Winter oceanographic observations in some New Zealand fiords

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1 New Zealand Journal of Marine and Freshwater Research ISSN: (Print) (Online) Journal homepage: Winter oceanographic observations in some New Zealand fiords B. R. Stanton To cite this article: B. R. Stanton (986) Winter oceanographic observations in some New Zealand fiords, New Zealand Journal of Marine and Freshwater Research, 2:2, , DOI:.8/ To link to this article: Published online: 3 Mar 2. Submit your article to this journal Article views: 53 View related articles Citing articles: 5 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 5 December 27, At: :9

2 New Zealand Journal of Marine and Freshwater Research, 986, Vol. 2: /86/22-299$2.5/ Crown copyright Winter oceanographic observations in some New Zealand fiords B. R. STANTON New Zealand Oceanographic Institute Division of Marine and Freshwater Science Department of Scientific and Industrial Research P. O. Box 2-346, Wellington North New Zealand Abstract The winter oceanography of four New Zealand fiords is discussed using data obtained from July 983, and comparisons are made with data collected on previous summers. Surface waters were more saline in winter than in summer and were everywhere cooler than the underlying water. Deep water renewal in the fiord basins was re-examined using all available data. For Milford Sound and the other deep silled fiords renewal appears to be related to the arrival of dense water at sill depth offshore by an as yet unknown advective or meteorologically forced process. In the more isolated Long Sound, Preservation Inlet, deep water renewal requires the subtle interplay of tidal flows and mixing processes over the shallow double sill. Complete renewal of Long Sound bottom water appears to be a slow process. Keywords oceanography; winter; fiords; salinity; temperature; freshwater inflow; Milford Sound; George Sound; Thompson Sound; Preservation Inlet INTRODUCTION The first winter physical oceanographic measurements in the New Zealand fiords were made in July 983 during cruise 48 of RV Tangaroa. Systematic surveys of Milford Sound, George Sound, Thompson Sound, and Preservation Inlet were carried out. All previously reported observations in the fiords have been made in summer between December and April. Stanton and Pickard (98) carried out a comparative survey of all the New Zealand fiords in December 977 and have Received 3 July 985; accepted 4 September 985 reviewed available earlier work. More recently, Stan ton (984) has reported on a survey of the above four fiords carried out in March/April 98 and included some current-meter observations in Milford Sound and Preservation Inlet. Lake McKerrow, a fiord lake, has been studied by Pickrill et al. (98). The four fiords selected for the 98 and 983 surveys (Fig. ) cover a representive range of the New Zealand fiords. Milford Sound has received the most scientific attention as it is the most accessible fiord and the principal site for meteorological and river flow data in the region. This fiord has the highest freshwater inflow relative to its surface area of any of the New Zealand fiords. George Sound is probably more typical of the other short fiords having a moderate freshwater inflow and a relatively deep entrance sill (7 m). Thompson Sound forms part of the interconnected Doubtful Sound/Thompson Sound complex and is somewhat typical of the larger interconnected inlets found further south (Fig. ). Preservation Inlet has been selected because it has a shallow entrance sill (26 m) and inner sill system (38 m) giving rise to a somewhat isolated deep basin in Long Sound. The present survey in July 983, along with the two earlier surveys in December 977 and March/April 98, provide the only available data for the three southern fiords, but data from seven other surveys of Milford Sound are available. For convenience the surveys will be referred to by the month and year in which they were done. THE JULY 983 SURVEY During the July 983 cruise, hydrocast stations were occupied at positions (Fig. 2-4) within the selected fiords matching those done on the two earlier cruises. Station procedures were the same as those described in the earlier reports (Stanton & Pickard 98: Stanton 984), but on this cruise no dissolved oxygen data were collected and no offshore stations were occupied. Station data are given in Greig (in press). The depth profile of Preservation Inlet (Fig. 4) has been revised in line with the bathymetric survey of Irwin and Main (984) and represents the deepest interconnections between the various basins. The main differences between this

3 3 New Zealand Journal of Marine and Freshwater Research, 986, Vol. 2 Fig. Map and longitudinal section of Milford Sound showing stations worked in July 983. Inset shows the location of the selected fiords surveyed on the south-west coast of the South Island, New Zealand. and the earlier profiles concern the nature of the double sill structure at the entrance to Long Sound. Freshwater inflow lowers the surface salinity and drives the estuarine circulation in the upper layer of the fiords. July is the month of minimum rainfall and minimum river inflow, as given in the rainfall statistics and Cleddau River discharge respectively, at Milford (Stanton & Pickard 98). Cleddau River gaugings were discontinued in June 98, but Milford rainfall (Table ) for July 983 was 387 mm, around % higher than the 353 mm average for this month. The lowest surface salinities were found in Long Sound with the minimum of 23.4 X 3 at Station S48 (Fig. 4). This midfiord minimum probably arose from the peak rainfall on July 8 when the estimated surface speed of this water in the five day travel down fiord was around 5 cm s". The surveys of the other fiords were under low freshwater inflow conditions when for most of the survey period daily rainfall was zero. Hence, river inflows would have been at base levels. Surface salinities were in the range x 3

4 Stanton Winter oceanography in some New Zealand fiords 3 S55 N - 5 S56 South WestArm ( S5757 S525' S58 A S5 5 km George Sound Fig. 2 Map and longitudinal section of George Sound showing stations worked in July 983.

