|
|
|
OCEAN
CURRENTS AND WIND Density
of air is controlled by temperature and moisture content Warm
air is less dense than cold air and moist air is less dense than dry air. Air
pressure is the weight of the air from Earth’s surface to the top of the
atmosphere and equals 1.04kg/cm2 (standard air pressure, one
atmosphere) at sea level. Low
pressure zone is where air density is lower than in surrounding areas because
the air is warmer or has a higher moisture content. High
pressure zone is where air pressure is higher than in surrounding area because
of cooling or lower moisture content. Heating
and Cooling of Air Fluids
(air and water) flow from areas of high pressure to areas of low pressure. Change
in pressure across a horizontal distance is a pressure gradient. Greater
the difference in pressure and the shorter the distance between them, the
steeper the pressure gradient and the stronger the wind. Movement
of air across a pressure gradient parallel to Earth’s surface is called a wind
and winds are named for the direction from which they come. In
contrast, ocean currents are named for the direction towards which they travel. Rotation
of the Earth strongly influences winds. Global
winds blow in response to variation in pressure. Coriolis
deflection is the apparent deflection of objects moving across Earth’s surface
to the right of direction of travel in the northern hemisphere and to the left
of direction of travel in the southern hemisphere. Three
major convection cells are present in each hemisphere. The
Hadley cell extends from the Equator to about 30o latitude. The
Ferrel Cell extends from 30o to about 50o latitude. The
Polar Cell extends from 90o to about 50o latitude. The
Coriolis effect causes wind in these cells to bend to the east or west to form
the westerlies, easterlies, and the trade winds. Wind-driven
currents are produced by the interaction between the wind and the water. As
wind moves across the water, collision of air molecules with water molecules
inefficiently transfers energy from the air to the water. Water
moves at about 3-4% of the wind speed. Pressure
gradients develop in the ocean because the sea surface is warped into broad
mounds and depressions with a relief of about one meter. With
time, wind-driven surface water motion extends downward into the water column,
but speed decreases and direction changes because of Coriolis deflection. The
current flow pattern in gyres is asymmetrical with narrow, deep and swift
currents along the basin’s western edge and broad, shallow slower currents
along the basin’s eastern edge. The
Sargasso Sea is a large lens of warm water encircled by the Water
at the surface is exposed to changes in salinity through evaporation or
precipitation and in temperature through cooling or heating. Once
water sinks and becomes isolated from the atmosphere, its salinity and
temperature are largely set for an extended period of time. Surface
water masses can be classified as Central waters (from 0 to 1 km), Intermediate
waters (from 1 to 2 km), and Deep and bottom waters (greater than 2 km). The There
are two principle ways to measure currents: Eulerian
method employs current meters fixed to the sea bottom. Langrangian
method employs drifters, drogues, or floats that are set loose at the sea
surface or at predetermined depths and tracked acoustically. Hurricanes,
which are called typhoons in the Western Pacific, evolve from tropical cyclones. In
order for hurricanes to form, sea-surface temperatures down to a water depth of
45m must be warmer than 27º C and upper-level winds must be weak. A
cross-sectional view shows that a hurricane consists of a central eye where
winds are light and skies are clear, surrounded by the eyewall, a thick band of
dense clouds with hurricane-force winds. Cross-Section
of a Hurricane |
|
|