How Do Oceans Work?

The oceans cover more than 70 percent of Earth's surface, yet they remain among the least understood parts of our planet. Oceanography is the study of the oceans, and it's a field that is growing in importance as we learn more about the role oceans play in our world.Oceans are important for many reasons. They help regulate Earth's climate by absorbing heat from the sun and releasing it back into the atmosphere. They also provide a habitat for many different types of marine life, including fish and other organisms that are important to humans. Oceans are also important sources of food and minerals, and they're a source of natural beauty.


In addition, oceans provide transportation routes for goods and people, as well as recreation opportunities.


In this article, we'll look at some interesting facts about oceans, including how they compare to lakes in terms of size and how much water there is on Earth. We'll also examine how ocean currents work and what causes tides to occur. Let's start by looking at how big oceans really are compared to lakes.­­­­­­­­­­


The Pacific Ocean covers an area of about 64 million square miles (168 million square kilometers), which is roughly equal to the combined areas of North America and South America [source: National Geographic]. The Atlantic Ocean covers about 48 million square miles (124 million square kilometers). The Indian Ocean covers about 30 million square miles (77 million square kilometers). The Arctic Ocean covers about 5 million square miles (13 million square kilometers). The Southern Ocean covers about 4 million square miles (10 million square kilometers).


The largest lake in the world is Lake Superior in North America with an area of 31,700 square miles (82,100 square kilometers). The largest lake in Africa is Lake Victoria with an area of 26,800 square miles (69,900 square kilometers). Lake Baikal in Russia holds roughly 20 percent of all the fresh water on Earth [source: National Geographic]. In comparison with these lakes, the ocean basins are enormous!





The average depth of all five oceans combined is 12,000 feet (3,658 meters) [source: National Geographic]. That means that if you put all five oceans together into one giant basin without any landmasses or islands in it -- just ocean water -- it would be 12,000 feet deep on average. If you put all five oceans together into one giant basin with Mount Everest sticking up out of it -- again without any landmasses or islands -- it would be 656 feet deep on average [source: National Geographic].


You can see from these numbers that most of Earth's surface is covered by water rather than landmasses like mountains or islands like Hawaii or Madagascar. In fact most people don't realize just how much water there is on Earth because most people live near landmasses rather than near open ocean areas. Even though there are only a few places where you can stand at sea level and look straight out at sea without seeing any landmasses or islands nearby -- such as Hawaii or parts of Australia -- most people have never seen what this looks like because they've never been to these places!


Now let's take a look at how ocean currents work next.


An ocean current is a continuous flow of water in one direction across Earth's surface (in contrast to wind currents). Ocean currents are caused by differences in temperature between different parts of Earth's surface as well as differences in salinity between different parts of Earth's surface (see below). These differences cause warm waters to move toward colder areas and salty waters to move toward fresher areas -- much like a river flows toward lower elevations or a stream flows toward lower elevations through valleys [source: NOAA].Ocean currents occur on three different scales: large-scale global circulation patterns (such as thermohaline circulation), mesoscale eddies (which are small-scale swirling motions) and local coastal currents [source: NOAA].


Large-scale global circulation patterns include thermohaline circulation (which moves warm waters from tropical regions to polar regions), western boundary currents (which move warm waters from tropical regions to higher latitudes) and eastern boundary currents (which move cold waters from polar regions to higher latitudes). Mesoscale eddies include features such as El Niño/La Niña events that occur every few years off South America's coast [source: NOAA]. Local coastal currents include features such as tidal bores along coasts where the tide is strong, and rip currents along coasts where the tide is weak [source: NOAA].


Ocean currents are important to the ocean's ecosystem because they are responsible for transporting heat, nutrients and dissolved gases (such as oxygen) across the globe. They also help to mix the ocean's layers and move nutrients from deep waters to surface waters. Ocean currents are also important to humans because they help to create favorable conditions for fishing and shipping. In addition, they play a role in global climate patterns by transporting heat from one part of Earth's surface to another. For example, warm waters carried by the Gulf Stream from the Gulf of Mexico up toward Europe help keep that continent warmer than it would be otherwise [source: NOAA].­­­­­­­­


Read on to learn about how salinity affects ocean currents.


Salinity is defined as the amount of dissolved salt in water. In general, saltier water is denser than fresher water -- so saltier waters sink below fresher waters [source: NOAA]. The density of seawater depends on its salinity and temperature (see diagram below). Because warmer water is less dense than colder water, warm water tends to float above cold water. Because saltier water is denser than fresher water, salty water tends to sink below fresher water. These two factors combine to create a vertical circulation pattern in which cold, salty waters sink at Earth's poles and warm, less-salty waters rise at Earth's equator [source: NOAA].


This circulation pattern helps drive global ocean currents.In addition to affecting global ocean circulation patterns, salinity also affects local coastal currents along Earth's coastlines. For example, along coastlines where there is a lot of freshwater runoff from rivers or melting snow (such as Alaska), there will be less salinity in coastal waters than there would be otherwise -- so coastal waters will be less dense and won't sink as easily as they would otherwise [source: NOAA]. This means that coastal currents will be weaker than they would otherwise be -- which can have a big impact on local ecosystems.





The density of seawater depends on its salinity and temperature. Because warmer water is less dense than colder water, warm water tends to float above cold water. Because saltier water is denser than fresher water, salty water tends to sink below fresher water. These two factors combine to create a vertical circulation pattern in which cold, salty waters sink at Earth's poles and warm, less-salty waters rise at Earth's equator [source: NOAA]. This circulation pattern helps drive global ocean currents.In addition to affecting global ocean circulation patterns, salinity also affects local coastal currents along Earth's coastlines. For example, along coastlines where there is a lot of freshwater runoff from rivers or melting snow (such as Alaska), there will be less salinity in coastal waters than there would be otherwise -- so coastal waters will be less dense and won't sink as easily as they would otherwise [source: NOAA].


This means that coastal currents will be weaker than they would otherwise be -- which can have a big impact on local ecosystems.

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