EVSC20004 Lecture Notes - Lecture 19: Boundary Current, Intertropical Convergence Zone, East Australian Current
21 Jun 2018
School
Department
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Professor
Ocean Basin Circulation
General:
Cold currents move from poles to equator
Warm currents from equator to pole
Warmer currents are the west of the ocean basins
Colder currents are the east of the ocean basins
This is because this is wind-driven
West to east winds (Westerly winds): Colder currents
East to west winds (Easterly winds): Warmer currents
Across the equator there is the Intertropical convergence zone: The region where we get
the most rainfalls and clouds
Rising air of the equator sucks air in (High pressure)
The winds (and gravity) drive the ocean currents
The deeper currents are driven in buoyancy and gravity, and changes in density (warmer
waters rise to the surface, colder waters sink)
What drives the winds?
Sunlight:
Temperature gradient drives atmospheric circulation
Solar heating varies with latitude
oSolar energy is maximised at the equator, and solar energy is diffused
at the poles
Why don’t tropical oceans boil and polar oceans freeze solid?
The ocean and the atmosphere are transporting that heat
Atmosphere of the ocean transmits the heat into our space- at night radiation is
emitted into space at night (earth cools down)
There is more heat coming in than there are going in at the equator
There is less heat coming than there are going in at the poles
Wind transfers this
Water vapour transports a lot of heat energy as well (Surplus goes through the north
during summer, then south- seasonal)
Heat moves around- doesn’t accumulate heat in one area and lack of heat in another
The rotation of the earth changes the movement of heat
Coriolis effect:
oIf the Earth did not rotate and remained stationary, the atmosphere would
circulate between the poles (high pressure areas) and the equator (low
pressure area) in a simple back and forth pattern. But because the Earth
rotates, circulating air is deflected.
oInstead of circulating in a straight pattern, the air deflects toward the right in
the Northern Hemisphere and toward the left in the Southern Hemisphere,
resulting in curved paths.
oEquator- highest speed
oPoles- zero speed
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oCoriolis effect is strongest at the poles and weakest at the equator
Variation in land masses also changes the movement of heat
Uneven surface heating
o‘Sea breeze’- warm air over land, cool air in the ocean
Land sea contrast causes differences in wind patterns- more sinking
air in the oceans, more rising air on land
oMost rising air: above the geographical equator on the intertropical
convergence zone (ITCZ)
There are alternating highs and lows because of locations of ‘belts’:
Uneven heating and Coriolis effect induces 6 major atmospheric circulation cells
Cells represent long-term mean behaviour, but circulation is highly variable in time
Land/water masses induce different effects due to thermostatic effect of water e.g.
ITCZ is 5-8o N
N/S Movement of ITCZ and land/ocean temp gradient gives rise to monsoons
Number of belts depends upon rotation rate and size of planet
Ocean layers and currents:
Deep ocean (80%), pycnocline, surface zone (2%)
Cold salty water in the deep ocean, warm less-salty water in the surface ocean
Pycnocline: Zone in which density increases steeply with increasing depth, isolates surface
water from denser water below
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Thermocline:Zone in which temperature decreases steeply with increasing depth, major
contributor to pycnocline
Wind-driven currents: Dominate in the surface mixed layer
Thermohaline circulation: Dominates in the deep layer (Very long- gravity and buoyancy
driven)
Western vs Eastern Boundary Currents:
Current strengths (flow rate) are measured in ‘sverdrups’= 1 million cubic metres per
second
The western boundary currents are warmer, stronger, deeper and narrow
The eastern boundary currents are cooler, weaker, shallower and broader
The asymmetry in the wester/eastern boundary currents are affected by the Coriolis
effect- as currents move to the western boundary (closer to the equator), they move
in a straight line. Water nearer the poles spreads out into the ocean (which is why it
is weaker)
Current temperatures and strength change with seasons:
Solar radiation varies seasonally
This causes the maximum heating location to move north and south of the equator
The belts move with it, thus causing the patterns of winds to shift north and south
also
This also affects the temperature and strength of the currents
Gulf Stream, Kuroshio, East Australian and the Peru current are important currents:
Gulf stream:
Warm water from the Gulf Stream stems a long way to the North Atlantic
Gulf stream has allowed countries southwest of England (Such as the Scilly Islands)
to be much warmer than it should be- allowing for vegetation and crops to thrive
(Mediterranean-like vegetation)
Kurushio current:
Impacts Asia- warm waters transferred to the north
Kurushio means ‘black current’ as it is warm and biologically unproductive
Interacts with cold current (Oyashio, the ‘parent current’, which has high biological
productivity)
Boundary between the two usually at about 40oN (Latitude)
Peruvian (or Humboldt) current:
Cold eastern boundary current off west coast of South America
Caused by movement of water from west to east in mid-latitudes, then striking coast
of South America, upwelling
As current turns to the left, draws up water from below. Cold currents are shallow,
can dig up cold water from below
Upwelled water is nutrient rich (Nutrients are in the bottom part of the ocean)
Current lowers SST by several degrees from the average at that latitude; within
100km of coast, upwelling lowers it by another 2-4 degrees
Important for fisheries and the Pacific climate systems
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Document Summary
Warmer currents are the west of the ocean basins. Colder currents are the east of the ocean basins. West to east winds (westerly winds): colder currents. East to west winds (easterly winds): warmer currents. Across the equator there is the intertropical convergence zone: the region where we get the most rainfalls and clouds. Rising air of the equator sucks air in (high pressure) The winds (and gravity) drive the ocean currents. The deeper currents are driven in buoyancy and gravity, and changes in density (warmer waters rise to the surface, colder waters sink) Solar heating varies with latitude: solar energy is maximised at the equator, and solar energy is diffused at the poles. The ocean and the atmosphere are transporting that heat. Atmosphere of the ocean transmits the heat into our space- at night radiation is emitted into space at night (earth cools down) There is more heat coming in than there are going in at the equator.
