Air-Sea Interactions I

I. Origin of Currents

Motion in the Sea -- Wind

air moves because less-dense air rises away from earth and more-dense air sinks toward earth

air that flows horizontally along the earth's surface is wind
air circulating in this manner forms convection cells

air becomes less dense when:

water vapor content increases (humidity);

when it is warmed; and

when atmospheric pressure decreases

air becomes more dense when:

water vapor decreases (dry air);

when it is cooled; and

when atmospheric pressure increases

amount of solar energy reaching earth's surface per minute (insolation rate), varies with transparency of environment (how many clouds), angle of sun above the horizon, and local reflectivity of the surface
thus, solar angle will be affected by latitude and season

sunlight striking polar latitudes spreads over a greater area, filters through more atmosphere, and approaches surface at a low angle, favoring reflection (so it solar energy there provides less heat)

in contrast, high solar angle in tropics distributes same amount of sunlight over a much smaller area; because it comes at almost a vertical angle, it passes through less atmosphere and reflection is minimized (happens between 23.5°N and 23.5°S)

mid-latitude heating is affected by season because earth's rotational axis is tilted (23.5°) -- called an orbital inclination; this affects which hemisphere is leaning towards or away from the sun during any one season

imagine an earth with no continents and no rotation, but heated like the natural earth

wind pattern is very simple: around the equator, air warmed from below rises and flows toward poles where it is cooled from below; it sinks to flow back towards the equator (forms atmospheric circulation cells)

BUT, due to unequal distribution of sun's heat over earth's surface, large amounts of heat and water vapor are transferred to the atmosphere around the equator; thus, equatorial regions are warm and wet while polar regions are dry and cold

in this model earth, surface winds of Northern Hemisphere blow from north to south (northerly) and upper winds of the atmosphere blow from south to north (southerly); surface winds of Southern Hemisphere blow from south to north (southerly) and upper winds of the atmosphere blow from north to south (NOTE: winds are named for the direction from which they blow)

BUT: the earth rotates, so if rotation is added, winds are displaced/deflected because they move over a spinning object

surface of the earth rotates from west to east, making the sun appear to rise in the east and set in the west
at the equator, earth's surface is far from its axis of rotation, so it moves rapidly (speed is equal to the diameter of the earth)

this is because the equator lies on the "fattest" part of the earth, so in order for that larger circumference to go 360° relative to the "thinner" part of the earth (near the poles, where the circumference is smaller), it must go faster

but at the poles, earth's surface is much closer to the axis of rotation, so it moves more slowly (~ half the speed of equator)
these differences in speed of rotation cause additional atmospheric cells and deflections:

get a pair of circulation cells on either side of equator (Hadley cells); at mid-latitudes, air descending at 30° latitude turns poleward and is joined by air returning from the north/south (Ferrel cells); at the poles air at the surface blows towards the equator and by 50°/60° latitude, it has taken up enough heat to rise -- however because of density differences it can't mix with air in Ferrel cells, so it turns poleward, creating a polar cell

in Northern Hemisphere, deflection is always to the right of the direction of air motion (or clockwise)

thus if air was moving northward, it is deflected to the east because the eastward motion is more rapid at lower latitudes from which the air is traveling, so the air gets entrained eastward
conversely, if air was moving southward from the poles, it is deflected to the west because it is moving from an area of slower motion to one of rapid motion and it lags behind

in Southern Hemisphere, deflection is always to the left of the direction of air motion (or counterclockwise)

thus if air was moving northward, it is deflected to the west
if moving southward, it is deflected to the east

for air moving parallel to the equator:

air moving to the east is moving more quickly than the earth beneath and is affected by a centrifugal force acting outward from the earth's axis of rotation that is stronger than the centrifugal force at the earth's surface -- this causes a deflection towards lower latitudes
air moving to the west is moving more slowly than the earth beneath and is affected by an outward-acting centrifugal force weaker than the centrifugal force on the earth's surface (which generates a force towards the axis of rotation in the direction of gravity) -- this causes a deflection toward higher latitudes

this deflection is called the Coriolis effect or Coriolis force; it is dependent on the rotation rate of the earth on its axis
the magnitude of the Coriolis force increases with:

increasing latitude -- the poles move more slowly than the equator
increasing speed of the moving air

it modifies the winds considerably