The terms ocean and sea are often used interchangeably. In geography however they are two different terms.
The ocean is one huge body of interconnected water, covering 71% of the earth’s surface. For the purpose of convenience the World Ocean or the Global Ocean is loosely categorised into five ocean bodies. They are The Pacific Ocean, the Atlantic Ocean, The Indian Ocean, the Southern or Antarctic Ocean and The Arctic Ocean. The Southern Ocean is basically the meeting point of all the other oceans. It surrounds the frozen landmass called Antarctica
The ocean holds 97% of the planet’s water. The water is saline and but is the principal source of evaporation that fuels the water cycle. Precipitation in the water cycle brings rain, hail and snow and other forms of fresh water to sustain life on the planet.
There are about 55 smaller bodies of water called seas spread out across the globe. Seas occur where the oceans meet the land. They are saline and partially enclosed by land. A person standing on the East Coast of USA faces the Atlantic Ocean as no land masses encloses the water body. On the Western Coast you can paddle in the Pacific Ocean. In Alaska, at the extreme Northern end of the Western Coast, you look out onto the Bering Sea as the ocean is enclosed here by the eastern extremity of the Asian land mass.
In India the Arabian Sea borders the Western Coast and the Bay of Bengal the East Coast. The two seas meet at the southern tip of the peninsular which looks onto the Indian Ocean.
The world’s largest ocean is the Pacific Ocean. It covers 64,186,000 square miles. The Mediterranean Sea is the largest sea. It covers 1,144,800 square miles. Despite its vast size, the Mediterranean is tiny compared to even the smallest ocean of the world, the Arctic Ocean. The Arctic Ocean occupies 5,427,000 square miles.
Both oceans and seas are defines by their salinity. A sea is partially enclosed by land and an ocean is a free moving body of water. By contrast a lake is a fresh water body enclosed by land.
There are some water bodies that lie in the category between a sea and a lake. The Caspian Sea is the world’s largest inland sea. It is an endorheic basin. This means that it is a body of salt water enclosed by land. The Caspian Sea is sometimes classified as a lake but its salinity gave it the name ‘Sea’. It is an elongated body of water that lies between Europe and Asia. It stretches 750 miles from north to south but the width never exceeds 200 miles.
The Dead Sea is also a hyper saline enclosed body of water. The salinity is caused by the super- fast rate of evaporation. There are many such saline lakes in the world. They occur when a fresh water river empties itself into an enclosed water body. The Aral Sea also falls in the category of saline lakes.
Depth is not a factor that indicates the difference between the ocean and a sea. The deepest sea the Caribbean Sea is 22,788 feet at its deepest point. The depth of the world’s deepest ocean, the Pacific ranges from 3,953 feet to 15,215 feet.
The seas being close to the land have a rich and diverse marine life. This abundance of living things is not found in the deeper oceans that support only basic life forms such as bacteria, microscopic plankton and shrimp.
Water that is not close to the bottom or touching a coast line is said to be in a pelagic zone. This zoning includes oceans, seas and lakes. The pelagic zone has various layers depending on the amount of sunlight that penetrates to the layer. The common names for the zones are the sunlit, twilight, midnight and lower midnight zones. Very little is known about the zone below the lower midnight zone.
The ocean is a driving force for weather and climate. Ocean currents transfer warm or cold air to different coastal regions. This impacts the climate on land.