User's Manual

Density Circulation Model ME-6816

*See the PASCO catalog or PASCO web site at www.pasco.com for more information.

Introduction

The Density Circulation Model consists of two one-liter reservoirs connected by two valves. Each reservoir has a milliliter (mL)

scale on one side and a centimeter (cm) scale on the other side for measuring volume and depth. The reservoirs feature pouring

spouts on three sides.

When the valve handles are vertical, the valves are closed. Turn a handle so it is horizontal in order to open the valve.

The Density Circulation Model is designed to allow students to investigate the circulation of fluids with different densities due to

conditions such as differences in temperature or differences in salinity.

Background

Temperature and Salinity

Two of the most important characteristics of Earth’s oceans are the temperature and salinity (salt content) of the water. Together,

temperature and salinity determine the density of the water, which is a major factor controlling the circulation and vertical move-

ment of ocean water. Density-driven circulation accounts for 90% of all the movement of water (currents) in the oceans. Water

movement of this type is also called thermohaline circulation (“thermo” for temperature and “haline” for salinity). Colder and salt-

ier water is heavier than warmer, fresher water. Water gets denser in higher latitudes due to the cooling of the atmosphere and the

increased salt levels, which result from the freezing of surface water. (Frozen water normally contains mostly freshwater, leaving

higher concentrations of salt in the water that remains liquid.) The polar regions have very dense, very cold, and very salty water.

Evaporation can also cause an increase in salinity. As sea water evaporates, the salt is left behind, increasing the salinity. (The

Mediterranean Sea is an example of an area where this happens.)

The global average for sea-water temperature is 3.51 °C. Although surface water temperatures can range from 10 °C to 25 °C,

about 77 percent of the water in the oceans is cooler than 4 °C. The global average for salinity is 34.72 grams (g) of dissolved sol-

ids per kilogram (kg) of seawater. Most of the water in the ocean is in the narrow salinity range of 33.8 to 36.8 g/kg.

Differences in water density generate slow moving currents due to the sinking of the colder, saltier water into deeper parts of the

oceans' basins and the displacement of lighter, less salty water upward. Deep water currents move very slowly, usually around 1

meter per hour (0.001 km/h), compared to wind-driven surface water currents that move at about 1 km per hour (0.6 miles per

hour). Dense water will sink until it reaches an area of even denser water. The water will then spread horizontally as more water

sinks behind it. For example, Antarctic Bottom Water, formed along the coast of Antarctica, will sink below less dense water as it

makes its way north toward the equator. As the dense water moves north, it is warmed by the surrounding water. As it warms and

becomes less dense, the water will rise toward the surface. This upwelling of water from the polar regions provides essential nutri-

ents for phytoplankton which live closer to the surface, and phytoplankton is the beginning of the food chain for many of the crea-

tures in the ocean.

Ocean Layers

The ocean has layers of water that differ in temperature. Overall, there is a three layered thermal structure in most of the ocean. The

lop layer of water is called the mixed layer. The sun-warmed surface water mixes with cooler, deeper water due to winds, breaking

waves, and turbulent currents. Below the mixed layer is the main thermocline that extends down about 1,000 m. The temperature

in this layer changes rapidly with depth. The main themocline separates the warmer mixed layer from the cooler deep layer that

extends to the ocean floor).

Recommended Equipment* Other Items

PASPORT Temperature Sensor (PS-2125 or equivalent) Water

PASPORT Salinity Sensor (PS-2195) Ice cubes

PASPORT® Interface Salt, reagent grade (non-iodized)

PASCO Data Acquisition Software Food coloring (or equivalent)