MET 102 Pacific Climates and Cultures

Lecture 13: Ocean Currents and Waves

MET 102 Pacific Climates and Cultures

Discussion Questions – Press and Siever 1999

• How can you increase the height of waves?• The height of the waves increases as:

1. The wind speed increases2. The wind blows for longer times3. The distance over which the wind blows the water increases

• What three characteristics do we use to describe waves?1. Wavelength – The distance between crests2. Wave height – the vertical distance between the crest and the trough3. Period – The time it takes for successive waves to pass

Discussion Questions – Finney 1959 • What are the three general forms of ancient Hawaiian surfing? Who surfed?• body-surfing (kaha nalu)• outrigger-canoe-surfing (probably called no ka pakaka ale)• surfboarding (he’e nalu)• Everyone, Men, Women, Children, Chiefs, Commoners – It’s the national sport of Hawaii

• What were the two different types of surfboards used in ancient Hawaii? Who were allowed to use them?• olo – a long, narrow board reserved exclusively for chiefs• alaia – shorter thinner boards for commoners

Ocean Current Layers

• Pycnocline – the layer between surface and deep waters; where a rapid change in temperature, salinity and density occur

• Deep Current – lower 90% of the ocean

• Surface Currents – upper 10% of the ocean; upper 400 m

TROPICS

Ocean Water Properties

•How they impact ocean currents• Temperature –

remember heat rises!

• Salinity – remember salty water sinks!

•Density – a function of temperature and salinity

Temperature and Ocean Currents

http://earth.usc.edu/~stott/Catalina/Oceans.html

Primary Ocean Current Forcing

• These Start the Water MOVING:• Sun/solar heating - causes water to expand

and move

• Winds - push the water; winds blowing for 10 hrs across ocean will cause the surface water to flow @ ~2% wind speed; wind has the greatest effect on surface currents

• Gravity - pull water downhill or pile against the pressure gradient (high/low); influences tides

• Coriolis Force/Effect – Rotational effect, pulling to the right in the Northern Hemisphere and to the left in the Southern Hemisphere

Wind Driven Ocean Currents http://www.bigelow.org/shipmates/hc_currents_lg.gif

Surface Currents• Surface current – with surface

circulation is less dense and influenced by winds

1. Warm surface currents: wind and Earth’s rotation

2. Cold surface currents: flow towards the equator

3. Upwelling current: cold, nutrient rich; result of wind

4. Western Boundary currents: warm & fast

5. Eastern Boundary currents: broad, slow, cool & shallow, associated with upwelling

• Ex: Gulf Stream = surface current that is the upper 20% of the ocean, western boundary current

Deep Ocean Currents

• Deep water – cold, dense, salty; move by density forces and gravity; move slower than layers above

• Thermohaline Circulation driven by: • differences in the density of the sea

water

• controlled by temperature (thermal) and salinity (haline).

• This slow (~0.1 m/s), but giant circulation has a flow equal to about 100 Amazon Rivers. • Ex: Global Conveyer Belt = deep current that is

the lower 20% of the ocean; takes 1,000 years to complete the cycle.

Ocean Conveyor Belt – Thermohaline Circulation

Ocean Gyres • Gyres – large mounds of water;

large circular currents in the ocean basin• Ex: South Pacific Gyre = consists of

4 separate currents – S. Equatorial, East Australian Current, Peru Current and the West Wind Drift.

WAVES

What is a Wave?• A Wave is a rhythmic movement that carries energy through

matter or space.• In oceans, waves move through seawater

Parts of a Wave

• Crest – highest point of a wave• Trough – lowest point of a wave•Wave Height – vertical distance between the crest and the trough•Wavelength – horizontal distance between two crests or two troughs

Wave Movement•When a wave passes through the ocean, individual water molecules move up and down but they do not move forward or backward.

Wave Movement

•When a wave breaks against the shore, the crest outruns the trough and the crest collapses.

• Called a breaker.

• In this case, water does move forward and backward.

• Waves Break in the Surf Zone

Types of Breakers• Spilling breaker

• Top of wave crest ‘spills over’ wave. Energy released gradually across entire surf zone.

• Plunging breaker• Crest ‘curls over’ front of wave. Energy dissipates

quickly. Common at shorelines with steep slopes

• Surging breaker• Never breaks as it never attains critical wave

steepness. Common along upwardly sloping beach faces or seawalls. Energy released seaward.

Wave Refraction

• The part of the wave crest closer to shore is in shallower water and moving slower than the part away from the shore in deeper water.

• The wave crest in deeper water catches up so that the wave crest tends to become parallel to the shore.

Idealized Wave Spectrum

•Waves come in different shapes and sizes with differences in wave height and wave period.

Wind Generated Waves

• Most of the waves present on the ocean’s surface are wind-generated waves.

• Size and type of wind-generated waves are controlled by:• wind velocity• wind duration• Fetch (distance over the sea surface)• original state of the sea surface.

• As wind velocity increases wavelength, period and height increase, but only if wind duration and fetch are sufficient.

CALM WIND

STORMY WIND

Fetch

• Fetch is the area of contact between the wind and the water and is where wind-generated waves begin.

• Seas is the term applied to the sea state of the fetch when there is a chaotic jumble of new waves.

• Waves continue to grow until the sea is fully developed or becomes limited by fetch restriction or wind duration.

Fetch and Wave Shape Swells: wave type found

outside the fetch.Chaotic seas inside fetch area.

Tides

• The rise and fall in sea level is called a tide.

•Caused by a giant wave.

•One low-tide/high-tide cycle takes about 12 hrs and 25 min.

• Tidal range is the difference in ocean level between high-tide and low-tide

Tides

•Two big bulges of water form on the Earth:• one directly under

the moon • another on the

exact opposite side

•As the Earth spins, the bulges follow the moon.

Tides

• Spring Tides• Earth, Moon, and Sun are lined up• High Tides are higher and Low Tides

are lower than normal

•Neap Tides• Earth, Moon, and Sun form right angles• High Tides are lower and Low Tides are

higher than normal

Global Tidal Patterns • Semidiurnal• two high and two

low tides of approximately equal size every lunar day.

• Diurnal• one high and one

low tide every lunar day.

• Mixed Semidiurnal• two high and two

low tides of different size every lunar day.

Hanauma BayPredicted Tides

• What type of tidal cycle do you see here?Semi-diurnal?

• The combination of diurnal, semi-diurnal and fortnightly cycles dominates variations in sea level throughout the islands.

Hawaii Island – Tidal Currents • Tidal currents result from tidal variations of sea level, and near shore

are often stronger than the large scale circulation.