5 32 New Zealand Journal of Marine and Freshwater Research, 986, Vol. 2 S53* Precipice Cove Bradshaw Sound 4 Fig. 3 Map and longitudinal section of Thompson Sound showing stations worked in July 983.

6 Stanton Winter oceanography in some New Zealand fiords 33 S486 Distance from head (km Fig. 4 Map and longitudinal section of Preservation Inlet showing stations worked in July 983. Lower curve shows depth mean tidal velocity squared, for the longitudinal section, plotted to an arbitary scale with little structure. In Thompson Sound (Fig. 3) surface salinity was around 28 X ~ 3 compared with more than 3 X ~ 3 in Bradshaw Sound showing the effect of the Manapouri Power Station outflow at Deep Cove in Doubtful Sound and the spreading of this water through Pendulo Reach into Thompson Sound. In Milford and George Sounds surface salinities were relatively high, generally greater than 32 X ~ 3 in Milford and greater than 33 X Hn George Sound. Surface temperature varied from 7.9 C (S48, Long Sound) to.5 C (S5, Gaer Arm). Surface waters within the fiords were everywhere cooler than the underlying water so that all stations exhibited a subsurface temperature maximum or temperature inversion surface to bottom. In Milford, George, and Thompson Sounds, temperature at the maximum was between 2. and 2.3 C and found at 5- m depths. In Preservation Inlet, the temperature maximum was shallower than in the other

7 34 New Zealand Journal of Marine and Freshwater Research, 986, Vol. 2 Table Winter 983 survey details and Milford rainfall data. Date (July 983) Survey Rainfall (mm) Q S467-S473 Preservation Inlet S479-S486 Preservation Inlet S53-S58 Thompson Sound S5-S52 Thompson Sound S55-S59 George Sound S525-S526 George Sound S528-S53 Milford Sound S53-S538 Milford Sound fiords, being found between 6 and 2 m. In Long Sound, temperatures at the: maximum were 2.2 to 2.4 C and in the outer fiord and Isthmus Sound the maximum was between.5 and.7 C. At the two outer stations (S467, S469) temperature increased monotonically from the surface to values between.4 and.7 C at the bottom. Clearly the water from m to the bottom (36 m) in Long Sound was warmer than water found elsewhere in the inlet. Sea surface temperatures have been measured at Anita Bay, near the mouth of Milford Sound, over a five year period ( ) as part of the NZOI coastal climate survey (N. M. Ridgway, pers. comm.). Monthly mean temperatures (Fig. 5) show a minimum in September but temperatures are close to the minimum throughout June to September. The daily data set shows that July 983 was cooler than the.7 C monthly average and the survey results probably reflect this throughout the selected fiords. In the deep zone, below the direct influence of the freshwater inflow, water properties changed more slowly with depth. Comparison of water properties in the deep basi ns of the selected fiords (Fig. 6) shows the broad similarities observed in the three northern fiords and the distinctly different properties in Preservation Inlet. Temperatures at, 2, and 3 m showed that Milford, George, and Thompson Sounds were similar with water of 2. C at m and.3-.7 C at 2 m. Preservation Inlet was slightly cooler at the outer stations (.4 C at m) and Long Sound deep water was Total warmer and exhibited a small temperature inversion with a minimum at m. Deep salinities exhibited weak subsurface maxima at around 5 m in Milford and 5-2 m in Thompson Sound. Salinity at the maximum was around 35. X 3. In George Sound, salinity values were similar but without the maximum and in Preservation Inlet deep salinities were markedly lower ( X " 3 ) than in the other fiords. The densities in Preservation Inlet were also markedly lower than in the other fiords (Fig. 6), largely as a result of these salinity differences. SEASONAL VARIABILITY Subject to the assumption that the data from the three surveys are typical of conditions in late spring (December 977), late summer (March/April 98), and winter (July 983) some general conclusions regarding seasonal variability can be drawn. Surface salinities were higher in winter than in other seasons. This was particularly noticeable in a high freshwater inflow fiord, Milford Sound, where July 983 values of around 32 X ~ 3 were much higher than the values of around 7 X ~ 3 observed on the two previous surveys. This trend correlates with the minimum freshwater inflow expected in July (Stanton & Pickard 98). Such seasonal trends were less noticeable in a fiord with lower inflow, George Sound, where surface salinity is generally relatively high. Surface salinities were generally higher in Thompson Sound and Preservation Inlet during the winter survey, although the

