Ocean Layering: Density, Temperature, Salinity, and CirculationSylvia Cole, Scripps Institution of Oceanography Maureen Quessenberry, University City High School

SummaryWe developed a 3-4 day unit on the verticalstructure of the ocean that gives students theopportunity to work with actual data, learn aboutthe global circulation, and learn about some ofthe smaller scale features that stir and mix theocean. The unit uses ship-based observations ona global scale and observations north of Hawaiifrom an ocean robot, the autonomous underwaterglider Spray, as application activities.

Lesson 1: Density and the Ocean Lesson 2: Deep Currents, Stirring, and Mixing Lesson 3: Glider Observations

Density & Salinity

Students investigate how salinityeffects density through a handson lab activity. The sink / float labincludes one object that will floatin salt water and sink in freshwater. Students conclude thatsalt water is denser than freshwater.

Stirring and Mixing

Students use clay to create a model of the ocean.Different colors represent different types of water:

Warm and cold waterFresh and salty waterPolluted and unpolluted waterWater full of organisms and water noneWater with nutrients and water without

A slice through the clay gives information aboutwhat happened to the clay. Similarly, a slicethrough the ocean gives information about whathas happened to the ocean.

So What?

Dense water sinks to the bottom of the ocean, slowly spreads out,and slowly rises back to the surface. This circulation is sometimesreferred to as the global conveyor-belt circulation.

Liquids can sink and float

A density column demonstration introduces students to the idea thatliquids can have different densities and can sink and float on top of eachother. The ocean is like a density column with the lightest water on topand the heaviest water on the bottom.

Learning Goals• Students will be able to describe vertical currentsand currents near the bottom of the ocean on a globalscale.

• Students will be able to explain vertical layers of theocean in temperature, salinity, and density.

• Students will be able to explain why temperature andsalinity change more near the surface than at depth.

Misconceptions• All ocean currents are caused by tides

• The deep ocean never changes anddoesn’t move

• The ocean has the same salinityeverywhere

Context for UseThis activity was designed for a 9th grade earth scienceclass, and would also be appropriate for middle schoolstudents. There are three 55-minute lessons and one20-minute quiz. This unit was taught after anatmospheric section and a short unit on the surfacecirculation of the ocean. Students were familiar with theconcepts of density, energy, waves, and theatmospheric circulation as well as the surface circulationof the ocean and what the ocean transports.

Atlantic Sections

Students complete a shortactivity using sections oftemperature and salinity in theAtlantic. Students see that not allof the water on the bottom is thesame, and can trace where waterhas come from using salinity.

Spray Gliders: autonomous ocean robots

Gliders have no propulsion mechanism anduse buoyancy to control their depth in thewater by pumping oil into and out of anexternal bladder. The glider moves forward inthe water as it dives up and down because ofthe wings. It is ~6 ft long and weighs ~100 lbs.

North Pacific Sections

16 sections of temperatureand salinity were obtainedfrom July 2007 - December2009 along a line north ofHawaii. It crosses throughthe center of the surfacegyre.

Science StandardsWe focus on teaching Californiascience standard 5d: “Studentsknow the properties of ocean water,such as temperature and salinity,can be used to explain the layeredstructure of the ocean, thegeneration of horizontal and verticalocean currents, and the geographicdistribution of marine organisms.”

Schematic of the globalconveyor-beltcirculation after thepopular one by W. S.Broecker*. Water sinksin a few spots near thepoles, spread outhorizontally, and risesup slowly everywhereelse.

*Broecker, W S, 1987. Thebiggest chill. Natural HistoryMagazine, Vol. 97, pp. 74-82.

Blue and yellow colors have beenstirred around, and green colorshave been mixed together.

Sampling the global ocean

One way oceanographers samplethe ocean is to take slices through itwith a ship, stopping every 50 kmand dropping instruments down tothe bottom. The R/V Melville,pictured at right, is one of the shipsused to sample the ocean.

From 1990-1998, several slicesthrough the ocean were taken as partof an international effort to observe theglobal ocean. Some of these sectionshave been observed prior to and afteras well.

Water is collected inbottles and brought backto the surface for analysis.

Above: sections of temperature and salinity in the Atlantic.Left: The red line shows where this section is located.

Gliders have a satelliteantenna in their wings, whichallows data to be sent back tothe scientists and instructionsto be sent to the glider. Theglider travels 5.5 kmhorizontally in 5.5 hours foreach 1000 m dive. The smallhorizontal spacing of divesallows smaller-scale featuresto be studied.

Students use six of the sections from different seasons and yearsto look at ocean layering and some of the smaller scale features.

Density Column Ocean

Salt water density >Object density

Object density >Fresh water density

Salt water density > fresh water density

Cold deep current

Warm surface current Latitude (°)

24 26 28 30 32 34