11. Temperature, salinity, and density overview

8 S. Afr. T. Nav. Antarkt., Deel 15, 1985

The South African SffiEX I Cruise to the Prydz Bay region, 1984:

11. Temperature, salinity, and density overview

CTD casts were made and water samples taken at 45 stations on the SIBEX I grid. The data are considered in terms of temperature, salinity and density of the surface mixed layer, winter water, mid-water mixing, mid-water density, and deep water density. A diagrammatic overiew is presented.

CTD-toerusting is by 45 stasies op die SIBEX !-rooster uitge- gooi en watermonsters geneem. Die gegewens word met ver- wysing na die temperatuur, soutinhoud en digtheid van die oppervlakmenglaag bespreek, asook met betrekking tot Win- terwater, Middelwatermenging en -digtheid, en Diepwater- digtheid. 'n Diagrammatiese oorsig word ook gebied.

Introduction

A study of the fields of temperature and salinity can provide much information on the large scale dynamics and on the ac- tive mixing processes. To this end, CTD casts were made and water samples taken at the 45 stations of the SIBEX I grid.

52 54 56 58

DEGREES EAST Fig. I. Grid of stations and track chart.

60

62

The N-S lines are positioned roughly as follows

stations 2- 8 64°E

stations 9- 15 62°E

stations 16- 22 60°E

stations 23- 29 58°E stations 30- 35 56°E

stations 86-41 54°E

stations 42- 45 and I 52°E

64

The two degree separation of N-S lines is approximately 60 nautical miles. The E-W positions of stations are roughly at 62°00'S, 62°40'5, 63°20'S, 64°00'5, 64°40'S, 65°20'5 and 66°00'S at a separation of approximately 40 nautical miles.

G.B. Brundrit Dept. Oceanography Univer sity of Cape Town , Rondebosch 7700

Bottle data are available from standard depths at stations 1 to 42, while CTD data are available for all stations except stations 3, 17,28 and 38.

The grid tableau can be used to provide information in a concise form from both the bottle and CTD measurements.

Rough contours can be drawn and conclusions drawn about the essential character of the upper 500 m at the grid sta- tions.

Reliability of the bottle and CTD data

The bottle data

.

provided a standard against which to base the uncalibrated CTD data. Thus, where absolute accuracy is required, the bottle data should be used. The CTD data can be used for relative measurements and for vertical de- tail.

The estimation of density (sigma-t) through the Inter- national Equation of State 1980 (Millero & Poisson 1981) uses the bottle data. The variation of sigma-t with depth at the stations was used to assess the vertical stability of the water column while the vertical variation in temperature and in salinity was used to establish the nature of the mixing pro- cesses at work at various depths. It should be noted that at these polar latitudes, it is the variation in salinity which is the major contribution to the stability in the sense that a change in salinity of 1 %o will lead to a change in sigma-t of 1, while a change in temperature of 1° will lead to a change in sigma-t of 0,1. The temperature and salinity data can also be used to characterise water masses and mixing between these masses.

Results and Discussion

The mixed layer

The surface mixed layer is well defined at all stations over the entire grid in the sense that the temperature and salinity, and hence density, do not vary with depth through the layer.

(It is assumed that the CTD data at stations J 9 and 16 are slightly suspect). The mixed layer was characterised by its thickness as well as its temperature and salinity. These de- tails are portrayed in succeeding tableaux with the thickness being taken from the CTD data (Fig. 2) and the. temperature (Fig. 3) and salinity (Fig. 4) from the bottle data.

The mixed layer thickness varies in a somewhat arbitrary manner between 50 m and 80 m over the grid. This suggests that it is caused by large scale meteorological disturbances passing through the grid. Further comment will be made when the stability of the base of the mixed layer is exam- ined.

The temperature of the mixed layer is obtained from the bottle data and at each station is constant through the layer to an error of at most +0,08

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S. Afr. J. Antarct. Res., Vol. 15, 1985

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52 54 56 58

DEGREES EAST

Fig. 2. CTD- mixed layer depths.

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Temperature isolines lie roughly east/west with a strong temperature front, a change of greater than 1

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stations 4 and 5, between stations 29 and 28 and between stations 41 and 40. The variation of mixed layer tempera- tures over the entire grid is 3 °C between 1,40

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at station 1 to -1,60

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at station 29.

Relatively speaking, there is slightly less variation in mixed layer salinity over the grid. The salinity measurements are constant to an error of -0,02. They vary from 33,83 %o at stations 1 and 36 to 34,07 at stations 30, 31 and 41. There is a general rise towards the south along each line with the maximum salinity in the south-west corner of the grid.

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Fig. 3. Mixed layer temperatures (bottle data) in

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52 54 56 58

DEGREES EAST

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Fig. 4. Mixed layer salinities (bottle data) in %o.

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Winter Water

Winter Water lies below the surface mixed layer in the north of the grid. It was characterised by its temperature, (Fig. 6) which was less than that in the mixed layer above (Fig. 7).

