A. The major currents in the oceans are driven by the tides, the wind, which generates the major surface currents, and differences in the density of seawater, which generate the major deep water currents.
B. First we'll look at the surface currents and then the deep water currents. Tides are discussed in another unit.
II. Surface Currents - In general the major surface currents are driven by the winds and are significantly influenced by the Coriolis deflection. Surface friction between atmosphere and seawater causes the winds to move the water at a speed of roughly 1/10 that of the wind.
A. The pattern of the surface seawater currents generally follows the pattern of the winds in a given region. The horizontal circulation cells in all the major oceans basins are called gyres. We will confine ourselves to mentioning the N. Atlantic, S. Atlantic, N. Pacific, S. Pacific, and Indian Ocean gyres.
1. The Trade Winds (southeast (S.Hem.) or northeast (N.Hem.)) drive westward flowing Equatorial currents. Review global wind patterns.
2. At higher latitudes (30-60), the prevailing Westerlies (i.e. blowing from the west) push the currents back across the ocean basins in the opposite direction (i.e. to the east).
3. This pattern results because of the right-hand deflection of surface currents (air and water) in the Northern Hemisphere and the left-hand deflection of surface currents in the Southern Hemisphere.
B. The intersections of these belts of surface currents moving in different directions result in regions in the oceans where water masses are converging‚ (i.e., coming together) and regions where they are diverging‚ (i.e., moving away from each other). These convergences and divergences have very important biological implications because:
1. Where surface waters diverge, deeper waters must come to the surface to replace them causing upwelling. These upwelling deep waters bring with them very nutrient-rich waters that have not been depleted of their nutrients by surface-layer photosynthesis. We will focus on this issue in the unit on Marine Productivity because these divergences support large fish populations.
a. Equatorial regions of all oceans
b. Antarctic divergence
2. Where surface waters converge, accumulating surface waters plunge downwards = downwelling.
D. Effect of Coriolis Deflection
1. The Coriolis deflection affects surface seawater currents just as it affects surface winds.
a. Because the Earth's rotational velocity differs with latitude, the amount of the Coriolis deflection also varies with latitude increasing at higher latitudes.
2. The deflection is to the right in the N. Hem. and to the left in the S. Hem.
3. Eckman Spiral
a. In the N. Hem. the surface seawater flows to the right of the wind direction. The competition between the effects of the Coriolis Effect and the actual direction of the wind results in the surface seawater in the upper few meters flowing at a 45o angle‚ to the right of the wind direction.
b. However, this surface water must drag the underlying water along with it, and because of the Coriolis Effect, this subsurface water is deflected more and more to the right. The lower levels of water are moved because of the internal connection between water molecules (i.e., those infamous hydrogen bonds again). At each successive depth the water is flowing more slowly and more to the right than the layer above it. Each layer will move in a direction 45o to the right (in the N.Hem.) of the one above it. At a particular depth (maybe ~100m) the flow direction, now extremely slow, is opposite to the wind direction. The net effect observed for the entire column of water affected by the wind is a net water transport 90o‚ to the wind direction. In other words, if you were to measure the direction of water transport in successively deeper layers, and calculated an “average” direction of motion the “net water transport” would be at a 90o to the wind direction. The resulting flow pattern is called the Eckman Spiral, named for the famous physical oceanographer.
c. In shallow water, the lower layers of the Eckman Spiral are missing and the water seems to travel more in a line with the prevailing wind direction. This explains why a northerly wind in Pamlico Sound or Tampa Bay blows the water to the south out of those bays.
d. In coastal regions this effect contributes to coastal upwelling. (Much more on this later) In the Northern Hemisphere the water transport is to the right of the wind direction. If the coast is to the left of the ocean (when looking downwind), the net water transport will be seaward and the departing surface waters will be replaced by upwelling, cool, nutrient-rich waters. (As along California coast)
e. Because the prevailing winds circle the Atlantic in a clock-wise gyre, the Ekman Transport of near-surface seawater makes currents flow into the center of the gyre. This gives rise to the “hill” of water known as the Sargasso Sea. The sea level of the Sargasso Sea is almost 3 feet higher than that along the coast of North America.
E. Geostrophic Flow - In the Northern Hemisphere the right-hand Coriolis deflection results in the accumulation of light surface waters on the right side of major surface currents. This causes the buildup of a mound of water on the right side of the current flow (i.e., a hill of water results). This buildup continues until the downhill force of gravity, tending to pull the water back off the mound, is equal and opposite to the Coriolis deflection of the flowing water up into the mound. When this happens the two forces balance and the current flows sideways along the mound in the wind direction. This type of current, which results from the balancing of gravity and the Coriolis deflection, is called geostrophic flow. Most ocean surface currents are a combination of wind-driven and geostrophic currents. The winds are the basic driving force, but inertia and geostrophic effects ensure that the currents continue to flow even during periods when the wind stops.
F. Western Intensification
1. Oceanographers have long observed that the so-called western boundary currents flowing along the western edges of the major ocean basins, are especially fast-moving and narrow compared to ocean currents elsewhere in the ocean basins. The Gulf Stream is an example of one of these western boundary currents.
2. There are a number of reasons for this all related to the Earth's rotation and atmospheric circulation. I'll mention two just to give you an idea of the processes at work here.
a. The Trade Winds blow equatorial surface water to the west causing it to pile up (i.e., higher sea surface elevation just like a hill) along western ocean boundaries. Here this excess water flows downhill to the north and south along the edge of the continents. This can be thought of as “squirting out the sides”. When moving “down the hill” along the continental edge, the water speeds up.
b. As I mentioned earlier the Coriolis deflection increases with increasing latitude (i.e., away from the equator). Therefore, the eastward return flow of the major currents at higher latitudes is strongly deflected southwards towards the equator. This compresses Equatorial currents and concentrates the westward-flowing water in a narrow band that squirts out forcefully when it reaches the western continental boundaries.
3. The Gulf Stream = Western boundary current
a. 50-75 km wide, 1-2 km deep, 3-10kph (velocity is fastest in the world’s oceans)
b. Warmed by the sun in the Gulf of Mexico, where seawater pumped in by the North Equatorial Current circulates, water must exit the Gulf by way of the narrow Straits of Florida. The Straits help to increase the velocity and force of this water, which “squirts” out into the Atlantic to provide the majority of the water to the Gulf Stream.
c. This crystal clear, azure-blue, warm, nutrient-poor water from the tropics moves up along the coast of North America gradually slowing down as it approaches Cape Hatteras and turns farther eastward into the Atlantic. The Gulf Stream’s flow is more than 100 times greater than the combined flow of ALL the world’s rivers.
d. Off the Grand Banks of Newfoundland, the warm Gulf Stream converges with the cold Labrador Current, running southward between Greenland and North America. The temperature change can be impressive. The two currents are sometimes so close that the bow of a boat may be in one while the stern is in the other. One boat reported a 22o temperature difference between its bow and stern.
1) This significant temperature difference causes heavy fog that often hides ice bergs that have broken off of the glaciers of Greenland.
e. Benjamin Franklin and his cousin, Timothy Folger, published the first chart of the Gulf Stream to try to speed up ship passages along the NE coast of the USA.
f. Along the NC coast the Gulf Stream is a dependable source of fish of all kinds and tropical seabirds often follow their food supply north along its path. Charter boat captains and commercial fishermen keep careful track of its position as it meanders around the western North Atlantic. The Gulf Stream may shift back and forth by as much as 1000 miles/week.
1) Organisms with tropical origins such as coconuts, multi-colored tropical fish, and Portuguese Men O’War jelly fish often wash up along the beaches of the Outer Banks – carried northward by the Gulf Stream.
g. Changes in position of the Gulf Stream often generate meanders that become cut off from the main flow of the current. These can trap within them cold or warm core rings of cold, nutrient-rich shelf water or warm, nutrient-poor Sargasso Sea water, respectively. Cold core rings move southeastward from the Gulf Stream into the waters of the Sargasso Sea. Warm core rings move northwestward up over the continental slope and shelf.
h. Thought of as wind-driven, about 65% of the driving force for the Gulf Stream is actually the wind and the rest is density-driven.
G. El Niño
1. Many people were talking about the unusual weather conditions of 1995 such as:
a. Torrential rains in California in January ($1.3 billion of damage)
1) Flooded rivers
2) 30-foot waves battered the coast
3) Waterspouts spun along the coast
4) Mudslides buried cars and closed roads.
5) In one northern California town 8” of rain fell in less than one day
6) By mid-January the Lake Tahoe basin in the Sierras had more than twice its normal snowfall.
b. At the same time New England had such warm temperatures that they had very little snow and ski resorts were suffering.
c. Temperatures in the USA were, on average, 6o above normal making it the warmest January on record.
d. Australia, the Caribbean, and parts of Central America experienced severe drought.
2. In the Pacific Ocean, where these effects are most pronounced, they are usually first observed around Christmas time. Therefore, the Spanish name for the Christ child, El Niño, is used to represent this phenomenon. All of the anomalies named above resulted from an El Niño, which is a warming of the equatorial Pacific Ocean that disrupts weather patterns across much of the globe. This results when the normal patterns of surface winds and currents is disrupted because the relatively constant Trade Winds decrease significantly in intensity for significant periods of time.
3. The change in the Trade Winds is a result of changes in atmospheric pressures on either side of the Pacific Ocean. This is called the Southern Oscillation. Remember that Earth’s surface winds move from areas of high atmospheric pressure into areas of low atmospheric pressure, so when the differences in pressure between these highs and lows decrease, the wind speeds diminish. This in turn diminishes flow of the Peru Current northward along the western coast of South America thus allowing warm, tropical Pacific Ocean water to drift back eastward.
4. As the warm, tropical water flows back eastward it suppresses the normal coastal upwelling observed off of western South America. (Remember that the Southeast Trades cause Ekman Transport of water to the west away from the S.Amer. coast because the Coriolis Deflection is to the left in the Southern Hemisphere.) It is this coastal upwelling of cold, deep, nutrient-rich water that supports a flourishing fishing industry off of Peru by supporting a thriving plant population.
5. Effects of El Niño
a. Floods and landslides in northwestern South America
b. Brush fires and droughts in Australia, India, Africa, Indonesia, and the Philippines
c. Heavy rains and severe storms along the coast of SW USA.
d. Significant declines in the number of anchovies found in coastal waters of northwestern S.Amer. This is the world’s largest commercial fishery. Even if you don’t like anchovies on your pizza, the decline in catch from this fishery affects your life. Anchovies are not a popular foodfish (also called table fish). Most of us don’t order them for dinner in a restaurant. However, they are widely used as a protein supplement in chicken and livestock feed. So when the price of anchovies goes up, the price of beef, pork, chicken, etc. increases.
1) During the 1982-1983 El Niño the catch from this fishery was only 5% of normal.
e. Seabirds in the area starve because their food supply dwindles.
f. Organisms that transmit disease also respond to such changes in climate. For example, the 1991 choler epidemic in Latin America is ascribed mainly to an El Niño event. In January 1991 an El Niño warming triggered a bloom of algae off the coast of Peru. Copepods (tiny shrimp-like creatures which are the most abundant marine animals) gorged on the algae and their population surged. Cholera bacteria live on the surface of copepods and when the populations of these carriers rose, so did the incidence of cholera. The cholera bacteria found their way into seafood and then into the people who ate the seafood. Human waste loaded with the bacteria seeped into the mostly unchlorinated water supply of Lima. About 350,000 people contracted the disease and 3.602 died.
g. The famous Galapagos Islands lizards starved in large numbers.
h. Coral reefs are negatively impacted. In fact a 1747 El Niño event has been recognized from coral reef limestone drilled from beneath a living coral reef.
6. These events used to happen only every 2-10 years but they appear to be happening more often these days. Because the effects of a strong El Niño can be devasting climatologists are doing their best to predict when these events are going to occur.
7. Some researchers suggest that Global Warming is increasing the frequency of El Niños.
III. Deep ocean currents
A. The driving force for the major subsurface currents that move the bulk of the ocean water (90% of it) is differing densities of adjacent water masses caused by differences in temperature, salinity and suspended load.
1. Deep water flows are much slower and much more voluminous than surface currents. A few tens of kilometers per year is a typical flow rate.
2. Because of slowness they are heavily influenced by Coriolis deflection.
B. Factors affecting seawater density (The first two are much less important than the last two and will not be given much time)
1. Sediment – increasing the suspended sediment carried by seawater increases its density. This is why turbidity currents flow down continental slopes and rises. When sediment is added to seawater near the shelf break, its density increases and it can eventually move down slope as a turbidity flow. This mechanism for driving deep ocean currents applies to only a small proportion of the water movement in the deep sea.
