Chapter 9 Oceanography

Coriolis effect determines the direction of the wind-driven surface water

In the northern Hemisphere it is 45 degrees to the right of the wind direction, in the southern hemisphere it is to the left of the wind-direction

Affects surface water only

• Surface current movement is caused by the wind

• Red arrows denote direction and speed of the surface current

• Speed of current decreases through the water column (decrease in length of red arrows)

• Displacement in relation to the wind direction increases with depth

• Surface starts at 45 degrees and in each subsequent deeper layer the displacement increases

  • If water movement is integrated from the top of the Ekman spiral to the bottom of the Ekman spiral, we find that the net transport is 90 degrees to the right of the wind direction

Winds named for direction they blow FROM

Northern hemisphere, two wind systems are the Westerlies and the Northeast trade winds

• Westerlies blow from the southwest to the northeast

• The resulting net Ekman transport pushed water towards the Southeast (see figure

• The northeast trade winds blow from the northeast and result in a net Ekman water transport towards the northwest (see figure)

• The result in the northern hemisphere is water being pushed towards the center, forming an elated convergence

  • Same reasoning for southern hemisphere (Coriolis effect deflects to the left in southern hemisphere)

The dashed red arrows show the resulting clockwise movement of the northern hemisphere gyres

Start with the black arrow. This is the net Ekman transport, pushing water into the center of the gyre as a result of the Coriolis effect. Fc denotes the Coriolis effect. We are in the northern hemisphere. A hill of accumulated water will form

Then the green arrows: The water on the hill will be pulled straight down the hill by gravity, resulting in a flow according to the green arrow

Then the red arrows: the water flowing downhill will be deflected to the right, by the Coriolis effect, giving a clockwise flow

Geostrophic clockwise flow results when the black and green arrows balance

The accumulated water in the center of the gyre actually cause a hill in the ocean surface

The warm colors in the figure denote areas where the sea surface is higher than the mean sea surface level

• The hill in the ocean surface can be as high as 100 m or 30 ft

Know names of ocean currents

Many currents follow same naming convention. Note how the northern sections od the currents in the North Atlantic and the North Pacific are simply called the North Pacific and the North Atlantic Current respectively

Close to equator, there are 3 currents with the same name in the Pacific, the Atlantic, and the Indian Oceans

Southern Section of the Southern Hemisphere gyres are all part of the West Wind Drift

It is only continuity currents, the western and eastern currents of the gyres that have own specific names

Occurs bc the Earth spins towards the east and ocean has inertia

• As Earth moves, water in oceans can’t quite follow and piles up on the western side of all ocean basins

• The pile pushes on the western continuity current, which becomes deeper, barrow and faster

• The opposite occurs on the eastern side where water has been removed - the eastern current are wide, shallow and slow-moving

The Arctic has its own gyre, which is not centered over the north pole but displaces to the west

Like rivers on land, ocean currents do not go straight but meander

• Eddies are formed from this meanders

Look at bottom 4 figures:

• A: the dark blue line with arrows that outlines the western boundary of the Gulf Stream will give you the spinning direction on the eddy

• B: the meander has become big enough that it has started pinching f two eddies

• C: outlining blue line in the W (warm) eddy gives you spinning direction

• D: the warm eddy pinches off and goes into the cold wter

  • The cold water eddy travels through the Gulf Stream and into the warmer water on the east side of the Gulf Stream

    • Outlining blue line of the C (cold) eddy gave the spinning direction

Langmuir cells are small scale patterns that occur in the surface mixed layer (SML)

Wind starts small circulation cells that have the diameter of the SML depth

• Where the circulation cells meet and go down, we have a small downwelling area

In coastal areas, the ocean surface often has some debris

• This debris will be concentrated in the downwelling area and causes wind rows

Convergence and divergence zones can occur from several different current patterns

• Top figure

  • Divergence means we have upwelling

    • Upwelling causes thermocline to bulge upwards

  • Convergence means we have down welling

    • Down welling causes thermocline to be depressed

Bottom figure

• Upper 3 figures show examples of current patterns that result in a DIVERGENCE zone

