Lecture 9+10: Buoyancy-driven flow, estuarine circulation, river plume, Tidal

mixing, internal waves, coastal fronts and biological significance

•

Thermohaline

circulation: the movement of water that takes place when its density is changed by a change of temperature or of salinity.

•

T: solar energy, weather or climate changes; S: Precipitation-EvaporationCoastal Ocean: River charge

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.)

Buoyancy-induced coastal jet

•River Plumes: The fresh water from river in the surface over shelf region•Estuarine plumes: The (fresh water+salty water) from estuary in the surface over the shelf.

a b

c d

(a) surface velocity vector (m/s), (b) salinity (psu) , (c) differences of surface elevation (m) and (d)surface velocity magnitude (m/s) between the cases with and without Pearl River discharge on day 30

Salinity (psu) and u (m s-1) as function of depth along the axis of plume (22.1N) on day 30.

s: 256

u: 256

v: 256

s: 338

u: 338

v: 305

Δ s: 256

Δu: 256

Δ v: 256

Δ s: 305

Δu: 305

Δ v: 305

Shantou

with river

without river

without river

with river

with river

+ADV.+mixingriver

without river with river

without river with river

Monsoonal wind

Day 10, NO3

Phyto.

Day 30, NO3

Phyto.

Zoo. Zoo.

NO3Phytoplankton

zooplankton

zooplankton > 1;phytoplankton > 2;NO3 > 4;

Day 30

22.1N

•River Plumes: The fresh water from river in the surface over shelf region•Estuarine plumes: The (fresh water+salty water) from estuary in the surface over the shelf.

Estuarine circulation

u

z

Offshore pressure gradient force

112

2

1

/)(have we

)(

ρρρ

ρρ

−=Δ==

Δ+=

hhPP

ghPhhgP

BA

B

A

To form an offshore pressure force, PA >PB at the depth z

12

1

ρρρ−Δ

<hz

• Dynamic balance in small estuary: The driving force of the circulation is the difference in the pressure gradient, which is balanced by the viscosity force if Rossby number is big so that Coriolis force can be ignored.

2

21dz

udKdxdP

e=ρ

•Mixing induced by vertical velocity shear and buoyancy

Ri=buoyancy restoring force/vertical mixing induced by vertical velocity shear

2)/(dzdu

dzdgRi ρ

ρ= Ri<0.25, unstable.

•Flush Time in an estuary: the time tF of an estuary can be defined as the time needed to replace its freshwater volume VF at the rate of the net flow through the estuary, which is given by the river discharge rate R:tF = VF / R

•Tidal fronts: the boundary between stratified and tidally mixed waters. (note: fronts are regions of strong gradients of T or other variables)

-Largely induced by the shallowness of waters

Formula:(1) )/(log 10 tDhE =

h: the height of water column, Dt : depth-integrated rate of dissipation energy of tides. E<1.9 well-mixed; >1.9 stratified

(2) ε10log=E E>-1.0, tidally mixed; E<-2.0, stratified;

dydhgfu

ghpdydpfu

−=

=

−=

ρρ1

2. Process-oriented model study in the PRE•

Model results and Discussion

(1) Surface salinity contour and sea surface elevation gradient

River-forced Without earth rotation River + Tide

River + Downwelling River + Upwelling Upwelling

2. Process-oriented model study in the PRE

•

Model results and Discussion(2) Distributions of the barotropic

current and the ratio of relative vorticity

to planetary vorticity

(color contours)

2. Process-oriented model study in the PRE

•

Model results and Discussion(3)

Contours of the salinity (psu, black contours) and AKv

( , color contours) on the vertical section along the axial of the PRE

sm /2

River-forced Without earth rotation River + Tide

River + Downwelling River + Upwelling River + Tide (at earlier time)

2. Process-oriented model study in the PRE

•

Model results and Discussion(4) Current field along the axial vertical section

River-forced Without earth rotation River + Tide

River + Downwelling River + Upwelling

•The effect of freshwater run-off on biological production in estuaries

Stratified: (+ effect) lead to the oxygen-depleted while nitrogen (e.g. from benthic organism) and phosphorus (from sediment particles )content rose;(-effect) limit the mixing between the surface and bottom

Mixing: (+ effect) replenishing the O2 at depth and upwelling nitrogen and phosphorus; (-effect) turbidity cause light to be limiting.

Alternating between these two process provides the conditions for very high primary production (e.g. York River Estuary). This ‘alternating’ can occur, for example, between spring and neap tides or other physic processes affected it.

(Continued of lecture 5)

Neap Tide bloom

•The biological effects of tidal mixing

Tidal front: the place where intensity of turbulent mixing was just enough to continuously overcome the barrier to mixing presented by the stratification.

