Current,Température and Salinity Observations in the Terra Nova Bay
171
During May-June, the température was close to the surface freezing point over
the entire water column. The salinity at 140 m was observed to increase to the
value of the HSSW, and the currents suggest the presence of two layers: from the
surface to about 400 m and from there to the bottom. At 402 m the salinity
reached the HSSW value with a delay of 1 month with respect to the upper record
(end of July, instead of end of June). In this period the surface haline mixing did
not reach this depth, probably due to the advection from the south of water fresher than HSSW or the mixing between the HSSW formed at the surface and the
bottom water [7].
The data show that the salinity mixing embraced the entire water column from
August to October, making the vertical density gradient minimum.
5 Normal Mode Décomposition
From time sériés presented in the previous paragraph, it is possible to ascertain
the presence of a density-homogeneous water mass from 140 m to the bottom
from August to October. The importance of continental shelf processes in the formation of dense water masses has been identified by many authors [2,8]. Négative
buoyancy forcing in polar régions is concentrated in areas of high ice production
[1-3], such as the polynyas. In the Terra Nova Bay polynya area, the katabatic
winds drive ice offshore, leaving the nearshore waters exposed to the atmosphère.
As new ice is formed, brine is rejected and acts as a sait flux [3] producing a négative buoyancy source and, as a conséquence, latéral density gradients in the shelf
waters. The saltwater formation, in turn, drives a baroclinie circulation involving
the locally created dense water, as well as the ambient water.
Although the current time sériés cannot be directly interpreted in terms of
HSSW formation, information on the nature of the water circulation and transport of dense water can be extracted from the empirical décomposition (EOF) of
the along-isobath components. The normalised eigenvectors of the first two
modes are shown in Fig. 4. The first mode (B) indicates a quasi-homogeneous
oscillation of the water masses over the water column. The second mode (C) indicates that the water above and below 300 m oscillâtes in opposite ways. The interprétation is that this second mode, extracting 12% of the total variance (with
respect to the 80% of the first mode), is représentative of the transport of HSSW
from the Terra Nova Bay polynya area to the Ross Sea. If this is true, the total
amount of HSSW drained out from the polynya area can be calculated, using a
mean horizontal dimension of about 35 km [1, 3], as equal to 0.85 Sv over the
entire column using a velocity of 4 cm/s. An approximate transport of 1 Sv is in
agreement with other calculations presented in this book [9].
The modal décomposition makes évident the presence of higher velocities in
the lower water column near the bottom. Numerical models applied to polynya
areas [10] show that the water mass circulation evolves from a geostrophic
adjustment to rapid eddies development and offshore eddy transport. The
numerical model shows that maximum along-shelf velocity can be attained at
depth.
171
During May-June, the température was close to the surface freezing point over
the entire water column. The salinity at 140 m was observed to increase to the
value of the HSSW, and the currents suggest the presence of two layers: from the
surface to about 400 m and from there to the bottom. At 402 m the salinity
reached the HSSW value with a delay of 1 month with respect to the upper record
(end of July, instead of end of June). In this period the surface haline mixing did
not reach this depth, probably due to the advection from the south of water fresher than HSSW or the mixing between the HSSW formed at the surface and the
bottom water [7].
The data show that the salinity mixing embraced the entire water column from
August to October, making the vertical density gradient minimum.
5 Normal Mode Décomposition
From time sériés presented in the previous paragraph, it is possible to ascertain
the presence of a density-homogeneous water mass from 140 m to the bottom
from August to October. The importance of continental shelf processes in the formation of dense water masses has been identified by many authors [2,8]. Négative
buoyancy forcing in polar régions is concentrated in areas of high ice production
[1-3], such as the polynyas. In the Terra Nova Bay polynya area, the katabatic
winds drive ice offshore, leaving the nearshore waters exposed to the atmosphère.
As new ice is formed, brine is rejected and acts as a sait flux [3] producing a négative buoyancy source and, as a conséquence, latéral density gradients in the shelf
waters. The saltwater formation, in turn, drives a baroclinie circulation involving
the locally created dense water, as well as the ambient water.
Although the current time sériés cannot be directly interpreted in terms of
HSSW formation, information on the nature of the water circulation and transport of dense water can be extracted from the empirical décomposition (EOF) of
the along-isobath components. The normalised eigenvectors of the first two
modes are shown in Fig. 4. The first mode (B) indicates a quasi-homogeneous
oscillation of the water masses over the water column. The second mode (C) indicates that the water above and below 300 m oscillâtes in opposite ways. The interprétation is that this second mode, extracting 12% of the total variance (with
respect to the 80% of the first mode), is représentative of the transport of HSSW
from the Terra Nova Bay polynya area to the Ross Sea. If this is true, the total
amount of HSSW drained out from the polynya area can be calculated, using a
mean horizontal dimension of about 35 km [1, 3], as equal to 0.85 Sv over the
entire column using a velocity of 4 cm/s. An approximate transport of 1 Sv is in
agreement with other calculations presented in this book [9].
The modal décomposition makes évident the presence of higher velocities in
the lower water column near the bottom. Numerical models applied to polynya
areas [10] show that the water mass circulation evolves from a geostrophic
adjustment to rapid eddies development and offshore eddy transport. The
numerical model shows that maximum along-shelf velocity can be attained at
depth.