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 Oceanography Lecture

Wind (Chapter 8):
51% of short-wave solar radiation is converted to heat (longer-wave radiation) upon striking the Earth's surface

Why is there less heating of the poles than the tropics even at the spring and autumn equinox when all areas on Earth are experiencing the same number of hours of sunlight?  
Reflection by ice and lower light in winters further contribute to cooler polar temperatures.

 As a result, there is a net surplus of heat near to equatorial regions and a net loss nearer to poles.
So why don't the equatorial oceans boil away and the polar oceans freeze solid?

Considering only unequal heating across the Earth's surface, how might air move over the global ?

Complications to this circulation pattern:

How will the rotation of the Earth affect the path (trajectory) of wind as it moves from the poles toward the equator (what will happen to a cannonball shot southward)?

The velocity of the Earth's surface increases toward the equator relative to the sideway velocity of the cannon ball (or the air mass of wind)

Hence the path will be deflected to the right in the nortern hemisphere and to the left in the southern hemisphere - the Coriolis effectWould ocean current on a large-scale be affected by this phenomenon?   Can you see this effect in your toilet bowl?

So air rising from the equator is deflected, but it also cools and looses moisture as it moves towards the polar, sinking at about 30° N and 30° S (Hadley cell).

Most of this sinking air as it reaches the Earth's surface moves back toward the equator, but some is deflected toward the poles (Ferrel cell).

This air mass encounters air that has been cooled over the pools and is warming as in moves toward the equator forcing air upward at 50°-60° latitude (polar cell).

Gobal patterns in air circulation are further complicated by seasonal differences in heating and the irregular distribution of continents and oceans (Do land and water have the same specific heat?)

(Chapter 9):

Surface Currents - horizontal circulation
What explains the pattern of circulation in surface currents shown in the figure below for the Atlantic Ocean?

Ocean water circulates in currents caused mainly by wind friction at the surface (10% of the world ocean.  i.e. mostly above the pycnocline).

The Coriolis effect and obstruction to flow by continental margins result in surface currents organized into huge circuits known as gyres. 

The Coriolis effect causes surface waters under ideal conditions to move ~45° to the right (in the northern hemisphere) of the direction of wind.   Each successive layer of water underneath is effected further by the Coriolis effect (but the foward movement of water is less) creating an Ekman spiral (image the path of a cannonball fired from a small cannon that sits upon a moving cannonball).   The net (sum) movement of this water column is theoretically ~90° to the movement of the wind (Ekman transport).

However, the movement of water in a gryre due to the Coriolis effect is opposed by a gravitation pressure gradient resulting from a build-up of water (higher elevation) toward the center of the gyre.  As a result Ekman transport is not greater than ~45°. Hence, gyres are referred to a geostrophic ("Earth turning") currents.

Some surface currents are rapid and riverlike, with well-defined boundaries, especially along the western gyre currents.  Western intensification is due to the Coreolis effect which is stronger closer to the poles.  Eastern boundary currents are braod, shallow and less defined, carrying cooler water toward the equator.


An example of a western boundary current is the Gulf Stream along the western North Atlantic  (55 million m3/s, 300X the flow of the Amazon, 160 km/d).  Why does England experience such a temperate climate given its high latitude?

Major currents meander significantly over periods of weeks (analogous to rivers meandering over geological time) resulting in formation of eddies.  Warm-core eddies rotate clockwise and cold-core counter clockwise.

What might be the
ecological significance of
these eddies?       

Wind-induced vertical circulation
Upwelling and downwelling describe the vertical movements of water masses. Upwelling is often due to the divergence of surface currents.  How might upwellings affect biological productivity of that area?   Downwelling is often caused by surface current convergence.

El Niño, an anomaly in surface circulation, occurs when the trade winds falter, allowing warm water to move eastward across the Pacific at the equator.   Why should currents reverse direction under these conditions?

Smaller scale, closer to home:

The Georgia Bight

Observation of cooler waters along the shores of Dayton Beach than at Savannah or Miami led to the discovery of summer upwellings from greater depths (cooler) waters of the Gulf Stream.  Three conditions are necessary:

The intruded patch of water can be over 100 km wide and persist for several weeks.  The water is nutrient-rich and when it reachs the photic zone induces phytoplanton growth on the continental shelf which in turn causes increases in zooplankton population sizes and species composition.

Georgia estuaries
Closer to shore, currents may develop from freshwater river inflows

Many estuaries along the US Atlantic coast
are flooded river valleys and low areas behind barrier islands. 
What has caused this flooding?

Thermohaline circulation
Circulation of the 90% of ocean water beneath the surface zone is driven by gravity, as dense water sinks and less dense water rises. What would cause the density of water to change? 

Where on the globe might these changes occur?

In the Wendell Sea of Antarctica, 20-50 million m3 of dense brine (future Antarctic Bottom Water) is formed every second.  Antarctic Bottom Water spreads northward taking hundreds of years to reach the equator and beyond.

Why is there a downwelling just west of the Mediterrean Sea? 

Since density is largely a function of temperature and salinity, the movement of deep water due to density differences is called thermohaline circulation.

The mixing of water masses in itself can cause vertical circulation

Why can water from Point a and b mix easily,
and what happens  when they do?

So where does dense water eventually go?

Why is productivity high off the coast of Antarctica?

Downwellings, upwellings, and surface currents result in a complex 'conveyor belt' that transfer heat, gases, nutrients and organisms worldwide.
Because they transfer huge quantities of heat, ocean currents greatly affect world weather and climateChanges in the Gulf Stream have already been detected.

Currents are the very heart of physical oceanography. Their global effects, vast masses of water, complex flow, and possible influence on human migrations make their study of particular importance.


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