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BEST – IST, 2006

Modelação do Ambiente Marinho

Ramiro Nevesramiro.neves@ist.utl.pt

www.mohid.com

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Is Ocean Physics controlling Biology?

• The next side shows a vertical slice of a aquatic system receiving light/heat from the top and nutrients from land.

• Heat generates thermal stratification and light stimulates primary production, converting dissolved nutrients into organic particles.

• Organic particles enter into the food web, growing in size. Zooplankton consumes smaller phytoplankton and is consumed by larger organisms, including fish.

• Part of the organic matter consumed (usually the largest part) is respired to supply consumer energy needs, regenerating mineral nutrients (ammonia in case of nitrogen) still in the photic zone. These nutrients are consumed again by primary producers generating the so called “recycled production”, that is mostly fed by nitrogen in the form of ammonia.

• The first generation of primary producers fed by “old” nutrients discharged from the continent or upwelled from deeper ocean layers is called “New primary production”.

• Organisms not consumed in the photic layer sink when they die being mineralized along their sinking path or on the bottom, carrying carbon and nutrients to the deeper ocean layers. This is called the “biological pump”.

• The fraction of the settling material hitting the bottom is high in the coastal (shallow) zones and lower in the deep ocean (only some 2% in the deep Atlantic).

• Why is physics controlling biology?

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BEST – IST, 2006

Marine Ecosystems

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Why is Physics controlling Biology?• Physics is controlling ocean biology because the effect of light and the effect of heat

push on different senses.• Light stimulates the production of organic matter that sinks in the water column

carrying nutrients from the top to the lower ocean layers. Heat increases surface temperature increasing buoyancy, contributing to retain warm water at the surface.

• The result is the decrease of nutrients at the surface because the water remains and nutrients are pumped to deeper layers.

• As a consequence ocean production occurs only where new land originated nutrients are discharged (e.g. Baltic Sea, Adriatic Sea, Southern region of the North Sea) or where ocean dynamics allows deep zone nutrients to move up to the surface.

• Along the continental shelf it is easier to recycle nutrients because particulate organic matter is mineralized closer to the photic zone. Along the deep ocean material is mineralized in deeper – dark - layers, recycling is more difficult and concentration of dissolved nutrients reach higher values.

• The previous slide shows a thermocline inhibiting vertical diffusion, the biological pump and the higher benthic activity on the bottom above the thermocline (i.e. in the shallow areas) where recycling of nutrients at the bottom can generate “recycled production”.

• The thermocline depth is the depth up to which the wind generated turbulence can mix water. The actual depth depends on the year season, but in average is located at the continental shelf depth and thus stratification over the shelf is low, easing nutrient recycling.

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Why are the “Oxygen Arrows” incorrect on the previous slide?

• Primary production promoted by nutrients generates organic matter, fixing carbon and generating O2, generating oversaturation and liberation to the atmosphere. When the organic matter is respired O2 is consumed and CO2 is regenerated.

• The final budget depends on the amount of carbon buried along the organic matter “life cycle”. Part of this carbon is used to generate calcium carbonates (e.g. in shells) and is buried . If Land discharges did not include organic matter, carbon burial would be enough to guarantee a net flux of O2 to the atmosphere and consequently a net flux of CO2 to the ocean. If organic matter is discharged from Land, the net flux of oxygen can be to the ocean.

• Any how the production and the consumption of Oxygen are out of phase and if phytoplankton biomass is too high it can happen that low concentrations of oxygen can be find in some shallow regions. In that case the trophic status is too high and nutrients must be controlled.

• Globally it is estimated that ½ of the CO2 from anthropogenic origin (mostly due to fossil carburant) is buried in the ocean. However in some regions of shallow seas as (the north Sea, the Baltic or the Adriatic) there are important areas with trophic levels much above the recommended values.

BEST – IST, 2006

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Ocean Primary Production• The next slide shows ocean primary production distributions. It shows that primary

production is higher in the cold polar regions and in upwelling regions located along the eastern ocean borders, both in the northern and in the southern hemispheres.

• Effects of Land discharges are seen only in Shallow, semi-enclosed seas as the Northern Sea or the Baltic sea and close to the Amazonas mouth or the estuary “Mar de la Plata”.

• One can thus say that globally ocean primary production is nourished by nutrient recycling in regions where physics allows deep nutrients to reach the surface, this includes the continental shelf, although it is higher if there is upwelling too.

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BEST – IST, 2006

Chlorophyll in the Ocean

http://public.wsu.edu/~dybdahl/lec10.htmlhttp://public.wsu.edu/~dybdahl/globeprod.gifhttp://public.wsu.edu/~dybdahl/tasmania.gif

Primary production is not directly related to anthropogenic discharges.

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Mixed Layer and Thermocline• Next slide shows a typical temperature vertical profile. It shows a mixed layer with higher

temperature close to the surface, a thermocline with high temperature gradiente and a lower temperature below the thermocline with a slower slope.

