Chapter 7 Oceaonography

The amount of solar radiation hitting the Earth varies with latitude

• Rays perpendicular to surface => more

  • Between Tropic of Cancer and Tropic of Capricorn

• Rays oblique to surface => less

• 35 units of solar energy immediately reflected to space

• 65 units of solar energy absorbed by Earth’s surface and atmosphere

  • Earth’s surface: 47.5 units

  • Atmosphere: 17.5

• Maintain heat balance: Earth must reradiate 65 units of energy back into space

  • Earth’s surface: 5.5 units reradiated

  • Atmosphere: 59.5 units reradiated

• Earth’s surface has a net gain of 42 units of heat while the atmosphere has a net loss of 42 units of heat

  • The loss of heat in the atmosphere is replaced by the heat gained by the Earth through evaporation, conduction, and re-radiation

• To understand the heat budget of a portion of the ocean

  • The total energy absorbed

  • The loss of energy due to evaporation

  • The transfer of heat (advectively) through currents (input and outflow of energy)

  • Warming or cooling of the overlying atmosphere due to energy at the sea surface

  • Heat re-radiated to space from the sea surface

• The intensity of solar radiation available at the Earth’s surface varies with latitude and time of year

• Solar radiation intensity at middle latitudes between about 40 degrees and 60 degrees N & S is highly variable annually

  • Angle of sun’s rays reaching the surface is slightly variable at these latitudes

• Solar radiation intensity is fairly constant through the year in tropics

• Polar latitudes are subject to severe changes in length of daylight

• Land and ocean respond differently to annual changes in solar radiation

  • Land has low heat capacity, gains or loses heat over short period of time (such as overnight)

    • Oceans have very high heat capacity, absorbing or releasing heat with very small changes in temperature (think about maritime vs. continental climates)

• Water has a higher heat capacity than land

  • More heat can be added or lost without a change in temperature

• The annual range in T will be smallest in the ocean than on land

  • Range = difference between smallest and highest T

• There is less land in the Southern Hemisphere than in the Northern Hemisphere

• The average annual range in sea surface temperatures is quite small because of water’s high heat capacity and the transfer of heat through the water by mixing

  • Sea surface temperature variations range from

    • 0 degrees to 2 degrees celsius in tropics

    • 5 degrees to 8 degrees celsius at middle latitudes

    • 2 degrees to 4 degrees at polar latitudes

• Ozone absorbs UV and changes wavelengths from light to heat (causes increase in temperature)

• Atmosphere is reasonably well-mixed envelope of gasses roughly 90 km (54 mi) thick

• 4 layers of atmosphere from lowest to highest elevation:

  • Troposphere

  • Stratosphere

  • Mesosphere

  • Thermosphere

• Density of the atmosphere decreases rapidly with increasing height

  • Roughly 90% of the mass of the atmosphere is found in the first 15 km (9 mi) and 99% of the mass in the first 30 km (18 mi)

• Refrigerant and propellant released into atmosphere

• Cause atmosphere to thin

  • CFC are broken down by strong UV radiation

  • Chlorine atoms are released that react with ozone molecules, depleting ozone layer

• Density of air is proportional to molecular weight of components in the air

  • Molecular weight of water vapor is much lower than the three major components of air

    • This means that increasing the amount of water vapor in the air decreases the density of the air

• 3 major permanent gasses of the atmosphere

  • Nitrogen

  • Oxygen

  • Argon

• Controlled by 3 variables:

  • Amount of water vapor: increased water vapor = decreased density

  • Temperature: as temperature increases air density decreases

  • Elevation above sea level = pressure

    • As elevation increases, density decreases

• Most of the carbon dioxide on Earth is in the oceans

  • Smallest amount of CO2 is found in the atmosphere

• The oceans buffer the increase of carbon dioxide in the atmosphere

• CO2 is stored in 4 reservoirs

  • 3 active reservoirs:

    • Atmosphere

    • Oceans

    • Earth’s terrestrial system

  • 1 inactive reservoir

    • Earth’s crust

• The CO2 concentration is increasing because of fossil fuel burning

  • Curve in chart is called Keeling curve after man who started collecting data about CO2 concentration in atmosphere

• Short-wavelength incoming radiation is not blocked by CO2 but re-radiated long wavelength energy is, and this warms the atmosphere

