Chapter 10 Ocean Currents and Gyres

The Ocean's Surface Circulation

• Ocean currents are generated by the transfer of energy from wind to water.
• Major surface currents are closely related to global patterns of prevailing winds.
• The distribution of landmasses and the Coriolis Effect also influence surface currents.
• Large, circular-moving surface currents (gyres) are located within each ocean basin.
• There are 5 main gyres: North Pacific, South Pacific, North Atlantic, South Atlantic, and Indian Ocean.
• The center of each gyre is located at approximately 30° north or south latitude, referred to as subtropical gyres.

Coriolis Effect

• Due to Earth's eastward rotation, free-flowing objects are deflected to the right of their apparent direction in the Northern Hemisphere and to the left in the Southern Hemisphere.
• This deflection is termed the Coriolis Effect.

Ocean Gyres

• North Pacific Gyre: consists of North Equatorial Current, Kuroshio Current, North Pacific Current, and California Current; takes approximately 6 years to complete.
• North Atlantic Gyre: consists of North Equatorial Current, Gulf Stream, North Atlantic Current, and Canary Current; takes approximately 3 years to complete.
• Large gyres develop a central area with no well-defined currents, such as the Sargasso Sea in the North Atlantic.
• South Pacific Gyre: consists of South Equatorial Current, East Australian Current, West Wind Drift, and Peru Current.
• South Atlantic Gyre: consists of South Equatorial Current, Brazil Current, West Wind Drift, and Benguela Current.
• Indian Ocean Gyre: consists of South Equatorial Current, Agulhas Current, West Wind Drift, and West Australian Current; influenced by monsoons.

Surface Currents and Climate

• Surface currents have a pronounced influence on climate.
• Ocean water accounts for ¼ of the total heat transport, while winds account for the remaining ¾.
• Poleward-moving warm currents warm the regions they flow near, such as the Gulf Stream and East Australian Current.
• Equatorial-moving cool currents cool the regions they flow near, such as the Canary Current and Peru Current.
• The California Current lowers temperatures in Southern California by nearly 11° F compared to the same latitudes on the US East Coast.

Upwelling and Deep-Ocean Circulation

• Surface currents generate vertical movements of water (upwelling and downwelling).
• Upwelling is the rising of cold water from deeper water to replace warmer surface water.
• Coastal upwelling occurs in regions when winds blow parallel to the coast and toward the equator.
• Upwelling brings higher concentrations of dissolved nutrients to the surface, promoting primary productivity.
• Deep-ocean circulation is responsible for the mixing of seawater, with a significant vertical component due to density differences.
• Thermohaline circulation begins in high-latitude areas where cold seawater freezes into sea ice, increasing the salinity and density of the surrounding water.

The Shoreline: A Dynamic Interface

• Shorelines are dynamic locations that vary in topography, geology, and climate.
• Shorelines are the transition zone of sediment deposition between the marine and continental environments.
• The shore is the interface of the air, land, and sea.
• Wave activity continually changes the shoreline, with some changes occurring rapidly and others slowly.
• Human activities, such as overdevelopment and unsustainable resource use, can also impact shorelines.

Ocean Waves

• Ocean waves are energy traveling along the interface between seawater and the atmosphere.
• Wave characteristics include wave height, wavelength, and wave period.
• The height, length, and period of a wave are dependent on wind speed, duration, and fetch.
• As waves approach the shore, the water becomes shallower, and the waves begin to "feel" the bottom.
• Waves eventually break, creating turbulent water (surf) and a turbulent sheet of water moving up the beach slope (swash).

Beaches and Shoreline Processes

• Beaches are accumulations of sediment along the landward edge of an ocean, marginal sea, or lake.
• Beaches are composed of material that is locally available.
• Beach material can come from erosion of beach cliffs or coastal mountains, or from rivers.
• Some beaches have a large biological component, such as shells and coral.
• Beaches are constantly moving along the shoreline due to wave activity.
• Wave erosion can be caused by pressure, wave impact forces, and abrasion.
• Breaking waves can cause sediment to move toward and away from the shoreline.
• Wave refraction (the slowing and bending of waves in shallow water) plays an important role in the distribution of wave energy along the shore.Here are the study notes for the text:

Beaches and Shoreline Processes

• Waves reaching the shore at an angle generate a swash that is not perpendicular to the beach face, resulting in a zigzag pattern of sediment movement along the beach face, known as beach drift.
• Non-perpendicular waves also generate a longshore current that flows parallel to the shore, which moves more sediment than the beach drift.
• The direction of longshore currents can vary daily and seasonally with the direction of the approaching waves.

