You may have noticed an “extra” ocean on some maps in 2021. This brings the number of oceans in the world to five: Pacific, Atlantic, Indian, Arctic and Southern.
The idea of sailing the “seven seas” has its roots in ancient trade routes, according to NOAA, but the bodies of water that were found in this set of seven varied by time and location. [I think it’s the number seven that people like, and in English, “seven seas” has a nice alliteration.]
Today, “seas” are not the same as “oceans”. By geographic definition, “seas” are smaller and more tightly bounded by land than oceans; most seas are subsets on the margins of oceans (like the Bering Sea in the North Pacific which is nestled between Alaska and Russia). There are also other divisions of oceans: for example, a “gulf” is a bay of an ocean (like the Gulf of Alaska which is nestled in the southeast curve of Alaska). Wikipedia lists more than 180 “seas” in the world, as well as many other gulfs, straits, saltwater bays, etc.
Essentially, oceans are bounded by their currents as much as they are by landmasses. The “newest” ocean, the Southern Ocean, surrounds Antarctica, with the continent in the middle and the Pacific, Atlantic and Indian oceans bordering the exterior of this current system.
The major currents in the northern and southern portions of the oceans that cross the equator rotate in opposite directions: on a large scale, the North Pacific and Atlantic oceans both rotate clockwise and the South Pacific oceans and Atlantic both rotate counter-clockwise.
The smaller northern part of the Indian Ocean also crosses the equator, although the Indian subcontinent interrupts the circular pattern in this section. So while the unhindered southern part of the Indian Ocean spins counter-clockwise like its neighbors, the northern part is a bit “swirled”.
Branches of the northern arcs of the Pacific and Atlantic Oceans feed into – and from – the Arctic Ocean, resulting in a complex current pattern in this northernmost ocean, dominated by the Atlantic side.
The Southern Ocean flows left to right around this large-scale continent, viewed from above with the South Pole below.
The oceans contain smaller systems. Driven by relative changes in temperature, density, bottom topography, or local winds, other currents break away, combine, plunge or overflow, grow or weaken within the larger framework of major ocean circulation patterns. .
At the largest scale, there is really only one planet-scale ocean, because all ocean waters are connected.
In 1992, a container of 28,800 small floating bath toys went overboard and opened up in the North Pacific Ocean. Later finds of the toys are documented and mapped by a small team led by Seattle oceanographer Curtis Ebbesmeyer. Several months later, yellow ducks began washing up on North and South Pacific beaches. About ten years after the spill, a few ducks began to appear on beaches on the east coast of the United States and in the United Kingdom. They got there by traveling thousands of miles across the North Pacific Ocean, the Arctic Ocean and the North Atlantic Ocean. Since the ducks float, they probably got stuck in arctic ice several times along the way.
(This unfortunate accident also provided an opportunity to study how the shapes of things can affect the movement of water and wind, as there were originally four different shapes of lost toys: yellow ducks, red beavers , blue turtles and green frogs. It turns out that the yellow ducks travel the farthest. The different colors fade differently over time, as the ducks and beavers have lost their colors while the turtles and frogs have kept theirs.)
With the distribution of the current direction between the North/South Pacific Ocean and the North/South Atlantic Ocean, the inclusion of the Arctic and Indian Oceans and the addition of the Southern Ocean, one could say that there is has, in effect, seven oceans that come together to become one.