E=lg(h/Dt)=1.9 is the place where front is located. (H is water depth, Dt is depth-averaged rate of dissipation of energy from tides.

Control by factors of (a) stratification; (b) mixing; (c) light

For Phytoplankton: Potentially, tidal mixing may have adverse effects on phytoplankton productivity more than compensated for by the increased nutrient flux to the water column from the sediments.

For Zooplankton:

Tidally mixing may delay warming of the water column due to the lack of stratification and prevent upward migration of a large biomass of adult and late stage copepods.

Tidally mixing waters tend to have a relatively slow growth of the zooplankton population

Large flux of nutrient from sediments and rivers in the tidally mixed water column

Poor penetration of light on account of the sediment load

• Biological effects of river and estuarine plumes

(a) Materials carried by the river on biological production in the plume;

(b) Entrainment and consequent upwelling of nutrient- rich water;

(c) Enhancement of the stability of water column (+: enhance productivity;-: inhibit vertical mixing and hence reduce primary productivity.

Mississippi plume in Gulf Mexico (effect a):

River-borne nutrients inputs enhance primary production and sinking of organic matter

The increased phytoplankton production and sinking of phytoplankton biomass increase bacterial activity and formation of zones of low oxygen (hypoxia) or zero oxygen (anoxia).

The plume in Amazon river (effect b)

The entrainment of salt water into upper fresh water layer lead to the compensatory shoreward flow of high nutrient bottom water as river plume moves offshore.

Algal blooms on the Amazon shelf receive 83% nitrogen, 69% of phosphorous and 59% of silicon.

Fresh water run-off in the coast of Iceland (effect c)

Fresh water input forms a great resistance for thermal stratification to be breakdown by the tidal or wind-induced mixing, which lead to earlier spring bloom.

Internal Waves

The convergence and divergence has significant biological impacts

The Biological Significance of Internal Waves

•

Internal waves as a nutrient pumps

Convergence: Cause floating organic matter to accumulate. Trajectories of water particles and breaking wave enhance mixing in the water.

The inorganic nitrate and chlorophyll are significant higher in the waters with internal waves.

Internal waves and phytoplankton production

•

Internal waves traveling the pycnocline

are likely both to increase turbulent transport of nutrients and to cause the phytoplankton to oscillate in depth, thereby increasing the light intensity experienced by light-limited phytoplankton cells at the lower layer.

Internal waves concentrate and transport planktonic

organisms

Concentration of organisms (no net advection of cells and no net shoreward movement of water)

•Increase of organism concentration and aggregations.

•Planktonic organisms become associated with the aggregations.

Aggregation and transport of organisms:

Shoreward accumulation of crab larvae and fish larvae during the

‘downwelling’

phase’

The breaking internal waves (or bores) will enhance vertical mixing.

Transport (upwelling and downwelling

) related to arrival of tide-induced internal wave (bore)

Fronts in the coastal waters: Physics and Biology

Fronts

•

Definition: Fronts: regions with enhanced horizontal gradients of hydrographic properties; regions where properties change markedly over a relatively short distance.

•

Classification: Tidal fronts (sea-shelf fronts), shelf-break fronts, upwelling fronts, plume fronts, estuarine fronts, fronts induced by geomorphic features.

The Physics of Fronts

(1) (2) convergence zone

Deep nutrient-rich water being advected

to the euphotic

layer

divergence zone

1. Form a jet at the surface with its velocity decreases with depth and reverses direction near the bottom.

2. Current on shoreward of front flows southward with fresher water on its right (in northern hemisphere).

3. Secondary circulations are created on both side of the front.

4. On the shore side, 2nd circulation (1)

is formed by the northward bottom friction in the southward jet , which creates a eastward Ekman

transport at the bottom

and compensating flow at the surface.

5. On the offshore side, 2nd circulation (2)

is formed because the jet direction is reversed (flows northward) and there is a westward Ekman

transport at the bottom

and compensating flow at the surface.

(a) Shelf break fronts•

The result of differences in hydrographic properties between the coastal ocean and the open sea

•

Geostrophic

flow formed by the pressure gradient from two different waters set up a boundary between shelf water and open ocean water and explains the name shelf break front.

•

Shelf break fronts are more or less stationary; their mean position is entirely controlled by the location of the shelf break.

Vert. well mixed well mixed well mixed

April, Rhode Island

Seasonal variation in northwest Europe

Cold cushion

Vertically well mixed

Scale of the front can be estimated by the internal or baroclinic

Rossby

radius

of deformation Rbc

. The baroclinic Rossby

radius is the length scale at

which disturbances grow in the oceanic circulation in the presence of stratification. For an ocean consisting of two layers it is given by

Rbc=1/f (g’D1

)0.5

where g = 9.8 m/s2

is gravity, f the

Coriolis

parameter, D1

is the upper layer thickness.

