Temporal Variability of Currents in the Ross Sea (Antarctica)
113
D, and progressively higher at H, B and C. Tidal amplitudes seem to increase
from région doser to the coasts or to the Ross Ice Shelf barrier to offshore ones.
3.2 Low Frequency Currents
On lower frequency bands, two well-known tidal periods, lunar fortnightly (Mf;
13.66 days) and lunar-solar fortnightly (MSf; 14.76 days) hâve been found in
datasets from B (both instruments), C (only Mf at 420 m), D (MSf at 55 m) and
H (Mf from both instruments). The peaks seen in the spectrum were particularly high and neat for C, H and B, even if the two components were in general
not well resolved, in agreement with relevance of signal noticed for other bands
above (Fig. 6).
Components with periods of ~9 and 11 days, typical of mesoscale activity,
and others spread in the range of a few days, were found in most spectra, with
peculiar strength and evidence for the 11 days component of mooring H.
Mesoscale activity interests the whole examined water column: important
meandering and rotations are observed simultaneously in ail the current métrés
in the same mooring. A noticeable feature cannot be ignored in the C 420-m
spectrum: at 6.8 days, i.e. about half the Mf period, there was a signal as high as
the Mf one, while it scarcely and weakly emerged in the 230-m spectrum from
the same mooring. Other peaks occurred at lower frequencies, particularly
between 16 and 33 days, but rather irregularly; they were very neat for A meters,
but with low energetic content.
4 Discussion
Seasonal cycle is mainly determined by the ice formation and melting processes
occurring at the surface. Ice formation in the Ross Sea Shelf generally starts by the
end of February, with observed interannual variability in the initiation [12], and
rapidly increases in concentration, reaching maximum coverage after about 5
weeks. In this area ice growth starts early in the eastern part, advancing west and,
from the coast, advancing southward, so that the northwestern sector of the shelf
can be considered the one with lower ice concentration. Ice concentration on the
continental shelf was found lower than in the open océan, probably due to the newly
formed ice exportation from strong continental winds. Ice decay starts in
December; summer minima are found from the end of January to the first 2 weeks
of February [13].
A seasonal signal due to these processes, détectable by température and salinity variations, also reaches the intermediate level with a certain delay, as a resuit
of strong vertical convective movements which mix the water column above.
Rapid température decreasing is observed in March in Terra Nova Bay polynya
(mooring D) where the absence of total ice coverage favours an intense exchange
with the strong atmospheric forcings, in May near the Ice Shelf barrier (moorings
A and F) and later moving north (B and H). Minimums of température, ranging
from -1.7 to -1.9 °C, are reached at moorings A, F and H in the period from late
113
D, and progressively higher at H, B and C. Tidal amplitudes seem to increase
from région doser to the coasts or to the Ross Ice Shelf barrier to offshore ones.
3.2 Low Frequency Currents
On lower frequency bands, two well-known tidal periods, lunar fortnightly (Mf;
13.66 days) and lunar-solar fortnightly (MSf; 14.76 days) hâve been found in
datasets from B (both instruments), C (only Mf at 420 m), D (MSf at 55 m) and
H (Mf from both instruments). The peaks seen in the spectrum were particularly high and neat for C, H and B, even if the two components were in general
not well resolved, in agreement with relevance of signal noticed for other bands
above (Fig. 6).
Components with periods of ~9 and 11 days, typical of mesoscale activity,
and others spread in the range of a few days, were found in most spectra, with
peculiar strength and evidence for the 11 days component of mooring H.
Mesoscale activity interests the whole examined water column: important
meandering and rotations are observed simultaneously in ail the current métrés
in the same mooring. A noticeable feature cannot be ignored in the C 420-m
spectrum: at 6.8 days, i.e. about half the Mf period, there was a signal as high as
the Mf one, while it scarcely and weakly emerged in the 230-m spectrum from
the same mooring. Other peaks occurred at lower frequencies, particularly
between 16 and 33 days, but rather irregularly; they were very neat for A meters,
but with low energetic content.
4 Discussion
Seasonal cycle is mainly determined by the ice formation and melting processes
occurring at the surface. Ice formation in the Ross Sea Shelf generally starts by the
end of February, with observed interannual variability in the initiation [12], and
rapidly increases in concentration, reaching maximum coverage after about 5
weeks. In this area ice growth starts early in the eastern part, advancing west and,
from the coast, advancing southward, so that the northwestern sector of the shelf
can be considered the one with lower ice concentration. Ice concentration on the
continental shelf was found lower than in the open océan, probably due to the newly
formed ice exportation from strong continental winds. Ice decay starts in
December; summer minima are found from the end of January to the first 2 weeks
of February [13].
A seasonal signal due to these processes, détectable by température and salinity variations, also reaches the intermediate level with a certain delay, as a resuit
of strong vertical convective movements which mix the water column above.
Rapid température decreasing is observed in March in Terra Nova Bay polynya
(mooring D) where the absence of total ice coverage favours an intense exchange
with the strong atmospheric forcings, in May near the Ice Shelf barrier (moorings
A and F) and later moving north (B and H). Minimums of température, ranging
from -1.7 to -1.9 °C, are reached at moorings A, F and H in the period from late