ch 10

Chapter 10: The Restless Ocean

The Ocean’s Surface Circulation

  • Ocean Currents

    • Generated by the transfer of energy from the wind to the water.

    • Major surface currents are closely related to the global pattern of prevailing winds.

    • Influenced by the distribution of landmasses and the Coriolis Effect.

  • Gyres

    • Large, circular-moving surface currents within ocean basins.

    • Five main gyres:

    • North Pacific Gyre

    • South Pacific Gyre

    • North Atlantic Gyre

    • South Atlantic Gyre

    • Indian Ocean Gyre

    • Centers of gyres located at approximately 30° north or south latitude, often called subtropical gyres.

The Ocean’s Surface Circulation (Illustrations)

  • Illustrated gyres show corresponding currents and their relation to prevailing wind patterns.

The Coriolis Effect

  • Definition: The deflection of free-flowing objects due to Earth’s eastward rotation.

    • Objects are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

  • Observations of the Coriolis Effect are facilitated by videos demonstrating its influence in both hemispheres.

The Ocean’s Surface Circulation (Gyres Detailed)

  • North Pacific Gyre:

    • Currents: North Equatorial Current, Kuroshio Current, North Pacific Current, California Current.

    • Time to complete: Approximately 6 years.

  • North Atlantic Gyre:

    • Currents: North Equatorial Current, Gulf Stream, North Atlantic Current, Canary Current.

    • Time to complete: Approximately 3 years.

  • Sargasso Sea:

    • Region in the North Atlantic with no well-defined currents.

  • South Pacific Gyre:

    • Currents: South Equatorial Current, East Australian Current, West Wind Drift, Peru Current.

  • South Atlantic Gyre:

    • Currents: South Equatorial Current, Brazil Current, West Wind Drift, Benguela Current.

  • Indian Ocean Gyre:

    • Currents: South Equatorial Current, Agulhas Current, West Wind Drift, West Australian Current.

    • Influenced by monsoons (detailed in Chapter 13).

Influence on Climate

  • Heat Transport:

    • Ocean waters account for ¼ of total heat transport; winds account for ¾.

    • Warm currents (e.g., Gulf Stream) warm regions they flow near.

    • Example: Western Europe’s moderated climate due to ocean currents.

    • Equatorial currents (e.g., Canary Current, Peru Current) cool nearby regions, e.g., Southern California being nearly 11°F cooler than its East Coast counterpart.

  • Desert Formation:

    • Example: Cool Peru Current reducing humidity in coastal Peru & Chile leads to stable air and low precipitation.

    • Similar effect noted in Namib, southwestern Africa.

Upwelling and Deep-Ocean Circulation

  • Surface and Vertical Currents:

    • Surface currents move horizontally, causing upwelling and downwelling vertically.

    • Upwelling: Rising of dense, cold water to replace warmer surface water.

    • Caused by winds blowing parallel to the coast towards the equator, facilitated by the Coriolis Effect.

    • Results in lower nearshore temperatures and higher nutrient concentration, promoting marine life.

  • Downwelling: Sinking of warm surface water to lower water column levels.

    • Not explicitly covered in the textbook.

Thermohaline Circulation

  • Definition: Deep-ocean circulation driven by density differences caused by temperature and salinity variations.

  • Causes of Increased Density:

    • Decrease in temperature.

    • Increase in salinity, significant in high latitude areas.

  • Thermohaline Process:

    • Initiated in high latitude areas where cold seawater freezes into sea ice, leaving the salt in the surrounding water.

    • Increased density causes the cold water to sink, forming the Antarctic bottom water which resurfaces very slowly (500 to 2000 years later).

The Shoreline: A Dynamic Interface

  • Characteristics of Shorelines:

    • Highly dynamic areas subject to changes in topography, geological makeup, and climate.

    • Serve as sediment deposition zones between marine and continental environments.

    • Change rate influenced by wave energy and climate change.

  • Natural Processes Modifying Shorelines:

    • Sea level rise, stream erosion, glaciation, volcanic activity, tectonic forces.

  • Human Impact:

    • Overdevelopment and unsustainable resource usage.

    • Example: Hurricane impacts illustrating the vulnerability of human structures along shorelines.

Ocean Waves

  • Definition: Energy traveling along the interface between seawater & atmosphere, often generated thousands of miles away.

