Oceans and Coastlines

Ocean Basins• Oceans are all connected, so it’s really just

1 big ocean!

• 5 ocean basins– Atlantic– Pacific– Indian– Antarctic (Southern)– Arctic (northern extension of the Atlantic)

Fig. 15.7a, p.380

Seawater Composition and Salinity– The salt content or salinity is around 3.5%– The dissolved material in sea water comes from

• Weathering of rock and transport to sea by rivers

• Volcanic activity

– Evaporation and freezing tend to concentrate salt in sea water

– Salt is removed by biological organisms– Sea water also receives an influx of fresh water

• from precipitation and river flow

3 Major Density Zones in the Ocean (controlled by temperature and salinity)

Current: continuous flow of water in a given direction

• Surface currents: caused by winds

• Winds blow in preferred directions due to differential heating of the earth’s surface

• Example of a surface current: Gulf Stream – 80km wide by 650m deep, 5km velocity (at fastest)

Gyres - circuit of currents around the periphery of an ocean basin

• Gulf Stream Current is part of the North Atlantic Gyre)

• Caused by Westerlies and trade winds

• Trade winds blow from the northeast in the or southeast in the tropics

• Westerly winds (westerlies) blow from the northwest or southwest in the mid-latitudes

Gyres

Gyres

• Gyres circulate clockwise in the northern hemisphere

• Gyres circulate counterclockwise in the southern hemisphere

Surface Ocean Currents

• Currents on the west side of the ocean basin (i.e. east side of continent) originate in the tropics and are warm-water currents

• Examples: Gulfstream, Brazil Current• Currents on the east side of the ocean basin

(i.e. west side of continent) originate at the poles and are cold-water currents

• Examples: California Current, Humbolt Current (cause of El Nino)

Surface Ocean Currents

The diagram shows the South Pacific during a normal year.• Strong S. Pacific Equatorial Current reinforces the northward-flowing

Humbolt Current• This promotes upwelling of cold, nutrient-rich water, which supports the

S. American fishing industry

An El Nino Event (ENSO)– Occurs every 3-7 years for a year– Weakens trade winds and S. Pacific Equatorial current– This in turn weakens the Humbolt Curent which reduces cold upwelling– This creates warmer surface water, depresses the fishing industry and

changes weather patterns in many places.

Consequences of El Nino

Anomalous warm water off the coast of South America

• Increased storm activity in many parts of the U.S and northwestern Europe

• Drought in Australia and S.E. Asia

Deep-Ocean Currents- Thermohaline Circulation

– caused by density differentials– Temperature: cold water is more dense than

warm and will sink– Salinity: saltier water is denser than fresh water

and will sink

– Deep-ocean currents are connected to the surface current system

– Increased freshening of polar waters is changing thermohaline circulation patterns. What is the effect on surface currents?

If the Labrador Current (Cold) is too fresh to sink, it may block passage of the N. Atlantic Drift (warm) which moderates the climate of Great Britain and

Scandinavia

Global warming and sea-level rise

• Sea-level has risen and fallen in the past– Past 40,000 yrs, 150m fluctuation– More recently, sea-level is rising ~3mm/yr– Warming temperatures raise sea level by

• Thermal expansion of water• Release of glacial water that was on land

– Some low lying coasts and islands are already feeling effects

Fig. 16-39, p.401

Rising Sea Level

Fig. 16.38a, p.423

Tides

• Both the Earth and the Moon revolve around a point located in the interior of the earth, near point A.

• The solid Earth is not affected, but the water forms 2 permanent bulges.

Tides

• Bulge “A” caused directly by gravitational attraction of moon.

• Bulge “B” caused by inertia of water as earth swings inward toward the moon (“centrifugal force”).

Tides

• As the earth rotates, shorelines pass through each bulge approximately 12 hours apart.

• Other influences include:– Gravitational effect of the Sun– Shape of the coastline

Spring and Neap Tides

• Spring tide – highest tides, occur when Earth, Sun & Moon are in alignment

• Neap tide – lower high tide, when Earth, Sun & Moon form a 90o angle

Spring and Neap Tide

Beach Drifting and Longshore Currents

Fig. 16-22, p.390