8 Stanton Winter oceanography in some New Zealand fiords 35 Fig. 5 Mean monthly sea surface temperature ( ) at Anita Bay, Milford Sound, and mean monthly air temperature (934-98) at Milford. Milford Sound minimum recorded in Long Sound was the lowest observed in this fiord on any of the surveys and illustrates the effect that short period storm inflow events can have on the observations. Surface temperatures were coolest on the July 983 survey. Daily sea surface temperature records from Anita Bay, Milford Sound, illustrate the seasonal trends in this region. The monthly means for the period (Fig. 5) show a seasonal range from a minimum of.5 C in September to a maximum of 4.9 C in March. However, the complete data set shows considerable variance at short time scales and the extreme temperature range observed was between 9.3 and 5.7 C. Throughout the fiords even greater temperature variations have been observed. The lowest surface temperature recorded was the 7.9 C in Preservation Inlet on the present survey and in February 969, Jillett and Mitchell (973) found surface temperatures up to 2.TC in Dusky Sound. Monthly mean air temperatures at Milford (Fig. 5) are generally lower than the mean sea temperatures, particularly in winter, so that near surface water would experience convective cooling by the atmosphere at this time of the year. In the July 983 survey, this cooling effect would account for the observed low temperature surface water overlying warmer subsurface water found at all stations. The temperature of the freshwater inflow will also affect the observed surface temperatures and in general it is difficult to distinguish these two cooling mechanisms. In the December 977 survey, sea surface temperatures were warmer than the underlying water in most places but where freshwater 6 7 Month 2 inflow was high (e.g., Milford Sound and Doubtful Sound) surface temperatures were cooler than the underlying water. This December survey was close to the time of minimum air-sea temperature differences in mid-summer (Fig. 5) at which time the heat flux may be from atmosphere to sea surface. On the March/April 98 survey, surface temperature patterns were somewhat between those observed on the other surveys as might be expected from the air-sea temperature cycle with the temperature difference increasing rapidly at this time (Fig. 5). Maxima and minima in the seasonal temperatures (Fig. 5) show the characteristic two monthly lag between sea and air temperatures which has been observed elsewhere on the west coast (Hessell 982). Water properties in the deep basins of the fiords exhibit both seasonal and non-seasonal changes and these are closely related to the deep water renewal processes discussed below. DEEP WATER PROPERTIES AND RENEWAL PROCESSES Replacement of deep water, below sill depth, in a fiord basin can only occur when the outside water at or above sill depth is more dense than the water within the basin. Consequently, the properties of the deep water found in fiord basins are closely related to the source water at the time of the last renewal but modified by the effects of mixing from above during the period since the replacement occurred. The extent of vertical mixing is determined by the upper estuarine circulation, the tidal

