Oceans and our Climate

 

I. The Earth's Heat Budget or Cycle

A. Traces the movement of heat from sources to sinks.

B. Different forms of energy (radiation) from the Sun

1. Ultraviolet - absorbed by ozone (O3) in the upper atmosphere

a. We can't detect this form of energy but it does damage to living tissue.

2. Infrared- absorbed by CO2 & H2O in atmosphere.

a. We sense this form of energy as heat.

3. Visible - reaches surface of the Earth.

a. We sense this form with our eyes as light.

C. As you can see from a diagram of the Earth's heat balance the Earth radiates back out into space as much energy as it receives from the Sun.

1. If the output didn't equal the input the Earth would be continuously heating up which it is not. 

2. As you can see about half of the incoming energy never reaches the surface but is reradiated back out into space or absorbed by the atmosphere. 

3. About half of the radiation that reaches the surface does so only after being scattered and somewhat altered by interaction with clouds and other components of the atmosphere (i.e., gases).

D.  The solar radiation that does strike the Earth's surface is largely absorbed by the oceans although some is reflected directly back into space. 

1. The energy reaching the surface that is not reflected is emitted back out into space by radiation, conduction, and evaporation of water from the world's oceans.

a. CONDUCTION - molecular process involving passage of rapid molecular motion from molecule to molecule.

1) Like the handle of metal spoon in hot liquid.

b. RADIATION - direct transmission of heat from the source such as the way the Sun's energy reaches us.

1) Like a quartz heater (can go thru vacuum)

c. EVAPORATION - water vapor back up into atmosphere takes latent heat energy with it.

E. The rate at which solar radiation reaches the Earth's surface varies depending on a number of things. Most importantly:

1. Over the course of hundreds to thousands of years - differing factors related to Earth's orbit

2. Season of the year  - differing position of the Earth with respect to the Sun, which influences whether the Northern or Southern Hemisphere is struck by the Sun's rays at a 90o angle.

3. Time of day - obviously when the Sun is "up" for a particular region more solar radiation reaches that portion of the Earth.

4. With latitude - this effect is less obvious but can be understood by remembering these facts

a. Heat spread out over a larger area near poles because light rays strike the surface at less than a 90o angle.

b. Heat passes through a thicker layer of atmosphere.

F. The diagram shows the relationship between the amount of incoming and outgoing solar radiation at various latitudes. More solar radiation strikes Earth's surface at the Equator than at the poles. More of this incident radiation leaves Earth's surface for outer space at high latitudes than at the Equator.  This suggests that temperatures at the equator should be steadily increasing and temperatures at the poles should be steadily decreasing. This is not the case, however, so significant poleward transport of the heat received at the equator must occur.

1. In the tropics the primary mechanism of heat removal from the surface of the Earth is evaporation. The heat that is removed is carried toward the poles by water vapor (as the latent heat of vaporization of water) and released at higher latitudes where condensation and precipitation occur. 

G. Review of Earth's Hydrologic Cycle

1. The Earth is such a habitable planet because of the abundance of surface water and its extraordinary properties.

2.  The hydrosphere is the fluid exterior covering of the Earth.  The hydrologic cycle describes the storage and movement of water within the hydrosphere.

a. Most of the water is in the ocean

b. Most of freshwater is in the Polar Ice Caps

c. Rivers mainly transport rather than store water.

H. Review the details of the variation in world-wide rates of evaporation and precipitation at low versus high latitudes.

1. The result of the predominance of evaporation over precipitation in the tropics is a poleward transport of heat received at the Equator. In fact, 20% of the heat received in the tropics goes to higher latitudes. This heat is transported:

a. mostly as the latent heat of vaporization because the air currents move from low to high latitudes and their is more evaporation than precipitation at low latitudes and more precipitation than evaporation at high latitudes.

b. to a much lesser extent by the major ocean surface currents (i.e., Gulfstream) = 2-4%

2. Looking in more detail remember that right around the Equator precipitation exceeds evaporation so that the world's great rain forests are in these latitudes. This is responsible for the low salinities of surface seawater in these latitudes. Farther north and south at about 30o north and south of the Equator evaporation exceeds precipitation and the world's great deserts exist in these latitudes. This is responsible for the high salinities of surface seawater in these latitudes.

