I. Global Winds and Ocean Currents

1
I. Global Winds and Ocean Currents
2
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

3
Planetary Winds

• Interaction of
• Mid-Latitude southwesterly winds
• Tropical northeasterly trade winds
• Produces Gyres

4
B. Relationship Between Oceanic Circulation and
General Atmospheric Circulation
5
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.

6
2. Currents flowing from higher latitudes are
cold and those flowing from lower latitudes are
warm.

• Warm Gulf Stream (North Atlantic Drift),
  Kuroshio Current
• Cold Labrador Current, California Current

7
3. Spinning Gyres in Subtropics

• Upper layer of water piles up in the gyres
  center.
• Sea level is 2 m higher than the surrounding
  ocean.
• Water flows outwards and is turned by Coriolis
• Continents form boundaries that contain flow in
  the ocean basins.

8
4. The Conveyer Belt

• Net northward transport of heat in N. Hemisphere
• Most circulates around the subtropical gyre
• Transfers heat to the atmosphere
• Above 50o N, large temperature contrast between
  ocean and atmosphere

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

9
5. Deep-Ocean Circulation

• 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

10
Thermoclines
Warm 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
11
b. Thermohaline Flow

• (i) Term is derived from the two processes that
  control deep water formation and influence the
  waters density.
• Thermo for temperature
• Haline for salinity
• From halite, the mineral name for salt

12
Salinity Increases Waters 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.

13
(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

14
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

15
North Atlantic Deep WaterThe 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

16
C. Effects on Climate
17
1. Moderating Effect of Warm Poleward Moving
Currents
18
The Gulf Stream
Hopedale, Newfoundland and Labrador Latitude
55.45o N Avg. Temp. 28.4o F (-2.0o C)
Stornoway, Scotland Latitude 58.22o N Avg.
Temp. 46.9o F (9.4o C)
19
2. Cold Ocean Currents

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

20
Effects of Cold Ocean Currents
21
Cold Ocean Currents Create Fog

• Fog and high relative humidity can result from
  air approaching its
• dew point temperature.
• - An example is the weather in Newfoundland
  from the Labrador Current.

22
The Labrador Current
23
So, how does water that sinks into the deep ocean
get back to the surface?

• Climate scientists really dont know the answer!

24
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 doesnt account
  for much of the return flow because its too slow

25
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

26
2. Equatorial Upwelling

• Trade winds push water away from the equator
• 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

27
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.
28
3. Coastal Upwelling

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

d) Upwelling brings to the surface greater
concentrations of dissolved nutrients (i.e.
nitrates and phosphates) that promote plankton
growth, which supports fish populations.
29
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.

30
Normal Conditions

• The trade winds and strong equatorial currents
  flow toward the west.
• The strong Peru Current causes upwelling along
  S. Americas 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.

31
Normal Pacific Ocean Conditions
32
El Niño (ENSO)

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

33
ENSO Pacific Ocean Condiations
34
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

35
Weather Related to ENSO

• Summers
• Wetter than average in U.S.
• Northwest,
• North-midwest
• North-mideast
• mountain regions

36
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

37
III. Global Distribution of Precipitation

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

38
Four Major Pressure Zones in Each Hemisphere
39
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

40
Between ITCZ and Subtropical High

• Influenced by both pressure systems which migrate
  seasonally
• Most precipitation in summer due to influence of
  ITCZ

41
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.

42
Cyclonic Storms Produce Most Precipitation in
Middle Latitudes
Satellite Image of a well-developed mid-latitude
cyclone over the British Isles.
43
Polar Regions

• Dominated by cold air with low moisture capacity.
• Little precipitation throughout the year.

44
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

45
B. Distribution of Precipitation over Continents

• Arid regions in the mid-latitudes dont 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

46
The Subtropics A notable anomaly

• Location of many of the worlds great deserts but
  also the location of regions with abundant
  rainfall

47
The Cause . . .Subtropical High Pressure Centers

• Have Different Characteristics on Eastern and
  Western Sides

48
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

49
Western sides of continents adjacent to these
lows are arid
50
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.