THE DYNAMIC OCEAN CHAPTER 15 1
Feb 24, 2016
THE DYNAMIC OCEAN
CHAPTER 15
1
Cape Hatteras, NC
2
LEARNING OBJECTIVES What is the force that drives the ocean’s
currents? What is the basic pattern of surface currents?
How do surface ocean currents influence climate?
What is thermohaline circulation? Why is the shoreline considered to be a
dynamic interface? What factors influence the height,
length and period of a wave? Can you describe the motion of water within a wave?
3
LEARNING OBJECTIVES How do waves erode? What are some typical features
produced by wave erosion and from sediment deposited by beach drift and longshore currents?
What are the local factors that influence shoreline erosion, and what are some basic responses to shoreline erosion problems?
How do emergent and submergent coasts differ in their formation and characteristic features?
How are tides produced? 4
OCEAN WATER MOVEMENTS Surface circulation
Ocean currents are masses of water that flow from one place to another
Surface currents develop from friction between the ocean and the wind that blows across the surface
Huge, slowly moving gyres
5
OCEAN WATER MOVEMENTS Surface circulation
Five main gyres North Pacific Gyre South Pacific Gyre North Atlantic Gyre South Atlantic Gyre Indian Ocean Gyre (mostly S. Hemisphere)
Related to atmospheric circulation Trade winds, westerlies Also influenced by major landmasses
6
GLOBAL SURFACE CIRCULATION
7
OCEAN WATER MOVEMENTS Surface circulation
Deflected by the Coriolis effect To the right in the Northern Hemisphere To the left in the Southern Hemisphere
Four main currents generally exist within each gyre ~6 years to make the loop in Pacific (Box 15.1)
Large central zone w/no well-defined currentsWest Wind Drift is the only one that
completely circles the Earth
8
OCEAN WATER MOVEMENTS Surface circulation
Importance of surface currents Climate
Currents from low latitudes into higher latitudes (warm currents) transfer heat from warmer to cooler areas
As N. Atlantic Current approaches W. Europe, kit splits; part of it carries warm air to Great Britain, Norway, Iceland
Canary Current - travels south (cool)
9
CORIOLIS EFFECT
10
A) On a non-rotating Earth, rocket would travel straight to its target.
B) Earth rotates 15/hr so even though rocket travels in a straight line, it follows a curved line that veers to the right of the target.
GULF STREAM
11
A) ~1769: Benjamin Franklin’s Gulf Stream chart
B) Satellite image of Gulf Stream (warm – orange)
OCEAN WATER MOVEMENTS Surface circulation
Importance of surface currents Climate
Influence of cold currents is most pronounced in the tropics or during the summer months in the middle latitudes
Cold currents travel equator, moderate climate
Aridity quite pronounced in w. South America, Africa Lower atmosphere chilled by cold offshore water Air is more stable, less likely to move upward and
form rain clouds
12
OCEAN CURRENTS INFLUENCE CLIMATE
13
ATACAMA DESSERT IS THE DRIEST ON EARTH
14
OCEAN WATER MOVEMENTS Surface circulation
Importance of surface currents Upwelling
The rising of cold water from deeper layers Most characteristic along west coasts of
continents Coastal winds + Coriolis Effect surface water
moves away from shoreBrings greater concentrations of dissolved
nutrients to the ocean surface
15
Coastal upwelling high photosynthesis
OCEAN WATER MOVEMENTS Deep-ocean circulation
Significant vertical movement (surface circulation is mostly horizontal)
A response to density differencesFactors creating a dense mass of water
Temperature – cold water is dense Salinity – density increases with increasing
salinityCalled thermohaline circulation
17
SEA ICE SURROUNDING ANTARCTICA
18
When seawater freezes, salt doesn’t become part of ice; water becomes denser and sinks
OCEAN WATER MOVEMENTS Deep-ocean circulation
Most water involved in deep-ocean currents begins in high latitudes at the surface
A simplified model of ocean circulation is similar to a conveyor belt that travels from the Atlantic Ocean, through the Indian and Pacific Oceans and back again
Warm water in ocean’s upper layers flows poleward Becomes dense and sinks Returns to equator as cold, deep water Eventual upwelling
19
IDEALIZED “CONVEYOR BELT” MODEL OF OCEAN CIRCULATION
20
THE COASTAL ZONE The land-sea boundary (interface)
Dynamic – topography, geology, climate vary greatly
Shoreline – contact between land and seaShore – area between lowest tidal level and
highest areas affected by storm wavesCoastline – the seaward edge of the coastBeach – accumulation of sediment along the
landward margin of the ocean
21
THE COASTAL ZONE
22
Crystal Beach TX (9/16/08), 3 days after Hurricane Ike. Most of the damage was caused by a storm surge. MAP
Sanibel Island, FL: beach is made of shells, shell fragments
Hawaii: beach is derived from dark volcanic rock
26
Green Sand Beach at Puu Mahana, Hawaii: green material is olivine sand ((Mg,Fe)2SiO4); dark gray material is basalt lithic sand.
