High-resolution observations from the curved tidal channel of the Otago Harbour shows secondary flows up to 20% of the primary flow and vertical velocity inferred from secondary flow up to 1% of the primary flow. This vertical velocity is inferred on a much finer scale than previous works. The spatial pattern of this vertical velocity is upward on the inside and downward on the outside of the bend, consistent with previous laboratory flume measurements. Linear regression, r p 5 0.95, shows the cross-channel distribution of the observed secondary flow can be resolved from the horizontal ADCP measurements well enough to be consistent with the cross-channel distribution of secondary flow derived from the observed primary flow using the model of Kalkwijk and Booij (1986). Linear regression, r p 5 0.80, shows the vertical velocity inferred from observed secondary flow is consistent with vertical velocity derived from the observed primary flow using the model of Kalkwijk and Booij (1986). This also shows the cross-channel distribution of the observed secondary flow is resolved well enough from the horizontal ADCP measurements to be able to infer the vertical velocity from mass continuity. The required horizontal resolution is made possible by Radial Basis Function (RBF) smoothing and spatial interpolation that allows for continuity of the spatial derivatives. Modeling trajectories using channel dimensions and velocity field values equivalent to the observations show that 3-dimensional secondary circulation forms a loose helical flow pattern.
Aotearoa New Zealand is a land and cultures surrounded by one of the planet's larger marine exclusive economic zones per capita. Understanding, living within and utilising this domain requires a well-developed knowledge of the physical oceanographyfate, provenance and transfer of ocean-water, material and energy. In the Aotearoa New Zealand context, understanding and utilising the environment through science also requires that Māori have a stake in the science and a pathway to connecting with their mātauranga. The pūtahitanga (intersection) of mātauranga Māori and western science perspectives brings challenges for implementation of the Vision Mātauranga framework for physical oceanographic science. Here we discuss the drivers and key themes for the pūtahitanga and propose a mahere kaupapa/plan bound by: (i) climate, (ii) decolonisation, (iii) shared language, (iv) data sovereignty, and (v) developing a cohort who can lead the field in the coming decades. TUHINGA-WHAKARĀPOPOTOKua karapotia te whenua me te ahurei o Aotearoa e te ohaoha aukatinga o te moana nunui tuarua o te ao mō ia ūpoko. Kia mārama, kia whakatīnana hoki ai tēnei tauwāhi me whakawhānui ngā mōhiotanga mō te mātai aumoanate tīmatanga me te whakawhitinga o te wai, ngā rauemi, me ngā pūngao. Nō roto mai te horopaki o Aotearoa, mā te putaiao ko māramatanga me te whakatīnanatanga i te taiao, nā tēnei āhua he wāhi mō Ngāi Māori ki te tūhono ō rātou matauranga ki ō rātou huanui hoki.Ko te pūtahitanga o ngā mātauranga Māori me ngā mātauranga pūtaiao Pākehā e wero tonu ana kia whakatīnana te pou tarāwaho o te Titirohanga Mātauranga mō te mātai aumoana. Koinei te matapakinga mō ngā pūtake me ngā mōraro matua mō te pūtahitanga e tūtohu ana he mahere: (i) purenga ihomatua, whakapurenga ngā ihomatua o te mātai aumoana, (ii) whakaahu he reo tohatoha, (iii) ko ētahi raraunga ka mau ake a Ngāi Māori (ā whānui nei), (iv) whakaahu te pito mata o tētahi rāngai kia arahi te whīra mō ngā tau e heke mai nei. ARTICLE HISTORY
The upwelling of nutrient‐rich bottom waters supports life at the oceans' surface. Secondary circulation can produce localized upwelling at headlands. Secondary circulation develops in the curved flow around headlands resulting in a loose helical flow pattern within the curved flow. The magnitude of secondary flow can be up to 20% of the depth average current. Moving vessel ADCP measurements were taken at Cape Saunders, Otago Peninsula, New Zealand. New radial basis function interpolation techniques for smoothing noisy data allows the weak horizontal secondary flow to be extracted from the stronger along shore flows. During peak floods of 1 ms−1 the measured strength of secondary flow is 0.2 ms−1. A region of secondary flow approximately 1500 m long and 800 m wide is observed downstream of the Cape. On the inshore edge this region, areas of vertical velocity inferred from horizontal ADCP measurements using mass continuity show a localized upwelling of up to 0.007 ms−1. Concurrent CTD measurements also show this tidally synchronized upwelling. Linear regression between upwelling from the CTD measurements and the inferred vertical velocity from the ADCP measurements show the data are well correlated, rp = 0.65. The upwelling is 4 m per hour so in this location with a mean depth of 25 m the entire water column is replacing itself with deep waters at least once during a tidal cycle. On a global scale, the cumulative upwelling from headlands and islands due to secondary circulation could be a significant source of nutrient delivery to coastal surface waters.
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