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177
Havets Dybde er meget ringe i Forhold til dets
horizontale Udstrækning. Den største Del af Bevægelsen
maa derfor foregaa i horizontal lletning, altsaa langs
Lige-dybde-Linierne (Isobatherne), ligesom Strømmen i Overfladen
langs Kysterne. Hvor Isobarerne gaa langs Isobatherne,
og ude i det frie Hav, kunne Formlerne faa Anvendelse,
forudsat at Isobarerne ikke frembyde sterkere Krumninger
og sterkere Variationer i deres indbyrdes Afstand. I
Dybderne 300 og 500 Favne opfyldes disse Betingelser tildels,
navnlig i Polarstrømmen. I de større Dybder ligge
Isobarerne jevnlig tvers paa Isobatherne. Dette sidste Tilfælde
er analogt med Vandets Bevægelse i en Elv, hvor
Gradienten peger i Elvens egen Retning, og Bevægelsens
Hastighed er betinget af Gradientens eller Heldningens og
Frietionens Størrelse.
I de større Dybder maa Bevægelsen for en meget
stor Del foregaa i Retninger, der danne smaa Vinkler
med Gradienterne. I de mindre Dybder kan, som
Isobar-systemerne indtil 500 Favnes Dyb vise, Bevægelsen
idet-heletaget foregaa cyclonisk omkring et Minimum af Tryk,
der ligger forholdsvis centralt i Havet. I de dybere Lag
derimod blive de mere radiale Bevægelser fra de Steder,
hvor Trykket er størst, henimod de Steder, hvor Trykket
er mindst, de overvejende. Men i Maximumspartierne maa
Bevægelsen være nedadstigende og i Minimumspartierne
opadstigende, og disse vertieale Bevægelser befordres i
fremtrædende Grad ved den nævnte radiale Retning af de
horizontale Bevægelser. Som en ligefrem Folge af Vandets
Continuitet ville igjen de vertieale Bevægelser i de dybere
Lag indvirke paa Bevægelsen i de højere Lag. De
nedstigende Bevægelser i Dybet drage de øvre Vandlag efter
sig, de opstigende ville drive dem til Side.
Vi have directe Bevis for disse Virkninger af
Trykkets Fordeling i de dybere Lag i Temperaturens og
Saltholdighedens Fordeling. Da Strømningerne i vort Nordhav
hovedsagelig foregaa i meridional Retning, er det
Vertical-snittene for Temperaturen og for den specifiskc Vægt langs
Meridianerne, der lade Aarsagsforbindelsen fremtræde klarest.
Man sammenstille Snittene XXVIII Pl. XIV og PI.
XXXVIII, der gaa langs Greenwich Meridian, med Kartet
PI. XLVI og Pl. XLVII. Ved Havbækkenets sydlige
Rand er der i disse Dybder, 1000 og 1500 Favne, et
Maximum af Tryk. Samtidig se vi Isothermerne sænke sig
og Saltholdigheden aftage fra et absolut Maximum. Der
er følgelig her en nedstigende Bevægelse af varmere og
saltere Vand. Denne Bevægelse vedvarer, efter
Isother-merues Vidnesbyrd, til 65°.5 N. Br.
I 68° Bredde eller lidt derover have vi et Minimum
af Tryk. Mod dette Parti løfte Dybets Isothermer sig og
de have her sine Toppe, medens Saltholdigheden viser et
absolut Minimum. Her have vi altsaa en opstigende Bevægelse
af koldere og mindre saltholdigt Vand.
Dell norske Nordhavsexpedition. II. Mohn: Nordhavets Dybder. Te:
The depth of the sea is very trifling compared to
its horizontal extent. Hence the motion must in greater
part take a horizontal direction, viz., along the lines of
equal depth (isobaths), as with the current at the surface
along the coasts. Where the isobars run parallel to the
isobaths, and out at sea. the formula} can be applied,
provided the isobars have no sharp curves or sudden
variations in their distance from each other. At the
depths 300 and 500 fathoms these conditions are partially
complied with, in particular throughout the Polar current.
In greater depths, the isobars lie as a rule straight across
the isobaths. This last case is analogous with the motion
of the water in a river, where the gradient points in the
same direction as the river flows, and the velocity of the
motion is determined by the steepness of the gradient or
the inclination, and by the amount of friction.
In the greater depths, the motion must to a very
great extent proceed in directions forming but small angles
with the gradients. In the lesser depths, the motion can, as
shown by the isobar-systems down to a depth of 500 fathoms,
proceed on the whole cyclonically, round a minimum of
pressure, that occupies a comparatively central position in
the sea. Throughout the deeper strata, on the other hand,
the more radial motions, from the localities where the
pressure is greatest to those where the pressure is least, will be
found to prevail. But in the maximum-regions the motion
must be downward, in the minimum-regions upward; and
these vertical motions are to a great extent furthered by
the radial direction of the horizontal motion. Again, as
a direct consequence of the water’s continuity, the vertical
motions in the deeper strata will act upon the motion in the
higher strata. The downward motions in the deep must
draw-after them the upper strata; the upward will force them aside.
We have direct proof of these effects of the
distribution of pressure throughout the deeper strata in the
distribution of temperature and salinity. The currents in
the North Ocean taking chiefly a meridional direction, it
is the vertical sections for temperature and specific gravity
along the meridians that give the clearest insight into the
causal connection.
Let us compare the sections XXVIII, Pl. XIV and
Pl. XXXVIII, extending along the meridian of
Greenwich. with the maps Pl. XLVI and Pl. XLVII. At the
southern margin of the sea-basin, we have in these depths.
1000 and 1500 fathoms, a maximum of pressure. At the
same time, the isotherms are found to descend, and the
amount of salt diminishes from an absolute maximum.
Hence, hereabouts there must be a downward motion of
warmer and salter water. This motion continues, judging
from the isotherms, up to lat. 65°.5 N.
On the 68th parallel of latitude, or a little higher,
occuis a minimum of pressure. Towards this part of the
sea, the isotherms of the deep are found to rise and to have
here their summits, whereas the amount of salt exhibits
an absolute minimum. Here, accordingly, we have an
upward motion of colder water containing a less proportion
of salt.
ratur øg Stramninger. ’ili
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