(b) Tidal or shelf-sea fronts•

The boundary set up by the heating from atmos. and mixing by the tidal flow is marked by shelf-

sea front.

Advance shoreward if tidal strength is weakened

Unstratified

due to tidal mixing as tides is approaching shallower shelf

•As one approaches the coastal sea from the deep ocean there comes thus a point where the stratification found in the deep sea can no longer be maintained against the increasingly vigorous tidal mixing.

•The front is associated with a density gradient and thus supports a geostrophic

jet along it,

which causes eddies to form and break off. Like all other fronts it is also linked with a convergence of the surface current.

(c) Fronts in estuaries•

Plume fronts form where relatively fresh water reaches the mouth region of an estuary and discharges into the oceanic environment. Front around the plume is strongly convergent and turbulent;

•

Fronts at the interface between tidally mixed and stratified waters (resembles sea-shelf fronts)

•

Estuarine fronts a miniature version of the shallow sea front in the sense that tidal mixing competes against buoyancy generated stability of the water column fronts

Dong, et al., 2004

(d) EddiesEddies: cause water to be exchanged across the front and contribute a significant flux of nutrients (FE ).

The eddies can be generated by the instability at the fronts.

CDgDFE ΔΔ

= 21

)(ρργ

γ=0.005,D is upper layer thickness,C is the nutrient.

Biology of Fronts1. The biology of shelf-break fronts

(a) Plankton biomass and production

•

Concentration of inorganic nitrate, chlorophyll-a and copepods are found to be much higher at the front than surrounding shelf and slope waters due to the convergence of the waters and upward motion at the front.

•

Shelf break fronts show that increased copepod abundance in the frontal region due to the daily augmentation of the nutrients which provides continuously increase in phytoplankton for the copepod population, which is in contrast to the situation at a tidal-mixing front, where the cycle of enhanced production follows the fortnightly cycle of spring and neap tides.

(b) Fish and birds•The distribution of fish larvae tend to be centered around the shelf-break front.

•The greatest carbon flux to the pelagic food web is found in at the shelf-break front (e.g. in the southeastern Bering Sea) and large concentrations of fulmars aggregate near the front.

•Internal waves often add energy to the mixed layer at the shelf-break fronts, causing a deepening with incorporation of nutrient-rich water from below the nutricline.

2. The biology of tidal fronts

(Cha=1.5 mgm-3) (Cha=0.5 mgm-3)

Subsurface maximum partly due to the passage of internal waves.

The persistent chlorophyll peak is caused by the offshore movement of front during spring tide which leads to the nutrients in the stratified water being brought to the surface. Thus, it forms bloom during

the subsequent neap tides.

Seasonal variation of the Chlorophyll0-a across a front

Remarks:The dense phytoplankton patches on the sea-shelf fronts is

induced by:

(1) Convergence flows which converges the surface high biomass of phytoplankton towards fronts,

(2) Transport of nutrients into the mixed layer of the stratified zone adjacent to the front by:

(a)Spring-neap tidal cycle.

Spring tide-strong mixing with high nutrients-offshore advance of fronts-Neap tide-weak mixing-onshore advance of fronts-bloom of previous high nutrient waters

(b) Baroclinic eddies.

‘Baroclinic’ means density-related. The transport of nutrient across front.

(c) Vertical transport.

KV : vertical eddy diffusivity (10-4 m-2 s-1); LW : the cross-fontal distance.

In general: effect (c)>effect (a)>effect (b)

WVV LZCKF )(

ΔΔ

=

3. The biology of upwelling frontsEnhanced biological productivity, high concentration of zooplankton

High nutrient, but light penetration is limited

Thus, across frontal nutrient transport will be crucial to form high productivity

5. The biology of fronts associated with geomorphic features

Enhanced phytoplankton growth in the regions with irregularities in the sea bed, coastline due to flow-geomorphic interaction.

Surface-dwelling planktonic

organisms tend to aggregate here.

Summary of mechanism that enhanced biological production at the fronts

• 1. The spring-neap tidal cycle. At a fixed point, water may be tidally mixed at one stage of tidal cycle and stratified at another. Nutrients are brought up during the mixing phase and utilized in the upper mixed layer during the stratified phase (see example before).

• 2. Cross-frontal transport. Mechanism that transfers nutrients from the tidally mixed side of the front (phytoplankton is light- limited) to the stratified side of the front. The mechanism can be due to baroclinic eddy.

• 3. Vertical transport. Frontal zone is favorable for the vertical transport of nutrients through the front to the stratified water above, which enhances phytoplankton in the immediate vicinity of the front.