar Semidiurnal Tide (M2)]. ''NIWA''. Accessed 21 November 2020.</ref> This is because the main [[M2 lunar tide]] component that happens about twice per day (actually 12.42 hours)<ref name=NASA2016 /> circulates anti-clockwise around New Zealand, and is out of phase at each end of the strait (see animation on the right). On the Pacific Ocean side the high tide occurs five hours before it occurs at the Tasman Sea side. On one side is high tide and on the other is low tide. The difference in sea level can drive tidal currents up to 2.5 metres per second (5 knots) across Cook Strait.<ref name="teara tides">Stevens, Craig and Chiswell, Stephen. [http://www.teara.govt.nz/EarthSeaAndSky/OceanStudyAndConservation/OceanCurrentsAndTides/2/en ''Ocean currents and tides: Tides'']. Te Ara: The Encyclopedia of New Zealand, updated 21 September 2007.</ref><ref name="benign" />

There are numerous computer models of the tidal flow through Cook Strait. While the tidal components are readily realisable,<ref>{{Cite web |url=http://gfs.sourceforge.net/examples/examples/tides.html#htoc12 |title=Lunar tides in Cook Strait, New Zealand |access-date=28 February 2009 |archive-date=3 April 2017 |archive-url=https://web.archive.org/web/20170403214043/http://gfs.sourceforge.net/examples/examples/tides.html#htoc12 |url-status=dead }}</ref> the residual flow is more difficult to model.<ref>Bowman, M. J., A. C. Kibblewhite, R. Murtagh, S. M. Chiswell and B. G. Sanderson (1983) Circulation and mixing in greater Cook Strait, New Zealand. Oceanologica Acta 6(4): 383–391.</ref> Probably the most prolific oceanographer to research the strait was [[Ron Heath]] based at the [[N.Z. Oceanographic Institute]].  He produced a number of studies including analysis of tides <ref>Heath, R. A., 1978. Semi‐diurnal tides in Cook Strait. New Zealand Journal of Marine and Freshwater Research, 12(2), pp.&nbsp;87–97.</ref> which identified the presence of a "virtual amphidrome" in the region. Heath also quantified a best estimate for the time of the "residual current" (i.e. net current after averaging out the tidal influence) in the strait.<ref>Heath, R. A., 1986. In which direction is the mean flow through Cook Strait, New Zealand—evidence of 1 to 4 week variability?. New Zealand journal of marine and freshwater research, 20(1), pp.&nbsp;119–137.</ref> This continues to be a topic of research with computer simulations combining with large datasets to refine the estimate.<ref>Hadfield, M. G. and Stevens, C. L., 2021. A modelling synthesis of the volume flux through Cook Strait. New Zealand Journal of Marine and Freshwater Research, 55(1), pp.&nbsp;65–93.</ref>

[[File:Cook Sampling.png|thumb|left|Historical ocean sampling locations within Cook Strait]]

[[File:Global surface elevation of M2 ocean tide.webm|thumb|upright=1.8|Global surface elevation of the M2 ocean tide (NASA).<ref name=NASA2016>[https://svs.gsfc.nasa.gov/4541 Ocean Tides and Magnetic Fields], ''NASA Visualization Studio'', 30 December 2016. {{PD-notice}}</ref> This computer animation shows the peaks and troughs of the M2 tides sweeping anticlockwise around New Zealand. When it is high tide on one side of Cook Strait, it is low tide on the other side. For this reason, the strait can experience exceptionally fast tidal flows.]]

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Despite the strong currents, there is almost zero tidal height change in the centre of the strait. Instead of the tidal surge flowing in one direction for six hours and then in the reverse direction for six hours, a particular surge might last eight or ten hours with the reverse surge enfeebled. In especially boisterous weather conditions the reverse surge can be negated, and the flow can remain in the same direction through three surge periods and longer. This is indicated on marine charts for the region.<ref>{{Cite web |url=http://www.linz.govt.nz/hydro/charts/nz202-chart-catalogue/catalogue.aspx?chart=463&page=20 |title=Chart of Cook Strait |access-date=28 February 2009 |archive-url=https://web.archive.org/web/20130212002918/http://www.linz.govt.nz/hydro/charts/nz202-chart-catalogue/catalogue.aspx?chart=463&page=20 |archive-date=12 February 2013 |url-status=dead }}</ref> Furthermore, the submarine ridges running off from the coast complicate the ocean flow and turbulence.<ref>Stevens, C. L., M. J. Smith, B. Grant, C. L. Stewart, T. Divett, 2012, Tidal Stream Energy Extraction in a Large Deep Strait: the Karori Rip, Cook Strait, Continental Shelf Research, 33: 100–109. {{doi|10.1016/j.csr.2011.11.012}}.</ref> The substantial levels of turbulence have been compared to that observed in the [[Straits of Gibraltar]] and [[Seymour Narrows]] in [[British Columbia]].<ref>Stevens, C. L., 2018. Turbulent length scales in a fast-flowing, weakly stratified, strait: Cook Strait, New Zealand. Ocean Science, 14(4), pp.&nbsp;801–812. {{doi|10.5194/os-14-801-2018}}.</ref>