 

II.  Moderating effects of the extensive amounts of water in  our hydrosphere.

A. These effects are clearly displayed in the climates of coastal cities and countries. Here is an example of the temperatures of two coastal cities in Canada compared to an inland city (all three at approximately the same latitude). As you can see the inland city is much colder in the winter and much hotter in the summer.

 

 

Victoria

Winnipeg

St. Johns

Mean January minimum

2

-22

-7

Mean July maximum

20

27

21

 

B. Also the surface temperature of the Pacific Ocean at 40 oN changes around 6-8oC during the year, whereas the seasonal average temperature fluctuation on the interior of the Asian continent at the same latitude is about 38oC.

C. Causes of this moderation

1. High heat capacity (or specific heat) of water. Remember this is the amount of heat required to raise the temperature of a gram of a substance by 1oC (1 calorie/gm/oC for water). This is about 6 times greater than that for most soils, so the land surface increases in temperature when heated by the Sun much more rapidly than does the water.  The land also, therefore, gives up its heat much more rapidly, so when the Sun goes down, the land cools off much more rapidly than does the water. 

a. The differing heat capacities of water & soil cause the sea breeze to come in during most afternoons in areas without strong prevailing winds.

2. Penetration - another reason for the greater temperature  fluctuations on land is that the heating and cooling takes place only in the surface layers while solar radiation  penetrates  much farther down into water. Therefore, the incoming solar radiation is spread out in a much greater volume of water than of sand and on land the heat is concentrated within a few centimeters of the surface.

3. Mixing - the surface layers of the oceans are very thoroughly mixed due to winds causing extensive wave action.  This enhances movement of solar radiation downwards in the water column and permits absorbance of a greater amount of energy.

4. Evaporation- we talked before of the high latent heat of  vaporization of water. That is the amount of heat energy necessary to actually evaporate water once its temperature has been raised to the boiling point. Once the energy has been input to get the molecules vibrating enough to raise their temperature to boiling, more energy is necessary to break the hydrogen bonds between the water molecules and free them from the water so they can go up into the air. The fast moving molecules that escape the surface of the liquid water and go into the vapor phase carry this energy (this latent heat of vaporization) with them.  This latent heat is stored with the water vapor until it condenses into precipitation. A rainfall of 1 cm releases more heat to the Earth's surface and atmosphere than does an entire day's sunshine over the same area.

D. Sea Ice formation - because of the large latent heat of fusion of water (like its large latent heat of vaporization), it requires the removal of large amounts of heat from seawater  to  create great thicknesses of ice.  The winters on Earth are neither long enough nor cold enough to remove the vast amounts of heat required to cause formation of a layer of sea ice much thicker than a few meters.  Also, ice is a great insulator, and once it forms on the surface of the sea it prevents the loss of heat in the water into the cold overlying air therefore preventing freezing to great depths.

1. The large, often thick (several hundred meters) ice bergs you hear about floating in the northern oceans don't form from sea ice but rather they break off of continental glaciers in places like Greenland and Antarctica when these glaciers move down to the edge of the ocean.

2. This sea ice formation helps moderate climate at high latitudes (high latent heat of fusion). Forms only where less saline (& therefore less dense) water is trapped near surface.  Average salinity (o/oo) of ice = 7 o/oo.

 

III. Composition and pressure characteristics of the atmosphere

A. The composition of dry air

 

Gas

Concentration (%)

Nitrogen (N2)

78.1

Oxygen (O2)

20.9

Argon (Ar)

0.9

Carbon Dioxide (CO2)

0.037

All others

trace

 

B. Water also makes up an important component of our atmosphere in humid climatic regions.

1. Under most Earth surface conditions because of the relatively low surface temperature of the Earth water is in the solid or liquid form because of the "stickiness" of water (i.e., the hydrogen bonding between water molecules).

