Lesson 7: Ocean Layers II Physical Oceanography

Last class we learned about salinity How is salinity measured? How does salinity affect the density of water? What other variable affects the density of water?

Today we’ll learn more about ocean layers There are three layers in the ocean Temperature typically gets colder as you move from the surface of the ocean down through the middle layer of the ocean. In the very deepest parts of the ocean, temperature and salinity tend to be uniform

Let’s take a journey down through the ocean’s layers The ocean has three layers The surface layer is on top, the thermocline in the middle, and the deep ocean on the bottom 1000 2000 deep ocean thermocline surface 50 Temperature (oC) 5 10 20 30 Depth (m) Teacher’s Note: The surface of the ocean istypically warmer than the water below. This is because the surface waters are closer to the sun and warm water is less dense. The thermocline is a narrow depth range in which temperatures decline significantly. The bottom waters tend to be colder, dense, and more uniform in temperature. The temperature plot shown in this slide depicts the surface layer and thermocline within the upper layers of the ocean.

Let’s take a journey down through the ocean’s layers The depth of each layer can change based on location and season Here, the thermocline ranges from about 50m-1,000m The majority of ocean water (by volume) can be found in the deep ocean below the thermocline 1000 2000 deep ocean thermocline surface 50 Temperature (oC) 5 10 20 30 Depth (m)

We’ll start our journey in the surface layer (Mid-Latitudes) We’ll start our journey in the surface layer (also known as the mixed layer) The upper surface of the ocean is called the mixed layer Wind and other forces stir or “mix” this upper layer of water to form a relatively constant temperature throughout Depending on weather, season, and latitude, the mixed layer typically ranges from 10 – 200 m in thickness 1000 50 Temperature (oC) 5 10 20 30 Depth (m) thermocline mixed Temperature Profile deep ocean

Let’s move down into the thermocline The thermocline is a layer of water where temperature changes rapidly with depth In the thermocline, the water quickly gets colder the deeper you go This layer separates the warm, surface layer from the cool, deeper waters 1000 50 Temperature (oC) 5 10 20 30 Depth (m) thermocline mixed Temperature Profile deep ocean

What happens as we move below the thermocline? Below the thermocline is the deep ocean Water here is cold, dense and salty But here in the deep ocean, salinity and temperature do not change much as we move downward and become nearly constant 1000 50 Temperature (oC) 5 10 20 30 Depth (m) thermocline mixed Temperature Profile deep ocean

Don’t get confused with “clines” Remember that thermocline refers to a “gradient” where temperature changes rapidly with depth As you study ocean ecosystems, you may see other terms that end with “-cline,” referring to other kinds of gradients Halocline is an area where salinity changes rapidly with depth (halo refers to salt)

What is the pycnocline? Pycnocline refers to an area where density changes rapidly with depth Seawater density is determined primarily by temperature and salinity, though the influence of temperature is often greater Just as temperature and salinity influence seawater density, the thermocline and halocline affect the pycnocline Temperature and salinity tend to remain constant below the pycnocline

Student activity Today we’ll graph data to determine how the thermocline changes with latitude and season