9 36 New Zealand Journal of Marine and Freshwater Research, 986, Vol. 2 flows, and meteorological effects (especially wind forcing). Biological activity determines the utilisation rate of the dissolved oxygen and in some situations can lead to anoxic conditions developing. The ease with which deep water replacement can occur is determined by fiord morphology, particularly the depth and nature of the sills, and the changes in the offshore water properties. The offshore water properties change both seasonally and non-seasonally when processes such as wind induced upwelling can produce conditions favourable for renewal to occur. The subtle interplay of these various factors have been reviewed by Gade and Edwards (98) and Farmer and Freeland (983). In this section the data from the present July 983 survey and earlier surveys are used in an attempt to understand something of the renewal processes in the selected New Zealand fiords. The changes in bottom water properties for the three surveys (Fig. 7) show the essential differences in the renewal process for the selected fiords. Milford (Fig. 7A), George, and Thompson Sounds exhibit relatively large temperature changes whereas deep salinities remain high and similar to oceanic values. In contrast, water below 5 m depth in Preservation Inlet (Fig. 7B) shows lower vertical gradients, smaller changes in temperature, and a gradual decrease in both salinity and density. These differences immediately suggest that renewal of bottom water occurs more readily and hence more frequently in the northern fiords. MILFORD SOUND Stirling Basin Milford Sound has been the most frequently sampled New Zealand fiord but on only five occasions have there been adequate sampling of the offshore waters for a comparison of the deep water properties inside and outside the fiord. In all these surveys, the deep water in Stirling Basin exihibited a temperature/salinity relationship somewhat similar to that of the offshore waters. The main difference was that salinity at the maximum inside the fiord was slightly lower than that outside. On all occasions temperatures sampled at and below sill depth were cooler in Stirling Basin than those found outside. However, the vertical temperature gradient in Stirling Basin was smaller than that offshore so that at deeper depths water of similar temperature (and salinity and density) could generally be found. The depths at which these water properties could be approximately matched is of interest and are shown in Table 2. For example, in 977, when density differences at sill depth (64 m) were minimal and the high dissolved oxygen levels (Stanton & Pickard 98) showed that deep water renewal had recently occurred, water similar to that found near the bottom of Stirling Basin was found 2 m offshore. In contrast, in March 98, when density differences at sill depth were relatively large, water of similar properties to that found at the bottom of Stirling Basin was found deeper (285 m) at the offshore station. At this time, bottom water in Stirling Basin was somewhat depleted in dissolved oxygen (Stanton 984) indicating that renewal of this water had not occurred for some time. Similar features are evident in the other instances shown in Table 2. From the density differences at sill depth, conditions were closest to those needed for deep water renewal at the time of the 957 (Garner 964) and 977 surveys and at these times water similar to the Stirling Basin bottom water was found at around 2 m and at other times such water was found some 8 m deeper. As the effect of tidal pumping on the deep water renewal process in this fiord is probably negligible (Stanton 984) the present study suggests that renewal occurs when dense water offshore is raised upward closer to sill depth or when the density structure offshore is suitably modified by seasonal or advective processes. The extent of changes in offshore density is largely unknown at present because of the lack of extensive winter data in this region. In 967, a summer and winter survey of temperatures over the continental shelf (Garner 969), showed that off the fiord coast, bottom temperatures were cooler in summer than in winter suggesting a strong advective influence in this region. On all surveys, the bottom water temperature in Stirling Basin has been between. and 2. C and consequently this can be taken as a marker for source water temperatures. On the 967 shelf water surveys (Garner 969), water of such temperatures was only found over the shelf in summer. It is, therefore, possible that deep water renewal is a summer feature in this area. The data studied (Table 2) suggest deep water renewal had recently occurred at the time of the 957 and 977 surveys. If the offshore waters at this time (Table 2) were similar in properties to the water that had been involved in the renewal event, then the smaller vertical temperature gradients within the fiord suggests that offshore deep water from around one level enters the fiord and spreads vertically giving rise to the cooler waters found inside the fiord as well as the lower vertical temperature gradient. However, if renewal had not occurred in the recent past, then it is possible that these temperature differences between fiord and offshore waters reflect the temperature structure of the offshore waters at the time of the last renewal event. It must be remembered that the data (Table 2) span only a small seasonal range and also at times

10 Stanton Winter oceanography in some New Zealand fiords a a. D 4-J Temperature ( C) _l_ i I j I I I.» :/ :/ :/ '/ I I / \ 'it \ :/' \*r V- /r // /// i-'i ' i / i 2 FT 3 _! - - Salinity(x" 3 ) 35 \ -I \ 'J \ \ i w 'r I ]j ': ( Density ((7 t) S533 Milford Sound S57 George Sound i S58 ClOO Preservation Inlet S467 Outer Sil Thompson Sound Preservation Inlet 25 "A \\ r>>*. \'-\ \ - I vl \'l >t \\ \ \- i: i: ri i i i : i i S469 Cavern Head 3 S479 Revolver Basin 4 - S483 Long Sound Fig. 6 Vertical temperature, salinity, and density profiles in July 983 for the deepest basins within the four selected fiords (top) and for selected stations in Preservation Inlet (bottom).

