Tuatara: Volume 20, Issue 3, November 1973
Present Knowledge of the Oceanic Circulation and Hydrology around New Zealand — 1971
Present Knowledge of the Oceanic Circulation and Hydrology around New Zealand — 1971
Syntheses of pioneering studies of the hydrology and circulation of the oceans surrounding New Zealand have been given by Garner (1961, 1962); Garner and Ridgway (1965) and Brodie (1960). However in the past decade there has been a marked increase of research into the physical properties of New Zealand waters, the major contribution being the analyses of temperature, salinity, depth data collected from a series of hydrological surveys by the New Zealand Oceanographic Institute (NZOI). The first of these surveys was made in the summer of 1963 and has been followed in the succeeding summers by surveys in different areas until, by 1970, the whole area of ocean up to 250 miles offshore has been covered around New Zealand (Garner 1967a, b, 1970a, Ridgway 1970a, in prep; Ridgway and Heath in press; Heath in press a). This paper provides a summary of the present knowledge of the physical properties of the sea water around New Zealand which is suitable for workers in fields which have some dependence on the physical environment.
Variations of Temperature and Salinity with Depth in the Upper 1000M
Around New Zealand there are five main water masses which have distinctive temperature-salinity characteristics. These are — the surface Subtropical and Subantarctic Waters, and from the surface down, Antarctic Intermediate water, Pacific Deep Water and Bottom Water. These water characteristics are illustrated by curves of temperature and salinity with depth and the salinity at a given temperature for a station immediately north of New Zealand (Fig. 1).
Subtropical and Subantarctic Surface Water:
Antarctic Intermediate Water:
Antarctic Intermediate Water is derived from the low salinity surface water south of the Antarctic Convergence which sinks at the Convergence near 55°S and travels northward giving rise to the lowest salinity sub-surface water around New Zealand. The salinity minimum which is formed has a salinity of about 34.3–34.4°% located at a depth of about 700m south of the Subtropical Convergence and 100m north of the Convergence around New Zealand (Heath 1972a).
Pacific Deep Water:
Pacific Deep Water which is found below the Antarctic Intermediate Water originates at the surface in the North Atlantic Ocean with some addition from the Mediterranean Sea. It travels southwards in the Atlantic Ocean, eastward in the Circumpolar Current then northward in the South-west Pacific Ocean and is characterised by a salinity maximum of about 34.75°% at a depth of approximately 3000m around New Zealand (Warren 1970).
Bottom Water which is found below the Pacific Deep Water originates mainly through winter cooling and sinking of the water around the Antarctic continental margin.
The Oceanic Circulation around New Zealand
The presence of New Zealand and its associated regions of relatively shallow bottom depths has a major influence on the circulation of the South Pacific Ocean for this landmass is athwart what would otherwise by a mainly zonal flow. New Zealand effectively splits this zonal flow such that, in general terms, the water flows in a clockwise direction around the northern half and anticlockwise around the southern half of New Zealand with the flow continuing eastwards east of New Zealand.page 128
A diagram showing the present knowledge of the circulation around New Zealand is shown in Fig. 2.
The circulation on the west coast of New Zealand is closely connected with that on the Australian east coast. The East Australian Current is deflected east at about 34°S after flowing down the east Australian coast (Hamon 1965). Anticlockwise rotating eddies cast off where this deflection occurs travel south, then east, forming the Subtropical Convergence where they meet the Subantarctic Waters of the West Wind Drift around the latitudes 40–45°S. On meeting the New Zealand coast this flow appears to branch near Jackson's Head, the southern component flowing through Foveaux Strait and south of Stewart Island to contribute to the northward flowing Southland Current on the east coast of the South Island (Garner 1969a) (Fig. 2). The other component flows north as the Westland Current and where it meets the deflected component of the East Australian Current northwest of the North Island, another convegence is formed — the mid-Tasman Convergence (Stanton 1969). Close inshore, north of Cape Egmont, the flow is usually towards the north although a southwards flow may occur in winter. The flow out of the Tasman Sea gives rise to the East Auckland Current which flows south-eastwards along the east coast of the North Island, between North Cape and East Cape (Barker and Kibblewhite 1965). Near East Cape the main flow of the East Auckland Current turns north (i.e. that part north of approximately 37°S) while the rest turns in a clockwise direction around East Cape giving rise to the southwards flowing East Cape Current. This current adjusts the temperature and salinity distribution such that a warm, saline tongue Figs. 3, 4) extends southward from East Cape (Heath, in press a).
