EARTH, PLANETARY, & SPACE SCIENCES 15 INTRODUCTION TO OCEANOGRAPHY

LABORATORY SESSION #6 Spring 2017

Ocean Circulation

The focus of the Lab this week is circulation of the ocean and atmosphere. Here, you will learn of the various processes that drive this circulation and of the various paths that winds and waters follow on Earth.

I. WIND-DRIVEN CIRCULATION

Surface circulation of the oceans is primarily driven by winds. When winds blow on the surface of the oceans, frictional drag between the atmosphere and the ocean causes the water at the surface to move. This process begins with the wind dragging on the surface and setting a thin layer of water in motion. This thin layer of water drags on the layer of water beneath it and sets it in motion. This process continues downward, transferring momentum to successively deeper layers. As a result, as much as a few hundred meters of the upper ocean can be set in motion as a current.

Because objects (e.g., masses of water) in motion are deflected by the Coriolis effect, surface currents actually move in a direction different from the direction of the wind. This effect is known as Ekman transport, by which the net transport of wind-driven ocean currents is at an angle (up to 90˚) to the prevailing wind (see Discussion on Figure 1). In the northern Hemisphere, Ekman transport is deflected to the right of the prevailing wind direction and in the southern Hemisphere Ekman transport is to the left.

Subtropical Gyres

The dominant components of surface circulation are large, crudely circular patterns of flow known as the subtropical gyres. These flow in a clockwise direction in the northern Hemisphere and counterclockwise in the southern Hemisphere (Figure 2). They extend from the Equator to about 45˚N and 45ºS. They are driven by the trade winds at low latitudes and the westerlies at higher latitudes. The trade winds cause surface currents to occur both north and south of the Equator. These North Equatorial and South Equatorial Currents flow westward where water tends to pile up against the continental margins. Some of this water flows back eastward as an Equatorial Counter Current (this is best developed in the Pacific), but most of it is deflected away from the Equator at the warm western boundary currents (e.g., the Gulf Stream, the Kuroshio Current). As the western boundary currents proceed to higher latitudes they are deflected by the Coriolis effect and driven by the westerlies to form an east-flowing current (Figure 3; e.g., the North Pacific Current), which is the high-latitude portion of the gyre. On the eastern margins of the oceans, cold eastern boundary currents flow toward the Equator. It is important to note that western boundary currents transport warm waters to high latitudes and eastern boundary currents return colder water to equatorial regions. Thus, equatorial surface waters tend to be coolest on the eastern sides of the oceans and warmest on the western sides.

Subpolar Circulation

Subpolar surface circulation is quite different between the northern and southern hemispheres. In the North Atlantic and North Pacific, small subpolar gyres (just north of the subtropical gyres) circulate in directions opposite to the clockwise subtropical gyres (i.e. counterclockwise). In the southern hemisphere there are no continents to block surface currents driven by the southernmost westerly winds, so the wind-driven Antarctic Circumpolar Current flows all the way around Antarctica (Figure 4). The Antarctic Circumpolar Current is the largest and strongest surface current in all of the oceans. It is so well developed that it influences circulation all the way to the ocean floor (5 km depth) and it is the primary connection between the Atlantic, Pacific, and Indian ocean basins.

2. GRAVITY-DRIVEN CIRCULATION

Deep-Ocean Circulation

Deep waters are separated from the surface waters by the pycnocline. The pycnocline is a region where large gradients in temperature and/or salinity separate less dense surface waters from more dense deep water. These gradients in temperature and salinity form a layer of water with rapidly changing density, which is how the pycnocline gets its name. Below the pycnocline, the deep waters usually move in sluggish currents that are not directly driven by the wind. This deep circulation is driven primarily by gravity. Differences in seawater density, which in turn are controlled primarily by temperature and salinity variations, cause dense water masses to sink under the force of gravity. This deep-ocean circulation is frequently called a thermohaline (thermo-heat; haline-salt) circulation.

Dense water forms at the surface of the oceans and then sinks to great depths and circulates through the ocean basins. The most important sources of the deep water of the oceans are in the polar-subpolar North Atlantic and in the Antarctic. Around Antarctica, particularly in the Weddell Sea (this is in the Atlantic Sector), the formation of sea ice plays an important role in formation of Antarctic Bottom Water (AABW). There, surface waters are strongly cooled until ice begins to form. As ice freezes, it incorporates only about 30% as much salt as is present in the seawater. The remaining salt is concentrated in the very cold, unfrozen water below, increasing its salinity and density, and lowering its freezing point. This cold, saline water is quite dense and flows down the shelf and slope of Antarctica, mixing with other water and thus forming the water mass known as Antarctic Bottom Water. AABW flows in an eastward direction around Antarctica, with the Antarctic Circumpolar Current. AABW also flows northward into portions of the Atlantic, Indian, and Pacific Ocean basins. The dense AABW formed at the southern latitudes composes about 59% of the world’s oceans. In general, AABW includes all water in the Indian and Pacific oceans with temperatures less than 3˚C and all water in the Atlantic less than 2˚C (except in the subpolar North Atlantic).

The other major source of deep water is in the North Atlantic, particularly in the Norwegian and Greenland seas. Here, low temperatures and high rates of evaporation of surface waters lead to production of a cold saline water mass known as North Atlantic Deep Water (NADW). NADW sinks to the bottom and flows south, eventually flowing over, and mixing with water around Antarctica. NADW is the major source of cold salty water that mixes with water from the Antarctic shelf to form AABW. Thus the flow of deep water can be considered as beginning in the North Atlantic, with the formation of NADW. This flows south and gets incorporated into AABW. The AABW then flows around Antarctica and flows into the Indian and Pacific Oceans. Figures 5 and 6 illustrate deep-water circulation.

III. CHEMICAL EVOLUTION OF DEEP WATERS WITH TIME

By far the greatest volume of deep-water formation is in the North Atlantic with the formation of NADW. As this water flows around Antarctica, this is just added to by smaller volumes of the cold, saline water in sources near Antarctica. The combination of these forms the AABW. When considering the formation and circulation of deep waters, it is reasonable to consider the North Atlantic to be the primary source region. Here, large volumes of surface water sink and form deep water that then flows through the oceans and is changed a bit by additions around Antarctica. As you have learned in Lecture, surface waters are typically rich in oxygen and depleted in nutrients like N, P, and Si. These are characteristics that are typical of deep water in the North Atlantic, which forms directly from surface waters. As this water flows south through the Atlantic it gets older, and more so, as it eventually flows into the Indian and Pacific Ocean basins. As it gets older, organisms in the water consume oxygen by respiration and produce carbon dioxide. As the carbon dioxide concentrations increase, deep waters get more acidic. Another effect is that over time, deep waters accumulate nutrients. These come from organisms in surface waters that produce materials that dissolve in deep waters. Thus, over time, deep waters become more CO2-rich, acidic, and rich in nutrients, in stark contrast to the O2-rich, nutrient poor deep waters of the North Atlantic.

Intermediate Water Masses

Near the high-latitude ends of the subpolar gyres, surface waters tend to converge and mix. This forms water of intermediate density that sinks below the surface, but is not dense enough to sink below NADW or AABW. These waters flow along isopycnals (surfaces of equal density) and are important in forming the pycnocline waters.

Another source of intermediate water is relatively warm, saline water that flows out of the Mediterranean Sea. This forms an important intermediate water mass in the north Atlantic. The Mediterranean water causes the north Atlantic to be the saltiest of all major ocean basins.

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Figure 6