11 38 New Zealand Journal of Marine and Freshwater Research, 986, Vol. 2 Table 2 Comparison of deep water properties at offshore stations and in Stirling Basin. Data at sill depth ( m) and at depths marked * have been found by interpolation. Data sources:, Garner & Ridgway (9); 2, Garner (964); 3, Stanton (978); 4, Unpublished NZOI data; 5, Stanton & Pickard (98); 6, Bowman (98); 7, Greig (983). Station Date A337(l) 3/2/57 N 97(3) 25/4/74 524(5) 7/2/77 Q463(6) 23//8 M725(7) 7/4/8 Depth (m) 2 28* 2 2* 285* Offshore Ternp. Sal. ( C) (x - 3 ) Density (sig-t) renewals of intermediate depth waters may occur when conditions do not allow the exchange of bottom water. Meteorological forcing of the waters within the fiord and offshore can be "important in deep water renewal dynamics (Gade & Edwards 98). Ship observations (Reid & Collen 983) give the frequency of winds greater than 34 knots (7.5 m s') as 5.7% for the ocean area adjacent to the southwest corner of New Zealand. Wind directions are predominantly from the westerly quarter in all seasons. Hence, south-westerly winds favourable to coastal upwelling and the raising of dense water to sill depths are common. However, as appropriate observations of the coastal upwelling system have not been made, definite conclusions cannot yet be drawn. Winds at the coast differ markedly from those measured offshore in this area (Neale & Thompson 978) and are generally weaker. In Milford Sound, winds tend to blow either up or down fiord with winds from the north-west predominating (Hessell 982). Wind driven currents, resulting from both the effects within the fiord and the effect of wind systems on the coastal waters, have been found to be the dominant current regime in many fiords (Svendsen & Thompson 978). Currents that are probably of this type have been measured in Milford Sound (Stanton 984) where intermittent currents up to.37 m s ~ ' were observed. These currents were not related to the tidal flows, which were small, nor to changes in the estuarine circulation as indicated by changes in the freshwater inflow. Garner (979) gives the data from two current meters moored in the entrance channel at a position km landward of the mooring reported by Station Date A329(2) 28//57 N24(4) 25/4/74 52(5) 7/2/77 Q468(6) 24//8 M592(7) 23/3/8 Stirling Basin Depth Temp. Sal. (m) ( C) (X -') Density (sig-t) Stanton (984). These records showed similar currents over the six day recording period in March 974 to those reported by Stanton (984) for December 977. At 9 m depth the current flowed for 45 h in the up fiord direction, reaching a peak speed of.33 m s. This was followed by a period of weak flows and then during the final 6 h the current reversed, reaching a peak speed of.26 m s". At 7 m depth, currents were weaker, reaching a peak speed of.4 m s~', and these flows appear to be only loosely correlated to those at the upper current meter. The lower current meter snowed a nett inflow over the recording period. At times of weak flow the tidal component of around.5 m s" was discernable in the records. At both current meters the strong non-tidal flows were bimodal in structure being either inflows or outflows. Similar bi-modal structure was observed by Stanton (984) but in this instance the mooring was too close to the the sharp bend in the fiord at Dale Point and distortions in the flow field gave currents that did not reverse along the channel direction. Deep Water Basin Deep Water Basin at the head of Milford Sound (Fig. ) is separated from the main reach by a channel about km long with a sill depth of 5. m. The Cleddau and Arthur Rivers enter this basin and on the present survey the surface water was cool (9.6 C) and of very low salinity (4.6 X 3 ) as a result. Temperature increased from the surface to 6 m depth, decreased slightly between 6 and 5 m, and then increased to a maximum of 2.2 C at the bottom. This bottom water temperature was.2 C

12 Stanton Winter oceanography in some New Zealand fiords 39 Temperature ( C) Salinity (x(t 3 ) Density (fft) I i i i I I ^' B ' ' ' Q Stirling Basin Milford Sound 52 Dec 977 M592 Mar 98 S533 July Long Sound Preservation Inlet 392 Dec M6 Mar 98 S533 July Fig. 7 Vertical temperature, salinity, and density profiles in December 977, March/April 98 and July 983 for A, Stirling Basin, Milford Sound; B, Long Sound, Preservation Inlet.

13 3 New Zealand Journal of Marine and Freshwater Research, 986, Vol. 2 warmer than the highest temperatures observed anywhere in Stirling Basin and this would suggest that deep water exchange was not occurring at this time, although some exchange of intermediate level water would have been possible. Salinities throughout the water column were around.8 X " 3 lower than at corresponding depths in Stirling Basin with the result that densities were also lower. However, the differences in water properties between Deep Water Basin and Stirling Basin were smaller than those found on previous surveys when the vertical profiles of temperature and salinity have been quite different. In 98 (Stanton 984) some marked layering of the water column was observed which along with almost anoxic conditions at the bottom showed that renewal of the bottom water was a rarer event than renewal of the intermediate layers, whereas in the upper layers intense mixing was probably occurring arising from a density inversion between 6 and 5 m. The temperature profile, with a positive gradient between and 6 m and negative gradient between 6 and 5 m, has been found on all three surveys of this basin. This characteristic is probably related to mixing processes in the basin and the nature of the exchange process through the relatively long and shallow channel connecting this basin to the main fiord. At present these processes are not clear from the limited data available. However, it is clear that exchanges do take place as illustrated by the fact that, in 983, salinity values at all depths were higher than those found in 98. GEORGE SOUND AND THOMPSON SOUND Although Milford Sound has been the most extensively studied fiord, the available data suggest that the other fiords with deep sills behave in a similar fashion. The general similarity of the vertical profiles of temperature and salinity seen in the 983 data (Fig. 6) for Milford, George, and Thompson Sounds has been observed on the other occasions when these fiords have been surveyed. As George and Thompson Sounds have deeper entrance sills they are probably more easily replenished with deep water than Milford Sound. The dissolved oxygen levels observed in 98 support this, because at that time low values were found in the bottom water of Milford Sound and high values were found in the other two fiords. Present data would suggest that these fiords do not go anoxic but Stirling Basin is known to have been critically low in dissolved oxygen on at least two occasions (in 955 and 98, Stanton 984) and may have gone anoxic for a period. The depths of the entrance sills are probably the controlling factor as Milford Sound has a sill at 64 m but those in George and Thompson Sounds are at 7 and 45 m respectively. Precipice Cove Precipice Cove with a sill at 4 m is partly cut off from Bradshaw Sound (Fig. 3) and has exhibited some effects of this isolation in the water properties below sill depth on past surveys (Stanton & Pickard 98; Stanton 984). In the 983 survey, bottom water (85 m) in Precipice Cove was cooler, more saline, and more dense than corresponding water in Bradshaw Sound, and had temperature/ salinity characteristics of water at 42 m in Bradshaw Sound. At around sill depths the water in Precipice Cove was more dense than water in the main reach hence renewal was not occurring. PRESERVATION INLET Preservation Inlet differs from the other fiords surveyed in having a shallow entrance sill (26 m) and a series of basins cut off to a greater or lesser extent by intervening sills (Fig. 4). The sill to Long Sound is the most constricting of these as a double sill structure is found in the narrow channel from Adam Head to Sandy Point. These sills will be referred to as the Adam Head sill (38 m) and the Sandy Point sill (39 m) with an intervening basin, the Narrows Basin. A shallow sill (38 m) is found at the entrance to Isthmus Sound. As a result of the constrictions on deep water exchange the water properties in Preservation Inlet change, from basin to basin, along the length of the fiord. This progression in water properties can be seen in the data from the present survey (Fig. 6B). Salinity was highest at Station S467, just outside the sill, and was progressively lower in Revolver Basin (S479) and Long Sound (S483). On this survey, longitudinal water property gradients were small in the reach from Cavern Head to Revolver Basin so that the Revolver Basin station (S479) could be taken as representative. However, on previous surveys some differences in water properties have been found along the basins in the outer reach of Preservation Inlet. In Long Sound, data from this survey and previous surveys have shown very small horizontal property gradients along this reach so that the deepest station (S483) can be taken as representative. The progression in deep water properties found in the basins along Preservation Inlet suggests that mixing occurs as water crosses the inter-connecting sills. Although the available data on the nature of deep water renewal processes in this fiord is limited, some aspects of the dynamics can be modelled from the known characteristics. The tides provide a source of kinetic energy available for mixing. This energy is proportional to the depth mean of the tidal velocity squared, which at a given cross-section is given by:

