Ocean Movements and Primary Production

Gulf Stream Current

Introduction

The Oceans cover 70% of the worlds surface.  Despite all of the Discovery Channel documentaries you may have seen, most of the ocean, particularly the central open ocean, is largely barren of life.   However, the shear size of these "wastelands," their subsequent effects on global weather patterns and atmospheric composition makes them the most important ecosystem in the world.

The open ocean is deep - over 12,000 ft deep on average and nearly 30,000 feet at its deepest. A verticle profile of the open ocean can be divided into a series of layers (based on temperature, light penetration, and the presence of a bottom.) These layers are in many respects independent of each other. They don't interact.

Life in the ocean is critically linked to the movement of currents. These currents transport heat and nutrients around the worlds oceans. There are two types of ocean currents moving water around the world.  The first is known as thermohaline circulation. These currents are driven by the changing densities of water that result from differences in salt concentration and temperature.  The second is know as wind circulation and its driving force, obviously, are surface winds.

Thermohaline Circulation

Thermohaline circulation (seen below) is the flow of water induced by differences in temperature (thermo-) and salinity (haline). These differences in water properties leads to density differences. Deep water forms when sea water entering polar regions cools and freezes. This process leaves colder, saltier and denser water that can sink to great depths and flow into the ocean basins. Surface water must replace the sinking water leading to a large-scale surface flow in the North Atlantic. Sometimes described as a global "conveyor belt" is a simple model of the large scale thermohaline circulation. Deep water forms in the North Atlantic, sinks, moves south, circulates around Antarctica, and finally enters the Indian, Pacific, and Atlantic basins. This water is upwelled and returned in a surface circulation (ADD figure). It can take a thousand years for water from the North Atlantic to find its way into the surface waters of the North Pacific. Click to watch a nice 3D animation of Thermohaline Circulation on YouTube.
The great ocean conveyor belt
The displacement of this cold water also has a major impact on global temperature.  It literally transports cold away from the poles and takes it towards the equator.  The resulting current helps to draw warmer waters from the mid-latitudes towards the poles.  The overall effect is to moderate much of the worlds climate. Click here to learn more about Thermohaline Circulation.

Wind Driven Currents

As wind moves across the water, the collision of air molecules and water molecules transfers some energy from the air to the water. As a result of this energy transfer, water moves at about 3–4% of the speed that the wind is blowing. As surface currents in the ocean are formed by interactions between wind and water, they are greatly influenced bythe Earth's rotation, and ocean basin geography.  These interactions form fairly stable patterns, called currents, that you see in the map below. Click to see and animation of the Ocean's Circulation Patterns on YouTube.

Warm surface currents invariably flow from the tropics to the higher latitudes, driven mainly by atmospheric winds, as well as the earth's rotation. Subtropical western boundary currents, such as the Gulf Stream and Kuroshio Currents, are warm, fast surface currents that transport a lot of water and heat of tropical origin to subpolar regions. The Kuroshio Current, for example, can travel between 25 and 75 miles a day, 1 - 3 miles per hour, and extends some 3300 feet down into the ocean's depths.  If you were to compare these currents to rivers in the ocean, you would find that the Gulf Stream moves 100 times as much water as all the rivers on Earth combined. This process, of warm currents flowing towards the poles and cold currents flowing toward the equator is extremely important for maintaining the Earth's overall heat balance. 

Cold surface currents come from polar and temperate latitudes, and they tend to flow towards the equator. Like warm surface currents, they are driven mainly by atmospheric forces and are influenced by the earth's rotation. The E. Greenland Current, Labrador Current, Malvinas Current, and Benguela Current are all important cold surface currents in the Atlantic Ocean. (Image from: Marinebio.org)

Ocean Current Map

The Coriolis Force

Air at the equator is constantly heated which causes it to rise.  As heated air in the upper atmosphere cools, it tends to sink.  This creates large global loops called Hadley Cells.  The Earth rotates from East to West, causing these large air movements to be deflected to the left (in the Northern Hemisphere) and the right (in the Southern Hemisphere).  Water moves in a similar pattern. This tendency for water and air masses to rotate is called the Coriolis Force.

The Earth is 40,000 kilometers (24,900 miles) around at its widest part, the equator. Because it spins on its axis once in 24 hours, a point on the Earth's equator is traveling about 1,700 km per hour (1,000 miles per hour) relative to its axis. However, the closer you get to the poles, the smaller the track a point takes in its daily rotation (the Earth moves slower at the poles). At 60° North or South latitude, the track is only half the distance that it is at the equator, and so a point travels only half as fast. Air (or water) moving from high latitudes to low then tends to lag, and a person on the surface would feel a wind blowing out of the east. On the other hand, air moving from low latitudes to high is deflected westwards.

Because of the Coriolis force, air rushing into a region of low pressure (which is what happens in a storm) will turn into a counterclockwise cyclone in the northern hemisphere and a clockwise one in the southern hemisphere.  Ekman Transport

Ekman Transport

In the early part of the 20th century, a Norwegian scientist, Fridtjof Nanson, noted that icebergs in the North Atlantic moved to the right of the wind.   This force driving these icebergs was described by the 19th-century French engineer-mathematician Gustave-Gaspard Coriolis. (image from: Wikipedia Commons; 1:Wind 2:force from above 3:Effective direction of the current 4: Coriolis effect)

Nanson's student, Walfrid Ekman, demonstrated that the earth's rotation caused this effect and in particular, that the Coriolis force was responsible.   One of the primary results of Ekman dynamics is that the net movement of water, forced by large-scale winds, are to the right of the wind in the Northern Hemisphere (and the left in the Southern Hemisphere).