  • Greenhouse effect

• Changing atmospheric chemistry can be monitored for past years by analyzing bubbles trapped in polar ice

  • Following Industrial Revolution, the concentration of CO2 has risen dramatically and continues to rise at an increasing rate

• Clear seasonal variation in CO2 related to increasing uptake by plants for photosynthesis in the spring and summer and increasing release through decay in the fall and winter

• Scientists estimated that greenhouse effect may produce a global warming of 2 - 4 degrees celsius over next 100 years

  • Could melt high latitude ice and raise sea level by as much as 1 m by the year 2100

• Greatest loss is over antarctica because the Antarctic winters are colder than Arctic winters

  • Ozone holes occur over poles

  • Ozone depletion is most severe in winter months

• Ozone in upper atmosphere absorbs UV light, which drastically decreases UV light reaching Earth 

  • If there are lower ozone concentrations in upper atmosphere, more UV light will reach Earth

    • Harmful to living organisms bc is has a short wavelength, which has more energy than long wavelength light

    • UV light is the highest-energy solar radiation that reaches Earth

• Ozone is very reactive and when it reacts with other chemicals ozone is destroyed

  • Chemicals reacting with ozone:

    • Chlorofluorocarbons (CFC) which were used in refrigerators, insulating foams, air conditioning systems (homes and cars)

    • Methyl bromide which is formed by single-celled organisms at surface of ocean, derived from pesticides, and released by industry and slow-smoldering burning of vegetation

• Ozone DOES NOT AFFECT GLOBAL CLIMATE

• Ozone is not a greenhouse gas

• An isobar is a line of connecting areas with the same air pressure

  • Isobars have the unit millibars in this figure

• The closer isobars are to each other, the stronger the winds

  • H is high pressure

    • H: Air is moving down = clear skies

  • L is low pressure

    • L: Air is moving up = precipitation

• Cross-hatching is rain

• Cold fronts occur when cold dense air wedges itself under less dense, warmer air

• Warm fronts are produced when less dense, warm air moves over denser cold air

• Earth’s surface heats more at equator than poles bc angle of sunlight

  • In a model with a non-rotating Earth, uniformly covered with water, this heat influx would produce a single, large convection cell in each hemisphere, extending from equator to the pole

    • Warm air rises at equator (forming a low-pressure system)

    • Cooled air sinks at the poles (forming a high-pressure system)

      • Rising equatorial air and the sinking polar air occurs in real life, and is the basis of Earth’s air circulation

  • Equatorial rising air cools as it ascends (because upper atmosphere is cold) causing water-vapor condensation

    • Condensed water vapor forms clouds and water drops fall as rain

      • Equatorial low-pressure area has a lot of rain

  • Upper atmosphere air is cold and dry, because all water vapor has been condensed and removed as rain

    • Polar areas have little precipitation

  • In general, low-pressure areas always have higher precipitation than high-pressure areas

• Surface winds blow from high-pressure areas to low-pressure areas

  • In model, surface winds would blow from the polar, high-pressure system towards the equatorial, low-pressure system

• As a result of the convection cell, high-altitude air will move away from the equator toward the poles

  • High altitude air releases heat as it more from the equator towards to poles

    • This way of moving heat energy is very important for global climate

• Winds are always named for the direction from which they blow

  • Different from ocean currents ( named for direction they are going towards)

• In model, all of the Northern Hemisphere would have surface winds from the north (north winds) and all of the Southern Hemisphere would have surface winds from the south (south winds)

Earth moves faster at wider areas because it has to make a circle at same time as thinner areas

• Earth’s surface locations rotate eastward at a speed that depends on latitude

• Air or water moving across Earth’s surface has a rotational speed at the origin point and it retains that rotational speed as it moves

• Deflection left in southern hemisphere; deflection right in northern hemisphere

  • It retains the rotational speed due to inertia

  • Compare this to how your body moves forward if you break quickly in your car - your body keeps moving forward due to inertia: your body moves forward with the speed it had before breaking

• If the air mass then moves closer to the equator, it will move over points on the surface that have a higher eastward velocity than it does

  • Consequently, to an observer on the surface, the air mass will appear to lag behind the eastward rotation of the planet, or it will appear to be moving westward 