14 Stanton Winter oceanography in some New Zealand fiords 3 U 2 = (B(x)/A(x) dh/dt) 2 where B(x) is the fiord surface area landward of the section, A(x) is the cross sectional area, x is the distance measured from the head of the fiord, and h is the tidal height Assuming that the tide is a forced standing wave of sufficient length that the instantaneous tidal height can be considered to be the same everywhere throughout the fiord, then the relative tidal velocity will be proportional to the ratio B(x)/A(x). This ratio was calculated for the key sections along the fiord, taking into account the major basins and sills. The resulting plot (Fig. 4) shows that the tidal energy and hence the tidal mixing are greatest over the double sill to Long Sound. Tidal mixing effects are much less over the shallower entrance sill because of its greater width. This calculation gives only the total tidal kinetic energy, not all of which will be available for mixing. Isthmus Sound Although Isthmus Sound is isolated from the main fiord by a sill at 38 m, tidal velocities are relatively low because of the small tidal compartment of this arm. The tidal energy over this sill is lower than that over the Long Sound Sill by a factor of.. On the 983 survey, profiles of temperature and salinity in Isthmus Sound were similar to profiles at adjacent stations in the main fiord and density at sill depth was slighty higher in the outside reach. This suggests that deep water in Isthmus Sound was not isolated at this time, unlike the situation in March 98 (Stanton 984) when low dissolved oxygen levels were observed along with contrasting temperature and salinity levels. Isthmus Sound has not been sampled on any other surveys but deep water renewal processes are likely to differ from those in Long Sound, even though sill depths are similar, because Isthmus Sound experiences much lower river inflow and considerably smaller tidal velocities, than those experienced in Long Sound. Long Sound During the July 983 survey, water in Long Sound between 4 m depth and the bottom was more than.6 C warmer than water anywhere else in the fiord (Fig. 6B). Consequently, such water cannot have been forming at this time by any mixing process with the other water masses in the inlet nor could it result from meteorological effects as this was a time of cooling from the surface. This shows that the deep water in Long Sound was somewhat isolated from water in the other parts of the inlet and that any replacement of this water, if it was occurring, must be a relatively slow process. Density structure was favourable to renewal occurring as water from 5m to sill depth and below in Revolver Basin was more dense than water at any depth in Long Sound (Fig. 6B). However, renewal would also require that this water could traverse the sill region and still maintain some density contrast despite mixing processes. On the earlier surveys of Preservation Inlet, the water properties showed some contrasting aspects of the renewal process. During the December 977 survey, renewal would have been possible (provided density contrasts were not lost by the mixing during passage over the sill) as density at sill depth and below in Revolver Basin were slightly higher than those at any depth in Long Sound. Dissolved oxygen values in Long Sound were high showing that renewal of the Long Sound deep water had occurred. On the March 98 survey, density below 5 m in Long Sound was greater than anywhere else in the fiord so that renewal was not possible below this depth, although intermediate level renewal to shallower depths may have been possible. The dissolved oxygen values in the bottom water were relatively low, around 3.8 ml ~' (Stanton 984), showing that this water had been isolated for some time possibly even for the whole inter-survey period. The profiles of water properties from all three surveys of Long Sound (Fig. 7B) show a trend which suggests that the isolation of the deepest water in Long Sound may have continued through to 983. Below 5 m the property gradients become very small. Density and salinity show a slight decrease from survey to survey and temperature shows a slight increase. Such a trend is consistent with slow diffusion from above, throughout , but this conclusion must remain speculative as no dissolved oxygen data were collected on the 983 survey. Such data would have shown whether the decrease in dissolved oxygen observed between 977 and 983 continued or whether bottom water renewal occurred in the 98 to 983 interval. Clearly the renewal of deep water in Long Sound depends on the passage of dense water over the shallow sill. The amount of mixing that occurs during this passage depends on the characteristics of the flow through the sill channel. One important aspect of this is the length of the constriction relative to the tidal excursion (Farmer 983). The tidal excursion over the inner sill to Long Sound was examined using closely spaced sections through this area. This detailed bathymetry (Fig. 8) was taken from the chart of Irwin and Main (982). The sill consists of a relatively long channel extending for about km from Adam Head and then broadening into the deeper Narrows Basin before the short sill at Sandy Point. The long channel has a somewhat constant cross sectional area as the landward deep-

