Geol 117 Home | Lectures | Review

ATMOSPHERE

Importance

.) O2 and CO2 for life processes; 2.) Greenhouse warming; 3.) Filters harmful radiation (ultraviolet); 4.) Ocean-atm. interactions; 5.) Heat Exchange; 6.)Winds drive ocean surface currents

Composition

Dry atmosphere
N2 78%, O2 21%, Ar 1%, CO2 0.036%
All other gases, (He, H2, etc) < 0.002%
Variable constituents
Gases: ozone (O), CO, CH4, N-O gases, S-O gases
Aerosols: dust, pollen, water droplets
Water vapor: "Humidity" depends on T

Human modification

Atmospheric Pressure P = F/A

"Hydrostatic" -- equal in all directions (like the oceans)
Pressure at sea level: 76 cm-Hg = 29.92 in-Hg = 1013 millibars (1 bar = 106 dynes/cm2)

Surface pressure -- changes with vertical air movement
high pressure = descending air
low pressure = ascending air

Atmospheric Zones

What differentiates the Troposphere from the Stratosphere?
Where is ozone found and why?

Global Prevailing Winds

Prevailing winds occur in 30 degree latitude belts

Trade Winds: Low latitudes, E --> W
Westerlies: Mid latitudes, W --> E
Polar Easterlies: High latitudes, E --> W

What accounts for this pattern?
1) Latitude radiation imbalance
2) Earth's rotation -- Coriolis effect

Radiation Imbalance

Equatorial zone (gain>losses): max. solar heating and high evaporation --> rising air masses.
Polar zones (losses>gain): min. solar heating and low evap. --> sinking air masses.
Net result: Global convection cells (Winds are surface components of circulation)

Coriolis Effect

Earth's rotation changes the initial direction of winds & currents:
N. Hem.: deflection always to the right
S. Hem.: deflection always to the left

Cause: Different latitudes rotate at different rates, meaning that the earth's Rotational velocity increases from poles to Equator

Winds and ocean currents have both an initial velocity and direction, and an initial rotational velocity that depends upon latitude.
As they cross latitudes, winds and currents are rotating at different velocities than Earth's surface.

Winds and currents are deflected a lot by the Coriolis Force because they are in continuous motion over long distances.

The Result is that the GLOBAL WIND SYSTEM has Major zones:

Trade Winds
Westerlies
Polar Easterlies

Complications

Convergent & divergent zones (surface and upper troposphere):

Convergent -- air masses coming together
Divergent -- air masses moving apart

Descending air creates zones of : 1.) high atm. pressure, 2). sinking dry air- nice weather
Ascending air creates zones of: 1.) low atm. pressure, 2.)high rainfall -- rising moist air


"Jet Streams" -- strong winds in upper atm. that lie above boundaries between the major zones

Other Regional Modifications

Differential heating of air and land resulting in Seasonal & Daily heating cycles

Oceans and lakes -- little T change
Land areas -- significant T change
  1. Daily cycle of winds in coastal areas
    Day: onshore winds (peak at mid-afternoon)
    Night: offshore winds (peak in early morning)
  2. Seasonal monsoons (India, southeast Asia)
    Summer: hot continent, rising air, onshore winds (and rain)
    Winter: cold continent (Tibet), offshore winds
  3. Seasonal changes in wind patterns and pressure zones
    over continents and oceans

Summer: Ocean (cool, high pressure) Land (warm, low pressure)

Winter: Ocean (warm, low pressure) Land (cool, high pressure)

Semi-permanent seasonal zones of different pressure

Sub-Regional High and Low Pressure areas move with the Global Prevailing Winds and Jet Stream

Winds (air flow) around pressure zones are deflected due to Coriolis effect:

Clockwise around (and out of) Hi-Pressure cells (N. Hem.)
Counterclockwise around (and into) Lo-Pressure cells (N. Hem.)

