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Shelf Fronts and Tidal Stirring in Greater Cook Strait, New Zealand

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Shelf Fronts and Tidal Stirring in Greater Cook Strait, New Zealand OCEANOLOGICA ACTA 1983- VOL. 6- N° 2 ~ -----~- Tidal stirring Fronts Shelf fronts and tidal stirring Stratification Mixing efficiency Cook Strait in Greater Cook Strait, Marée Fronts Stratification Mélange New Zealand Détroit de Cook M. J. Bowman •, A. C. Kibblewhite b, S. M. Chiswell •, R. A. Murtagh a a Marine Sciences Research Center, State University of New York, Stony Brook, New York 11794, USA. b Physics Department, University of Auckland, New Zealand. Received 9/8/82, in revised forrn 15/11/82, accepted 3/12/82. ABSTRACT Numerical tidal simulations and hydrographie data from two austral summer (January 1980 and 1981) cruises were used to study tidal mixing variations, upwelling and circulation in the shelf seas of central New Zealand. Four classes of fronts were identified; the Cape Farewell upwelling front, two tidal mixing fronts spanning Cook Strait, a plume front bounding an intrusion of subtropical water driven into Cook Strait from the northwestern approaches, and a shelf break front east of Cook Strait. Bulk stratification correlated well with the hfu 3 stratification index (r,..,0.6); potential energy deficit and surface to bottom temperature differences less well (r"'. 53 and. 38, respectively); potential energy deficit showed sorne tendency to follow the bathymetry. A critlcal value of the stratification index s"' 1. 5 (c.g.s. units) appeared to separate mixed from stratified water at the tidal mixing fronts, in good agreement with results obtained in other semi-enclosed shelf seas in temperate latitudes. The data suggest that prevailing patterns of stratification can be considerably modified by the variable and often strong winds that frequent the region. Oceanol. Acta, 1983, 6, 2, 119-129. RÉSUMÉ Mélanges par la marée et fronts hydrologiques sur le plateau continental du détroit de Cook, Nouvelle-Zélande. Des simulations numériques de la marée et les données hydrologiques recueillies lors de deux campagnes de l'été austral Uanvier 1980 et 1981), ont été utilisées pour étudier les variations du brassage par la marée, le phénomène d'upwelling et la circulation dans les mers du plateau continental du centre de la Nouvelle-Zélande. Quatre types de fronts ont été mis en évidence : le front de l'upwelling du Cap Farewell, deux fronts transversaux dans le détroit de Cook associés au mélange par la marée, un front en panache limitant une intrusion d'eau subtropicale en provenance du Nord-Ouest, et un front au bord du plateau continental à l'est du détroit de Cook. Le gradient vertical moyen de densité est bien corrélé avec l'indice de stratification (hju 3), ce qui n'est pas le cas du déficit en énergie potentielle et de l'écart de température entre la surface et le fond. Le déficit en énergie potentielle tend à suivre la bathymétrie. Aux fronts de mélange par la marée, la séparation entre les eaux bien mélangées et les eaux stratifiées correspond à des valeurs critiques de l'indice de stratification s-1,5 (unités c.g.s.). Cette valeur de l'indices est en bon accord avec les résultats obtenus pour d'autres mers semi-fermées à des latitudes tempérées. Les résultats suggèrent que les traits généraux de la stratification peuvent être facilement modifiés par les vents variables et souvent violents de cette région. Oceanol. Acta, 1983, 6, 2, 119-129. 0399-1784/1983/119/S 5.00/ «:>Gauthier-Villars 119 M. J. BOWMAN et al. INTRODUCTION summer and autumn phytoplankton production in the Celtic Sea and English Channel (Pingree, 1978; Pingree Tidal stirring variations in energetic shelf seas and sorne et al., 1978), in explaining phytoplankton patchiness at shallow estuaries can exert an important influence on a morphological group levet in a major moderately the seasonal onset, progression and final distribution of stratified estuary (Bowman et al., 1981) and the summer water column stratification. distribution of major herring spawning areas (Iles, Although any study on the control of thermocline Sinclair, 1982). development must obviously include the important Results of several physical oceanographie studies are effects of wind stirring and advection, a number of summarized in the Table. The stratification index is 3 investigators have demonstrated that a predictable expressed as s=log10 (hfku ) in c.g.s. units where pattern of summer stratification can often be identified k- . 002 5 is the dimensionless bottom drag coefficient with tidal mixing variations. These studies, first made and u3 is the mean cubed value of the depth averaged 3 in the shelf seas of the United Kingdom (Simpson, M 2 current. The ku fh can be identified as the kinetic 1976; Simpson, Hunter, 1974; Simpson et al., 1977; energy dissipation rate per unit mass of water column. 