27
Gulf St. Vincent, southern Australia: all quartz (SiO2)
28
Basalt cobble beach at Yaquina Head, coastal Oregon, 8/3/12
OCEAN WATER MOVEMENTS Waves
Energy traveling along the interface between ocean and atmosphere
Can transfer energy 1000s of kmDerive their energy and motion from wind Shape/modify shorelines that must absorb
energyParts
Crest Trough
29
OCEAN WATER MOVEMENTS Waves
Measurements of a wave Wave height – the distance between a trough
and a crest Wavelength – the horizontal distance between
successive crests (or troughs) Wave period – the time interval for one full wave
to pass a fixed position
30
CHARACTERISTICS AND MOVEMENT OF A WAVE
31
OCEAN WATER MOVEMENTS Waves
Wave height, length, and period depend on Wind speed Length of time the wind blows Fetch – the distance that the wind travels
As the wave travels, the water passes energy along by moving in a circle Waveform moves forward At a depth of about one-half the
wavelength, the movement of water particles becomes negligible (the wave base)
32
OCEAN WATER MOVEMENTS Waves
Swells – waves that have traveled away from area of originationSea waves seen from shore are
usually a mix of swells from faraway storms and waves created by local winds
33
CHANGES THAT OCCUR WHEN A WAVE MOVES ONTO SHORE
34
OAK ISLAND PIER, NC
36
OAK ISLAND PIER, NC
37
OAK ISLAND PIER, NC
38
OAK ISLAND PIER, NC
39
WAVE EROSION Wave erosion
Caused by Wave impact and pressure Breaks down rock material and supplies sand to
beaches Abrasion – sawing and grinding action of water
armed with rock fragments
40
SAND MOVEMENT ON THE BEACH Beaches are composed of whatever
material is available Some beaches have a significant biological
component Material does not stay in one place
Wave energy moves large quantities of sand parallel and perpendicular to the shoreline
41
BEACHES AND SHORELINE PROCESSES Wave refraction
Bending of a waveWave arrives parallel to shore Results
Wave energy is concentrated against the sides and ends of headland
Wave erosion straightens an irregular shoreline
42
WAVE REFRACTION ALONG AN IRREGULAR COASTLINE
43
BEACHES AND SHORELINE PROCESSES Longshore transport
Beach drift – sediment moves in a zigzag pattern along the beach face
Longshore current Current in surf zone Flows parallel to the shore Moves substantially more sediment than beach drift
44
BEACH DRIFT AND LONGSHORE CURRENTS
45
SHORELINE FEATURES Erosional features
Wave-cut cliffWave-cut platform Marine terracesAssociated with headlands
Sea arch Sea stack
46
SEA ARCH
47
A SEA STACK AND A SEA ARCH
48
SHORELINE FEATURES Depositional features
Spit – a ridge of sand extending from the land into the mouth of an adjacent bay with an end that often hooks landward
Baymouth bar – a sand bar that completely crosses a bay
Tombolo – a ridge of sand that connects an island to the mainland
49
AERIAL VIEW OF A SPIT AND BAYMOUTH BAR ALONG THE MASSACHUSETTS COASTLINE
50
SPIT
51
TOMBOLO
52
SHORELINE FEATURES Depositional features
Barrier islands Mainly along the Atlantic and Gulf Coastal Plains Parallel the coast Originate in several ways
53
STABILIZING THE SHORE Shoreline erosion is influenced by the local
factors Proximity to sediment-laden rivers Degree of tectonic activity Topography and composition of the land Prevailing wind and weather patterns Configuration of the coastline
54
STABILIZING THE SHORE Responses to erosion problems
Hard stabilization – building structures Types of structures
Groins – barriers built at a right angle to the beach that are designed to trap sand