Shoreline Features

• Shorelines vary in terms of features, which are the result of rock types, wave intensity, coastal currents, and whether the coast is stable, sinking, or rising.
• Features developed from erosion are called erosional features, while those developed from sediment deposition are called depositional features.
• Erosional features:
  • Headlands are eroded as the shoreline retreats, leading to the formation of wave-cut cliffs.
  • Sea caves can form at the base of cliffs, which can eventually erode to the opposite side, forming a sea arch.
  • If erosion continues, the top of the arch may collapse, producing a sea stack.
• Depositional features:
  • Beaches are the most prominent feature of depositional shores.
  • Spits are linear ridges of sediment that extend from land into deeper water near the mouth of a bay.
  • Tidal currents can keep the mouth of a bay open, but if not, the spit may completely cut off the bay from the open ocean, forming a bay barrier or bay-mouth bar.
  • A tombolo is a ridge of sand that connects an island or sea stack to the mainland.

Barrier Islands

• The US Atlantic and Gulf Coasts are relatively flat and have a gentle slope, and barrier islands are common in these regions.
• Barrier islands are defined as low ridges of sand that parallel the coast at distances from 2 to 20 miles offshore.
• These features develop in various ways, including:
  • Spits that were cut off from the mainland by wave erosion or by a rise in sea level.
  • Deposits of sand piled up by turbulent waters.
  • Sand dunes that developed during the last glacial episode and were cut off from the mainland by rising sea waters.

Stabilizing the Shore

• Humans have utilized coastal areas inappropriately, leading to erosion and property damage.
• Factors affecting erosion include:
  • Proximity to sediment-laden rivers.
  • Degree of tectonic activity.
  • Topography and composition of the land.
  • Prevailing winds and weather patterns.
  • Configuration of the coastline and nearshore areas.
• Hard stabilization structures, such as:
  • Jetties.
  • Groins.
  • Breakwaters.
  • Seawalls.
• Alternative solutions:
  • Beach nourishment: adding large quantities of sand to the beach.
  • Relocation: removing or relocating storm-damaged property and letting nature reclaim the beach.

Contrasting America's Coasts

• The Pacific, Atlantic, and Gulf Coasts of the United States have different geological characteristics.
• The west coast is the leading edge of the North American plate, experiencing deformation and uplift.
• The east coast is tectonically inactive.
• Barrier islands absorb a good portion of a storm's energy, causing extensive modification of these islands.

Tides

• Tides are daily changes in the elevation of the ocean surface.
• The gravitational attraction between the Earth, moon, and sun generates a bulge on both sides of the Earth.
• The Moon's position changes slightly during the day, resulting in two high tides and two low tides each day.
• The lunar bulges migrate as the Moon revolves around the Earth, resulting in a 50-minute delay in tidal cycles each day.
• The Sun's tide-generating effect is only about 46% that of the moon.
• Spring tides occur during the new and full moon, resulting in a large tidal range.
• Neap tides occur during the ¼ and ¾ moon phases, resulting in a lower tidal range.
• Tidal patterns: diurnal, semidiurnal, and mixed.

Tidal Currents

• Tidal currents are the horizontal flow of water associated with the rise and fall of the tide.
• Flood currents move into a coastal zone as the tide rises, while ebb currents move out of a coastal zone as the tide falls.
• Slack water is the term used to describe periods of little or no current.
• Tidal flats are areas affected by these tidal currents, which can vary greatly in size and shape.
• Tidal deltas are created by tidal currents, either as flood deltas landward of an inlet or as ebb deltas on the seaward side of an inlet.

Learn about the components and characteristics of the North Atlantic Gyre and South Pacific Gyre, including their currents and notable features.