ATMO 102 Pacific Climates and Cultures Lecture Ocean

ATMO 102 Pacific Climates and Cultures Lecture: Ocean Currents and Waves

Ocean Water Properties • How they impact ocean currents • Temperature – remember heat rises! • Salinity – remember salty water sinks! • Density – a function of temperature and salinity

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

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). • Ex: Global Conveyer Belt = deep current that is the lower 20% of the ocean; takes 1, 000 years to complete the cycle. • This slow (~0. 1 m/s), but giant circulation has a flow equal to about 100 Amazon Rivers.

• 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. Ocean Gyres

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

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

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.

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: • • CALM WIND 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. 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 Chaotic seas inside fetch area. Swells: wave type found outside the fetch.

Tides • The rise and fall in sea level is called a tide. • Caused by a giant wave. • Tidal range is the difference in ocean level between high-tide and lowtide • One low-tide/high-tide cycle takes about 12 hrs and 25 min.

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

• 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. Global Tidal Patterns

Hanauma Bay Predicted Tides • What type of tidal cycle do you see here? Mixed 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.