Ocean Circulation and Currents

Surface Circulation

* Horizontal currents in upper few 100 meters; speed = about 1 m/s
* Driving force: Global Prevailing winds initiate currents
* Modifying factors: deflection by ...

1) Coriolis effect [right in N.Hemisphere, left in S.Hemisphere]
2) Continents
General pattern -- rotary circulation (gyres); major gyres centered in sub-tropics

Some important major surface currents

  1. Gulf Stream
  2. Equatorial currents and counter-currents
  3. West wind drift - circles Antarctica
    Very important- connects Atlantic and Pacific

How does ocean circulation change as continents drift? or What was Global climate 100 million years ago?

WATER TRANSPORT AT SURFACE AND DEPTH (model of V. W. Ekman)
Surface currents are at 45° to wind direction (due to Coriolis effect)
Successive deeper layers- angle is greater and greater
This is the "Ekman spiral" of water movement
- to a depth of about 150 m
"Ekman transport" - - average direction of this layer is 90° from wind direction

Western Intensification of currents in the gyres

Because of the details of winds and the Coriolis effect:
-Western currents: strong, narrow, deep (Gulf Stream)
- Eastern currents: slow, broad, shallow (Canary Current)
- This effect not very important in S. Hemisphere (positions of land masses)
 
UPWELLING AND DOWNWELLING INDUCED BY EKMAN TRANSPORT
Ekman transport can:
  1. Drive surface waters apart creating zones of upwelling,
  2. Force them together creating zones of downwelling
  3. Drive surface waters away from coasts (upwelling)
  4. or force them onto coasts (downwelling)
Regions of important upwelling and downwelling
1. Equatorial upwelling
2. Upwelling along west side of continents (especially South America and Africa)
* Upwelling and downwelling along coasts may change with seasons due to seasonal wind changes
Importance of upwelling: Brings nutrient-rich deep waters close to surface creating regions of high productivity
 
What are EDDIES?

What are El Niño (ENSO) conditions?

El Nino is a change in ocean surface and atmospheric conditions in Equatorial Pacific about every 3-7 years.

Deep Circulation

* Driving force: creation of dense water masses at the surface of high-latitudes oceans (particularly the Atlantic).
1) Coriolis effect
2) Continents and mid-ocean ridges
* General pattern -- sinking, spreading, eventual upwelling and mixing

Where does this mixing occur? New data shows that a lot of mixing occurs in the Scotia Sea near the Antarctic Pennisula where the deep water is pushed up over undersea ridges and seamounts.

Review: Temperature and Salinity control the density of surface sea water and thus its tendency to sink.

Density-driven circulation is "thermohaline" circulation.
T and S are determined by processes occuring at the surface:
- - Exchange of heat with atmosphere (T) (conductive and convective cooling)
- - Exchange of water with atmosphere (S) (evaporation)

Densest water masses of world ocean are formed at high latitudes in the Atlantic:
Highest latitude, coldest surface T
High salinity because of Evaporation of already saline waters from Mediterranean and Caribbean

Surface and deep circulation are coupled in the "Global Conveyor Belt"

Moves heat and salt Globally

- - Transport of surface waters to high latitudes (North Atlantic and Antarctic)
- - Sinking of dense cool saline water and flow at depth
- - Upwelling and mixing -- return to surface

Jargon: Antarctic Bottom Water (AABW), North Atlantic Deep Water (NADW)

Q.Where does Pacific and Indian Deep water come from?

INTERMEDIATE WATER MASSES- moderately dense; typical depth 1 km.; examples:

Q. Has the global conveyor system always worked as now?

Q. how long does it take for a block of water to complete the circuit?

Waves

What are waves?