  • Types of Waves:

    • Wind-generated waves are pivotal in modifying shorelines.

  • Key Characteristics:

    • Wave crest: Highest part above average water line.

    • Wave trough: Lowest part below average water line.

    • Wave height: Vertical distance between crest and trough.

    • Wavelength: Horizontal distance between successive crests or troughs.

    • Wave period: Time taken for one wavelength to pass a fixed point.

Changes in Waves Near Shore

  • Wave Behavior:

    • As waves approach shallower waters, interaction with the seabed causes changes in wave shape and behavior (circular orbits become elongated, decreasing wavelength).

  • Surf and Swash:

    • Breaking waves generate surf and swash as they approach the beach.

  • Backwash: The return flow of water back toward the surf zone after swash.

Beaches and Shoreline Processes

  • Definition of a Beach: Accumulation of sediment along the landward edge of oceans, seas, or lakes.

  • Composition and Formation:

    • Derived from local erosion or rivers, which affects the texture (coarser from cliffs, finer from rivers).

    • Some beaches contain biological materials (shells, coral, marine remnants).

    • Varieties of colors based on geological makeup (e.g., green from olivine, black from basaltic lava).

  • Wave Erosion:

    • Minimal during calm weather, significantly increased during storms due to wave pressure and force.

    • Erosion causes significant geological changes over time; historical instances of coastal retreats (e.g., British coast erosion by 2-3 miles in 2,000 years).

Sediment Movement

  • Movement Dynamics:

    • Longshore currents and the resultant longshore drift influence sediment movement along beach profiles.

  • Wave Refraction:

    • The bending of waves as they approach the shore affects energy distribution, leading to erosion in headlands and sediment accumulation in bays.

Shoreline Features

  • Erosion and Deposition:

    • Features shaped by erosional processes consist of cliffs, caves, and wave-cut features.

    • Depositional features include beaches, spits, bay barriers formed from sediment accumulation.

  • Barrier Islands:

    • Common along the U.S. Atlantic and Gulf Coasts; defined as low ridges of sand, often off 2-20 miles from shore and separated by lagoons.

  • Formation Processes:

    • Development due to past glaciations or disturbance methods such as currents or sea level rise.

Stabilizing the Shore

  • Human Impact on Coastlines:

    • Improper usage and overdevelopment of coastal areas increase risks of erosion and destruction from natural disasters.

  • Erosion Influencers:

    • Proximity to sediment-rich rivers, tectonic activity, coastal topography, prevailing winds, and coastal configurations all affect erosion rates.

  • Hard Stabilization Techniques:

    • Jetties, groins, breakwaters, and seawalls designed to prevent coastal erosion but often lead to unintended consequences.

  • Alternatives to Hard Stabilization:

    • Beach nourishment (temporary) and relocation of properties as potential environmentally sustainable solutions.

Contrasting America’s Coasts

  • Geological Differences:

    • The Pacific and Atlantic/Gulf Coasts function differently due to tectonic activity (Pacific is geologically active while Atlantic is relatively stable).

    • Coastal development along both varies due to the understanding of barriers and shore dynamic processes.

  • Emergent vs. Submergent Coasts:

    • Emergent shores result from uplifts or sea-level drops (e.g., coastal California).

    • Submergent shores develop from subsiding regions or rising sea levels.

Tides

  • Definition: Daily changes in ocean surface elevation driven by gravitational attractions between Earth, Moon, and Sun.

  • Gravitational Effects:

    • Gravitational pull results in tidal bulges; the Moon’s effect is dominant due to its proximity.

  • Tidal Cycle: Tidal cycles migrate due to lunar movements, resulting in time shifts for high and low tides.

Types of Tidal Patterns

  • Diurnal: Single high tide and low tide per lunar day.

  • Semidiurnal: Two high tides of equal heights, two low tides of equal heights each lunar day.

  • Mixed: Two high tides and two low tides of unequal heights each lunar day.

Tidal Currents

  • Definition: Horizontal water movement associated with the rise and fall of tides.

  • Types: Flood currents (incoming) and ebb currents (outgoing).

  • Slack water: Transitional periods between the two current types.

  • Tidal flats: Areas affected by tidal currents, often shaped into deltas.