Making water move

• Vertical transport & mixing achieved by differences in density of different seawater masses

• What effects the density of seawater?– Temperature– Salinity

• Density affected more by

How it is mixed & transported

temperature than salinity, because the former is more variable in the ocean

Vertical mixing in the ocean

• Remember the thermal profiles with depth at tropic, temperate and polar latitudes– Polar regions essentially isothermal to the bottom, with coldest temperatures near the surface in winter

– In temperate & tropical regions a warmer surface layer lies above a deep, cold layer

• Warm water less dense than cold

• In a density-stratified water column, warm surface water floats on colder water below

• Temperate & tropical regions stable, polar regions-density unstable

Density profile with depth at tropical and polar latitudes

Dense surface waters at the poles sink

• At the poles in winter, cold surface water is more dense than underlying warmer water

• Moreover, formation of sea ice at high latitudes concentrates salinity further contributing to increased density

• The greater density of cold, saline polar waters causes them to sink and form the water at the bottom of all the world’s oceans at all latitudes by moving towards the equator

• It takes hundreds of years for a water mass sinking at the poles to complete its journey

Deep Ocean Circulation• All circulation below 1000 m is accomplished principally by density differences of polar water masses and their transport both down in the water column and to lower latitudes

• The density of a particular water mass determines what layer, or depth beneath the surface it resides at

Surface ocean circulation• What moves

water horizontally across the ocean?– ?– ?

Surface ocean circulation

• What moves water horizontally across the ocean?– Wind– Coriolis effect (due to the earth being a sphere that rotates from west to east)

• The Coriolis force is that of the earth’s rotation on the movement of particles

Coriolis force• A particle at the equator travels east at 1613 km/h to complete a full rotation in one day

• Particles nearer to the poles travel eastward much more slowly because the circumference is less

• So, particles traveling from the equator towards the poles move from areas of high to low eastward velocity– They will have a relative deflection further east

• Particles moving toward the equator move from low to high eastward velocity; this lag deflects them to the west

Coriolis force

• Whether moving towards or away from, the equator, the effect is the same:– In the northern hemisphere, parcels of water are defelcted to the RIGHT

– In the southern hemisphere, parcels of water are deflected to the LEFT

Combining the effects of planetary winds and Coriolis in the movement

of oceanic surface waters• Planetary wind system driven by differential heating of earth’s surface– Solar insolation is greatest near the equator and least near the poles• A given amount of sunlight spread over a larger area if it strikes at an angle: therefore, lower density of energy

• Solar radiation must pass through more atmosphere at poles than at equator

– Because of earth’s tilt, this pattern varies seasonally

Building a model of oceanic circulation

Planetary wind system driven by differential heating of earth’s surface

•Solar insolation is greatest near the equator and least near the poles

-A given amount of sunlight spread over a larger area if it strikes at an angle: therefore, lower density of energy-Solar radiation must pass through more atmosphere at poles than at equator

•Because of earth’s tilt, this pattern varies seasonally

Building a model of oceanic circulation

•Warm air rises, cold air sinks•Air at equator heated, so rises

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Building a model of oceanic circulation

•As surface air rises, it is replaced by surface air flowing from north & south•Risen equatorial air expands as pressure is less high in the atmosphere•Expanded air cools and starts to descend but is pushed north and south by air rising behind it.•As air high in the atmosphere is deflected north or south, coriolis acts on it pushing it eastward; consequently it sinks to the surface at ~30° latitude•This air warms as it descends (takes up moisture which is why the world’s deserts predominate at these latitudes

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Building a model of oceanic circulation

•The first cell, or tropical cell, drives the other 2 cells which are more or less passive

•Blue arrows represent the winds blowing across the earth’s surface (on globe showing effect of coriolis force on wind with red arrows)

•As wind blows over the surface water, the water (because of Coriolis force) does not travel in the same direction as the wind•Water is deflected at an angle of 45° relative to the wind

Ekman Spiral

• Friction of the surface water layer acting on layers beneath it, causes underlying layers to be dragged in motion.

• They too are deflected by the coriolis force

• As the influence of wind is greatest at the air-sea interface, the effect decays exponentially with depth and eventually ends

45°

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Surface current

Ekman Transport

• The sum of these vectors over all depths results in a direction of net transport of water of 90° to that of the wind

• 90° to the right of the wind in the northern hemisphere

• 90° to the left of the wind in the southern hemisphere

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Surface current

Ekman transport

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Building a model of oceanic circulation

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•Blue arrows represent Ekman transport of water•D=Divergence (planetary upwelling of water)•C=Convergence (water masses converging)

Oceanic cross-sectionAt depth surface

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Upwelled waters

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Building a model of oceanic circulation

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•Subsurface flow of water from high to low areas of oceanic pressure•Water masses deflected, as always, by Coriolis force•Results in mid-latitude, high pressure oceanic gyres in both northern & southern hemispheres due to the presence of land masses.

Comparisons between ocean basins

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• Both oceans have equatorial currents that flow west• An equatorial countercurrent that runs east along the equator• Oceanic high pressure systems at 30° N&S that generate gyres in the ocean• Polar current systems

Differences between N & S hemispheres

• More land in northern hemisphere results in smaller polar current systems in both Pacific & Atlantic compared to southern hemisphere– In southern hemisphere, the west wind drift is circumpolar because virtually no land in its path at 60°S latitude; not only surface moving east, but sub-surface geostrophic flow as well.

• Western intensification- weaker in south Pacific because of a leaky boundary associated with Indonesia

• Eastside boundaries - at 60°S latitude, swiftly, moving open ocean to east because there is no land, except area between Palmer Peninsula of Antarctica and South America; – a string of islands that that act as a weir blocking free flow of water

– This pile up of water is deflected north and becomes the exceptionally strong Humboldt (or Peru-Chile) current

Polar wind speed and wave height differences