The above image shows major surface ocean currents around the world. Note that despite geographical differences, some currents in each ocean in each hemisphere follow the same general pattern, flowing east to west in the tropical latitudes, toward the poles on the western edge of ocean basins, west to east at mid-latitudes, and finally toward the equator on the eastern edges. These circular currents are known as subtropical gyres. For example, the Gulf Stream is the western poleward current in the north Atlantic subtropical gyre and the California current the eastern current toward the equator in the north Pacific tropical gyre. These exist largely due to the Earth's prevailing winds.
The prevailing winds at the Earth's surface fit into a larger dynamic atmospheric pattern. The greater heating of the tropics as compared to the polar regions and the rotation of the Earth lead to the formation of three atmospheric cells in each hemisphere. The winds in these cells rotate due to the Coriolis effect (in essence the fact that straight paths appear to curve from the viewpoint of an observer on a rotating planet), producing east to west winds in the tropics and polar regions, and west to east winds in the mid-latitudes. Look back at the subtropical gyres on the map of currents. Notice that the currents labeled "equatorial" follow the trade winds, and the north Pacific, north Atlantic, etc. currents follow the prevailing westerlies. The west and east boundary currents then "complete the circle" and close the flow. This is no coincidence. It is friction between air and water that drives subtropical gyres: the force of wind tends to make water flow in the same direction. Another important example is the Antarctic circumpolar current, the largest in the world. Since there are no landmasses between roughly 50°S and 60°S latitude, the westerlies drive a current unimpeded that stretches all the way around the continent of Antarctica. Many other currents in the global diagram are responses to these main gyres, or are connected to prevailing winds in more complicated ways.
This system of ocean currents has profound impacts on weather and climate. Due to the Gulf Stream, north Atlantic current, and its northern extension, the Norwegian current, temperatures in northwestern Europe are several degrees warmer than they would otherwise be.
The above image illustrates one of the influences of ocean currents on weather. It shows all tropical cyclone tracks (hurricanes, typhoons, etc.) worldwide from 1985-2005. Since tropical cyclones need warm ocean surface waters to develop, the cold California current helps to suppress eastern Pacific hurricanes north of 25° N or so. In contrast, the warm Kuroshio current in the western Pacific allows typhoons to regularly affect Japan, which is at a higher latitude. Note also the presence of cyclones in the southwest Pacific and the lack of any formation in the southeast Pacific (though very cold surface waters are only one of several factors in this).
Despite their vast impact, which goes well beyond the examples listed, all of the currents considered so far are surface currents. Typically, these currents exist only in the top kilometer of the ocean, and the picture below this can look quite different.
The above image gives a very schematic illustration of the global three-dimensional circulation of the oceans, known as the thermohaline circulation. The first basic fact about this circulation, especially the deep ocean circulation, is that it is slow. Narrow, swift surface currents such as the Gulf Stream have speeds up to 250 cm/s. Even the slower eastern boundary currents often manage 10 cm/s. In contrast, deep ocean currents seldom exceed 1 cm/s. Their tiny speed and remoteness makes them extremely difficult to measure; in fact, rather than directly charting their course, the flow is inferred from quantities called "tracers" in water samples. Measurements of the proportion of certain radioactive isotopes, for example, are used to calculate the last time a given water sample "made contact" with the atmosphere.
The above graphic illustrates the age of deep ocean water around the world. The age (in years) is how long it has been since a given water parcel came to equilibrium with the surface. Note that the thermohaline circulation occurs on timescales of over 1000 years. This information indicates that deep water formation (when water from the surface sinks) takes place in the North Atlantic but not the North Pacific, as indicated in the first graphic. This is because all the deep waters of the Pacific are quite "old". Deep water formation also occurs in the Southern Ocean, near Antarctica. In both cases, the mechanism is similar: exposure to frigid air near the poles makes the surface waters very cold, and therefore dense. Further, in winter, sea ice forms in these cold waters, leaving saltier water behind (since freshwater was "taken away" to form sea ice). This salty, cold water is denser than the ocean around it and it sinks. The newly formed deep water can flow near the bottom of the ocean for hundreds of years before coming back to the surface.
One climatological influence of this phenomenon is the ocean's increased ability to take up carbon dioxide. Most of the carbon dioxide emitted by human industry since the late 1800s has dissolved in the oceans. Since deep water takes so long to circulate, increased CO2 levels are only now beginning to penetrate the deep ocean. Most ocean water has not "seen" the anthropogenic CO2 so it will continue to take up more of the gas for hundreds of years. Without this, there would much more carbon dioxide in the atmosphere, and likely faster global warming.
The network of mechanisms driving ocean currents and the thermohaline circulation is quite intricate, and we have only touched on some of them here: weather systems, prevailing winds, differences in density, etc. There are many more subtleties as to why the ocean circulates the way it does. The study of these nuances is essential for fully understanding the Earth's weather and climate.
Sources: Atmosphere, Ocean, and Climate Dynamics: An Introductory Text by John Marshall and R. Alan Plumb, https://www.britannica.com/science/ocean-current, http://www.seos-project.eu/modules/oceancurrents/oceancurrents-c01-p03.html