2. In general as the temperature of an air mass increases its ability to hold water increases. This is because at higher temperatures the water molecules move faster and collide more forcefully making it more difficult for them to stick together and condense into the liquid form.

3. The amount of water vapor that air can hold depends on the temperature of the air and increases almost by a factor of two for every 10oC increase in temp.

C. Describing the water content of the atmosphere

1. Definitions

a. Air is saturated if it is holding as much moisture as it can. The amount of water an air mass can hold at saturation depends on the temperature of the air mass.

1) For example air at 20oC with 2.3% water vapor is saturated.

b. If a saturated air mass is rapidly cooled so that precipitation does not fall from it immediately, it is said to be supersaturated and eventually the appropriate amount of water will condense and precipitate out of it to bring it down to saturation.

c. Absolute humidity- number of grams of water vapor per m3 air.

d. Relative  humidity-the percentage of the maximum possible water content that a given air mass is holding.

1) For example, as you can see if a m3 of air at 25oC contains 37 gms of water vapor then it is saturated and the relative humidity is 100%. If that same air mass held only half as much water vapor/m3 the absolute humidity would be 18.5 gms/m3 and the relative humidity would be 50%.

e. Dew point is the temperature to which a given air mass must be lowered in order to saturate it (i.e., in order for it to achieve 100% relative humidity)

D. Effects which result from the world-wide differences in evaporation and precipitation rates.

1. Over the oceans surface EVAP > PRECIP and generally over the continents PRECIP > EVAP.

a. This excess of precipitation over land is triggered by the fact that air rises as it moves up over the continent, which causes it to cool (Because the source of the heat to the air mass is the surface of the Earth). This cooling decreases air's ability to carry water and precipitation is triggered. This causes rain shadows downwind from large mountain ranges.

E.Temperature and atmospheric composition and their relationship to the density of an air mass and the local atmospheric pressure

1. Atmospheric pressure is the force with which a column of overlying air presses down on an area of Earth's surface. 

                                 Force

          P R E S S U R E  =  -------------

                               Unit area

a. In the case of atmospheric pressures the force is due to the weight of the column of air sitting over top of a given area of the surface of the Earth.

1) Pressure decreases at higher elevations above the sea because there is less air above the top of a mountain than above a valley thousands of feet below it.

b. Where atmospheric pressure is greater, air (which after all is a gas and, therefore, highly compressible) is correspondingly denser.

1) High pressure zones = denser air

2) Low pressure zones = less dense air

2. Higher temperature = lower density because it causes a given air mass to expand thereby increasing the volume

3. Composition

a. Higher water vapor content = lower density because H2O is lighter than the N2 and O2 molecules, which make up the largest portion of our atmosphere.

4. Differential heating by the sun causes low- and high- pressure regions to develop on the surface of the Earth due to changes in these three factors.

a. In equatorial regions, low-pressure areas are generated because air rises up and out of the region. This decreases the mass of the column of air near the surface of the Earth, thereby, generating the low- pressure region at the surface. The air rises for two reasons:

1) High input of solar radiation warms the air at the Earth's surface causing it to become less dense and, therefore, to rise.

2) Less importantly, since there is lots of evaporation at low latitudes the air there contains more of the light water molecules and is, therefore, less dense.

b. Conversely, cooler, drier air masses are denser and sink, thereby, generating high-pressure areas.

5. The average yearly pattern of high and low-pressure areas is shown in your textbook.

a.  This concept of high and low-pressure areas is so important because surface winds blow from areas of high pressure to areas of low pressure.

 

IV. Atmospheric Circulation (Redistribution of heat drives it)

A. Air moves because of the existence of high- and low-pressure areas and areas of denser and less dense air. Around the equator hot, humid air rises away from the surface of the Earth and in high latitudes cool, dry air sinks down towards the surface of the Earth. The sinking air displaces air where it settles and the rising air mass leaves a "gap" behind, which must be filled by some other air mass moving in along the surface of the Earth.

