I. Global Winds and Ocean Currents

A. Origin of Ocean Currents

1. Drag exerted by winds flowing across the ocean causes the surface layer of water to move.

2. Winds are the primary cause of surface ocean currents

Planetary Winds

❑ Interaction of: ➢ Mid-Latitude

southwesterly winds ➢ Tropical northeasterly

trade winds

❑ Produces Gyres

B. Relationship Between Oceanic Circulation and General Atmospheric

Circulation

1. North and South Equatorial Currents

a) North and south of the equator and are westward moving currents. b) Derive energy from the trades winds c) Affected by the Coriolis Effect (clockwise spiral in Northern Hemisphere and

counterclockwise in the Southern Hemisphere) d) Found in each of the major ocean basins and centered around the subtropical

high pressure systems.

2. Currents flowing from higher latitudes are cold and those flowing from lower latitudes are warm.

a) Warm: Gulf Stream (North Atlantic Drift), Kuroshio Current b) Cold: Labrador Current, California Current

3. Spinning Gyres in Subtropics

a) Upper layer of water piles up in the gyre’s center. b) Sea level is 2 m higher than the surrounding ocean. c) Water flows outwards and is turned by Coriolis d) Continents form boundaries that contain flow in the ocean basins.

4. The “Conveyer Belt”a) Net northward

transport of heat in N. Hemisphere

b) Most circulates around the subtropical gyre

c) Transfers heat to the atmosphere – Above 50o N, large

temperature contrast between ocean and atmosphere

a) Warm northward flowing salty water cools and sinks north of Iceland between N. America and Greenland a) This cold water flows south at 2 to 4 km depths.

5. Deep-Ocean Circulation

a) Thermocline: (i) A zone of rapid temperature change

between: • Warm upper layers • Cold water of deeper ocean basins

(ii) Two Thermoclines • Deeper permanent portion • Shallower portion

(iii) Changes as a result of seasonal heating by the Sun

ThermoclinesWarm poleward flow is balanced by sinking cold water at high latitudes that moves towards the equator (conveyer).

Thermohaline flow: Term for this over-turning circulation

b. Thermohaline Flow

(i) Term is derived from the two processes that control deep water formation and influence the water’s density.

• “Thermo” for temperature • “Haline” for salinity

– From halite, the mineral name for salt

Salinity – Increases Water’s Density

• Dissolved salts • Average 35 parts per

thousand (o/oo) by mass. • 3.5% denser than

freshwater • Evaporation increases

salinity • Salt Rejection at high

latitudes – Sea Ice is freshwater – Salt left behind and

dissolves in sea water.

(ii) Deep Waters Sink Due to Increased Density

• Cooling – Increases the density due to a decrease in

volume • This Causes

– Warm water to be carried poleward into cooler regions

– Cold air masses to move to lower latitudes

Sources of Deep Ocean Water• High latitude North Atlantic ocean and the Southern Ocean, near Antarctica • Pacific Ocean high latitudes are not a source because surface waters are not dense enough due to low salinity

North Atlantic Deep Water The Part of the “Conveyer” that Returns Water to Lower Latitudes

• Occurs north of Iceland and east of Labrador • Fills Atlantic between depths of two and four kilometers • Flows southward with a total volume 15x greater than the combined flow of all the streams on Earth

C. Effects on Climate

1. Moderating Effect of Warm Poleward Moving Currents

The Gulf Stream

Stornoway, Scotland Latitude = 58.22o N

Avg. Temp. = 46.9o F (9.4o C)

Hopedale, Newfoundland and Labrador Latitude = 55.45o N

Avg. Temp. = 28.4o F (-2.0o C)

2. Cold Ocean Currents

a) Influence temperature b) West coast deserts become more arid because the cold air is more stable and does not rise. Examples: - Peru Current - Benguela Current

Effects of Cold Ocean Currents

Cold Ocean Currents Create Fog

c) Fog and high relative humidity can result from air approaching it’s dew point temperature. - An example is the weather in Newfoundland from the Labrador Current.

The Labrador Current

So, how does water that sinks into the deep ocean get back to

the surface?

Climate scientists really don’t know the answer!