Fig. 3: Isotherms (°C) at the sea surface around New Zealand contoured from data collected by the N.Z. Oceanographic Institute in a series of block hydrological surveys conducted in February/March of the successive years from 1963 to 1971.
Current measurements in Cook Strait have shown that the main flow is tidal and highly variable (Olsson 1955; Gilmour 1960) with page 131 current speeds of up to 7 knots (Hydrographic Department 1958). These strong tidal flows result from the difference in tidal height at either end of the Strait. The mean circulation in the Strait has been deduced from drift card records (Brodie 1960; Heath 1969) and changes in water characteristics (Garner 1953, 1954, 1959a, b, 1961; Heath 1971). Warm saline Subtropical Water in the D'Urville Current sweeps into Cook Strait from the northwest (Brodie 1960; Heath 1969) — this current is derived from the Westland Current flowing northwards along the west coast of the South Island. The water of the Southland Current in Cook Strait mixes with water from the D'Urville Current flowing in from the north and water over the Cook Strait Canyon from the East Cape Current. Mixed water derived from all three currents travels eastward across Cook Strait and around Cape Palliser to meet the water of the Southland Current, that has diverged seaward between Kaikoura and Cook Strait (Heath, in press a, b). The Southland Current turns eastward and back southwards south of Hawke Bay (usually near Cape Turnagain) combining with the East Cape Current. The combined Southland and East Cape Current water, after flowing south to about the latitude of Cape Palliser, turns east then north to form the outer arm of the East Cape Current System. Where the East Cape Current turns north a large permanent anticyclonic eddy is formed (Sdubhundhit and Gilmour 1964; Garner 1967a; Heath 1968, in press a, b). Small eddies, which are probably periodically shed off from the permanent eddy, are guided by the bottom topography towards Kaikoura where they raise the temperatures and salinity above the seasonal mean (Garner 1953; Houtman 1965; Bradford 1972). These eddies also disturb the northward passage of the Southland Current. The 50–70 day periodicity of these eddies is probably linked to a similar periodicity in the East Australian Current (Heath in press a).
Surface Temperatures and Salinity Distribution
The distribution of surface temperature and salinity around New Zealand in February/March drawn from data collected in a series of NZOI block surveys are shown in Figs. 3, 4 respectively. The surface temperature and salinity ranges from 25°C and 35.8°% in the south. The seasonal range of surface temperatures around New Zealand is about 6.7°C in Subtropical Water and the seasonal salinity range is about 0.3–0.4°%, the maximum values occurring in February and the minimum values in August. Seasonal changes in the subsurface temperature structure of the water are illustrated by the differences between the temperature distribution on the continental shelf for summer and winter found by Garner (1969b).
Garner (1969b) made temperature observations around the entire New Zealand coastline in the summer and winter of 1967 in water of an average depth of 100m. In summer he found the isotherms were near horizontal with a well-defined thermocline developed north of Castlepoint on the east coast and Hokitika on the West Coast. The surface temperature ranged from 20°C in the north to 13°C in the south. In winter the isotherms were predominantly vertical with a thermocline developed and the surface temperature ranged from 16°C in the north to 9.5°C on the north Canterbury coast. However, in some areas of the west coast, temperatures near the bottom were warmer in winter than summer indicating that advective effects associated with inflow of water derived from the East Australian Current may over-ride the seasonal cycle of warming and cooling.
Bottom Temperatures and Salinities:
Ridgway (1969) has contoured the distribution of bottom temperature and salinity in the area bounded by latitudes 24°S, 57°30′S, longitudes 157°E, 167°W with bottom depths greater than 500m from extrapolated values from hydrological stations. The bottom salinity within this region was found to vary little from 34.7°%. Temperatures less than 1°C were found below depths of 5000m south of 35°S and 4000m south of 45°S, these temperatures being characteristic of Bottom Water. Between these depths and 2000m bottom temperature ranged from 1° to 2°C, temperatures which are characteristic of Pacific Deep Water. Above 2000m the bottom temperature range was relatively large and was characteristic of Antarctic Intermediate Water from 2000 to 1000m and surface water above 1000m. The marked difference between these bottom temperatures and salinities compared to the surface values and their lack of much seasonal variation emphasises the need for caution in correlating biological distribution with values of temperature and salinity other than those at the actual depth where the specimens are found.
Where the different water masses meet in the vertical plane — areas of convergence — the horizontal gradients of temperature and salinity and current speeds usually increase. There are several of these convergence areas around New Zealand.
