Download Characteristics of coastal and estuarine circulation

Characteristics of coastal and
estuarine circulation
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• Characteristics of shelf and coastal ocean
• Characteristics of estuaries
• Classification of estuaries
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™Definitions
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Shelf sea
Coastal ocean
Shelf break, shelf slope
Estuary
Coastal zone
Marginal sea
Mediterranean Sea
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•Shelf Sea:
A sea area between a continental shelf break with a water depth of
about 200 m and the land shore.
About 70% of global fish resources spend part of their life cycle in
the coastal seas.
Accounts for 90% of marine biomass yield
Great influence of freshwater and nutrients from the land
Chemical reactions occur there rapidly between substances from the
land as they encounter substances from the ocean
Boundary layer of kinetic energy exchange with open ocean
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•Coastal Ocean
Coastal ocean as those shelf sea regions that display the
dynamics of the deep ocean, modified by shallow water depth
and the presence of the coast.
The driving force for water movement is the wind stress and that
thermohaline forces (the influence of temperature and salinity
changes on the stratification) are of secondary importance.
•Shelf Break:
The location where the continental shelf joins the flanks of the
continents near the 200 m depth
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Contour of bathymetry over the continental shelf of North South China Sea
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•Estuary:
A semi-enclosed coastal body of water which has a free
connection with the open sea and within which seas water is
measurably diluted with fresh water derived from land drainage
The place where land and sea meet.
Water movement is controlled primarily by thermohaline forcing
Freshwater from land sends salt water upstream far beyond the
river mouth
Vigorous mixing
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Pearl River Estuary
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•Coastal Zone:
Shallowest part of the shelf sea (immediate vicinity of the
coastline)
Strong sediment movement
Strong effect from bottom friction, boundary layer
dynamics
Wave breaking
•Marginal (Adjacent) Sea:
A part of the ocean that is separated from the major ocean
basins by topographic features
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Surface Cha. has large concentration at the coastal
ocean over continental shelf
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Dynamics: shallow water (shelf, coastal ocean) and deep sea
e.g. South China Sea
•Mediterranean Sea
Ocean that has only limited communication with the major
ocean basins
Circulation is mainly driven by thermohaline forcing
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™: Characteristics of Coastal
Ocean:
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• Shallow water depth:
(1)Friction effect: besides wind-induced surface boundary
(Ekman) layer, bottom friction forms bottom friction
(Ekman) layer
-The nutrients released by detritus accumulated at the
bottom may be carried to the surface waters and used in
phytosynthesis.
(2) Amplification of tide and tidal current and intensified
tidal mixing
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Contours of bottom
depth (m) in the coastal
Waters along Oregon
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Sea Surface Temperature
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Surface and bottom boundary layers (Vertical diffusive values)
Surface friction (Ekman) layer
Bottom friction (Ekman) layer
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•Presence of coastline:
Resulting convergence and divergence when flow
arrives or departs the coast, generating upwelling or
downwelling
Sea level increase/decrease, leads to variation of
pressure gradient, surge (or storm surge) .
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Mass accumulation and pressure-gradient set up
at the coast
Surface elevation
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Surface elevation
Depth-integrated velocity
Loss of mass nearshore, set-up of pressure gradient and current as a
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result of the presence of coast
as wind blowing southward.
Gain of mass nearshore, set-up of pressure gradient and current as a
result of the presence of coast
as wind blowing northward.
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The topography (in meters)
in the northern
South China Sea
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The selected crossshelf sections (dashed
lines) are marked by
their grid numbers.
372
338
305
zoomed area in the region
between Guangdong and Xiamen
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The location where
shelf isobaths protrude
shoreward is marked
by SCI (shoreward
convex isobaths).
(a)
(b)
(c) depth-integrated velocity magnitude (m s-1) and (d) surface
elevation (m).
Daily mean (a) surface, (b) bottom velocity vectors (m s-1) ;
(c)The
(d)as red and blue contour lines,
30 m and 50 m isobaths are shown
respectively.
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Bottom
Surface
Day 10
Day 10
Day 30
Day 30
Daily average surface (left column) and bottom (right column) σθ
(kg m-3) on days 10 and 30, showing the strong shoreward
advection near widened shelf and the subsequently eastward advection
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of dense bottom waters between
Shanwei and Shantou during upwelling.
v
Cross-shelf
transport of
materials
density
NO3
phytoplankton
zooplankton
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•Terrestrial influences
River discharge, buoyancy-driven
force
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A sketch of the circulation on the shelf produced by
freshwater input into the coastal zone. The diagram
is valid for the northern hemisphere. In the southern
hemisphere the current direction is reversed.
(An x in a circle indicates a current going into the
page; a dot in a circle indicates a current coming out
of the page.)
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Buoyancy-induced coastal jet
River Discharge
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Physical Environment in the Pearl River Estuary and Adjacent
Continental Shelf
4. Seasonal variation of river plume
Time average distribution of surface chlorophyll concentration
(mg m-3) from MODIS data during June 29 to July 7, 2007. (From
Liang L.)
Ocean true color. (From Gan, J.)
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bulge
Eastward current increased
by pressure gradient set-up by the
buoyancy of river plumeENVR608
Seaward current increased
by the stratification in the plume which
enhance the efficiency of wind stress
•Open ocean influences (across-shelf transports)
Large scale circulation (e.g. Kuroshio intrusion into South
China Sea)
Eddies and other meso-scale disturbances
Internal waves
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Positive values refer to
shoreward transport
from SCS basin
Transport across
1000 m isobath,
indicating
exchanges
between open
ocean and
coastal waters
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Onshore propagation
Shoreward propagation of solitary waves, showing the
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influence of open ocean disturbances
on coastal waters
Internal
solitary
waves
™(Empirical) classification of Estuaries
• Salt wedge estuary
•Highly stratified estuary
•Slightly stratified estuary
•Vertically mixed estuary
•Inverse estuary
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™Estuary and Estuarine Circulation
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Salinity in a salt wedge estuary
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Salt wedge estuary
• R/V is large (R: fresh water volume; V:
tidal volume)
• front between fresh water and oceanic
water at the surface near the estuary
mouth
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Salt wedge in the Mississippi River
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highly stratified estuary
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Highly stratified estuary
• R/V to 0.1 – 1, stronger tidal current at the
bottom
• Create instability and internal wave
• a net upward transport of mass and salt
(entrainment)
• Intensified circulation induced by
entrainment
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Intensification of circulation in a highly stratified estuary
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Highly stratified estuary, Hardanger
Fjord, Norway
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slightly stratified estuary
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slightly stratified estuary
• R/V 0.005 - 0.1, much strong tidal current
• Strong turbulent everywhere, mass and
salt vigorously mix between two layers
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Slightly stratified estuary
Chesapeake Bay, USA
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vertically mixed estuary
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Verically mixed estuary
• R/V <0.005
• Efficient turbulent mixing
• No distinction between upper and lower
layers
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inverse estuary
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Inverse estuaries Myall River
channel in New South Wales,
Australia
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Inverse estuary
• No river input of fresh water
• The evaporation and salinity water
accumulation at the head of estuary lead
to vertically anti-clockwise circulation
• salinity in the upper layer decreases
towards the sea; the salinity in the lower
layer increases towards the estuary
head
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