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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:
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