15 32 New Zealand Journal of Marine and Freshwater Research, 986, Vol. 2 N 2 Distance from Adam Head(km) Fig. 8 A Bathymetry (m) of the sill to Long Sound, Preservation Inlet showing selected sections used to calculate tidal excursions. B VariatiiDn of tidal velocity through sill region as a result of variations in cross sectional area and up-fiord tidal compartment. ening of the channel is largely compensated by the narrowing of the channel width. Using the key cross sections shown in Fig. 8, the tidal excursion of water through the sill region was simulated numerically. The distance travelled by a water particle in time t, is given by: X = j U(x,t) dt o where U(x,t) is given by the above tidal equation. This integral was calculated by time stepping through a sinusoidal tidal cycle and evaluating the B(x)/A(x) function at each point required using linear interpolation between the selected key sections (Fig. 8). The results show that for average neap or spring tidal ranges (.3 and.8 m respectively) the tidal excursion is only over the shallow part of the sill but not through the whole sill length. However, for tides of 2 m range it is possible for water to traverse the whole distance over the Adam Head sill through the Narrows Basin and over the Sandy Point sill. Such large tides probably occur on occasions as it is known that tides in the region have

16 Stanton Winter oceanography in some New Zealand fiords 33 a strong declination as shown in current-meter records from this sill (Stanton 984). Stanton (978) has reported tides of up to 2.3 m in Caswell Sound. The density stratification at stations closest to the sill, as measured by the Brunt-Vaisala frequency, N, decreased sharply from a near surface maximum of around. s~' to around.5 s~' at sill depth with a depth averaged value of around.2 s". Using these stratification values along with the tidal velocities and tidal excursions, the dynamics of the flow over the sill can be classified along the lines given by Farmer (983). The Densimetric Froude Number, F, defines the balance between kinetic and potential energy in the baroclinic field. For first mode internal waves this is given by: F = U/C ~ (L/H) (w/n) where U is depth average tidal speed, C is first mode internal wave speed, L is tidal excursion, H is channel depth away from the sill, w is frequency of the dominant semi-diurnal tide. As (w/n) is small the flow falls within the quasi-steady regime on the characteristic diagram (Farmer 983, Fig. 4). Consequently there is insufficient kinetic energy in the upstream flow to raise the deeper fluid up to the sill crest and therefore blocking will occur at some depth. However, over the sill crest the maximum tidal velocities are sufficient to generate lee waves similar to those that have been observed in Knight Inlet, British Columbia (Farmer & Smith 98). At the times of maximum tidal velocity, the flow regime might be expected to be similar to the Mode response observed in Knight Inlet in winter, when the stratification is somewhat similar to that found in Preservation Inlet. The extent to which tidal velocity shear can mix the water column against the stabilising effect of the stratification is given by the Richardson Number: Ri = N 2 /(du/dz) 2 The current-meter observations over the sill (Stanton 984) showed little vertical shear down to 27 m depth (.3 S" at most). Consequently the sill flow will not in general mix out the near surface low salinity layer (Ri of the order of 9) but in the bottom m, where the shear is larger and N relatively low (Ri of the order of.), considerable tidal mixing will occur around the time of the peak tidal flows. The total extent of mixing through the sill region will arise not only from vertical shear and internal wave response discussed here but also from local effects resulting from the sinuous nature of the channel and the roughness and complexity of the double sill region. These latter effects will undoubtedly increase the amount of mixing. However, these calculations only give the tidal component of the circulation. Tidal flows can enhance the two layer transport through a constriction (Stigebrandt 977; Farmer & Freeland 983), so some knowledge of the estuarine circulation and other flow components is required before a quantitative assessment of the renewal process can be made. The current-meter data from the mooring on the Adam Head sill (Stanton 984) showed that the tidal currents dominated at that time. Assuming that the other current components can be neglected in the sill region, then the present calculations would suggest that water from Revolver Basin cannot traverse the sill without some mixing and tidal excursions suggest that direct intrusions of water only occur on unusually large tides. Consequently the volume of new water introduced to Long Sound over a tidal cycle is probably small relative to the volume of this deep basin. Hence, the complete flushing of Long Sound could be expected to take a considerable length of time. At times when the tidal excursion is less than the total length of the double sill then introduction of new water to Long Sound would rely entirely on suitably dense water being formed during the tidal mixing in the sill region or on tidal enhancement of the estuarine circulation. ACKNOWLEDGMENTS The author gratefully acknowledges the work of R. A. Pickrill and the scientists and crew of RV Tangaroa for collecting and making available the July 983 oceanographic data. REFERENCES Bowman, M. J. 98: Oceanography of central New Zealand waters. 9 January-5 February 98. A data report. University of Auckland, Maui Development Report Environmental Study. Report no. 8-. Farmer, D. M. 983: Stratified flow over sills. In : Gade, H. G.; Edwards, A.; Svendsen, H. ed., Coastal oceanography. New York, Plenum Press, p Farmer, D. M; Freeland, H. J. 983: The physical oceanography of fiords. Progress in oceanography 2: Farmer, D. M.; Smith, J. D. 98: Tidal interaction of stratified flow with a sill in Knight Inlet. Deep-sea research 27(3/4a):