• The thermocline depth is determined by the wind mixing capacity, i.e., by the turbulent kinetic energy that can be used to generate potential energy. In fact when colder water is pushed upward to replace local warmer water, there is a potential energy increase. That energy must be supplied by wind generated turbulence.

• This process also involves mechanical energy dissipation and consequently the depth up to which the mixed layer can increase depends on the initial stratification and on the wind speed.

• In winter the mixed layer deepens, reaching some 200 meter at moderated latitudes and in summer shallower thermoclines are created inside the mixed layer, the shallower being typically some 20 m deep.

• In winter the mixed layer is deeper than the photic zone and some nutrients are diffused up to the photic zone permitting the spring phytoplankton bloom. As these nutrients are consumed, the surface layer is warmed and a upper thinner mixed layer is formed.

• (see also http://forecast.weather.gov/jetstream/ocean/layers_ocean.htm)

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BEST – IST, 2006

Water column structure

Temperatura Profile

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Vertical Oxygen and nutrient profiles in the Ocean

• The next slide shows vertical profiles of dissolved oxygen and nutrients in the ocean (in the region of Madeira archipelago).

• The Oxygen profile shows a small increase between the surface and 100 meters and then it decreases up to 800 meters deep, to increase again towards the bottom. The concentration closer to the bottom.

• At the free surface the concentration is in equilibrium with the atmosphere. The maximum at 100 meters is a consequence of primary production that is maximum at the lower limit of the photic zone as a consequence of the nutrient limitation.

• The decrease up to 800 meters is a consequence of the consumption (respiration) of the settling organic matter produced at the surface.

• Up to 2000 meters the oxygen concentration increases again and then it becomes constant and higher than the surface concentration. The detail of the oxygen profile depend on the consumption and on the vertical diffusion, but suggests that most organic matter is mineralized above the 2000 meters.

• The higher concentration below 2000 meters must be due to deep water formation in regions where the surface water density is higher than the bottom density (grosso modo in areas where surface temperature is lower than bottom temperature). This occurs in the polar regions for the Oceans and in specific areas of other seas, as is the case of the Gulf of Lyons in the Med Sea).

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Vertical nutrient profiles

• Vertical nutrients profiles are consistent with the oxygen profile. IN the upper layer above 100 meters nutrient concentrations are very low due to primary production consumption. Below that zone the concentrations start to increase up to 800 meters, suggesting that mineralization is particularly important up to that depth. Below 800 meters concentration of nitrate and phosphate is constant suggesting that production is small compared with diffusion. Silicate increase up to the bottom suggesting that particulate silicates deposited at the bottom are the most important source of silicates of the water column.

• Vertical nutrients profiles show that the biological pump is effectively an important mechanism transferring nutrients to the deep ocean and that the formation of deep water is critical to maintain the aerobic processes in the deep ocean.

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12

WOA05 data (NOAA)

Oxygen

Nutrients

http://www.nodc.noaa.gov/cgi-bin/OAS/prd/text/query

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Thermo haline Circulation• The next slide illustrates the thermohaline circulation in the ocean. Even if the ocean

was horizontal in the “beginning of the times”, heating in the tropical zones and cooling in the polar regions would create a surface gradient responsible for a surface flow from the tropical regions to the polar regions.

• This flow would increase the level in the polar regions, would increase the pressure and would generate a return flow at a lower depth. The depth at which the flow is generated depend on the density of the cooled water.

• If the water is cooled up to a density identical to the bottom water, the flow would return close to the depth.

• In this area vertical density gradients will be low and vertical diffusion is possible. As a consequence nutrients concentration close to the surface will increase and primary production will be possible. The high productivities in polar regions is a consequence of the vertical mixing.

• The sinking water is saturated in oxygen and has higher concentrations than in warmer regions because oxygen solubility incr4eases when temperature decreases. This explains the high oxygen concentration in the deep ocean and the fact that in the Region of Madeira the bottom concentration is higher than the surface concentration.

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Equator Pole

Ocean Circulation

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Regions of deep and intermediate water formation according tohttp://earth.usc.edu/~stott/Catalina/Deepwater.html

Upwelling zones close to the continents and along the equator are explained by the wind fields and coriolis. The upwelling zone close to Antartida

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Antartida upwelling

The southern current flowing from the Atlantic to the Pacific is responsible for strong upwelling in Antartida.

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The coriolis effect (meridian movement)

• The next slide aims to illustrate the coriolis force acting mechanism. If one would throw a stone from the pole to a target in the equator in a standing earth the stone would move along a meridian and to hit the target would be just a matter of power. In a rotating earth the target would move rightwards and the stone would maintain its meridian movement. An observer would rotate with the earth and would get the impression that the stone was deviating its trajectory to the right.

• An observer repeating the experience in the southern pole would see the stone moving in the same direction, but because his head is up side down he would see it moving to its left side.

• This is the coriolis effect. It is an apparent force that deviates the movement to the right in the northern hemisphere and to the left in the southern hemisphere.

