Лекция: Ocean circulation

(1) General

Comparisons between the structure and dynamics of the oceans and the atmosphere have long been made, particularly in respect of their behaviour above the permanent thermocline and below the tropopause – their two most significant stabilizing boundaries. Within these two zones, fluid-like circu­lations are maintained by meridional thermal energy gradients, dominantly directed polewards, and acted upon by the Coriolis force. Prior to the last quarter of a century, oceanography was studied in a coarsely averaged spatial-temporal framework similar to that applied in classical clima­tology. At the present day, however, its similarities with modern meteorology are more marked. The major behavioural differences between the oceans and the atmosphere derive from the greater density and viscosity of ocean waters and the much greater frictional constraints placed on their global move­ment. Macroscale characteristics of ocean dynamics that invite comparison with atmospheric features include the general circulation, major oceanic gyres (similar to atmospheric subtropical high-pressure cells), major jet-like streams such as sections of the Gulf Stream, large-scale areas of subsidence and uplift, the stabilizing layer of the permanent thermocline, boundary layer effects, frontal discontinuities created by temperature and density contrasts, and water mass ('mode water') regions. Mesoscale characteristics inviting atmospheric analogues are oceanic cyclonic and anticyclonic eddies, current meanders, cast-off ring vortices, jet filaments, and circulations produced by irregulari­ties in the North Equatorial Current.


(2) Macroscale

The most obvious feature of the surface oceanic circulation is the control exercised over it by the low-level planetary wind circulation, especially by the subtropical oceanic high-pressure circulations and the westerlies. The oceanic circulation also displays seasonal reversals of flow in the monsoonal regions of the northern Indian Ocean, off East Africa and off northern Australia. As water moves meridionally, the conservation of angular momentum implies changes in relative vorticity, with poleward-moving currents acquiring anticyclonic vorticity and equatorward-moving currents acquiring cyclonic vorticity.

The more or less symmetrical atmospheric subtropical high-pressure cells produce oceanic gyres with centres displaced towards the west sides of the oceans. The gyres in the southern hemisphere are more symmetrically located than those in the northern, due possibly to their connection with the powerful West Wind Drift. This results, for example, in the Brazil Current being not much stronger than the Benguela Current. The most powerful southern hemisphere current, the Agulhas, possesses nothing like the jet-like character of its northern counterparts.

Equatorward of the subtropical high-pressure cells, the persistent trade winds generate the broad North and South Equatorial Currents. On the western sides of the oceans, most of this water swings polewards with the airflow and thereafter increasingly comes under the influence of the Ekman deflection and of the anticyclonic vorticity effect. However, some water tends to pile up near the equator on the western sides of oceans, partly because here the Ekman effect is virtually absent, with little poleward deflection and no reverse current at depth. To this is added some of the water that is displaced northwards into the equatorial zone by the especially active subtropical high-pressure circulations of the southern hemi­sphere. This accumulated water flows back eastward down the hydraulic gradient as compensating narrow surface Equatorial Counter-Currents, unim­peded by the weak surface winds. Near the equator in the Pacific Ocean, upwelling raises the thermocline to only 50-100 m depth, and within this layer there exist thin, jet-like Equatorial Undercurrents flowing eastwards (under hydraulic gradients) at the considerable velocity of 1 to 1.5 m s-1.

As the circulations swing polewards around the western margins of the oceanic subtropical high-pressure cells, there is the tendency for water to pile up against the continents, giving, for example, an appreciably higher sea level in the Gulf of Mexico than that along the Atlantic coast of the United States. This accumulated water cannot escape by sinking because of its relatively high temperature and resulting vertical stability, and it consequently continues polewards in the dominant direction of surface airflow, augmented by the geostrophic force acting at right angles to the ocean surface slope. As a result of this movement, the current gains anticyclonic vorticity, which reinforces the similar tendency imparted by the winds, leading to rela­tively narrow currents of high velocity (for example, the Kuroshio, Brazil, Mozambique-Agulhas and, to a less-marked extent, the East Australian Current). In the North Atlantic, the configuration of the Caribbean Sea and Gulf of Mexico especially favours this pile-up of water, which is released pole­wards through the Florida Straits as the particularly narrow and fast Gulf Stream. These poleward currents are opposed both by their friction with the nearby continental margins and by energy losses due to turbulent diffusion, such as those accompanying the formation and cutting off of meanders in the Gulf Stream. These poleward western boundary currents (e.g. the Gulf Stream and the Kuroshio Current) are fast, deep and narrow (i.e. approxi­mately 100 km wide and reaching surface velocities greater than 2 m s-1), contrasting with the slower, wider and more diffuse eastern boundary currents such as the Canary and California (i.e. approxi­mately 1,000 km wide with surface velocities gener­ally less than 0.25 m s-1). The northward-flowing Gulf Stream causes a heat flux of 1.2 * 1015 W, 75 per cent of which is lost to the atmosphere and 25 per cent in heating the Greenland-Norwegian Seas area.

