In-Class Activity 3
5 October 2006
Sea Ice and Density-Driven Ocean Circulation

1. The temperature-salinity diagram at the right shows four water masses:
A = warm and fresh
B = warm and salty
C = cold and fresh
D = cold and salty

a. (0.5 pts) Which of these water masses is
most dense?

Answer:  Water mass D (which is cold and salty).  On the
graph, water mass D has density slightly greater than
1.029 grams per cubic centimeters.

b. (0.5 pts) Which two water masses are closest in
density? ________ and ________

Answer: Water masses B and C.  Note that the density of
B is about 1.026 grams per cubic centimeter and the
density of C is about 1.0265 grams per cubic centimeter.
The warm, fresh water (A) has much lower density
(about 1.021 grams per cubic centimeter), and the cold,
salty water (D) has higher density (about 1.029
grams per cubic centimeter).

c. (0.5 pts) How might water mass B form? Where
does this happen in the ocean today?

Water mass B is very salty and relatively warm.  This
water mass might form by evaporation of warm seawater.
As warm seawater evaporates, salt is left behind and
concentrated in the remaining liquid water.  This process
occurs today in the Mediterranean Sea.

2. Consider two water masses that form in the ocean today.
North Atlantic Deep Water (NADW) – forms by cooling (but not freezing) water at the surface of the
North Atlantic Ocean
Antarctic Bottom Water (AABW) – forms by freezing water at the surface of the oceans near the coast
of Antarctica

Sketch A - showed AABW covering the bottom of the Atlantic Ocean in the southern hemisphere and NADW covering the bottom of the Atlantic Ocean in the northern hemisphere.

Sketch B - showed NADW covering the bottom of the entire Atlantic Ocean, with the AABW sitting on top of the NADW.

Sketch C - showed AABW covering the bottom of the entire Atlantic Ocean, with the NADW sitting on top of the AABW.

a. (0.5 pts) Which of the sketches below best describes how these two water masses are distributed in the Atlantic Ocean?

Sketch C

b. (1.0 pts) Describe why you chose the answer that you did. (Use the back of this sheet for your answer).

AABW is very cold and very salty and forms when the ocean freezes, produce some sea ice, and leaving behind very cold, very salty liquid water.  NADW is cold because it forms when the atmosphere cools the ocean (but not enough to freeze it).  NADW is therefore less salty than AABW and AABW is more dense than NADW.   AABW will therefore sink below NADW, and the correct configuration in the diagram is with the heavier (more dense) AABW on the bottom of the Atlantic, and the less dense NADW sitting on top of it.