B. If the Earth did not rotate, and if there were no continents, the resultant circulation pattern would be quite simple.

1. The resultant model involves two large circulation cells; one in the Northern Hemisphere and one in the Southern Hemisphere. In the Northern Hemisphere the surface winds would blow from the north (NORTHERLY), whereas, in the Southern Hemisphere the surface winds would blow from the south (SOUTHERLY).

C. The Effects of Rotation

1. Consider next a model of a rotating water-covered Earth. Because the Earth spins it essentially moves out from under a surface seawater current or an air current in the atmosphere.

a. Imagine the situation of an observer standing on a stationary turntable. If she throws a ball it will move in a straight line and hit the target. However, if the turntable is rotating counterclockwise the observer will see an apparent deflection of the ball to the right.  Actually the ball travels in a straight line but the turntable moves out from under the path of the ball causing the apparent deflection.

b.  Analagously, when viewed from above the North Pole the Earth rotates in a counterclockwise direction and, therefore, this deflection of moving objects to the right occurs in the Northern Hemisphere.  The apparent deflection is reversed (to the left) in the Southern Hemisphere.

2. Gravity holds the Earth's atmosphere captive, but the atmosphere is not rigidly attached to the Earth's surface. There is little or low friction between the Earth's surface and the atmosphere so the atmosphere moves somewhat independently of the Earth's surface; like the ball mentioned in the example above.

3. For example,a parcel of air that appears to be stationary above a point on the equator is actually turning with the Earth and moving eastward at a speed of about 1700 km/hr  (1000 mph).

4. If a south wind blows this parcel of air due north, it is moved across circles of latitude with progressively smaller circumferences.  At these higher latitudes, points on the Earth's surface move eastward more slowly.

a. At 60 oN the eastward speed of a point on the Earth's surface due to rotation is only half the speed at the equator.

5. For this reason, a parcel of air that was originally moving northward away from the equator carries with it its initial eastward speed, so that it is now moving eastward at a speed that is greater than the eastward speed of the Earth's surface at this higher latitude. Therefore, the air parcel appears displaced to the east, relative to the Earth's surface.

 

IN THE NORTHERN HEMISPHERE THIS DEFLECTION IS TO THE RIGHT OF THE ORIGINAL DIRECTION OF AIR MOTION.

 

6. If this same situation occurs in the Southern Hemisphere with a parcel of air moving southward from the equator due to the north wind, the deflection is still to the east relative to the Earth's surface, however, this deflection is now to the left of the direction of the air motion.

7. If an air parcel moves southward toward the equator in the N. Hemisphere, it moves from a latitude where a position on the Earth has a low eastward speed south to a latitude where the Earth has a higher eastward speed. This relationship causes the southward-traveling air to fall behind the position on the Earth, so that the air is moving westward relative to the Earth.

a. The deflection is still to the right of the direction of motion of the air mass.

8.  This effect is called the CORIOLIS EFFECT.  You may have heard this phenomenon referred to as the Coriolis Force, but it is not a true force, but rather an effect due to the rotation of the Earth and our choice of a frame of reference.

9. Eastward and westward-moving air is also deflected to the right in the North. Hem. due to differing strengths of gravitational fields and "centrifugal forces".  These effects supplement the effects of Coriolis.

10. The net effect of the Coriolis Deflection is to constantly deflect currents (wind and ocean) to the right in the North. Hem. and to the left in the South. Hem.

a. Actual amounts of deflection are dependent upon latitude and speed of currents.

D. The Resultant Wind Bands

1. The Coriolis Effect short-circuits the single-cell circulation system described on the earlier stationary-Earth model.

2. We'll concentrate our effort on the North. Hem., but similar wind patterns arise in the South. Hem. because of this effect.

3. The cold, dense, dry sinking air moving south from the North Pole along the surface of the Earth is continually deflected to the right by the Coriolis Effect.  This generates the POLAR EASTERLIES.

a. As this air moves south it picks up heat and water vapor and becomes less dense so that it rises around 60oN.