A Widely Accepted Explanation

• Deep water gradually mixes into the central ocean basins • Moves slowly upward along the thermocline into warmer waters • Recent measurements

– Show that this upward diffusion doesn’t account for much of the return flow because it’s too slow

D. Upwelling

1. Mechanism a) Initiated by surface winds b) Assisted by the Coriolis Effect c) Intermediate depth water moves upward to

replace surface water that has been pushed away by winds

2. Equatorial Upwelling

a) Trade winds push water away from the equator b) Warm surface water moves

– Northward in the N. Hemisphere – Southward in the S. Hemisphere

c) Cooler water moves upwards from below to replace the surface water

3. Coastal Upwelling

a) Common along the coasts of California, Peru, and West Africa.

b) Winds flow toward the equator parallel to the coast (i) The Coriolis effect directs surface water away from shore. (ii) Surface water is replaced by water that slowly rises from below (from 50 to 100 meters). (iii) This water is cooler than the surface water it replaces.

3. Coastal Upwelling

d) Upwelling brings to the surface greater concentrations of dissolved nutrients (i.e. nitrates and phosphates) that promote plankton growth, which supports fish populations.

c) This water is cooler than the surface water it replaces

El Niño• The sudden warming of a vast area of the equatorial

Pacific ocean surface. • Typically starts off Peru and works up the coast to

western Mexico and California • Occurs in a three to seven year cycle. • See-Saw Pattern from normal to El Niño conditions is

called the Southern Oscillation • ENSO sometimes used for El Niño Southern Oscillation.

Normal Conditions

• The trade winds and strong equatorial currents flow toward the west. • The strong Peru Current causes upwelling along S. America’s west coast.

• High air pressure between the eastern and western Pacific causes surface winds and warm equatorial waters to flow westward. • Warm water piles up in the western Pacific.

Normal Pacific Ocean Conditions

El Niño (ENSO)

• Pressure over the eastern and western Pacific flip-flops • This causes the trades to weaken and warm water to move eastward.

ENSO Pacific Ocean Condiations

Weather Related to ENSO

• Winters – Warmer than normal in northern U.S. and Canada – Cooler than normal in the Southwest and Southeast – Eastern U.S.

• Wetter than normal conditions – Indonesia, Australia, Philippines

• Drought conditions – Suppression of the number of Atlantic Hurricanes

Weather Related to ENSO

• Summers – Wetter than average in U.S.

• Northwest, • North-midwest • North-mideast • mountain regions

La Niña After an ENSO Episode

• Water Temperature – Water temperature returns to normal – Colder water temperatures in the eastern Pacific

• Trade winds may become especially strong, causing increased upwelling • Typical La Niña weather patterns

– Cool conditions over the Pacific Northwest – Especially cold winter temperatures in the Great Plains – Unusually dry conditions in the Southwestern and Southeastern U.S. – Increased precipitation in the U.S. Northwest – Increased Atlantic hurricane activity

III. Global Distribution of Precipitation

A. Precipitation on a “uniform” Earth without considering variations caused by land and water

Four Major Pressure Zones in Each Hemisphere

Annual Global Distribution of Precipitation

• Dry Conditions: In regions influenced by high pressure – Subsidence and divergent winds

• Ample Precipitation: In regions influenced by low pressure – Converging winds and ascending air

Between ITCZ and Subtropical High

• Influenced by both pressure systems which migrate seasonally

• Most precipitation in summer due to influence of ITCZ

Mid-latitudes

• Most precipitation from traveling cyclonic storms

• Dominated in winter by the Polar Front which generates cyclones is in this region

• In summer, dominated by subsidence from the dry subtropical high.

Cyclonic Storms Produce Most Precipitation in Middle Latitudes

Satellite Image of a well-developed mid-latitude cyclone over the British Isles.

Polar Regions

• Dominated by cold air with low moisture capacity.

• Little precipitation throughout the year.

Seasonal Changes in Precipitation Patterns in the Mid-latitudes

• Results from seasonal shifts in insolation • Summer

– Dominated by subsidence associated with the dry subtropical high • Winter

– Polar front moves equatorward – Precipitation from numerous cyclones

B. Distribution of Precipitation over Continents

• Arid regions in the mid-latitudes don’t conform to the ideal zonal patterns – Desert regions in southern South America (Patagonia) result from the

orographic effect of a mountain barrier. • Other differences result from the distribution of continents and

oceans

The Subtropics: A notable anomaly

• Location of many of the world’s great deserts but also the location of regions with abundant rainfall

The Cause . . . Subtropical High Pressure Centers

Have Different Characteristics on Eastern and Western Sides

Eastern Side of a Subtropical High

• Subsidence creates stable air – Upwelling of cold water along the west coasts of adjacent continents

cools the air from below, adding to the stability on the eastern side of the low.

– Results in arid conditions

Western sides of continents adjacent to these lows are arid

Western Side of a Subtropical High

• Convergence and uplifting on the western side – Air travels over a large expanse of ocean and acquires moisture. – Eastern regions of subtropical continents receive ample yearly

precipitation. • A good example is Southern Florida.