Breakwaters – barriers built offshore and parallel to the coast to protect boats from breaking waves
Seawalls – Armors the coast against the force of breaking waves
Often these structures are not effective
55
STABILIZING THE SHORE Responses to erosion problems
Alternatives to hard stabilization Beach nourishment by adding sand to the beach
system Relocating buildings away from beach
Erosion problems along U.S. CoastsShoreline erosion problems are different
along the opposite coasts
56
MIAMI BEACH BEFORE BEACH NOURISHMENT
57
MIAMI BEACH AFTER BEACH NOURISHMENT
58
STABILIZING THE SHORE Erosion problems along U.S. Coasts
Atlantic and Gulf Coasts Development occurs mainly on barrier islands
Face open ocean Receive full force of storms
Development has taken place more rapidly than our understanding of barrier island dynamics
59
STABILIZING THE SHORE Erosion problems along U.S. Coasts
Pacific Coast Characterized by relatively narrow beaches
backed by steep cliffs and mountain ranges Major problem is the narrowing of the beaches
Sediment for beaches is interrupted by dams and reservoirs
Rapid erosion occurs along the beaches
60
COASTAL EROSION “FIELD TRIP”
Solana Beach to Del Mar, San Diego County, California
61
COASTAL CLASSIFICATION Shoreline classification is difficult Classification based on changes with
respect to sea level Emergent coast
Caused by Uplift of the land, or A drop in sea level
62
COASTAL CLASSIFICATION Classification based on changes with
respect to sea level Emergent coast
Features of an emergent coast Wave-cut cliffs Marine terraces
63
COASTAL CLASSIFICATION Classification based on changes with
respect to sea level Submergent coast
Caused byLand adjacent to sea subsides, or Sea level rises
Features of a submergent coast Highly irregular shoreline Estuaries – drowned river mouths
64
MAJOR ESTUARIES ALONG THE EAST COAST OF THE UNITED STATES
65
TIDES Changes in elevation of the ocean
surface Caused by the gravitational forces
exerted upon Earth by the Moon, and to a lesser extent by the Sun
66
IDEALIZED TIDAL BULGES ON EARTH
67
TIDES Monthly tidal cycle
Spring tide During new and full moons Gravitational forces added together Especially high and low tides Large daily tidal range
68
EARTH-MOON-SUN POSITIONS DURING THE SPRING TIDE
69
EARTH-MOON-SUN POSITIONS DURING THE NEAP TIDE
70
TIDES Monthly tidal cycle
Neap tide First and third quarters of the Moon Gravitational forces are offset Daily tidal range is least
Tidal patterns Many factors influence the tides
Shape of the coastline Configuration of the ocean basin Water depth
71
TIDES Tidal patterns
Main tidal patterns Diurnal tidal pattern
A single high and low tide each tidal day Occurs along the northern shore of the Gulf of Mexico
Semidiurnal tidal pattern Two high and two low tides each tidal day Little difference in the high and low water heights Common along the Atlantic Coast of the United States
72
TIDES Tidal patterns
Main tidal patterns Mixed tidal pattern
Two high and two low waters each day Large inequality in high water heights, low
water heights, or bothPrevalent along the Pacific Coast of the United
States
73
TIDES Tidal patterns
Tidal currents Horizontal flow accompanying the rise and fall of
tides Types of tidal currents
Flood current – advances into the coastal zone Ebb current – seaward moving water
Sometimes tidal deltas are created by tidal currents
74
FEATURES ASSOCIATED WITH TIDAL CURRENTS
75
RIP CURRENTS
Break the Grip of the Rip
76
END OF CHAPTER 15
77