4. At the same time the hot, moist, "light" air rising away from the equator and moving northward as an upper level wind is gradually cooled and dried out (by precipitation of the water it contains) so that its density increases and it sinks around 30oN.  This initiates the surface wind movement from north to south towards the equator, which is deflected to the right thereby generating the NORTHEAST TRADES.  This lower latitude circulation cell is called the Hadley Cell.

5. The rising air at 60oN and the sinking air at 30oN drives an intermediate cell which causes air to move northwards along the Earth's surface from 30oN to 60oN.  This wind, which is initially moving northwards, is deflected to the right by the Coriolis Deflection and generates the WESTERLIES.

E. High and Low Pressure Areas

1. This pattern yields high-pressure areas at the poles and 30oN and 30oS and low-pressure areas at the equator and 60oN and 60oS.

2. These high and low-pressure areas are determined by the location of rising and falling air masses and in turn determine the climate (weather conditions averaged over 30 years) at various latitudes.

a. 30oN and 30oS = Horse latitudes.

Sinking air masses

Low rainfall

Air warms as it falls and can, therefore, hold more water

High evaporation

High atmospheric pressure

No wind because air movement is vertical

Spanish ships got stuck in no wind-no drinking water regions and threw their horses overboard. Madeira Islands (NW Africa) populated by Spanish horses.

              Note location of world's deserts.

b. Equator = doldrums

Rising air masses

High rainfall

Air cools as rises and cannot hold as much water so precipitation occurs

Low evaporation

Low atmospheric pressure

No wind because air movement is vertical

Ships got stuck but at least had water

c. 60oN and 60oS

Rising air masses

High rainfall

Low evaporation

Low atmospheric pressure

No wind because air movement is vertical

F. Seasonal modifications of the wind bands

1. Indian Monsoon - During summer near the Indian Ocean the continent of India heats up more than the water of the Indian Ocean (Remember, the water has a higher heat capacity than the land.) As a result, air masses over India are heated, become less dense and rise. The gap they leave at the surface is immediately filled by air blowing in off the cooler Indian Ocean to the south of the continent. These air masses also bring with them moisture from the Indian Ocean causing the wet, summer monsoon for which India is famous. However, during the winter the situation is reversed. In that season, the land mass cools faster than the Indian Ocean so the air is heated over the Ocean and not over the land. As a result, air over the Ocean rises and is replaced by air blowing off the land mass that is north of the Ocean. This is the cool, dry winter monsoon.

2. Other changes in the locations of high- and low-pressure areas around the world occur seasonally. This causes the prevailing wind direction in some regions to change direction with the seasons. This effect is much less pronounced in the Southern Hemisphere (The Water Hemisphere) where there are many fewer continents at Earth's surface.

G. Jet Stream - Narrow, rapid, eastward-moving "river" of air that runs just below the top of the troposphere centered at an altitude of about 10 km. It meanders north and south and affects how far south the cold, polar fronts reach. It can also steer a warm, tropical air mass far to the north. It was first discovered during World War II in the Fall of 1944 during the first massive air strike on Tokyo. This raid involved 100’s of bombers that dropped 1000’s of bombs. Only 48 hit the city. They figured out later that as the pilots turned around Mt. Fuji and started back over the city to actually drop their bombs they were suddenly traveling about 140 mph faster than their theoretical maximum speeds. This was because they had gotten caught up in the Jet Stream.

H. Hurricanes, cyclones, typhoons – These intense tropical storms start in the Tropics and migrate north and west in the Northern Hemisphere and south and west in the Southern Hemisphere. At the equator strong low pressure regions are created by the pumping of lots of moisture and heat into the air. When condensation occurs and this heat is liberated strong winds develop around these low pressure areas. They generally follow a curved path westward in response to the Northeast and Southeast Trade Winds and the Coriolis Effect.

