Density and Ocean Circulation

Temperature and salinity control seawater's density. The weight of seawater determines whether it sinks or floats, helping to drive deep ocean circulation. At the sea surface, wind and waves move seawater — and the heat it carries — around our globe.
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Understanding salinity helps us understand global patterns, including the circulation of water in the deep ocean. Satellite salinity measurements are complementary to existing programs that monitor sea surface temperature because, together, salinity and temperature control density at the ocean surface. The resources on this page illuminate the connection between density and the ocean's motion, and the consequences on our climate.

Water Density Experiments
The density of water is controlled by its temperature and salt content.
Hear from a Scientist
Dr. Stephen Riser tells us why measuring ocean salinity matters.
Density-Driven Circulation
Density makes the ocean go 'round.
The Ocean in Motion
Watch density-driven circulation in action in this computer simulation.
Layers of Water
Explore how layers of water are formed from water of differing densities.
Hear from a Scientist
Dr. Susan Lozier gives us the big picture of ocean circulation.
Surface and Deep Currents
In addition to deep circulation, the ocean is moved by winds and currents.
Convection and Heat
Learn how the ocean transports heat.
The Ocean Moderates Temperature
Moving heat around the world makes our planet habitable and comfortable.
The Importance of Salinity
The ocean has major impacts on weather and climate, affecting us miles from the shore.
Go With the Flow
This game tests your knowledge of convection, density, sinking and floating.

FAQs

Salinity changes a lot as you go deeper into the ocean. The satellite can only "see" the ocean surface, a very thin layer around a few centimeters.
Yi Chao, Aquarius Project Scientist, NASA Jet Propulsion Laboratory in Inspire Chat with Yi Chao, 2010 [Host: Jim Gerard]
Studies indicate that this could be the case. A group of scientists have looked at data from Aquarius and SMOS taken over the Amazon river plume in the western tropical Atlantic during the time of hurricane Katia.
Simon Yueh, Aquarius/SMAP Project Scientist, NASA Jet Propulsion Laboratory, 2014 [Interviewer: Chris Howell]

I think with the information we are now learning from Aquarius, we are learning to go back and re-assess, to re-evaluate the role of vertical mixing and horizontal advection in the ocean dynamics. So, even for a region we think is dominated by evaporation, or for some reason, dominated by precipitation, now the general picture is we can only grasp these as hypotheses. Horizontal advection, vertical mixing, evaporation, precipitation, all these play an important role at different times, at different scales and resolutions.

For example, another very interesting science phenomena discovered by Aquarius is what we call the tropical instability wave across the Pacific and the Atlantic. This is a new discovery by Aquarius. That kind of information has been seen in the past in the ocean surface temperature images a few degrees north of the equator. But Aquarius is providing a new picture of information that, these kinds of waves also exists near the equator, but can only be seen in the salinity data set.

The other example is the river outflow. Standard thinking has been that the fresh river outflow from the Mississippi River would only affect the coastal region near Texas, Mississippi and Louisiana. The Aquarius data has now shown that the fresh water on the surface can reach all the way to the southern end of the Gulf of Mexico. In the winter, when there's not much precipitation in the region, then in the summer when there is precipitation, we can see that salinity cycle in the upper ocean through the Aquarius data.

Another example I'd like to tell you is pretty interesting, it's something that we've just started to understand. It's a combination of how useful it is to combine the whole motion image salinity together. One of the interesting regions for the oceanographer or the climate scientist is the ocean surrounding the Indian Subcontinent. Why is that region so important? They have a monsoon season, right? In September, they get a lot of rain. Then, in the Indian sub-continent, around Tibet, they also get lots of snow. So the heavy precipitation and river run-off dump vast quantities of fresh water in the Bay of Bengal. We also have heavy precipitation over the open ocean in the same region. When we start to look at the soil moisture changes over the continent, and the salinity changes in the ocean, we can see their time correlation. Then, we can see when the fresh water will actually reach the southern point in the Bay of Bengal. But there's something else, more than just the fresh water coming into the Bay of Bengal. There's also a current in the Bay of Bengal, and that current can change direction in winter and summer. So, we can see how everything plays together, with fresh water coming out of the Bay of Bengal, and then the ocean current actually moving the fresh water around.

Simon Yueh, Aquarius/SMAP Project Scientist, NASA Jet Propulsion Laboratory, 2014 [Interviewer: Chris Howell]