The Subtropical Convergence:
The Subtropical Convergence is a region formed where the warm saline Subtropical Water meets the cool less saline Subantarctic Water. It is marked at the surface by an increased horizontal temperature and salinity gradient. The general position of the Convergence in New Zealand waters has been described by Deacon (1937, 1945), Fleming (1944), Garner (1954, 1959b, 1967a, b), Wyrtki (1960, 1962) and Heath (1968, in press a, b). East of New Zealand the Convergence is formed where the warm saline water of the page 133 East Cape Current meets the cool less saline water of the West Wind Drift over the Chatham Rise near latitude 43°S (Fig. 2). A northwards extension of cool water (the Southland Current) may be present as far north as 41°S but will be broken near Kaikoura by the offshore turning component of the Southland Current (Garner 1959b; Heath, in press a); the position and structure of this whole northern extension is highly variable and dependent on the circulation further offshore. East of the Chatham Islands the Subtropical Convergence initially extends toward the southeast (Ridgway, in prep; Heath, in press e) before turning towards the northeast in the middle of the Pacific Ocean (see e.g. Neumann and Pierson 1966).
The vertical structure of the Convergence is illustrated by salinity profile in Fig. 5. The isotherms, isohalines and isopycnals generally slope downwards towards the north in at least the upper 600m. A sub-surface tongue of higher salinity water may extend southwards from the Convergence into the lower salinity Subantarctic Water. This higher salinity tongue was found between 80m and 200m by Deacon (1937) and between 100m and 300m by both Garner (1967a) and Heath (1968). On the east coast of New Zealand the position of the Convergence is determined by the shallow depth of the Chatham Rise which limits the southwards flow of Subtropical Water (Heath, in press a) and consequently the position of the Convergence does not vary much seasonally compared with its seasonal latitudinal range in the open ocean, as reported by Deacon (1937).
There has been some conjecture as to the position of the Subtropical Convergence west of New Zealand (see e.g. Garner 1969b, 1967a; Heath, in press a). Recent analyses have shown, however, that the water off the southwest coast of the South Island is of subtropical nature (Garner 1967b; Heath, in press a) with the Convergence extending on to the Snares Shelf south of Stewart Island (as shown in Fig. 1). Some of this conjecture probably arises because the Convergence, here on the west coast, is not as easily recognisable as it is on the east coast because it is not confined by a topographic feature such as the Chatham Rise. On the west coast it may vary its position seasonally although probably the diversion of the Subtropical Water southwards by the land mass of the southern part of the South Island will restrict this seasonal variation more than in the open ocean.
Garner (1959b) found that the Subtropical Convergence usually parallels approximately the 15°C surface isotherm in summer, the 10°C surface isotherm in winter and the 34.7°% surface isohaline in both seasons. More recent analyses have confirmed the range of values for the position of the Convergence.
The Southland Front:
Fig. 5: Meridional salinity cross-section across the Subtropical Convergence near the Chatham Rise at longitude 178°E (Heath 1968, fig. 4).
In the Southwest Pacific Ocean the Tropical Convergence is the region where the westerly directed Trade Wind Drift meets the easterly directed flow derived from the East Australian Current (Fig. 2), Garner (1955) found a region north of New Zealand where the temperature changed by 2°–3°C near latitude 30°S in summer and 22°S in winter. This region was associated by Wyrtki (1960) with the position of the Tropical Convergence determined from surface currents. Stanton (1969) showed that at the Tropical Convergence north of New Zealand, the isotherms slope downwards towards the north in the upper 200m, the Convergence being noticeable mainly as a temperature boundary only rather than as a temperature and salinity boundary and was most strongly developed in winter. A summer hydrological survey between latitudes 28°S and 35°S, longitudes 175°E and 175°W by Ridgway and Heath (in press) showed that the Tropical Convergence was not well-developed there and it therefore seems that the Convergence will be better defined north of North Cape where the eastward flow from the Tasman Sea is confined to a narrow latitudinal extent than it will be to the northeast of New Zealand.
Mid Tasman Convergence:
The possibility of the existence of another convergence in the Tasman Sea was suggested by Garner (1959b) and evidence for its presence has been given by Stanton (1969). It is thought that this Mid-Tasman Convergence arises as the boundary between the flow of water from the East Australian Current that turns eastward near latitude 34°S on the Australian Coast and modified water also derived from the East Australian Current, that moves south from 34°S in a series of eddies on the Australian coast (Hamon 1965) then north-eastwards in the Tasman Sea. The structure and position of this Convergence between essentially two Subtropical Waters is probably extremely variable but its existence may explain why the Subtropical Convergence has been considered by several authors to extend towards the North Island of New Zealand rather than towards the south of New Zealand.page 136
The temperature and salinity distributions considered so far have only been averages of what might be found at any particular time and will be markedly affected, especially in coastal areas, by changing meteorological conditions (Ridgway 1970b), coastal run-off and upwelling.