17 34 New Zealand Journal of Marine and Freshwater Research, 986, Vol. 2 Gade, H. G.; Edwards, A. 98: Deep water renewal in fiords. In: Freeland, H. J.; Farmer, D. M; Levings, C. D. ed., Fiord oceanography. New York, Plenum Press, p Garner, D. M. 964: The hydrology of Milford Sound. In: Skerman, T. M., Studies of a New Zealand fiord. New Zealand Oceanographic Institute memoir 7: p : The seasonal range of sea temperatures on the New Zealand shelf. New Zealand journal of marine and freshwater research 3(2) : : Ocean current-meter records, New Zealand coastal waters University of Auckland, Physics Department. Physics report 79-. Garner, D. M.; Ridgway, N. M. 9: Hydrology of New Zealand offshore waters. New Zealand Oceanographic memoir 2 : 62 p. Greig, M. J. N. 983: New Zealand Oceanographic Institute Hydrology Station data in press: New Zealand Oceanographic Institute Hydrology Station Data Greig, M. J. N.; Bradford, J. M. 98: New Zealand Oceanographic Institute Hydrology Station Data 978. Hessell, J. W. D. 982: The climate and weather of Westland. New Zealand Meteorological Service miscellaneous publication 5(). 44 p. Irwin, J.; Main W. de L. 984: Preservation Inlet bathymetry :2 (2 sheets). New Zealand Oceanographic Institute chart miscellaneous series no. 58. Jillet, J. B.; Mitchell, S. F. 973: Hydrological and biological observations in Dusky Sound, south-western New Zealand. In : Fraser, R. ed., Oceanography of the South Pacific 972. Wellington, New Zealand Commission for UNESCO. 524 p. Neale, A. A.; Thompson, G. H. 978: Surface winds in coastal waters off Westland. New Zealand Meteorological Service technical note p. Pickrill, R. A.; Irwin, J.; Shakespeare, B. S. 98: Circulation and sedimentation in a tidal influenced fiord lake: Lake McKerrow, New Zealand. Estuarine, coastal and shelf science 2: Reid, S. J.; Collen, B. 983: Analyses of wave and wind reports from ships in the Tasman Sea New Zealand areas. New Zealand Meteorological Service miscellaneous publication p. Stanton, B. R. 978: Hydrology of Caswell and Nancy Sounds. In: Glasby, G. P. ed., Fiord studies: Caswell and Nancy Sounds, New Zealand. New Zealand Oceanographic Institute memoir 79: : Some oceanographic observations in the New Zealand fiords. Estuarine, coastal and shelf science 9: Stanton, B. R.; Pickard, G. P. 98: Physical oceanography of the New Zealand fiords. New Zealand Oceanographic Institute memoir p. Stigebrandt, A. 977: On the effect of barotropic current fluctuations on the two-layer transport capacity of a constriction. Journal of physical oceanography 7(): Svendsen, H.; Thompson, R. 978: Wind-driven circulation in a fiord. Journal of physical oceanography 8(4):