What is the Difference Between a Sea and an Ocean?

Caitlin Dempsey

Updated:

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.  Geographically, the oceans cover the majority of the Earth. About 71% of the Earth’s surface is covered by oceans.

The world’s oceans supply at least half of the world’s oxygen and store about 50 times more carbon than the atmosphere.

Oceans also influence the Earth’s climate through a constant transfer of heat from the Equator towards the poles. Evaporation from the ocean’s surface brings rain to much of the Earth’s land surfaces.

What is the Definition of Ocean and Sea?

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


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Shaded relief map of the world with just the five major oceans labeled.
The five major oceans of the world. Patterson map projection. Map: Caitlin Dempsey.

Size of oceans versus seas

An ocean represents a far larger body of open water than a sea.  Oceans are vast bodies of water that cover over 70% of the Earth’s surface, with an average depth of 3,800 meters.

By definition, a sea is a smaller part of an ocean and is typically partially contained by an area of land.  Almost all seas are found in areas where the ocean and land meet.

Only sea in the world without a land boundary

One exception to this definition is the Sargasso Sea. The boundaries of the Sargasso Sea are defined by ocean currents, rather than physical features such as coastlines or islands.

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

Location map showing where the Sargasso Sea is located. The base map is a tinted topography map showing the surrounding continents and a blue ocean.
Map showing the general location of the Sargasso Sea. Map: Caitlin Dempsey using Natural Earth data.

If the Sargasso Sea has no land boundaries, how is it a sea?

The Sagasso Sea is bounded on the east by the Gulf Stream, on the north by the North Atlantic Current, on the west by the North Atlantic Subtropical Gyre, and on the south by the Canary Current.

These currents create a circular pattern of water circulation, which traps water in the center of the Sargasso Sea and prevents it from mixing with the surrounding waters.

What is the Sargasso Sea named after?

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

Oceans are deeper than seas

Oceans are also the deepest water bodies on the planet. For instance, the Pacific Ocean is the largest and deepest ocean in the world, with an average depth of 4,028 meters and a total area of 60 million square miles.

A photograph of a view from space showing the Earth's limb, or horizon, from the International Space Station as it orbits above the Pacific Ocean off the coast of Chile. Photo: NASA
A view of the Pacific Ocean off the coast of Chile. Photo: NASA astronaut Mike Hopkins, iss064e006316, public domain.

In contrast, the Mediterranean Sea, which is one of the largest seas globally, has an average depth of 1,500 meters and covers an area of approximately 2.5 million square kilometers.

What is the difference between a sea, a bay, and a gulf?

Bays and gulfs are part of seas and oceans. Bays, gulfs, and seas are all marginal water bodies. This means that a part of the water feature is bounded by a land mass or another barrier.

Shaded relief map of the Florida and Central America area showing gulfs and seas.
A sea is an area of open water that is partially bounded by a land barrier while a gulf is an area of water indented into a land mass. Map: Caitlin Dempsey.

While there are many exception to these definitions, bays and gulfs can be generally defined as follows:

Gulfs are bodies of water that are formed by a large inlets that intrudes into a landmass.

Bays are similar to gulfs but their intrusion into a landmass is not as narrow. Bays are usually smaller and less enclosed than gulfs.

Examples of Ocean and Seas on Maps

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.

Shaded relief map showing the Indian Ocean and nearby seas.
Map of the Indian Ocean. Map: Caitlin Dempsey.

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.

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.

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

This article was originally written on August 23, 2020 and has since been updated.

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.