99 Surface and Deep Circulation

Chamberlin and Dickey, 2008 Exploring the World Ocean Questions to Consider What are the patterns of the surface circulation? What drives the surface circulation? What are the patterns of the deep circulation? What drives the deep circulation? How are the surface and deep circulation interconnected? What are the patterns of the surface circulation? What drives the surface circulation? What are the patterns of the deep circulation? What drives the deep circulation? How are the surface and deep circulation interconnected?

Chamberlin and Dickey, 2008 Exploring the World Ocean Figure 9-3

Chamberlin and Dickey, 2008 Exploring the World Ocean Figure 9-12

Chamberlin and Dickey, 2008 Exploring the World Ocean Western Boundary Currents Currents that occupy the western edge of subtropical gyres are known as western boundary currents. The best known and most well-studied is the Gulf Stream in the North Atlantic and the Kuroshio off Japan. Western boundary currents tend to be the warmest, fastest moving and the most energetic of all surface currents. They also tend to shed eddies, sometimes called rings. Currents that occupy the western edge of subtropical gyres are known as western boundary currents. The best known and most well-studied is the Gulf Stream in the North Atlantic and the Kuroshio off Japan. Western boundary currents tend to be the warmest, fastest moving and the most energetic of all surface currents. They also tend to shed eddies, sometimes called rings.

Chamberlin and Dickey, 2008 Exploring the World Ocean The Gulf Stream The Gulf Stream extends from the tip of southern Florida to the coast of Newfoundland. It transports warm water along the eastern seaboard of the US and is responsible for the relatively mild winters and humid summers. The Gulf Stream often appears like a “river” in the ocean. With maximum speeds approaching 5 knots (~ 6 mph) and a transport volume of 31 Sverdrups, the Gulf Stream generates the flow of more than 300 Amazon Rivers! Standing on the beaches near his home town of West Palm Beach, Florida, Professor Sean could see the undulations of the Gulf Stream just a mile offshore. The Gulf Stream makes its closest approach to Florida here. The Gulf Stream extends from the tip of southern Florida to the coast of Newfoundland. It transports warm water along the eastern seaboard of the US and is responsible for the relatively mild winters and humid summers. The Gulf Stream often appears like a “river” in the ocean. With maximum speeds approaching 5 knots (~ 6 mph) and a transport volume of 31 Sverdrups, the Gulf Stream generates the flow of more than 300 Amazon Rivers! Standing on the beaches near his home town of West Palm Beach, Florida, Professor Sean could see the undulations of the Gulf Stream just a mile offshore. The Gulf Stream makes its closest approach to Florida here.

Chamberlin and Dickey, 2008 Exploring the World Ocean One of the earliest maps of the Gulf Stream, a western boundary current, was compiled by Benjamin Franklin and his whaling captain cousin, Timothy Folger. Figure 9-1

Chamberlin and Dickey, 2008 Exploring the World Ocean Norman Kuring, Modis Ocean Team Two views of the Gulf Stream. The first image represents SSTs. The second image represents chlorophyll. These two images illustrate the close coupling between physical and biological processes in the world ocean.

Figure 9-17

Chamberlin and Dickey, 2008 Exploring the World Ocean Figure 9-18

Chamberlin and Dickey, 2008 Exploring the World Ocean Figure 9-18

Chamberlin and Dickey, 2008 Exploring the World Ocean A view of the forces that cause geostrophic flow. Winds indirectly supply energy that results in Ekman transport that sets up a horizontal pressure gradient. As surface flows respond to the horizontal pressure gradient, they are deflected by the Coriolis effect. A steady state is reached between the pressure gradient and the Coriolis effect resulting in geostrophic currents and the gyre circulation observed in the world ocean. Figure 9-6

Chamberlin and Dickey, 2008 Exploring the World Ocean An Ekman spiral. Note that this is not an eddy or a whirlpool. The arrows indicate speed (longer=faster) and direction. Each layer moves like a stack of 3 x 5 cards moving in different directions and at a different speed, with the fastest at the top and the slowest at the bottom. Mean Ekman transport Figure 9-4

Chamberlin and Dickey, 2008 Exploring the World Ocean Figure 9-10a (upper) Coastal upwelling occurs as northerly winds move surface waters offshore in the Northern Hemisphere. Note that this image has been corrected from the erroneous one in the textbook.

Chamberlin and Dickey, 2008 Exploring the World Ocean Coastal downwelling occurs as southerly winds move surface waters onshore in the Northern Hemisphere. Note that this image has been corrected from the erroneous one in the textbook. Figure 9-10a (lower)

Chamberlin and Dickey, 2008 Exploring the World Ocean Figure 9-9a Trade winds blowing north and south of the equator causes surface waters to diverge. Colder water flows upwards to take its place. Equatorial upwelling cools the atmosphere and contributes to cloudiness along the equator.

Chamberlin and Dickey, 2008 Exploring the World Ocean Stommel’s model of gyre circulation without the Coriolis effect (upper) and with the Coriolis effect (lower) illustrates the importance of the Coriolis term for explaining western intensification. Figure 9-15

Chamberlin and Dickey, 2008 Exploring the World Ocean Professor Dickey captured this Ekman spiral when a hurricane passed over the Bermuda Testbed Mooring. The x-axis indicates time while the z-axis indicates depth. The lines illustrate direction and speed; a larger “loop” corresponds to a faster current. Colors on the lines indicate seawater temperature. If you study this graph carefully, you will see that current speeds decrease with depth and that current direction is offset for each depth at a given time. Figure 9a

Chamberlin and Dickey, 2008 Exploring the World Ocean Purple and blue colors represent upwelling of cold water along the coast of California. Coastal upwelling stimulates blooms of phytoplankton by providing dissolved nutrients from below the euphotic zone. The blooms of phytoplankton nourish coastal food webs and make these waters among the most productive in the world ocean. We’ll cover this topic in greater detail in Chapter 14 Figure 9-10a

Chamberlin and Dickey, 2008 Exploring the World Ocean The Great Ocean Conveyor Figure 9-24

Chamberlin and Dickey, 2008 Exploring the World Ocean Figure 9-23 Some of the many processes that may influence the deep circulation. Identifying the dominant forces is a major goal of physical oceanographers.

Chamberlin and Dickey, 2008 Exploring the World Ocean Quick Review 10. The deep circulation is likely an important factor in a) submarine warfare b) transport of petroleum and goods c) distribution of surface-dwelling fishes d) climate change 10. The deep circulation is likely an important factor in a) submarine warfare b) transport of petroleum and goods c) distribution of surface-dwelling fishes d) climate change

Chamberlin and Dickey, 2008 Exploring the World Ocean For Further Exploration Check out Exploration Activity 9-1, Exploring Ocean Circulation Through Scientific Investigation. Check out the resources for this chapter at Watch “Blue Planet: Open Ocean”, an excellent video on marine life in oceanic gyres and regions affected by upwelling. Check out Exploration Activity 9-1, Exploring Ocean Circulation Through Scientific Investigation. Check out the resources for this chapter at Watch “Blue Planet: Open Ocean”, an excellent video on marine life in oceanic gyres and regions affected by upwelling.