In the two upper intervals down to the bottom boundary of the sharp upper
thermo-halocline, there are two (stations 2698 and 2699) to three (station 2700)
intervals with the negative density ratio related to the positive stability parameter as
well as to positive ΔS and negative ΔT gradients. At the equator (station 2699)
there is a distinct interval with R ρ < 0 at the depths of the salinity maximum (60–
85 m) and at the northern station 2700, at depths from ∼40 to 65 m. The corresponding positive values of E θ, S are extremely high here. Such combination of
parameters ΔT < 0, ΔS > 0, E θ, S > 0, R ρ < 0 determines absolute, convectively
stable stratification. The effects of double-diffusion convection are excluded. The
same conclusion is related to station 2698, where the effect is less pronounced.
There are several thin intervals, where R ρ is positive and E θ, S > 0, and ΔT < 0,
ΔS < 0 (60–80 m at station 2698; 30, 55, and 65 m at station 2699; and 15, 30, 60,
and 80 m at station 2700). These conditions define convectively unstable stratification, but owing to the fact that R ρ is too high (∼4–8) the probability of the
development of the salt fingering double diffusion is very low within the intervals.
Now we address the depths of the main thermo-halocline below its sharp upper
part (deeper than 120 m) with the development of stepwise structures and oscillating thermohaline stability E θ, S (Fig. 5).
As seen from Fig. 5, the peaks of E θ, S “oscillations” that exactly correspond
(Fig. 5) to the thin, well-developed high-gradient portions (“sheets”) of the stepwise
structures over the salinity, temperature, and density profiles are also associated
with the low positive values of density ratio 1 < R ρ < 2, while some of them are
within 1 < R ρ < 1.5.
Pronounced examples of such thin intervals with peaks of positive stability and
low density ratio, related to the “sheets” of the steps can be found at the depths
shown in Table 1.
We can conclude that these minimum positive values 1 < R ρ < 2 and <1.5 are
found against the background of stable stratification E θ, S > 0, when both temperature and salinity decrease downwards with sharp negative gradients; hence, such
intervals meet the strict conditions of the development of convective instability and
double diffusion convection of the salt finger type. The intervals of possible fingering are from 4 to 14 m thick. The most intense manifestation of the process
occurs at the southern station 2698 and at the equator (station 2699). At the northern
station 2700, the manifestation of fingering is least expressed. We show in Table 1
the depths of possible fingering at this station but only the interval 4 m thick
between 155 and 159 m with R ρ = 1.62 may be regarded here as reliable.
It is important to note that the approximate coincidence of the most clearly
pronounced fingering intervals at depths of 300 and 375 m at stations 2698 and
2699 located 30 miles apart may show the possible great horizontal scales of such
processes.
In the rest part of the main thermocline within 100–600 m, as was shown above
and seen from Fig. 5, there are thick intervals of well-mixed portions of steps
related to low positive to low negative values of stability E θ, S (intervals close to 212
and 219 m at station 2698 are examples of slightly negative E θ, S ). This implies
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T. A. Demidova
thermo-halocline, there are two (stations 2698 and 2699) to three (station 2700)
intervals with the negative density ratio related to the positive stability parameter as
well as to positive ΔS and negative ΔT gradients. At the equator (station 2699)
there is a distinct interval with R ρ < 0 at the depths of the salinity maximum (60–
85 m) and at the northern station 2700, at depths from ∼40 to 65 m. The corresponding positive values of E θ, S are extremely high here. Such combination of
parameters ΔT < 0, ΔS > 0, E θ, S > 0, R ρ < 0 determines absolute, convectively
stable stratification. The effects of double-diffusion convection are excluded. The
same conclusion is related to station 2698, where the effect is less pronounced.
There are several thin intervals, where R ρ is positive and E θ, S > 0, and ΔT < 0,
ΔS < 0 (60–80 m at station 2698; 30, 55, and 65 m at station 2699; and 15, 30, 60,
and 80 m at station 2700). These conditions define convectively unstable stratification, but owing to the fact that R ρ is too high (∼4–8) the probability of the
development of the salt fingering double diffusion is very low within the intervals.
Now we address the depths of the main thermo-halocline below its sharp upper
part (deeper than 120 m) with the development of stepwise structures and oscillating thermohaline stability E θ, S (Fig. 5).
As seen from Fig. 5, the peaks of E θ, S “oscillations” that exactly correspond
(Fig. 5) to the thin, well-developed high-gradient portions (“sheets”) of the stepwise
structures over the salinity, temperature, and density profiles are also associated
with the low positive values of density ratio 1 < R ρ < 2, while some of them are
within 1 < R ρ < 1.5.
Pronounced examples of such thin intervals with peaks of positive stability and
low density ratio, related to the “sheets” of the steps can be found at the depths
shown in Table 1.
We can conclude that these minimum positive values 1 < R ρ < 2 and <1.5 are
found against the background of stable stratification E θ, S > 0, when both temperature and salinity decrease downwards with sharp negative gradients; hence, such
intervals meet the strict conditions of the development of convective instability and
double diffusion convection of the salt finger type. The intervals of possible fingering are from 4 to 14 m thick. The most intense manifestation of the process
occurs at the southern station 2698 and at the equator (station 2699). At the northern
station 2700, the manifestation of fingering is least expressed. We show in Table 1
the depths of possible fingering at this station but only the interval 4 m thick
between 155 and 159 m with R ρ = 1.62 may be regarded here as reliable.
It is important to note that the approximate coincidence of the most clearly
pronounced fingering intervals at depths of 300 and 375 m at stations 2698 and
2699 located 30 miles apart may show the possible great horizontal scales of such
processes.
In the rest part of the main thermocline within 100–600 m, as was shown above
and seen from Fig. 5, there are thick intervals of well-mixed portions of steps
related to low positive to low negative values of stability E θ, S (intervals close to 212
and 219 m at station 2698 are examples of slightly negative E θ, S ). This implies
442
T. A. Demidova