1978; Pingree et al.,· 1978) and later in the Bay of The value of hfku3 in c.g.s. units is roughly equal to the Fundy (Garrett et al., 1978), the Bering Sea (Schumacher numerical value of hfu3 in S.I. units preferred by sorne et al., 1979) and Long Island-Block Island Sounds workers where u is now the amplitude of surface spring (Bowman, Esaias, 1981) were based on comparing tidal currents. the hfu 3 stratification index (where h is water column Critical values of s ranged from 0.5 to 2.2, with a depth and u3 the mean cubed tidal current; Simpson, suggestion of an increase with increasing latitude. In Hunter, 1974) with hydrographie data. higher latitudes one would expect a lower value of u in The basic premise behind the stratification index is that water of a given depth to provide sufficient dissipation two criteria must be met in the vicinity of a steady state to prevent the establishment of stratification. Sorne- of tidal mixing front. First the power demand by a water these experiments allowed the estimation of the mixing column to maintain a given stratification while efficiency, E, the ratio of the kinetic energy used for experiencing a surface buoyancy flux (arising from mixing to that dissipated. Values ranged between 1.2 insolation) must be matched by the available power for and 3. 7 x 10-3 with the exception of the Bering Sea 3 mixing derived from the tides. Second, the stratification estimate which was much higher (20 x 10- ). must be marginal. The first condition states that the We designed a study in the coastal ocean of central water column is in a state of equilibrium (or perhaps New Zealand (Fig. 2), a semi-enclosed shelf sea with slowly changing over a seasonal or biweekly cycle). The strong tides, a great diversity in bottom depths, variable second defines the boundary between the mixed winds and the presence ot water types of both 3 3 (low hfu ) and the stratified (high hfu ) water. A subtropical and subantarctic origin. This provided an contribution to mechanical mixing can of course be opportunity to test whether the stratification index bas provided by the winds. applicability to a southern hemisphere shelf sea of 4 2 These ideas are contained in the potential energy intermediate scale ( -4 x 10 km ), and whether or not deficit V (relative to the mixed state) equation, tidal mixing fronts could be identified in the presence developed by Simpson et al. (1978): of upwelling fronts, water mass boundary fronts and meteorologically forced circulation patterns. dV r::tgQh 3 -d =---+Ekpu3 +5kaPaW, v ~o. (1) t 2c The left band side is the power deficit. This is the difference between the demand for (1st term rhs), and the supply of (2nd and 3rd terms, rhs) power from the tides and winds to maintain a steady density profile. In principle, V could be obtained for a given location by integrating equation (1) from the beginning of the heating season with a knowledge of the time histories a of Q, u and W (see Fig. 1). Thus at a location of frontogenesis as depicted in Figure 1 b, both dVfdt and V .-0 during the summer months. Renee, in the absence of winds, FALL b SPRING SUMMER (2) Figure 1 Schematic diagram of mixing energetics in: a) a stratifying water column; and b) a frontal water column. The power demand curve derives from the seasonal insolation cycle. The undulations in the power supply defines the location of the front if the right band side curve represent fluctuations in energy dissipation rates due to wind is considered constant for a given locality. events and spring-neap cycles of tidal stirring. The shaded area is a measure of the work, V, required to mix away the stored buoyancy in The stratification parameter has also proven to be a the stratified water column. Typical density profiles are shown for the useful biological index in understanding the spring, stratifying column. 120 FRONTS AND STIRRING IN GREA TER COOK STRAIT GEOMORPHOLOGY The bottom topography of the region includes the broad, flat, western continental shelf of South Taranaki TASMAN Bight (Heath, 1974a; Ridgway, 1980) exposed to strong SEA prevailing westerly winds, and the shallow sheltered / embayments of Golden and Tasman Ba ys (Heath, 1973; 1 ... / 1914a; Ridgway, 1977) protected by Farewell Spit, a ,tOO.: bar over 40 km in length built by easterly littoral drift around Cape Farewell. Cook Strait canyon rises steeply from over 2 000 rn, penetra ting westward through the Cook Strait narrows; the 200 rn contour extends westward more than 100 km from the east coast. This submarine canyon is an extension of the 5 000 rn Hikurangi Trench, a major feature of the island arc system of the southwest Pacifie plate margin. The Marlborough Sounds are a series of drowned river valley estuaries which form a dendritic system of navigable waterways. TIDES The strong cooscillating tide in Greater Cook Strait is a consequence of the degenerate amphidrome on which ~··· ····· New Zealand is centered, the semi-diurnal tide rotating counter clockwise around the land mass as a resonant Figure 2 trapped shelf wave (Bye, Heath, 1975).
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