84 INTRODUCTORY DYNAMICAL OCEANOGRA

84 INTRODUCTORY DYNAMICAL OCEANOGRAPHY
absolute value of either Vx or V2 we will know the absolute value of the other.
There are several possibilities:
(1) assume that there is a level or depth of no motion (reference level) e.g. that
V2 = 0 in deep water, and then calculate Vx for various levels above this
(the classical method);
(2) when there are stations available across the full width of a strait or ocean,
calculate the velocities and then apply the equation of continuity to see if
the resulting flow is reasonable, i.e. complies with all facts already known
about the flow and also satisfies conservation of heat and salt;
(3) use a "level of known motion", e.g. if surface currents are known or if the
currents have been measured at some depth(s) by current meters or
neutrally buoyant floats (preferably while the density measurements for
the geostrophic calculations were being made). In the future, it is
possible that satellite measurements of the surface slope may enable the
surface currents to be calculated, at least in regions of strong currents.
Note that the above techniques are the "classical" ones for obtaining
absolute velocities; a more recent technique, the "beta-spiral" approach, is
described in Section 8.9.
Since surface velocities are important and can be inferred quickly from the
slope of the sea surface (which is assumed to be isobaric) it is common to plot
the geopotential (or "dynamic") topography of the sea surface relative to some
deeper surface, if a sufficient grid of station data is available. The relative
current directions will be parallel to lines of constant geopotential and relative
speeds will be inversely proportional to the spacing of the lines (i.e. close
spacing = steep slope = large speed, e.g. Figs. 8.8 or 11.4(b)). It is also possible
to plot the geopotential topography of subsurface isobaric surfaces to deduce
the motion there.
Remember that these geopotential topography plots are usually based on
some assumed level of no motion, generally in the deep water, unless adequate
direct current measurements are available which is rarely the case. If the
average upper-layer currents are much larger than the average deep currents,
which is often the case, we may get quite good values for them even if the deep
currents are not exactly zero. Note, however, that although a small current
which is neglected (or not known) in the deep water may not affect the
calculated velocities in the upper layer very much, it may make a substantial
contribution to the total volume transport when integrated over the full depth.
For instance, an average current of 10cms"1 in the upper 1000 m based on
zero current assumed at (and below) 1000 m would give a volume transport
from surface to 4000 m of 100 m3 s "* for each metre width of the current. If the
actual current from 1000 to 4000 m were 2 cms~* in the same direction as that
in the upper layer, this would give rise to a 20% error for the upper layer



 
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velocity but the total volume transport from surface to 4000 mm would be
180 m 3 s - 1 or 80% more than that assuming zero current below 1000 m.
Notice that there is one known velocity region which cannot be used as a
level of no motion—the sea bottom. The reason why this level cannot be used is
that the velocity tends to zero there because of the action of friction, a force
which was deliberately assumed to be negligible when deriving the geostrophic
equation. Remember then that the geostrophic equation does not apply in
regions where friction is important.
8.6 Relations between isobaric and level surfaces
Classically the basis for the assumption of a level of no motion was the belief
that velocities are small in deep water. Observations in recent years with
86 INTRODUCTORY DYNAMICAL OCEANOGRAPHY
Swallow floats* have indicated that this belief may not always be correct, and
ripple marks on sand bottoms recorded in photographs in deep water suggest
that bottom currents of 0.5 m s~1 or more may occur. However, it is possible
that these indicate only local and/or transient currents and in many regions
there are indications from distributions of water properties that speeds
averaged over several months or more, or over tens or hundreds of kilometres,
are probably small in deep water, and the selection of a level of no motion at
about 1000 m depth may give quite good results for geostrophic calculations.
In the Pacific, the uniformity of properties in the deep water suggests that
assuming a level of no motion at 1000 m or so is reasonable, with very slow
motion below this. In the Atlantic, there is evidence of a level of no motion at
1000-2000 m (between the upper waters and the North Atlantic Deep Water)
with significant currents above and below this depth. A selection of relations
between isobaric and constant geopotential or level surfaces is shown in Fig.
8.9. Figure 8.9(a) is typical of the west Pacific and Fig. 8.9(b) of the west Atlantic
(Gulf Stream region) with the characteristics described above. Figure 8.9(c)
would indicate little motion at the surface but increasing speed into the deep
water, which is unlikely in the real ocean. Figure 8.9(d) shows a situation where
all the isobaric surfaces are parallel and equally inclined to level surfaces—the
so-called "slope current" situation. In this case, the application of the
geostrophic calculation would yield zero relative velocity at all depths which
would be correct although the absolute velocity would not be zero. This

Hasil (Bahasa Indonesia) 1:

