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Upper Ocean Circulation

C. Chen

General Physical OceanographyMAR 555

School for Marine Sciences and TechnologyUmass-Dartmouth

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MAR555 Lecture 4: The Upper Oceanic Circulation

The Oceanic Circulation Types

The upper circulation The deep circulation

The upper circulation→The thermocline circulation (temperature) or the wind-driven circulationIt principally exists in the upper few hundreds of meters and mainly drivenby surface wind stress.

The deep circulation→The thermohaline circulation (salinity)prevails in the deep ocean and mainly driven by the buoyancy forcingassociated with cooling (heated) by the cold (warm) air, and modified bysources and sinks of the fresh water.

Observed currents: a result of the combined effects of wind- and buoyancy-driven forcings.

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60o

Polar High

Subpolar Low

60o

30o

0o

30o

0o

north-east trades

westerlies

sinking air

rising air

rising air

sinking air

sinking air

rising air

sinking air

heavy precipitation

large evaporation

heavy precipitation

heavy precipitation

large evaporation

Polar High

30o

30o

easterlies

Subtropical High south-east trades

Equatorial Low

Subtropical High

westerliesSubpolar Low

easterlies 60o

90o N

90o S

60o

The Global Wind Field:

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60o

60o

0o30ow60o90o120o150o180o150o120o90o60oE

30o

30o

15o

15o

45o

45o

0o

Wind Field on the northern hemisphere July

ITCZ

ITCZ: Inter-tropical convergence zone

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Wind field on the northern hemisphere January

ITCZ

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N. Equatorial CurrentEquatorial Counter CurrentS. Equatorial Current

Wintertime upper layer circulation

Kuroshio Gulf Stream

Subtropic gyre

Subpolar gyre

The Antarctic circumpolar current

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1. The Atlantic Circulation System

In the northern hemisphere:

• Subtropic Gyre• The Gulf Stream and Rings• Subpolar Gyre

In the southern hemisphere:

• Subtropic Gyre• The Atlantic circumpolar circulation

Equatorial current patterns are very similar in the Atlantic and Pacific Oceans,which will be described later.

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Sargasso Sea

Subtropic Gyre

Cape Hatteras

Guyana Current

North Equatorial Current

North AtlanticDrift

Subpolar Gyre

90o 30o 0o

20o

40o

40o

20o

The Gulf Stream: A strong narrow current flowing northward along the continental slope through theFlorida Strait and leaves to the interior of the North Atlantic Ocean at Cape Hatteras. Bowing to popularusage, oceanographers also include its extension part between Cape Hatteras and Grand Banks ascontinuous flow of the Gulf Stream.

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West East

�

!x

Wind stress

Total wind-induced southward volume transport in the interior of the ocean isbalanced by the a narrow northward flow on the western boundary

Wind-induced subtropic gyre flowGulf Stream

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Ekm antransport

Surface w indstress

Ekm anpum ping

Subtropic gyreW esternBoundaryC urrent

Limit the upward nutrient flux from the deep ocean to the euphotic layer

Good light condition, mixed layer depth: 100-200 m in summer, 400 m in winter

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excretion

Deep NO3 pool Vertical profile of NO3

Sea Surface

Mixed layer P grazing

NH4

Nutroclines Turbulent diffusion

High nutrients

uptake

NO3

Z

N

Main Sources of nutrients’ supply in the mixed layer:

1) Nutrient recycling among the food web— a nutrients-phytoplankton-zooplanktonsystem;

2) Upward flux through the thermoclines—turbulent diffusion and meso-or smallscale eddy interaction.

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Global Distribution of Chl-a Concentration

Subpolar gyre: 15-150 mg/m2

In the divergence zone near the equator: 15-30 mg/m2

Subtropic gyre: 5-25 mg/m2

Biological Desert

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The Gulf Stream:

• The warm and saline water;• Steeply sloping isotherms (temperature) and isohalines (salinity);• Cross-shelf~ 100 km;• A vertical scale of ~ 1000 m;• The surface current ~ 100 cm/s

400

800

20

1200

1600

Dep

th (m

) 12

16

84

400

800

1200

1600

Dep

th (m

)

36.5

35.5

100 km

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Cape Hatteras

The position of thermal front of the Gulf Stream taken from the satellite imagesand surface tracking drifters

30 Sv

80 Sv

150 Sv

1 Sv (Sverdrup) = 106 m3/s

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QS: Why does the Gulf Stream transport increase when it movenorthward along the slope and after it enters the interior North AtlanticOcean?

Recirculation and rings !

Sargasso SeaSubtropic Gyre

CapeHatteras

GuyanaCurrent

North Equatorial Current

North AtlanticDrift

Subpolar Gyre

90o 30o 0o

20o

40o

40o

20o

18 degree water

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Surface0

10

20

30

700 m

10

5

0-5

-5

-5

02000 m

45oN 40o 35o 30o

5

0

-5

-10

4000 m

Zona

l Vel

ocity

(cm

/s)

Latitude (degree)

A direct current measurement on anorth-south section of 55o W(Hogg et al. 1986)

Blue: The recirculation flow

Red: The eastward flow

Solid black line: Measurement uncertainty

Recirculation increases with depth.

At ~4000 m,

it can reach ~10 cm/s

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A chart of the topography of the 150 isothermal surface showing the GulfStream, cold-core rings and warm-core rings

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50 100 150 (km)

0

50 km

100

50100150200

Depth (m

)

upwelling

26o

24o

22o

20o

18o

16o

Upwelling advects nutrients upward to the euphotic layer: producing a near-surfacephytoplankton bloom. The euphotic layer depth: > 20 m in the outer shelf.

Instability of the Gulf Stream front: eddy formation at the shelf break.

Inner

shelf

Mid-she

lf

Outer-s

helf

L ~ 50-60 km

Eddy scale: