Objectives
In some near-shore areas of the ocean, coastal orientation, prevailin
Objectives
In some near-shore areas of the ocean, coastal orientation, prevailing winds, and Earth’s rotation combine to influence the vertical circulation of the surface ocean. In these regions, wind and the Coriolis Effect transport water from the surface ocean away from the coast and then cooler water from deeper in the water column rises to the surface to fill this gap. This process, called coastal upwelling, brings nutrient-rich water to the sunlit surface, spurring ocean productivity. In other places, the wind transports surface water toward the coast. The build-up of water pushes surface water deeper into the water column in a process called coastal downwelling. It thickens the surface layer of nutrient-deficient water, reducing biological production.
After completing this investigation, you should be able to:
•Demonstrate the causes of coastal upwelling and downwelling.
•Describe the influence of the prevailing wind and Coriolis Effect on upwelling and downwelling.
•Explain the use of “ocean color” to remotely monitor ocean productivity.
•Describe how and why upwelling and downwelling enhance or suppress ocean productivity.
Upwelling and Downwelling
Coastal upwelling and downwelling require key characteristics to occur in the ocean. Depending on which hemisphere and the wind direction, near-surface water flow produces either upwelling or downwelling. The resulting processes can be described by visualizing the characteristics of (a) Hemisphere (Northern or Southern) and (b) wind (southerly or northerly), which affect (c) near-surface ocean flow (east or west), and cause (d) offshore upwelling and downwelling.
As a reminder, winds are described by the direction from which they blow. Hence an arrow of wind pointing toward the north, which is from the south, is described as a south or southerly wind.
In Figure 8A-1, the blue upper surface of the block diagram represents the east-west cross-section of an ocean basin in the (a) Northern Hemisphere with (b) a southerly wind (blowing from the south toward the north). The wind is the black arrow, the near-surface flow is yellow, the downwelling is red, and the upwelling is blue. The color arrows depend on hemispheres and wind directions. Coasts are identified according to their related landmasses.
Figure 8A-1. Northern Hemisphere ocean basin with a southerly wind. The coasts are relative to their continents.
1.In Figure 8A-1, using the cardinal directions in the upper left and the coast labels, the western boundary of the ocean basin is the land’s ______ coast. The eastern boundary of the ocean basin is the adjacent land’s ______ coast.
a.eastern … western
b.western … eastern
2.In Figure 8A-1, imagine you are in a boat positioned at the end of the wind arrow, looking in the direction the wind is blowing. At that location, the wind is blowing toward the ______ and the direction of the near-surface ocean flow is toward the ______.
a.south … east
b.north … east
c.south … west
d.north … west
3.Surface water set in motion by the wind is deflected clockwise by Earth’s rotation in the Northern Hemisphere, which is known as the Coriolis Effect. (See Investigation 5A.) Compare the direction of the wind and the near-surface flow. The near-surface flow is 90° to the ______ of the direction the wind is blowing. This water flow is called Ekman transport.
a.right
b.left
4.This forces ocean water toward the western coast, where it piles up and sea level rises, which produces ______. At the same time, the near-surface water flows away from the eastern coast, the western ocean basin, which lowers the sea level and causes upwelling as water rushes upward to replace it.
a.downwelling
b.upwelling
Figure 8A-2 models an ocean basin in the (a) Northern Hemisphere with (b) northerly wind (blowing from the north toward the south).
Figure 8A-2. Northern Hemisphere ocean basin with a northerly wind. The coasts are relative to their continents.
5.Ekman transport causes the ______ near-surface flow under a northerly wind in the Northern Hemisphere.
a.eastward
b.westward
6.In Figure 8A-2, the coastal downwelling occurs offshore of the ______ while coastal upwelling would be happening offshore of the opposite coast.
a.eastern coast
b.western coast
In the Southern Hemisphere the Coriolis Effect is the direction opposite of that in the Northern Hemisphere. In the Southern Hemisphere, Ekman transport flows 90° to the left of the wind. From what you’ve already learned of the wind in the Northern Hemisphere, how do you expect upwelling and downwelling to play out in the Southern Hemisphere?
Figure 8A-3 models an ocean basin in the (a) Southern Hemisphere with (b) northerly wind (blowing from the north toward the south).
Figure 8A-3. Southern Hemisphere ocean basin with a northerly wind. The coasts are relative to their continents.
7.In Figure 8A-3, the south wind prompts the near-surface water to flow to the ______, or to the left of the direction the wind is blowing.
a.west
b.east
8.Coastal ______ is produced along the western coast, in the east of the ocean basin.
a.downwelling
b.upwelling
Figure 8A-4 models an ocean basin in the (a) Southern Hemisphere with (b) southerly wind (wind blowing from the south toward the north).
Figure 8A-4. Southern Hemisphere ocean basin with a southerly wind. The coasts are relative to their continents.
9.In Figure 8A-4, the southerly wind produces Ekman transport of ocean water that results in ______ off the western coast, in the east of the ocean, and ______ off the eastern coast.
a.downwelling … downwelling
b.downwelling … upwelling
c.upwelling … downwelling
d.upwelling … upwelling
10.Coastal upwelling of nutrient-rich water stimulates the growth of marine autotrophs (e.g., phytoplankton) that support marine food webs and fisheries. In the (a) Northern Hemisphere, a (b) ______ wind would produce upwelling and increase productivity along western coast of Africa.
a.southerly
b.northerly
11.In the (a) Southern Hemisphere a (b) ______ wind would produce downwelling along western coast of South America. When the wind transports near-surface water toward a coast, the warm surface layer thickens and sinks, decreasing ocean productivity.
a.southerly
b.northerly
12.Along the coast of central and northern California, prevailing surface winds blow from north to south in the summer and from south to north in the winter. Based on the model, the season when cold coastal upwelling water cools the overlying air to saturation and could produce frequent fog is ______.
a.summer
b.winter
Upwelling also occurs where winds diverge. The wind entrains water in the surface ocean. Figure 8A-5 shows how this scenario leads to equatorial upwelling in the Pacific Ocean. Near the equator, the Coriolis Effect deflects water initially flowing toward the equator to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This creates upwelling along the equator, where cold, nutrient-rich water from deeper within the water column rises to the surface.
Figure 8A-5. Equatorial upwelling in the Pacific Ocean. This same scenario also occurs to a lesser extent in the Atlantic and Indian Oceans. [SEOS, Link 8A-1]
The trade winds blow from the east to the west in both hemispheres. In the Northern Hemisphere’s tropical latitudes, the trade winds blow from the northeast toward the southwest, and in the Southern Hemisphere, the winds blow from the southeast toward the northwest.
13.Wind stress on the ocean surface produces Ekman transport of water 90° to the right of the direction of motion in the Northern Hemisphere and toward the northwest, and 90° to the left of the direction of motion in the Southern Hemisphere and toward the southwest. This results in surface ocean water flowing ______ the equator.
a.parallel to
b.away from
c.toward
14.The surface water flows described in #13 indicates that during trade wind conditions the surface waters along the equatorial Pacific are ______ and, consequently, the equatorial Pacific experiences ______.
a.diverging … upwelling
b.diverging … downwelling
c.converging … downwelling
d.converging … upwelling
The movement of surface currents in the equatorial ocean diverges, allowing cold, nutrient-rich water to rise to the surface. Because the Sun remains fairly uniform year-round in the sky, this area should be marked by high primary productivity.
Ocean Productivity
Upwelling transports cold, nutrient-rich waters into the sunlit upper reaches of the ocean (the photic zone). Nutrients and solar energy provide conditions that favor increased populations of primary producers, such as the phytoplankton in Figure 8A-6, which occupy the lowest trophic levels of marine food webs. On the other hand, downwelling lowers primary production as nutrient-poor surface waters move shoreward and downward, limiting ocean productivity.
Figure 8A-6. Examples of phytoplankton shells, diatom (left) and coccolithophore (right). Every species produces a different variation on the shell pattern and formation. [(left) R. Femmer, USGS, Link 8A-2, (right) NEON_ja, Wikipedia, Link 8A-3]
Phytoplankton thrive with high nutrient concentrations and sunlight, and produce shells so they remain buoyant and stay within the photic zone, where photosynthesis is possible. The part of phytoplankton that absorbs sunlight, to manufacture organic carbon, is chlorophyll.
Special satellite sensors monitor reflected radiation sensitive to the color of organic carbon, specifically the green color of chlorophyll. The visible radiation reflected to space by the ocean (ocean color) provides a measure of the concentration of chlorophyll in surface waters and thereby an estimate of primary production. As the concentration of phytoplankton increases, the ocean becomes greener.
With the data from Earth-orbiting satellites that monitor the color of the ocean, we can examine worldwide chlorophyll concentration, and thus ocean productivity. Figure 8A-7 is a map of ocean color in 2018. Chlorophyll concentration is displayed using a color scale where red indicates the highest annual productivity and the blues and violets indicate the lowest productivity. To access ocean color images, go to Ocean Studies Maps & Links, Biological, “Ocean ‘Color’ (Productivity)” or click Link 8A-4. More ocean color maps are available at Link 8A-5.
Figure 8A-7. Composite view of Aqua MODIS chlorophyll concentration of data collected during two orbits of the Suomi-NPP/VIIRS instrument on 10 DEC 2018. [NASA, Link 8A-4]
15.In Figure 8A-7, coastal areas tend to be ______ productive than the open ocean.
a.more
b.seasonal
c.less
Particularly productive coastal regions are located at the mouths of major rivers, which deliver large amounts of nutrients to the ocean similar to upwelling delivery of nutrients to the surface.
16.In Figure 8A-7, the coastal areas at the mouths of the Mississippi River, the Amazon River, the Platte River, the Congo River, and the Indus and Ganges Rivers have ______ concentrations of chlorophyll, indicating they have ______ productivity.
a.low … high
b.low … low
c.high … high
d.high … low
Beyond the nutrients from upwelling and rivers, solar variability also affects productivity. For example, at high latitudes, such as in the Arctic and Southern Ocean, productivity is impacted by solar variability.
17.Figure 8A-7 also shows that in the broad expanses of the Pacific Ocean, ocean productivity along the equator is ______ ocean productivity in the subtropical ocean basins to the north and south.
a.the same as
b.somewhat higher than
c.somewhat lower than
18.The band of high productivity along the equator is due to ______.
a.seasonal solar variability
b.seasonal nutrient variability
c.downwelling
d.upwelling
19.Regions of low primary productivity can be found ______.
a.around the poles
b.along the coasts
c.at the center of northern and southern ocean basins
The productivity zone along the equator in the Indian Ocean is less productive than the Pacific Ocean because the Pacific Ocean is much larger so both the winds and the Coriolis Effect have more time and space to create surface water divergence.
Nutrients transported by wind-driven coastal upwelling into the photic zone result in high concentrations of phytoplankton. While coastal upwelling only accounts for about 1% of the global ocean area, these waters account for up to 50% of the fish harvest. In fact the world’s richest fisheries are typically found along coastal upwelling zones. One example is a 25,900-km2 (10,000-mi.2) region off the west coast of Peru, which yields one of the richest fishing areas in the world.
20.From Figure 8A-7, rich fisheries are located ______.
a.off the northwest coast of Australia
b.off the northwest coast of Africa
c.off the eastern coast of South America
Figure 8A-8 shows variations in ocean productivity accompanying changes in wind direction and speed associated with the Indian monsoon. The left composite image is from May to June, when winds were light and productivity in the Arabian Sea was low. The right composite image stretches from September to October, when winds were stronger, blowing from the southwest from the Horn of Africa, across the Arabian Peninsula and Sea, and into western India.
Figure 8A-8. Ocean productivity in the Arabian Sea in (a) May to June and (b) September to October. [NASA GSFC, Link 8A-4]
21.The right image in Figure 8A-8 suggests the September-October winds transport coastal surface waters ______ producing ______.
a.onshore … upwelling
b.offshore … downwelling
c.offshore … upwelling
22.The ocean productivity is ______ from September to October.
a.high
b.low
23.Comparing both images in Figure 8A-8, you can see that ocean productivity ______ seasonal in the Arabian Sea.
a.is
b.is not
The impact of winds on the vertical circulation of ocean water, especially in coastal regions, is a prime example of ocean-atmosphere interaction.
Summary
The combination of the Coriolis Effect and Ekman transport can result in upwelling or downwelling, which is strongest where prevailing winds blow parallel to a coast or at open ocean divergences or convergences. Upwelling is associated with the western coast of the continents in each hemisphere. Most upwelling areas are among the ocean’s most productive regions, supporting rich fisheries. Seasonal winds also affect coastal upwelling and downwelling.