• Lower 3 figures show examples of current pattern that result in a CONVERGENCE zone

Map shows global convergence and divergence zones formed from the surface current patterns shown in the previous figure

Notice how the naming convention is pretty simple

• Start at tropical convergence path and go south

• In order, we find the tropical divergence, the subtropical convergence, and the Antarctic convergence

Then look at the northern hemisphere and notice that we have the same names for the convergence and divergence zones

• Only difference between the hemispheres is the Antarctic divergence in the southern hemisphere

• There is no Artic convergence

In this figures we are looking at the same coastal section during summer and winter. There is a north wind (the black arrow labeled W) in the summer and a south wind in the winter

• Note again how the winds are named for the direction they are blowing FROM

Figure A

• In the initial stages, the surface current NET water transport (T) is at a 90 degree angle to the right of the direction of the wind, as predicted bu the Ekman spiral

• However, the flow of water moves water away from the coast creating a sort of resistance and at steady state the net transport will be 45 degrees to the right away from the shore

• The last figure shows how the movement of water away from the hore affects water movement at the coast

  • We have moved water away from the shore and continuity of flow means this water has to be from the shore and continuity of flow means this water has to be replaced

    • This occurs through near coastal upwelling

      • Note how water surface is sloped, reflecting the movement of water away from the coast

Figure B

• In the winter, we have a south wind

• Initially, the Ekman transport is at 90 degrees to the right of the wind

• At steady state, water transport will be at 45 degrees to the right of the wind

  • Water is moving towards the coast, it piles up, has to go somewhere and downwelling occurs

    • Note the slope of the surface, reflecting that water that has piled up against the coast

Also good illustration of how important the equatorial upwelling area is the largest and most powerful in the world

Note how the summer upwelling occurs further north than the winter upwelling

• Also not how there are some strong upwelling areas close to the coasts of Africa and India in the winter time

  • The unit in this figure is cm/day = centimeters per day

SUPER IMPORTANT FIGURE

• World Water Flow is also called the Global Conveyor Belt

The whole flow is powered by a single area - downwelling of water from the Gulf Stream that is cooled as it enters the North Atlantic. Look at the area between Europe and Greenland, this is where you see the yellow band turn down into green deep water

• Recognize the North Atlantic Gyre

The water that sinks in the North Atlantic forms North Atlantic deep water, which flows south in the Atlantic. There is a small amount of mixing between deep and surface water off the north coast of South America and the west coast of Africa. However, most of the water continues its path around the globe as deep water.

There is mixing between surface and deep water in the striped areas on the map - the east coast of North America and south of South America in the West Wind Drift, but most of the deep water stays in the deep

The deep water finally surfaces in the North Pacific and starts its return path as surface water

• Will go through the gyres in the Pacific, enter the gyres of the Atlantic and finally return to the North Atlantic as part of the Gulf Stream

  • There it will sink again and the journey starts ove

  • Whole circulation takes about 400 years

N=> warm, dry

S=> cold, wet

Tree ring data

• 30 oscillations since Columbus

Variations in current patterns occur and have a profound effect on local climate

• North Pacific oscillation has a limited spatial extent, but each state of the oscillation lasts for decades

  • Yellow oscillation has a mostly northerly current flow and results in warm dry weather

  • The red oscillation has an easterly, southeasterly current flow and results in cold, wet weather

• Trees grow faster when they get a lot of rain and by collecting tree ring data scientists have learned that there has been about 30 oscillations since Columbus

Cold water pool circulating in North Atlantic

Mixes with water in Gulf Stream and Norwegian Current => affects T in N. Europe

• Oscillation in the figure started in 1968 north of Iceland and made a full circle in about 13 years

  • As cold eddy passed close to land, the nearby landmass would experience colder weather

    • For example, Scandinavia had very cold snowy winters in 1977-78