Lecture 9+10: Buoyancy-driven flow, estuarine circulation, river plume, Tidal mixing, internal waves, coastal fronts and biological significance

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-Evaporation Coastal 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 s: 338 u:256 u:338 v: 256 v: 305

Δ s: 256 Δ s: 305 Δu: 256 Δu: 305 Δ v: 256 Δ v: 305

Shantou

with river

without river with river without river with river river +ADV.+mixing

without river with river

without river with river

Day 10, NO3 Day 30, NO3 Monsoonal wind Phyto. Phyto. Zoo. Zoo.

Day 30 NO3 Phytoplankton 22.1N zooplankton zooplankton > 1; phytoplankton > 2; NO3 > 4;

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 z u Offshore pressure gradient force

1 1 2 2 1 / ) ( have we ) ( ρ ρ ρ ρ ρ = Δ = = Δ + = h h P P gh P h h g P B A B A To form an offshore pressure force, P A >P B at the depth z 1 2 1 ρ ρ ρ Δ < h z

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. 1 dp ρ dx = K e 2 d u 2 dz Mixing induced by vertical velocity shear and buoyancy Ri=buoyancy restoring force/vertical mixing induced by vertical velocity shear Ri g dρ ρ dz du 2 = /( ) Ri<0.25, unstable. dz

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) E = ( h / D ) log 10 t h: the height of water column, D t : depth-integrated rate of dissipation energy of tides. E<1.9 well-mixed; >1.9 stratified

(2) E = log 10 ε E>-1.0, tidally mixed; E<-2.0, stratified;

fu p fu = = = ρ gh 1 ρ g dp dy dh dy

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 2 (3) Contours of the salinity (psu, black contours) and AKv ( m / s, color contours) on the vertical section along the axial of the PRE 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

(Continued of lecture 5) 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.

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 nutrientrich 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 divergence zone Deep nutrient-rich water being advected to the euphotic layer

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

Vertically well mixed Cold cushion Seasonal variation in northwest Europe

Scale of the front can be estimated by the internal or baroclinic Rossby radius of deformation R bc. 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 D 1 ) 0.5 where g = 9.8 m/s 2 is gravity, f the Coriolis parameter, D 1 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 shelfsea front.

Unstratified due to tidal mixing as tides is approaching shallower shelf Advance shoreward if tidal strength is weakened

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

F E gdδρ γ ( ) ρ 1 2 = γ=0.005,d is upper layer thickness, C is the nutrient. DΔC (d) Eddies Eddies: cause water to be exchanged across the front and contribute a significant flux of nutrients (F E ). The eddies can be generated by the instability at the fronts.

Biology of Fronts 1. 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. ΔC F V = KV ( ) L Δ Z K V : vertical eddy diffusivity (10-4 m -2 s -1 ); L W : the cross-fontal distance. In general: effect (c)>effect (a)>effect (b) W

3. The biology of upwelling fronts Enhanced 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 Surface-dwelling planktonic organisms tend to aggregate here. Enhanced phytoplankton growth in the regions with irregularities in the sea bed, coastline due to flow-geomorphic interaction.

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 lightlimited) 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.