Wind blowing on the surface of the ocean has the greatest effect on the surface. However, for the lower layers of the ocean to move they must be pushed by the friction between the layers of water above. Consequently, the lower layer moves slower than the layer above. This leads to the spiral affect known as Ekman Transport seen in the following diagram.

Partially due to Ekmann Transport, ocean currents circulate in large circles called gyres. The oceans are filled with huge clockwise-running currents in the North Pacific and North Atlantic, and counterclockwise-running currents in the South Pacific, South Atlantic, and Indian Oceans.   Often eddies break off from the main flow of currents (much like they would in a large river, the direction of flow in these smaller eddies is also determined by the Coriolis force.

Gyres will actually create "hills" of water in the ocean by constantly pushing water in.  In the below link the colors correspond to sea surface height (above or below the oceanic average).

To learn more about this and to see animation's of what this looks like to a satellite go to:  Dynamic Ocean Topography

Coastal Upwelling and Downwelling

As a result of Ekman Transport winds blowing along a coastline can cause phenomena known as upwelling or downwelling.  The waters moved offshore by the wind are replaced by waters from the depths below. For example a wind blowing from the north along a western coastline will cause water to be pushed out to sea.  To replace the water moving offshore waters are brought to the surface from the ocean bottom. These waters are normally very cold and rich in nutrients. Areas of coastal upwelling are typically areas of high productivity. Click to watch an animation of coastal upwelling. (image from: Nasa)

upwellingPrimary Production

It is difficult to seperate a discussion of the currents, particularly upwelling currents from a discussion of primary production. Currents move water, but they also move nutrients.  When organisms in the ocean die, gravity causes them to sink to the bottom. Oceanographers refer to this a "marine snow." Click on the link to watch a YouTube video of Marine Snow.

The ocean contains an incredible network of decomposers that break down marine snow into the basic building blocks of life. As once living organisms (or their fecal material) sink to the bottom of the ocean, they are picked up by deep ocean currents.  When upwelling events bring them to the surface, these nutrients provide one of the two key ingredients (the other being sun) for planktonic life.  The satellite image below (image from Nasa) shows chlorophyll concentration in the worlds oceans.  Its an indicator of plankton abundance.  Notice that the large open ocean gyres are nearly devoid of life, whereas coastal upwelling areas show high concentrations (yellows and oranges) of plankton.

When sunlight and nutrients are combined together it creates an explosion of planktonic life often called a "bloom" To learn more about seasonal plankton blooms go to:What causes plankton blooms? To watch an animation of it, check out Ocean Color Animation.Chlorophyll Concentration

Nearly all primary production in the ocean takes place in the epipelagic layer. The layer that makes up the top 200 meters of the ocean, where light can successfully penetrate. Tiny phytoplankton which include the diatoms, dinoflagellates, blue-green algae, coccolithophores, cryptomonads, silicoflagellates; make up the bottom of all ocean food webs.  These phytoplankton will be consumed by herbivores called zooplankton such as copepods,  krill, amphipods, pteropods, and jellies.  As the food web progresses up, zooplankton are consumed by members of the nekton, which are larger, stronger swimmers (fishes, marine mammals, squids, turtles, sea snakes, penguins, etc.) that are capable of swimming against a current.

Questions to Research:

  1. Read more about Thermohaline Circulation, identify the two places in the world that generate the thermohaline "conveyor belt" of the world's oceans through cold water sinking.
  2. Using the term Coliolis Force, explain why hurricanes rotated counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. If you are stuck, watch a YouTube video on how huricanse work.
  3. Use the blank current map that you were given in class.  Label  different currents from the numbers on the map.  Using a blue marker (or pen) trace out 4 or 5 of the ocean's cold currents. 
  4. On the same map, using a red marker, trace 4 or 5 of the worlds warm water currents.
  5. Define the term oceanic gyre. On your current map, label five of the large ocean gyres.
  6. Go to the link for Ocean Color animations , and watch the examples of plankton blooms in different parts of the world during different months. Next, go to the link - What causes plankton blooms?. Explain what a plankton bloom is and what conditions lead to such events.  
  7. Watch an animation of the Earth's "Primary Production."  This animation records concentration of chlorophyll (the pigment that plants and algae use to photosynthesis.  In this animation purples and blues are low concentrations, while greens to reds are high concentration.   What parts of the ocean show the most productive, what parts show the least?
  8. Explain how offshore blowing winds create upwellings, and how upwellings create ideal conditions for plankton blooms.
  9. Net Primary Production Click on the link and examine the graph. The average level of primary production (per square meter) in the open ocean is very low. Nonetheless, the open ocean's make up nearly one quarter of the entire Earth's primary production? Expain the apparent contradiction.
  10. Do huricanes cause plankton blooms? Click on the link, watch the animation, and explain the mechanisms of how this works.