Ocean circulations are very complex and can travel in different directions depending on how deep you look in the ocean, but the main source that drives all ocean circulations is the energy from the sun. Two important by-products of the sunās energy that affect circulation are wind and ocean density.
How does wind affect ocean circulation?
|Figure A: Ocean Currents|
Surface ocean currents are primarily affected by wind patterns. Trade winds can push water along the top of the ocean and aid in the formation of surface currents. One example of a wind-driven circulation affecting an ocean current is the Gulf Stream. The Gulf Stream takes very warm water from the Gulf of Mexico and parts of the Caribbean Sea and transports it northward. During the winter, the Gulf Stream can have a great effect on storm systems along the East Coast of the United States. For example, norāeasters can strengthen and grow over the Gulf Stream and bring heavy snow or rain, strong winds, and damaging beach erosion to the East Coast. It used to be thought that the flow of the Gulf Stream alone helped keep Europe warm in winter, but it has been recently shown that the temperate climate of Great Britain comes from warm air moving along with the Gulf Stream over the Atlantic Ocean. This keeps temperatures along the western coast of Europe milder than continental areas farther east away from the coast.
|Figure B: The Flow of the Gulf Stream|
|Image from the NC State Climate Office|
How does ocean density affect ocean circulation?
Deep ocean currents are primarily driven by ocean density. In order to understand how density can affect ocean circulation, we need to understand what density is. Density for salt water depends on two things: temperature and salinity (how salty the water is). A more common name for this kind of circulation is known as a thermohaline circulation. In Greek, āthermosā stands for heat, while āhalineā means salinity (the amount of salt in water). Although warmer ocean water near the surface of the ocean is less dense and contains less salt than colder, deeper ocean water, warm water actually is capable of holding more salt than cold water.
So why is the deeper, colder water saltier than the warmer, surface-based seawater? One answer: It is due to freshwater runoff. At the oceanās surface, fresh water from rivers or precipitation falling from the sky mixes with the ocean water and dilutes it. As a result, the water at the ocean surface is less salty than the water below it. Another answer: formation of ice at high latitudes. In the northern Atlantic and Pacific Oceans, when the air gets cold enough for ice to form on the surface of the ocean, the dissolved salt is excluded from the ice that forms, leading to cold and extra salty water forming at these latitudes as the fresh water becomes ice. This very dense water sinks to deep levels in the ocean and spreads out from there. The bottom topography of the ocean and the arrangement of the continents helps steer this deep salty water around the globe.
From this, we can get underwater currents that can be thousands of feet below the surface of the ocean. In a way, the thermohaline circulation can work together with wind-driven circulations and create a conveyer belt that circulates all around the world. Thus, the combined thermohaline circulation and wind-driven circulation has also been dubbed the āGreat Ocean Conveyor Belt'.
|Figure C: The Conveyor Belt|
|Image from NOAA|