Historically the world was thought of having 4 oceans the Pacific, Atlantic, Indian and Arctic. Today we have five oceans (adding the Southern (around Antarctica)) covering over 71 percent of the earth’s surface and over 97 percent of the earth’s water. (National Geographic)
The term ‘Seven Seas’ has referred to bodies of water along trade routes, regional bodies of water, or exotic and far-away bodies of water.
In Greek literature (which is where the phrase entered Western literature), the Seven Seas were the Aegean, Adriatic, Mediterranean, Black, Red and Caspian seas, with the Persian Gulf thrown in as a “sea.”
In Medieval European literature, the phrase referred to the North Sea, Baltic, Atlantic, Mediterranean, Black, Red and Arabian seas.
As trade picked up across the Atlantic, the concept of the Seven Seas changed again. Mariners then referred to the Seven Seas as the Arctic, the Atlantic, the Indian, the Pacific, the Mediterranean, the Caribbean and the Gulf of Mexico.
Not many people use this phrase today, but you could say that the modern Seven Seas include the Arctic, North Atlantic, South Atlantic, North Pacific, South Pacific, Indian and Southern Oceans. (NOAA)
Oceans have currents, the continuous, predictable, directional movement of seawater driven by gravity, wind and water density. Ocean water moves in two directions: horizontally and vertically. Horizontal movements are referred to as currents, while vertical changes are called upwellings or downwellings. (National Geographic)
Ocean currents exist both on and below the surface. Some currents are local to specific areas, while others are global. And they move a lot of water. The largest current in the world, the Antarctic Circumpolar Current, is estimated to be 100 times larger than all the water flowing in all the world’s rivers.
A characteristic surface speed is about 2 to 20 inches per second. Currents generally diminish in intensity with increasing depth.
Vertical movements exhibit much lower speeds, amounting to only a few meters per month. As seawater is nearly incompressible, vertical movements are associated with regions of convergence and divergence in the horizontal flow patterns. (Britannica)
All of this moving water helps more stationary species get the food and nutrients they need. Instead of going looking for food, these creatures wait for the currents to bring a fresh supply to them.
Currents also play a major role in reproduction. The currents spread larvae and other reproductive cells. Without currents many of the ocean’s ecosystems would collapse. (Ocean Blue Project)
Ocean surface currents tend to form ring-like circulation systems called gyres. A gyre is a circular ocean current formed by a combination of the prevailing winds, the rotation of the Earth, and landmasses.
Continents interfere with the movement of both surface winds and currents. Gyres form in both the northern and southern hemispheres.
Gyres in the Northern Hemisphere travel in clockwise directions while gyres in the Southern Hemisphere travel in counter-clockwise directions. It takes about 54 months for water to travel the circuit of the North Pacific gyre, while only 14 months in the North Atlantic gyre.
In the Northern Hemisphere near the equator, trade winds drive currents westward, forming a North Equatorial Current (NE), which moves at about 1 m/sec. At the western boundary of an ocean basin, the water turns and flows towards the North Pole, forming the western-ocean boundary currents.
Western boundary currents are very strong. Two examples are the Gulf Stream (GS) that runs in the Atlantic ocean basin and the Kuroshio Current in the Pacific ocean basin.
They are narrower, but deeper and swifter, than the other currents in the gyre. For example, speeds of 2 m/sec have been measured in the Gulf Stream. These currents, as deep as 1 km, generally remain in deeper water beyond the continental shelf. Western-ocean boundary currents carry warm water from the equator north.
The broad, gentle pitch of the continental shelf gives way to the relatively steep continental slope. The more gradual transition to the abyssal plain is a sediment-filled region called the continental rise. The continental shelf, slope, and rise are collectively called the continental margin. (Britannica)
Eventually, the western boundary currents fall under the influence of the westerly winds and begin flowing to the east, forming the North Atlantic Current (NA) and North Pacific Current (NP).
When they approach the eastern-ocean boundaries of continents, they turn and flow south, forming the eastern-ocean boundary currents. Eastern-ocean boundary currents are shallower and slower than western-ocean boundary currents.
Current flow over the continental shelves (the edge of a continent that lies under the ocean), close to shore, carrying colder waters from the north to the south. Two examples are the California Current (Cal) in the Pacific Ocean basin and the Canary Current (Can) in the Atlantic Ocean basin.
One major current, the equatorial Countercurrent (EC), appears to be an exception to the circulation pattern set up by the gyres. This countercurrent forms just north of the equator in the region between the north equatorial current and the south equatorial current and flows in the opposite direction. (Hawaii-edu)