Transcript Ocean Currents and El Niño

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.
a)
b)
Currents flowing from higher latitudes are cold
and those flowing from lower latitudes are warm.
Warm:
Cold:
Gulf Stream (North Atlantic Drift), Kuroshio Current
Labrador Current, California Current
3. Spinning Gyres in Subtropics
a)
b)
c)
d)
Upper layer of water piles up in the gyre’s 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.
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
• 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.
– Above 50o N, large
temperature contrast
between ocean and
atmosphere
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
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
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
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)
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)
b)
Trade winds push water away from the equator
Warm surface water moves
–
–
c)
Northward in the N. Hemisphere
Southward in the S. Hemisphere
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
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.
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.