2. Pressure - water is largely incompressible so very large changes in pressure cause only small changes in density.
a. Increased pressure = increased density
b. Pressure increases in the ocean at the rate of 1 atmosphere for every 10 meters of depth.
3. Temperature - Along with salinity changes the most important factor causing density differences in seawater is temperature.
a. Increased temperature = expansion = decreased density
b. 99% of seawater falls within a temperature range of -2 to 30oC.
c. 75% of seawater falls within the temperature range 0-5oC.
4. Salinity -
a. Increased salinity = increased density
b. 99% of seawater falls within a salinity range of 33-37o/oo.
c. 75% of seawater falls within a salinity range of 34-35o/oo.
C. Density is a complex function of temperature and salinity. Therefore, graphs can be made to show the density that results from given combinations of temperature and salinity. You can study examples of these graphs in your text.
1. Sigma-tee (σt) notation - Seawater densities generally vary from 1.024 to 1.028 gms/cm3. The first two digits are always the same and most of the variation is in the last few decimal places. As a result, the first two digits are ignored, to simplify notation, and the last few numbers only are used. Therefore, the sigma-tee (σt) of the above seawaters is 24 and 28, respectively. This notation is used on the diagram shown in class, but in your text densities are displayed in gms/cm3.
2. Temperature and salinity can be measured electronically with a CTD (conductivity-temperature depth) device and the density determined from a diagram such as the one discussed in class and in your textbook.
3. As small as these variations in density might seem they cause large changes in the positions of masses of seawater.
D. The changes in density of seawater with depth at a given locality can be plotted on such a T-S diagram to yield some very useful information. In lecture you will see a plot from a station in the ocean showing how temperature, salinity and density of the water vary with depth.
1. If you trace the change in temperature with depth you will see that it decreases from the surface to 1 km, stays relatively constant from 1-2 km and decreases again from 2-4 km.
2. If you trace the change in salinity with depth you will see that it decreases from the surface to 1 km, increases sharply from 1-2 km, and decreases a little from 2-4 km. It clearly shows a layer of anomalously low salinity sandwiched between layers with higher salinities.
3. So temperature and salinity can either increase or decrease as depth increases.
E. NOTE HOWEVER that density always INCREASES as depth increases. The denser water is always at greater depths.
F. By studying the changes in temperature, salinity and density with depth at thousands of stations in Earth’s oceans, oceanographers have
drawn longitudinal profiles through the oceans (from surface to bottom) that reveal a distinctly stratified water column. These profiles have been divided into a series of water masses, which are parcels of water characterized by a uniform relationship between temperature and salinity. These water masses acquire their temperature and salinity characteristics at the ocean’s surface where interaction with the atmosphere 1) removes or adds heat energy and 2) evaporates or precipitates fresh water. The density of the water mass determines its flow depth.
G. Water masses maintain their T-S characteristics after they sink and even after traveling 1000’s of km from their place of origin through the depths of the ocean.
IV. Major water masses – see diagram from your Course Pack for the Atlantic, Pacific and Indian Oceans
1. Atlantic Ocean – Deep and bottom waters of the Atlantic are dominated by cold, dense water.
a. Antarctic Bottom Water (AABW) is the densest of the major deep water and is produced principally in the Weddell Sea in Antarctica. At such high latitudes the density of surface seawater is increased by heat loss to the atmosphere and formation of sea ice. This water sinks to the bottom of the Atlantic and flows northward, moving as far north as the equator and up into the western North Atlantic Ocean. The Mid-Atlantic Ridge prevents AABW from getting into the eastern side of the North Atlantic.
b. Antarctic Deep Water (AADW) forms in less extreme latitudes and is slightly warmer and less salty than AABW so it sits on top of AABW.
c. North Atlantic Deep (NADW) and Bottom Water (NABW) are produced in the North Atlantic in the Norwegian and Greenland Seas. These cold, dense waters sink to fill most of the Atlantic Ocean. The Gulf Stream and North Atlantic Current pump surface water northward to replace the
surface water that sinks to form NADW and NABW. The moderately cold and salty NADW is produced by winter cooling and evaporation. It is the most common type of water in the oceans.
d. Antarctic Intermediate Water (AAIW) forms at the Antarctic Convergence (~50oS) and sinks to about 1 km of depth. It is traceable north of the equator where it meets the southerly moving AIW.