Periodic oscillations, orbital motion of water
Generated by forces: wind, earthquakes, etc.
Restoring force: usually gravity
therefore Waves transmit energy

Origins and Types

Wind-generated waves -- most commonly observed
Tsunamis -- earthquake-generated, common in Pacific
Tides -- gravity of Moon, Sun; forces in Earth's rotation
Seiches -- standing waves, oscillating water levels and currents

Wave Parameters

Crest and Trough, Height, H, Wavelength, L, Steepness, H/L, Period, T
Velocity of an individual wave, C = L/T, Velcoity of a wave group, V

Orbital Motion of Water in Waves

Diameter decreases with depth -- no motion at D > 1/2 L (depth of wave action)
Wavelength vs water depth
D > 1/2 L -- circular, wave does not "feel" bottom
-- "Deep-water" waves
D < 1/20 L -- elliptical, wave motion is impeded by the bottom -- Shallow-water waves
Relation between L and D distinguishes deep water waves from shallow water waves

WAVES IN THE OPEN OCEAN

1. Wind-generated waves

Deep-water waves -- D >> L
H = 1 - 15 m
L = 50 - 500 m
T = 5 - 20 sec
C = 30 -100 km/hr

Wave theory:
L = (g/2)*T*T
Since C=L/T, then
C = 1.56*T and C = 1.25*L = 1.25 * sqrt[L]
(T is in the units of seconds, L is in meters, and C is in meters/sec)

Note: both C and L increase with increasing T

Controls on wave parameters -- Energy transfered by wind

Wind speed (the most important factor)
Wind duration (how long the wind blows in a constant direction)
Wind fetch (the distance over which wind blows in a constant direction)

Increase speed, duration, fetch --> increase H, T, L, C

Dispersion

Waves with long T and L have highest speed (C)
Sort themselves out as they travel from storm center
Accounts for "swell" at sea -- long-T waves

2. Tsunami

T = 10 -20 min (600-1,200 s)
L = 100 - 200 km (100,000 - 200,000 m)
H = 1 - 2 m

H/L very low -- not detectable in open ocean!

These are Shallow-water waves: L >> depth of ocean

Speed controlled by depth only (wave theory)
C = [gD] = [gD]1/2 = 200 m/s (400 mph)
Travel at maximum allowed speed in ocean

H increases to 20+ m as they move onshore!
Energy focused by bottom topography and man-made barriers

ENCROACHMENT OF WIND-GENERATED WAVES

(1) Changes in wave characteristics -- "feel bottom" in shallow water (D < L/2). "Deep-Water" --> "Shallow Water" waves

C & L decrease, T remains constant, H increases (wave energy confined to smaller area)
When D < L/20, C & L controlled only by depth.
Top of wave advances faster than deep part --> "breakers," "surf" when H/L > 1/7 (D ª H)

(2) Changes in wave direction -- refraction. Bending of wave "fronts" (crests) and "rays" (^ to fronts, direction of wave energy)

.... waves approach at oblique angle (not parallel)
.... irregular topography of sea floor and coast

Why does refraction occur? -- speed C of different parts of wave front changes as waves advance through water of different depth.

Shallow waters:
C decreases a lot
Fronts (energy) focused toward shallows, e.g. headlands

Deeper waters:
C decreases less
Fronts (energy) bend away, e.g., embayments

Consequences:
Erosion of headlands -- focus of wave energy
Deposition in bays -- little wave energy
Current parallel to coasts

(3) Longshore transport

Wave fronts and onshore "swash" -- oblique to beach front
"Backswash" ^ to beach front
Resultant = longshore transport, water and sediment
Erosion & transport -- strong wave action
Deposition -- low wave energy.
spits, quiet bays
convergence in bays

Q. What is a rip current and where does it usually appear?

Natural Beach Processes

Beach = accumulation of sediments from rivers, headland erosion
Beach dynamics -- continuous sediment movement due to waves and longshore currents.
Summer -- gentle waves, onshore transport & deposition
Winter -- storm waves, erosion and seaward transport
Longshore transport -- depends on wave energy