The Winter Water formed a wedge extending into the grid from the north-west corner. It was thickest at station 1 and became progressively thinner towards the south and east. It was absent in the south-east corner of the grid (Fig. Sa).

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Fig. Sa. CTD- Winter Water thicknesses/depths. A= absent.

Superimposition of CTD temperature traces for the line of stations 23 to 27, demonstrated that the layer of Winter Water became shallower to the south where it is entrained into the turbulent mixed layer. Figure Sb shows that the mixed layer then becomes colder. A similar pattern in the salinity traces is also shown.

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Mid-water mixing

Below the surface water, and particularly below the Win- ter Water, the temperature rises to a maximum, which is reached at depths near to 500 m (Fig. 8). This water can be gravitationally unstable unless there is a large enough com- pensating increase in salinity. Technically, the layer is then diffusively stable. This is the situation at most of the sta- tions, particularly those where there is a layer of Winter Water. There is little evidence of mixing in the water column in these cases, except for thin layers around 150 m where in- stability arises through decreases in salinity.

At some stations, however, there is extensive evidence of mixing and interleaving. This is particularly true of stations 2, 4, 13, 16, 29, 41 and 42. Station 42 is the only station where there is extensive mixing and interleaving taking place below Winter Water. The manner in which the salinity and temperature at separated depths match suggests that the in- terleaving is the after effect of an earlier nearby overturning event (Ruddick & Turner 1979).

52 54 56 58

DEGREES EAST

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Fig. 7. CTD-interface temperature drop (surface-winter water) in

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The situation is illustrated for station 41 in Figure 9 where interleaving layers below the unstable mixed layer are strongly stable, diffusively stable, stable, strongly diffusive (even unstable), salt fingering (tending to instability), strongly diffusive, salt fingering and stable.

Mid-water density

Density calculations from the bottle temperature and sa- linity values and the International Equation of State 1980 re- veal the existence of a strongly stable layer below the surface

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S. Afr. T. Nav. Antarkt., Deel15, 1985

SALINITY

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Fig. 9. CTD- mid-water mixing and interleaving. Temperature and salinity traces, station 41. U = unstable; F = fingering - unstable; D = diffusive-stable; S = stable.

mixed layer. This layer is quite thin but has changes in sigma-t of 0,3 in less than 40 m. Such strong stability can en- sure that wind mixing does not deepen the surface mixed layer. Using the methods of Pollard et a/ (1973) as developed by Dr G Hughes, it can be shown that it would require a wind stress of 1,5 Nm-² (a wind speed of approximately 25 ms-') to deepen this layer. Conversely such a strong wind

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S. Air. J. Antarct. Res., Vol. 15, 1985 11

must have originally formed the surface mixed layer. The

Summary

size of such storms leads one to conclude that it formed the mixed layer at all stations in the grid.

Density sections 0- 500 m along N-S lines of stations (Fig.

10) reveal that there is a gentle upward tilt of the isopycnals towards the south as far as latitude 64°40'. For most stations this coincides with the presence of overlying Winter Water.

All the extensive midwater mixing lies to the south of the midwater upwelling. The dynamic topography relative' to 500 db is similar to that found in other studies (Gordon et al.

1981) and reproduces the variation in the sigma-t section.

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DEGREES SOUTH Fig. 10. Density sections 0-500 m along N-S line of stations.

Deep Water density and dynamic heights

The density structure from the bottle data in the Deep Water does not reveal much variation in the density (sigma- t) of the Deep Water. (Here it is assumed that the salinity data at the deep levels of station 18 are incorrect and should be higher.)

The dynamic topography relative to 3000 db is shown in Figure 11 and is also without much structure. It would be better to use a deeper level as a level of no motion. The sur- face velocity which is calculated from this dynamic topogra- phy is not consistent over the grid and is of magnitude much less than 10 cm s-^1•

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Figure 12 provides a convenient overview of the sit-uation and emphasises the boundary at about 6SOS.

52 54 56 58 60 62 64

DEGREES EAST

Fig. 12 .

I. Temperature contours 0.0°C: -0.5°C: - 1.0°C: - I.5°C 2. W = Winter Water. A= absence of Winter Water 3. S = stable mid-water; M = active mixing mid-water 4. Arrows indicate extent of upwardly inclined isopycnals .

References

MILLERO, F.J. & POISSON, A. 1981. International one-atmos- phere equation of state of sea water. Deep-sea Res. 28A: 625- 629.

RUDDICK, B.R. & TURNER, J.S. 1979. The vertical length scale in double diffusive intrusions. Deep-sea Res. 26A: 903-913.

POLLARD. R.T, RHINES, P B & THOMPSON, R.O.R.Y.

1973. The deepening of the wind mixed layer. Geophysical Fluid Dynamics 4: 381-404.

GORDON, A.L., MARTINSON, D.G. & TAYLOR, H.W. 1981.

The wind driven circulation in the Weddell Enderby Basin. Deep-sea Res. 28A: 151-163.