84 OSEANOGRAFI DINAMIK PENGANTARnilai absolut Vx atau V2 kita akan tahu nilai absolut yang lain.Ada beberapa kemungkinan:(1) berasumsi bahwa ada tingkat atau kedalaman tidak ada gerakan (referensi tingkat) misalnya yangV2 = 0 di dalam air, dan kemudian menghitung Vx untuk berbagai tingkat di atas ini(metode klasik);(2) ketika ada stasiun yang tersedia di seluruh lebar penuh Selat atau laut,menghitung kecepatan dan kemudian menerapkan persamaan kontinuitas untuk melihat apakahaliran yang dihasilkan wajar, yaitu sesuai dengan semua fakta yang sudah dikenaltentang aliran dan juga memenuhi konservasi panas dan garam.(3) menggunakan "tingkat dikenal gerak", misalnya jika permukaan arus dikenal atau jikaarus telah diukur di depth(s) beberapa meter saat ini atauapung netral mengapung (sebaiknya sementara pengukuran kepadatan untukperhitungan geostrophic sedang dilakukan). Di masa depan, itu adalahmungkin bahwa satelit pengukuran permukaan lereng memungkinkanarus permukaan harus dihitung, setidaknya di daerah arus kuat.Catatan bahwa teknik di atas adalah yang "klasik" untuk mendapatkanmutlak kecepatan; teknik yang lebih baru, pendekatan "beta-spiral", adalahdijelaskan dalam bagian 8.9.Karena permukaan kecepatan penting dan dapat disimpulkan dengan cepat darilereng permukaan laut (yang dianggap isobaric) umum untuk plotgeopotential (atau "dinamis") topografi permukaan laut relatif terhadap beberapadeeper surface, if a sufficient grid of station data is available. The relativecurrent directions will be parallel to lines of constant geopotential and relativespeeds will be inversely proportional to the spacing of the lines (i.e. closespacing = steep slope = large speed, e.g. Figs. 8.8 or 11.4(b)). It is also possibleto plot the geopotential topography of subsurface isobaric surfaces to deducethe motion there.Remember that these geopotential topography plots are usually based onsome assumed level of no motion, generally in the deep water, unless adequatedirect current measurements are available which is rarely the case. If theaverage upper-layer currents are much larger than the average deep currents,which is often the case, we may get quite good values for them even if the deepcurrents are not exactly zero. Note, however, that although a small currentwhich is neglected (or not known) in the deep water may not affect thecalculated velocities in the upper layer very much, it may make a substantialcontribution to the total volume transport when integrated over the full depth.For instance, an average current of 10cms"1 in the upper 1000 m based onzero current assumed at (and below) 1000 m would give a volume transportfrom surface to 4000 m of 100 m3 s "* for each metre width of the current. If theactual current from 1000 to 4000 m were 2 cms~* in the same direction as thatin the upper layer, this would give rise to a 20% error for the upper layer GAMBAR 96BOLA DUNIA  kecepatan tapi total volume transportasi dari permukaan 4000 mm akan180 m 3 s - 1 atau 80% lebih dari itu dengan asumsi nol arus bawah 1000 m.Perhatikan bahwa ada satu wilayah dikenal kecepatan yang tidak dapat digunakan sebagaitingkat tidak ada gerakan — dasar laut. Alasan mengapa tingkat ini tidak dapat digunakanbahwa kecepatan cenderung nol ada karena tindakan gesekan, kekuatanyang sengaja diasumsikan dapat diabaikan ketika berasal geostrophicpersamaan. Ingat kemudian bahwa persamaan geostrophic tidak berlaku didaerah mana gesekan penting.8.6 hubungan antara isobaric dan tingkat permukaanKlasik dasar asumsi tingkat tidak ada gerakan adalah kepercayaankecepatan kecil di dalam air. Pengamatan dalam beberapa tahun terakhir dengan86 OSEANOGRAFI DINAMIK PENGANTARBurung layang-layang mengapung * mengindikasikan bahwa kepercayaan ini tidak selalu mungkin benar, danmenunjukkan tanda-tanda riak di pantat pasir yang tercatat dalam foto di dalam airyang bawah arus 0,5 m s ~ 1 atau lebih mungkin terjadi. Namun, mungkinini menunjukkan hanya lokal dan/atau sementara arus dan di berbagai daerahada indikasi dari distribusi properti air yang kecepatanrata-rata selama beberapa bulan atau lebih, atau atas puluhan atau ratusan kilometer,mungkin kecil di dalam air, dan pemilihan tingkat tidak ada gerakan disekitar 1000 m kedalaman dapat memberikan hasil yang cukup baik untuk geostrophic perhitungan.Di Pasifik, keseragaman properti di air yang dalam menunjukkan bahwadengan asumsi tingkat tidak ada gerakan di 1000 m atau lebih wajar, dengan sangat lambatgerak di bawah ini. Di Atlantik, ada bukti tingkat tidak ada gerakan di1000-2000 m (antara hulu dan air dalam Atlantik Utara)dengan arus penting diatas dan dibawah kedalaman ini. Sejumlah hubunganantara geopotential isobaric dan konstan atau tingkat permukaan ditunjukkan pada gambar.8.9. gambar 8.9(a) khas Barat Pasifik dan gambar 8.9(b) Atlantik Barat(Gulf Stream wilayah) dengan karakteristik yang dijelaskan di atas. Gambar 8.9(c)menunjukkan sedikit gerakan di permukaan, tetapi peningkatan kecepatan dalamair, yang tidak mungkin di Samudera yang nyata. 8.9(d) gambar menunjukkan situasi dimanaSemua permukaan isobaric paralel dan sama-sama cenderung untuk tingkat permukaan —situasi apa yang disebut "lereng saat ini". Dalam kasus ini, aplikasiperhitungan geostrophic akan menghasilkan kecepatan relatif nol pada semua kedalaman yangakan benar walaupun kecepatan mutlak tidak akan nol. Ini