Currents

Currents are the motion in the ocean! While tides involve water moving up and down; currents are the back-and-forth movement of water. Two main components of currents are their speed and direction. A simple way to measure a current is to toss an object into the water and time how long it takes the object to travel a fixed distance. Technology allows us to be a little more accurate and sophisticated in our measurements. For example, the object in the water might be a buoy that is equipped with Global Positioning System technology and satellite communications to relay data and information about its change in position over time in the water.

Physics of Currents

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Water is constantly moving. The engines driving surface and sub surface currents throughout the world ocean are wind and water density. To understand the dynamics of the global system of ocean currents, you must understand the effect that wind, air and water temperature, salinity, and Earth's rotation have on ocean currents.

Wind is the flow of air between areas of high and low pressure. If Earth did not rotate, air in the atmosphere would basically circulate in a simple back-and-forth pattern between the poles (high pressure areas) and the equator (a low pressure area). The direction of surface currents would then align with this general wind pattern. But because Earth rotates, circulating air is deflected toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere, resulting in curved paths. This deflection is called the Coriolis Effect.

Global winds drag on the water’s surface. Just as Coriolis deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, it also results in the deflection of major surface ocean currents to the right in the Northern Hemisphere (in a clockwise spiral) and to the left in the Southern Hemisphere (in a counter-clockwise spiral). These major spirals of ocean-circling currents are called gyres and occur north and south of the equator.

One particularly powerful western boundary current is the Gulf Stream. The Gulf Stream, paired with the eastern boundary Canary Current, flanks the North Atlantic gyre. The Gulf Stream originates in the Gulf of Mexico, exits through the Strait of Florida, and follows the eastern coastline of the United States and Newfoundland. It influences the climate of the east coast of Florida, keeping temperatures warmer in the winter. Since it also extends toward Europe, it warms western European countries as well.

The location of modern-day currents exist because of the shape of the ocean basins. This has not always been the case. The long-term positions of currents have changed over millennia due to plate tectonics, climate, and periodic astronomical events such as asteroid impacts.

Winds drive ocean currents in the upper 100 meters of the ocean’s surface. However, ocean currents also flow thousands of meters below the surface. These deep-ocean currents are driven by differences in the water’s density, which is controlled by temperature (thermo) and salinity (haline). This process is known as thermohaline circulation.

In Earth's polar regions, ocean water gets very cold, forming sea ice. As a consequence the surrounding seawater gets saltier, This happens because salt is left behind when sea ice forms. As the seawater gets saltier, its density increases, and it starts to sink. Surface water is pulled in to replace the sinking water, which in turn eventually becomes cold and salty enough to sink. This initiates the deep-ocean currents driving the global conveyer belt.

The Global Conveyer Belt is the main avenue by which high temperatures are distributed around the globe moderating extremes and giving Earth more even air surface temperature.

Coastal Currents

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Waves do not typically reach the beach perfectly parallel to the shoreline. Rather, they arrive at a slight angle. This angled impact with the coastline sets up a current that moves parallel to the shore. This is called a long-shore current. Longshore currents, which are very powerful during storms, erode sediments off a beach and transport them in the direction of the current. This process is known as longshore drift. Longshore drift influences the shape and composition of the coastline. It can change the slopes of beaches and create long, narrow shoals of land called spits, that extend out from shore. Longshore drift may also create or destroy entire barrier islands.

Click here to view an animation of longshore currents.

Another type of coastal current, upwelling, forms when water below the surface rises to replace surface waters that have been transported out of an area by strong winds.

Click here to see an upwelling image.

Rivers can affect coastal currents when their sediment deposits build up and redirect water flows. Shifting deposits in major navigable waterways require constant monitoring and dredging to maintain safe passage for vessels and keep navigation channels open. As a result of coastal currents, people have created structures to divert currents and their capacity to erode shorelines. Jetties, sometimes hundreds of feet long made of wood, stone, or concrete extend into the ocean forming barriers to deflect longshore currents. These structures are not always effective in preventing longshore drift and may divert the currents to other shorelines, which are then more heavily impacted by increased erosion or deposition. Constantly shifting sand bars and deposits are an ongoing concern for marine traffic. Many a vessel has run aground and been shipwrecked on uncharted sand bars.

Open Ocean Currents

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Hydrodynamics is the study of water in motion. It explains how water flows and forms currents. The hydrodynamics of the ocean are very complex with thousands of miles of open ocean that can be thousands of feet of depth, however, there are a few basic patterns which can be easily explained.

Open ocean currents are minimally influenced by tides. They flow in complex patterns affected by wind, the water’s salinity temperature, and Earth's rotation. In essence, open ocean currents are the steady flow of surface ocean water in a prevailing direction. Think of them as streams of water moving within the larger ocean body of water, either along the surface, or at depth. Though open ocean currents have no rigid borders (other than continents and other geographic features), their locations can be definite and have occurred continuously for thousands of years. Ocean currents are so large that they are measured in Sverdrup (Sv), where 1Sv is equivalent to a volume flow rate of 1,000,000 m3 (35,000,000 cu ft) per second.

Without open ocean currents, heat would build up in the tropics, polar regions would be colder, and the climate in every corner of the planet would be very different. For example, without the warming influence of the Gulf Stream, the United Kingdom and parts of Northern Europe would be colder and snowier like other countries at that latitude. Ocean currents redistribute heat around the planet, replenish nutrients in surface waters from upwelling of deeper waters, and provide transportation routes for ships.

Currents: Measurements and Data

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The existence of ocean currents provided early navigators like James Cook the ability to explore new worlds. Understanding the ocean's currents created an explosion in travel and trade to distant lands. Instead of overland travel from Europe to Asia and Africa, travel times were reduced exponentially by utilizing ocean routes and developing instrumentation to increase accuracy of determining position.

Today, drifting buoys equipped with multiple oceanographic instruments provide information on open ocean currents. Most are equipped with GPS and satellite communications technology to relay their positions to observers on land.

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