e. Mediterranean Intermediate Water (MIW) forms due to the intense evaporation affecting Atlantic Ocean water that enters the Strait of Gibraltar and flows eastward across the surface of the Mediterranean Sea. Evaporation raises the salinity of this water to >36.5 o/oo, which makes it dense enough to sink to 2-3 km deep when it reenters the North Atlantic and flows westward as a distinctly warm and salty tongue of water.
f. Arctic Intermediate Water (AIW) is formed in the subarctic and slides southward under the warm saline water of the Sargasso Sea.
g. Arctic Bottom Water never gets out of the Arctic Ocean because of continents and sills.
2. Pacific Ocean
a. Due to the shallowness of the Bering Strait between North America and Asia very little water flows into the Pacific from the arctic region. Also the production of AADW and AAIW in the South Pacific is not large so deep water in the Pacific is of relatively low density. Therefore, the Pacific is only weakly layered and subsurface circulation is relatively slow making it hard to track water masses in the Pacific.
b. The main water type in the Pacific is called Common Water (CoW), which represents a mixture of AABW and NADW. CoW flows mostly eastward into the Indian Ocean eventually making its way into the Pacific.
c. Pacific Subarctic Water (PSW) fills most of the depths of the Northern Pacific.
d. AAIW in the South Pacific and North Pacific Intermediate Water (NPIW) form by poorly understood mixing processes in subsurface waters rather than forming in surface waters.
e. Contrasting with the high-volume exchange in the Atlantic Ocean, the rate and volume of water transport between the North and South Pacific is so low that it’s hard to measure. These water masses have low dissolved oxygen content (3.5-4.5ppm in Pacific compared to 4.5-6.5 ppm in Atlantic), which indicates that they are older than deep water from the Atlantic and, thereby, must move much more slowly.
3. Indian Ocean
a. CoW fills most of the Indian Ocean with a layer of AAIW sandwiched between CoW and the surface water mass.
b. Red Sea Intermediate Water (RSIW) having a typical salinity >40o/oo penetrates into CoW at a depth of about 3 km.
VI. Linkage between surface circulation, deep-water flow and climate
A. Surface and deep-water currents are not independent of one another; they are all part of the larger global ocean. The "Ocean Conveyer Belt", as it is sometimes called, flows as one fairly cohesive unit throughout Earth's oceans. In fact, although surface currents are most strongly affected by the winds, they are also partly density-driven. Your text includes a diagram that shows the path that a single water molecule might follow on a thousand-year trek through the sea. A molecule that falls to the sea surface in the North Atlantic to become part of NADW will eventually make its way to the South Atlantic, where it will mix with AABW. The molecule then moves eastward through the Southern Indian Ocean and on into the Pacific, eventually making its way to Alaska. Off Alaska, water gets pulled up into the surface circulation system, perhaps making a few cycles around the North and South Pacific gyres, before ultimately returning to the North Atlantic Ocean. This rising and sinking of water has a major impact on life in the sea. Where surface water sinks, it takes oxygen to the depths to support animals. Where deep water rises it brings nutrients back to surface waters to support plants. There would be much less life in the oceans without this exchange between the surface and the depths.
B. Strangely enough, global warming could cause significant cooling in the eastern U.S. and Europe. The ocean conveyor warms climates along the East coast of the U.S. and Europe just as it cools climates such as that of California. Currents like the Gulf Stream pick up heat from the atmosphere at low latitudes and carry it to higher latitudes, where it is gradually released. The conveyor system is largely driven by the pull of sinking water in the cold North Atlantic. The formation of NADW is crucial to the movement of both density-driven and surface currents. Research reveals that near the end of the last Ice Age (~12,000-14,000 years ago) warming generated additional precipitation, and melting of sea ice and glaciers. The flood of low-density freshwater into the far North Atlantic generated low-density water that floated instead of sinking to form NADW. The conveyor shut down, the Gulf Stream essentially stopped, and for more than 1000 years Europe experienced extremely cold temperatures. The present global warming could initiate similar events. Archaeologists have studied the impact of such climate changes on human history. Their work suggests the decrease in rainfall accompanying cooling has destroyed civilizations. When countries accustomed to producing their own food experience drought and no longer can, widespread political instability often results.