Lesson 6.03 – Deep-Ocean Circulation

Standards:  ES5.a Students know how differential heating of Earth results in circulation patterns in the atmosphere and oceans that globally distribute the heat.

ES5.b Students know the relationship between the rotation of Earth and the circular motions of ocean currents and air in pressure centers.

ES5.d Students know properties of ocean water, such as temperature and salinity, can be used to explain the layered structure of the oceans, the generation of horizontal and vertical ocean currents, and the geographic distribution of marine organisms.

ES6.b Students know the effects on climate of latitude, elevation, topography, and proximity to large bodies of water and cold or warm ocean currents.

INTRODUCTION

Deep-Ocean Circulation

In contrast to the largely horizontal movements of surface currents, deep-ocean circulation has a significant vertical component. It accounts for the thorough mixing of deep-water masses.

Density Currents

Density currents are vertical currents of ocean water that result from density differences among water masses. Denser water sinks and slowly spreads out beneath the surface. An increase in seawater density can be caused by a decrease in temperature or an increase in salinity. Processes that increase the salinity of water include evaporation and the formation of sea ice. Processes that decrease the salinity of water include precipitation, runoff from land, icebergs melting, and sea ice melting. Density changes due to salinity variations are important in very high latitudes, where water temperature remains low and relatively constant.

High Latitudes

Most water involved in deep-ocean density currents begins in high latitudes at the surface. In these regions, surface water becomes cold, and its salinity increases as sea ice forms. When this water becomes dense enough, it sinks, initiating deep-ocean density currents. Once this water sinks, it is removed from the physical processes that increased its density in the first place. Its temperature and salinity remain largely unchanged during the time it is in the deep ocean. Because of this, oceanographers can track the movements of density currents in the deep ocean. By knowing the temperature, salinity, and density of a water mass, scientists are able to map the slow circulation of the water mass through the ocean.

Near Antarctica, surface conditions create the highest density water in the world. This cold, salty water slowly sinks to the sea floor, where it moves throughout the ocean basins in slow currents. After sinking from the surface of the ocean, deep waters will not reappear at the surface for an average of 500 to 2000 years.

Figure 5 Sea Ice in the Arctic Ocean When seawater freezes, sea salts do not become part of the ice, leading to an increase in the salinity of the surrounding water. Draw Conclusions How does this process lead to the formation of a density current?

Evaporation

Density currents can also result from increased salinity of ocean water due to evaporation. In the Mediterranean Sea conditions exist that lead to the formation of a dense water mass at the surface that sinks and eventually flows into the Atlantic Ocean. Climate conditions in the eastern Mediterranean include a dry northwest wind and sunny days. These conditions lead to an annual excess of evaporation compared to the amount of precipitation. When seawater evaporates, salt is left behind, and the salinity of the remaining water increases. The surface waters of the eastern Mediterranean Sea have a salinity of about 38‰ (parts per thousand). In the wintermonths, this water flows out of the Mediterranean Sea into the Atlantic Ocean. At 38‰, this water is more dense than the Atlantic Ocean surface water at 35‰, so it sinks. This Mediterranean water mass can be tracked as far south as Antarctica. Figure 6 shows some of the different water masses created by density currents in the Atlantic Ocean.

Figure 6 This cross section of the Atlantic Ocean shows the deepwater circulation of water masses formed by density currents.

A Conveyor Belt

A simplified model of ocean circulation is similar to a conveyor belt that travels from the Atlantic Ocean through the Indian and Pacific oceans and back again. Figure 7 shows this conveyor belt model. In this model, warm water in the ocean’s upper layers flows toward the poles. When the water reaches the poles, its temperature drops and salinity increases, making it more dense. Because the water is dense, it sinks and moves toward the equator. It returns to the equator as cold, deep water that eventually upwells to complete the circuit. As this “conveyor belt” moves around the globe, it influences global climate by converting warm water to cold water and releasing heat to the atmosphere.

Figure 7 This “conveyor belt” model of ocean circulation shows a warm surface current with an underlying cool current.

PRACTI CE

1. Take notes on the above information.
2. Click on the Conveyor Belt map above to listen to an audio file about it.

ASSESSMENT

1. Turn in your notes.
2. Take the 6.03 Quiz.