What is the Difference Between a Sea and an Ocean?

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The majority of the Earth’s water is stored in its oceans, seas, and bays which make up 96.5%  of the 1.36 billion tons of water.  

While some use the terms ocean and sea interchangeably, there is a difference in the geographic definitions of those two terms.

Tabular icebergs in the Southern Ocean. Photo: Mike Goebel, NOAA NMFS SWFSC Antarctic Marine Living Resources (AMLR) Program, 1992, public domain.
Tabular icebergs in the Southern Ocean. Photo: Mike Goebel, NOAA NMFS SWFSC Antarctic Marine Living Resources (AMLR) Program, 1992, public domain.

An ocean represents a far larger body of open water than a sea.  

By definition, a sea is a smaller part of an ocean and is typically partially contained by an area of land.  

Therefore, all seas are found in areas where the ocean and land meet. Seas are typically partially enclosed by land.

The map below shows the location of the Chukchi Sea, Bering Sea, and the Sea of Okhotsk in the northern Pacific Ocean. All three seas are partially enclosed by land masses.

Map showing the location of various seas in the northern Pacific Ocean. Map: NOAA, public domain
Map showing the location of various seas in the northern Pacific Ocean. Map: NOAA, public domain

In this example below, the Indian Ocean is shown as the open body of water.  The two areas of water that are partially enclosed by land are named the Red Sea and the Oman Sea.

Map showing the Indian Ocean, Oman Sea, and the Red Sea. Map: Epmistes, MediaWiki Commons.
Map showing the Indian Ocean, the Oman Sea, and the Red Sea. Map: Epmistes, MediaWiki Commons.

The Sargasso Sea is the Only Sea Without a Land Border

One exception to this definition of a sea is the Sargasso Sea. The Sargasso Sea is a sea within the open ocean.

The Sargasso Sea is the only sea in the world without a land boundary.

Map showing the location of the Sargasso Sea. Image: MediaWiki Commons
Map showing the location of the Sargasso Sea with arrows showing the direction of the currents. Image: MediaWiki Commons, public domain

What makes the Sargasso Sea a sea are the ocean currents. Located off the eastern coast of the United States, the sea is found within the  the Northern Atlantic Subtropical Gyre. The sea is bounded to the north by the North Atlantic Current, to the east by the Canary Current, and to the south by the North Atlantic Equatorial Current.

The Sargasso Sea is named after a free-floating seaweed called Sargassum. This type of seaweed is ‘holopelagi’ which means it reproduces out in the open waters instead of reproducing on the sea floor.

The Sargasso Sea is a spawning site for threatened and endangered eels, as well as white marlin, porbeagle shark, and dolphinfish

What are the Seven Seas?

The Sevens Seas is a historic term naming the dominant trade routes and regional bodies of waters.  The definition of what those Seven Seas are has changed over time.  The term is believed to have first appeared in 2,300 BCE Hymn 8 of the Sumerian Enheduanna to the goddess Inanna (Meador, 2001).  The ancient Greeks named the seven seas as Aegean, Adriatic, Mediterranean, Black, Red, and Caspian seas, and the Persian Gulf.

While no longer a common phrase, the modern day Seven Seas are the Arctic, North Atlantic, South Atlantic, North Pacific, South Pacific, Indian, and Southern Oceans.

References

Meador, Betty De Shong, ed. (2001). Inanna, Lady of Largest Heart: Poems of the Sumerian High. Translated by Betty De Shong Meador. University of Texas. ISBN 0-292-75242-3.

What are the seven seas? NOAA

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