• If the experience was repeated the way around, i.e. if the target was placed in the northern (or southern) pole and the stone was thrown from the equator the same deviation would occur. IN this case the cause of the movement would be the initial tangent velocity of the stone. In fact in this case, the stone would have an initial velocity that is where R is the earth radius at the equator. When the stone moves to a pole, it passes along places of decreasing R and consequently with lower tangential velocity. Because the stone has a higher tangential velocity, the observer sees the stone moving eastward, i.e. moving to the right. Again, that is the coriolis force.

• What would happen if the stone was thrown along a parallel?

Rv

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BEST – IST, 2006

Earth Rotation: Coriolis effect

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Coriolis effect (zonal movement)

• If the stone was sent eastward, it would have a tangential velocity higher than the earth tangential velocity at the same place. As a consequence it would be submit to a higher centrifugal force. As a consequence it would be deviated towards the equator, i.e. to the right in the northern hemisphere and to the left in the southern one.

• If the stone was thrown westward its tangential velocity would be smaller and then the centrifugal force would me smaller and the stone would be deviated towards the pole.

• The stone only feels the difference between its own velocity and the earth velocity because the earth is deformed due to the centrifugal force. Due to this deformation the gravity force that acts along a line linking the place to the center of the earth can be decomposed in two forces, one perpendicular to the earth surface and another pointing to the pole This two forces and the centrifugal force balance. Additional movement tangential movement deviates the system from equilibrium.

• Anticyclonic and cyclonic movement in the high and low atmospheric pressure regions is a consequence of coriolis and the coastal upwelling in the ocean generated by the wind is also a consequence of the atmospheric pressure.

• The high and low temperatures in oceanic eddies according to the sense of rotation and its size variation when submit to meridian movement as well.

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Coastal upwelling• The next slide illustrates the effect of coastal upwelling generated by equator ward

winds (northern winds in the northern hemisphere) along the eastern ocean margin. • These winds generate a friction force equator ward that is balanced by friction

inside the water column (Ekman flow) and by coriolis, originating a surface flow with a velocity component off the coast perpendicular to the wind at the surface. This velocity generates a surface level gradient with lower level at the coast which generates underwater flow to the coast.

• This circulation pattern is visible at the surface through a temperature decrease but also through a nutrient concentration increase and subsequent increase of phytoplanktonic primary production.

• Some crab larvae use this flow pattern to migrate. During their first life stage they remain close to the surface and are transported off the coast and southward and at a letter stage they dive to lower layers and are transported coastward.

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Coastal UpwellingVento

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Global wind and ocean circulation

• The next slide shows the global wind circulation over the oceans (white arrows). These arrows show anticyclonic circulations associated to the high pessure over the oceans (the Azores cyclone in the Northern Atlantic).

• The anticyclonic winds push the ocean surface along the wind circulation and the coriolis force drive upwelling along ocean margins and along the equator. The temperature decrease along the eastern margin is very clear in the Atlantic and in the Pacific.

• Along the western coast the upwelling effect is masked by the much warmer water surface carried by this circulation from the equator region to the western coast.

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BEST – IST, 2006

Winds, Sea Surface Temperature and upwelling

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Coastal upwelling and downwelling• The next slide shows temperature distributions off the Iberian coast in summer and

in winter. • In Summer northern winds are predominant and strong upwelling events are

frequent.• In winter upwelling is less important and the slope current can be identified at the

sea surface. In this period the maximum temperature is registered next to the coast, contrasting with the summer situation.

• The slope, current visible in winter, is generated by the meridian density gradient in the presence of a zonal bathymetry density.

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BEST – IST, 2006

Surface Temperature (in upwelling and downwelling conditions)

Agosto 98

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The Slope current and the JEBAR effect

• The JEBAR hypotheses is due to J. Huthnance in 1984. The hydrostatic pressure is given by:

• Let’s assume a uniform vertical distribution of density and a meridian gradient. In that case, in regions of large depth (H) and low depth (h) one the shelf one will have:

• Without a free surface slope we would get flow oriented southward, with higher velocity in the deeper region. This flow would generate a free surface slope and the flow would stop when the pressures at both latitudes would have balanced, i.e. when:

•

z

gdxp 3

gHp

gHp

xxxxH

xxH

ghpghp

xxxxh

xxh

x

Hx

xg

xgH

xHg

xgH

xgH

0

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JEBAR• The equation shows that the free surface gradient increases with the depth of the

water column. This means that between to parallels the free surface slope is smaller on the shelf than on the deep waters. If the levels where the same at lower latitude, at higher latitude the level would be higher on the shelf, generating a current from the shelf towards deeper ocean.

• This current would be deflected northwards and would generate the slope current.• The next slide shows the slope current schematically on the left and on the right

shows a surface flow distribution where the slope current is easily identifiable along the 200 meters depth isobath.

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JEBAR, Slope Current

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Tide generation• For detail please consult:• http://forecast.weather.gov/jetstream/ocean/tides.htm

A

B

C

D

FC

FC

FC Fg Fg

Fg

Fg

Fgx

Fgx

Fr

Fr

Fr

Fr

Terra

Lua