1. Beneath the eye of the storm (that is in the low pressure region at its center) atmospheric pressures are low enough to allow sea level to rise. This is in part responsible for the severe coastal flooding that often accompanies these storms when they make landfall. Coastal flooding is exacerbated by waves pushed by the storm winds. This flooding is called the “Storm Surge” or “Storm Tide”, but be aware it is not caused by the same factors that generate Earth’s tides.

2. In the Northern Hemisphere the winds circulate in a counter-clockwise direction around the eye. In the Southern Hemisphere the circulation is in a clockwise direction. You can see the reason for this if you draw a picture on a piece of paper of a low-pressure region surrounded by high-pressure. As winds move from the surrounding high-pressure region in toward the low-pressure in the center of the storm they are deflected to the right by the Coriolis Effect.  This develops the counter-clockwise circulation seen in the Northern Hemisphere.

3. Within the eye of the storm winds are minimal, there is very little rain and often few clouds. However, around the eye in the eye wall the winds reach their maximum which may be upwards of 150 mph.

4. These storms are classified on the basis of wind speed using something called the Saffir-Simpson Scale:

    

Saffir-Simpson Scale

Category

Wind Speed (mph)

Tropical Depression

<39

Tropical Storm

40-73

I

74-95

II

96-110

III

111-130

IV

131-155

V

>155

 

a. The five numbered categories are hurricanes.

5. There are usually 10-100 hurricanes/ year on Earth.

6. They must be over water to gain the energy to keep them going so after they make landfall they begin to weaken and finally disappear all together.

7. The right semi-circle of a hurricane in the Northern Hemisphere is the most destructive because on that side of the storm circulating wind speed is augmented by the forward motion of the storm. Places located on this side of the storm are the ones that most need to be evacuated, so it is very important for forecasters to be able to predict the location at which the hurricane will make landfall.

8. One of the most destructive hurricanes in American history was Andrew. It passed over central Florida from east to west causing about $40 billion worth of damage. Andrew was a category V storm in Puerto Rico where its winds reached >200 mph. Although the wind speed had slowed by the time it reached Florida it traveled so slowly across the state that it was doing damage for a very long time. This storm also had devastating impacts on the barrier islands of Louisiana in the Gulf of Mexico. This low-lying coast is eroding at a rate of about 60 ft/year even without hurricane damage and Andrew totally destroyed some of these islands.

I. Antarctic Ozone Hole - Ozone gas (O3) is abundant high in Earth's atmosphere in the stratosphere. It absorbs UV radiation from the Sun, preventing most of it from reaching Earth's surface. Excessive exposure to UV radiation causes cataracts and skin cancer in humans and can seriously damage organisms living near the water surface in streams, lakes and the ocean. In the 1970's scientists became concerned about the steady decreases in ozone concentration in the stratosphere. Over the time period from 1969 to 1986 levels over Antarctica appeared to have declined by as much as 10%. By the late 1980's depletions in some regions may have reached 40% below their normal levels. Declines were less severe in the Northern Hemisphere. The cause of this depletion was increasing levels of synthetic compounds called chlorofluorocarbons, or CFC's. One of the best known of the CFC's is Freon (DuPont's trade name for the compound). At Earth's surface these compounds are nearly chemically inert. A few decades ago they were extensively used as propellants in aerosol sprays and as coolants in refrigerators and air conditioners. When they escape into the upper atmosphere they remain there for many decades where they are broken down and release their chlorine atoms. These chlorine atoms act as catalysts, which destroy ozone molecules without being consumed themselves. It is estimated that a single chlorine atom can destroy 100,000 ozone atoms during its tenure in the stratosphere. In the extreme cold of the Antarctic stratosphere, clouds remove nitrogen from the stratosphere. Nitrogen compounds normally inactivate chlorine and reduce ozone depletion, so when nitrogen is depleted ozone depletion progresses. By 1995 Antarctic ozone depletion bottomed out and ozone levels began to increase. This was a result of the phaseout of industrial use of CFC's.