Upwelling may result from water rising in a compensatory flow when surface and near-surface water is driven offshore by the wind, or when water passes over an area of decreasing depth or round a headland. Wind-derived upwelling caused by southerly winds has been observed on the West Coast of the South Island at Capes Foulwind and Farewell by Garner (1959b, 1961) and Stanton (1971). These upwellings were shown by Stanton (1971) to be enhanced by the curved flow around these promontories. Wind-derived upwelling has been observed on the east coast of the South Island between Kaikoura and Cape Campbell, the mean rate of upwelling for a period in November/December being 0.11cm sec-1 (Heath 1972b). From an analysis of 12 months wind records (Heath 1972c) has shown that wind-derived upwelling would occur most frequently near Cape Campbell in summer.
Areas of upwelling around New Zealand which appear to result from the water flowing over a decreasing depth have been observed near Three Kings Islands (Garner 1954, 1959b, 1961; Stanton 1969, pers. Comm.), near East Cape (Garner 1959b, Heath, in press a), in the Mernoo Gap (Heath 1970, in press a, b), and in Cook Strait (Hydrographic Department 1958; Garner 1953; Heath 1971).
Localised hydrological studies have been made of some of the many embayments around New Zealand. In most of these the non-seasonal variations in temperature and salinity are large and therefore any discussion of these studies would be misleading unless the data were considered in their entirety. Because of this only reference to these studies is made.
Variations of surface temperatures within many New Zealand harbours have been recorded in association with biological studies and reference to a few of these are listed below: in Wellington by Ralph and Hurley (1952), Skerman (1958); Otago by Skerman (1958), Hurley (1959), Hurley and Burling (1960), and in Auckland, Lyttelton, Timaru and Bluff by Skerman (1958). More localised studies of the Hauraki Gulf have been made by Cassie (1960), Slinn (1968), Paul (1968) and Jillett (1971) and for the Otago Harbour by Slinn (1968). Any summary of circulation in harbours should also include the limitations which the original author placed on the data and the reader is therefore best referred to the original paper. The page 137 circulation in Lyttelton Harbour has been analysed by Garner and Ridgway (1955) and in Wellington Harbour by Brodie (1958).
In two of the larger bays, Hawke and Tasman Bays, however the results have allowed the mean circulation to be deduced.
Drift card investigations of the surface circulation in Hawke Bay were made by Ridgway (1960, 1962). More recently, Ridgway and Stanton (1969) occupied a dense grid of hydrological stations in the bay which allowed them to derive the circulation from the salinity distribution. These two studies have each shown much the same non-tidal circulation. Water flows westwards into the bay along its mid-line, the flow bifurcating to produce one current flowing along the shore northwards and the second southward with the water leaving the bay at its southern (Ridgway 1962) and northern extremities. This circulation appears to be controlled by the currents outside the bay, the northwards wind-derived surface flow of the Southland Current and the southwards flowing East Cape Current further offshore (Ridgway 1960). Saline water entering the bay is modified by freshwater run-off and this generally lowers the salinity by about 0.4°%.
Investigations of the circulation in Tasman Bay have been made from drift card measurements by Heath (1969) and Baker (pers.comm.) and from drift card and hydrological measurements by Heath (in press c). This last study has shown that the surface circulation is controlled by the prevailing wind velocity with the surface flow being towards the head of the bay during northerly winds, which are most frequent from October to March, and out of the bay when the winds are from the southerly quarter, which are most frequent from April to August. The mean circulation within this variability consists of an inflow of Subtropical Water from the D'Urville Current on the western side of the bay around Farewell Spit, a counter-clockwise circulation in the bay, and an outflow on the eastern side of the bay near D'Urville Island. In summer the surface temperature increases towards the head of the bay and the thermocline is stronger inside the bay than outside, both effects resulting from solar heating. The greatest amount of coastal dilution occurs in the western part of the bay in Golden Bay.
Thanks are expressed for helpful discussion of the manuscript with Messrs J. W. Brodie, N. M. Ridgway and Dr D. E. Hurley, all of the N.Z. Oceanographic Institute and Miss P. Lawrence also of NZOI for drawing the figures.
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