On the poleward sides of the subtropical high-pressure cells, westerly currents dominate, and where they are unimpeded by land masses in the southern hemisphere they form the broad and swift West Wind Drift. This strong current, driven by unimpeded winds, occurs within the zone 50 to 65°S and is associated with a southward-sloping ocean surface generating a geostrophic force, which intensifies the flow. Within the West Wind Drift, the action of the Coriolis force produces a convergence zone at about 50°S marked by westerly submarine jet streams reaching velocities of 0.5 to 1 m s-1. In the northern hemisphere, a great deal of the east­ward-moving current in the Atlantic swings north­wards, leading to anomalously very high sea temperatures, and is compensated for by a south­ward flow of cold arctic water at depth. However, more than half of the water mass comprising the North Atlantic Current, and almost all that of the North Pacific Current, swings south around the east sides of the subtropical high-pressure cells, forming the Canary and California Currents. Their southern-hemisphere equivalents are the Benguela, Humboldt or Peru, and West Australian Currents.

(3) Mesoscale

Mesoscale eddies and rings in the upper ocean are generated by a number of mechanisms, sometimes by atmospheric convergence or divergence or by the casting off of vortices by jet-like currents such as the Gulf Stream. These vortices are generated by the transfer of warm water from low to high latitudes. Oceanographic eddies occur on the scale of 50–400 km diameter and are analogous to atmospheric low- and high-pressure systems. Ocean mesoscale systems are much smaller than atmospheric depressions (which average about 1,000 km diameter), travel much slower (a few kilometres per day, compared with about 1,000 km per day for a depression) and persist from one to several months (compared with a depression life of about a week). Their maximum rotational veloci­ties occur at a depth of about 150 m, but the vortex circulation may be felt to depths of several thou­sands of metres. Some eddies move parallel to the main flow direction, but many move irregularly equatorwards or polewards. In the North Atlantic, for example, this produces a 'synoptic-like' situa­tion in which up to 50 per cent of the area may be occupied by mesoscale eddies. Cyclonic rings are commonly three times as numerous as anticyclonic eddies, having
a maximum rotational velocity of about 1.5 m s-1.



Упражнение 2.

Ответьте на следующие вопросы:

1. В чем причина основных различий в поведении между океаном и атмосферой?

2. Какие общие макро- и мезомасштабные детали характерны для циркуляции атмосферы и океана?

3. В чем заключается эффект Экмана?

4. Почему уровень моря в Мексиканском заливе выше, чем у Атлантического побережья США?

5. Какой наклон в южном полушарии имеет поверхность океана?

Упражнение 3.

Найдите в тексте термины, соответствующие следующим выражениям.

especially apparent because of yet
like show own close
originate shift wide  


Упражнение 4.

Словам в левой колонке подберите антонимы в правой колонке.

1. long 2. below 3. similarity 4. most 5. stronger 6. sink 7. shallow 8. move a. difference b. weaker c. raise e stay f. short g. deep h. above i. least



Упражнение 5.

Переведите следующие слова на русский язык.

however for example like towards between
such as consequently within about over
dominantly some almost commonly also



Упражнение 6.

Из слов в правой и левой колонке образуйте цепочки существительных:

ocean layer

energy mass

boundary dynamics

density meander

water contrast

ring gradient

trade vortex

current wind

Упражнение 7.

Образуйте причастия 1 и 2 рода из следующих глаголов. Найдите примеры таких причастий в тексте из упражнения 1.

Derive, invite, display, imply, produce, posses, generate, tend, raise.


Упражнение 8.

Прочитайте текст. (Контрольное время – 10 минут)


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