HUMAN INTERVENTION IN BEACH/COASTAL PROCESSES

(1) Dam coastal rivers [water supply, flood control, recreation]
Reduced sediment input to coasts
Longshore tranport continues
"Downstream" beaches are eroded
(2) "Groins" to minimize loss of beach sand
Upstream deposition -- current speed reduced
Downstream erosion -- current resumes and regains
its suspended load of sediment
(3) "Jetties" to protect harbors
Same pattern of deposition and erosion
(4) Breakwaters to protect harbors
Reduce wave energy --> reduce longshore currents
--> deposition in harbor
Sand deposition requires continuous dredging
Erosion of downstream beaches as longshore transport resumes

Tides

Definition: Periodic rise and fall of water level along coastlines related to the phases of the Moon.
Cause: balance of two celestrial forces:
1. Gravitational attraction between Earth and the Moon (and Sun).
2. Centrifugal forces in the rotation of the Earth around the center of mass (center of gravity) of the Earth-Moon system.

Types

Tidal periods and inequalities -- Equilibrium Theory of Tides.

Explaining types of tides from Equilibrium Theory:

Semi-diurnal tides (2x per day) . . occur as the Earth rotates beneath the tidal bulges.
Any location on Earth should experience 2 high tides and 2 low tides per revolution (per day).
Expected T = 12 hrs exactly -- the case for solar semi-diurnal tides.
Lunar semi-diurnal tides: T = 12 hr 25 min. because
Moon revolves as Earth spins.
Any location on Earth's surface must rotate a little further each day (about 50 min. more rotation) to keep up.
"Lunar tidal day" = 24 hr 50 min.
Period of the lunar semi-diurnal tide is one-half of that, or 12 hr 25 min.
Diurnal tides and semidiurnal mixed tides . . occur because the Moon and the Sun are not directly overhead at the Equator, but at a different latitude, the"Declination"

Sun's declination -- 23.5 deg N to 23.5 deg S, T = 1 year
Moon's declination -- 28.5 deg N to 28.5 deg S, T = l8.6 years (the "lunar cycle").

Tidal bulges are in both the N. and S. Hemispheres when the moon is not over the equator. Resulting tides as Earth rotates:
Diurnal (once per day) immediately beneath the bulge
Semi-diurnal at the Equator and elsewhere depending on phase in the lunar cycle.
Semi-diurnal mixed elsewhere depending on phase in the lunar cycle.
 

The sun has the same effect on tides as the moon, but weaker.

  1. If the sun is close to the moon in the sky (new moon), the sun's tidal effect strengthens that of the moon- highs and lows are augmented (Spring tides)
  2. If the sun is opposite the moon (full moon), the sun's tidal effect strengthens that of the moon (remember, there is a bulge on the side of the earth away from the moon)
  3. Halfway in between these two cases (first or last quarter), the sun's tidal effect works against the moon's- highs and lows are weakened (Neap Tides)

Result is Two-week variations in tidal ranges (Spring and neap tides)

Actual tides in ocean basins and coastal areas

Each ocean basin and each coastal area responds uniquely to tidal forces. Heights and periods of actual tides do not exactly follow the "equilibrium" model.

1. Tides are shallow-water waves (both "standing" and moving)
Cannot keep up with tide-raising forces (i.e., position of Moon)
Direction and speed are altered:
- - friction with bottom
- - reflection from continents and continental margins
- - refracted as they move into shallow, coastal waters
2. Continental barriers interrupt the passage of tides.
3. Tides are subject to the Coriolis effect
Large distance and long duration of tidal motions
Crests of tide waves "rotate" around central point like a stationary (standing) wave
 
Tidal prediction at any location (based on the l8.6-year lunar cycle)
1. Record observed tidal variations
2. Compare E-M-S motions to observations
3. Determine the specific components of E-M-S tide-raising
forces that contribute to tidal periods and heights
4. Determine local factors influencing tides
("dynamic responses" of ocean basin and coastal area)