• Wind/water moving east in relation to Earth’s surface will have a higher speed than Earth’s surface and will be deflected towards an area where the winds have equal speed, i.e. regions with larger circumference

• Wind moving west in relation to Earth’s surface, will be deflected toward areas with equal speed, i.e. regions with smaller circumference

• Earth’s rotation causes deflection of large-scale paths

• Objects in frictionless motion will appear to be deflected to the right of their direction of movement in the Northern Hemisphere and to the left in the Southern Hemisphere

  • This apparent deflection is called the Coriolis effect after Gaspard Gustave de Coriolis (1792-1843), who solved the problem of deflection in frictionless motion when the motion is referred to a rotating body and its coordinate system 

• As we move from the equator to the north we fall ahead of the eastward rotation of the earth

• As we move from the equator to the south we fall ahead of the eastward rotation of the earth

• The Coriolis effect only work on large spatial scales

• Contrary to popular belief, the Coriolis effect does not cause water to move in opposite directions in the Northern vs. Southern hemisphere

• Remember the model with one large convection cell in each hemisphere?

  • The coriolis effect divides the one convection cell into three separate convection cells in each hemisphere

    • Again, think of these cells as pillows going all around Earth

• Jet streams are strong winds always flowing from west to eat

  • Strongest het stream are the Polar jets, at around 7-12 km (23,000-39,000 ft) above sea level, and the higher and somewhat weaker Subtropical jets at around 10-16 km (33,000 - 52,000 ft)

    • Jet streams in the northern hemisphere are indicated by the small loops above the Mid-latitude cell and between the Hadley cell and the Mid-latitude cell

• Winds are named by the direction that are coming from

• Deflection short circuits the large convection cell that covers the whole area from equator to pole => several small convection cells are created

  • They are roughly 30 degrees

• You can derive the whole pattern on this figure by knowing that air rises at the equator and that each convection cell is ~30 degrees

• Go through each convection cell, the direction and the wind names. A wind is named for the direction is blows FROM

• Incredible and extreme rains are a result of climate change

  • Strong storms and winds

• Energy released into atmosphere adds energy to the jet stream, causing it to dip down and stay in place

• If air moves downwards, towards Earth’s surface, a high-pressure area forms

• If air moves upwards, away from Earth’s surface, a low-pressure area forms

• Wind direction around high-pressure systems in Northern Hemisphere

• Wind direction around low-pressure systems in Northern Hemisphere

• Wind direction around high-pressure systems in Southern Hemisphere

• Wind direction around low-pressure systems in Southern Hemisphere

• The polar jet stream results from a series of high and low pressure areas surrounding the Arctic Ocean. If you follow the red line of the wind directions in the Polar Jet Stream you will see that they coincide with the wind directions in the high and low pressure areas

• The location shown is the average location, but the Polar Jet Steam can be displaced north or south, and the meanders can also change shape. If the Polar Jet Stream over the continental US is displaced south, the temperature in the north will be dramatically lower. The amount of energy carried by the Polar Jet Stream is immense - it has been estimated that 1% would cover all of Earth’s present energy needs

• Flying along the Great Circle connecting two points is always the shortest route

  • However, if there are strong wind currents it can be faster to fly with this air current

  • This is the case when flying eat within a jet stream

    • The figure shows the flight routes between Tokyo and LA

• Two important concepts:

  • Sinking air pushes against the surface and forms high-pressure air system. Rising air moves away from the surface and formas a low-pressure air system

  • Land has lower heat capacity than the ocean, which causes land to heat faster than the ocean

• Top figure:

  • Average pressure in July (summertime in the northern hemisphere (NH) and wintertime in the southern hemisphere (SH))

  • In July, NH gets more solar radiation than the SH

    • Land will heat faster than water, which causes air over land to rise, forming a low pressure system

    • In contrast, ocean heats slower than land and air ocer ocean will sink forming a high-pressure system

• Bottom figure:

  • Average air pressure in January (wintertime in NH and summertime in SH)

  • In January, the SH gets more solar radiation than NH

    • Land will heat faster than water, which causes air over land to rise, forming a low pressure system

    • In contrast, ocean heats slower than land and air over ocean will sink forming a high-pressure system

• Sometimes called oceanographic equator

  • Low pressure system along the equator in air-circulation model, ITCZ is that low-pressure area and follows the outline of where the circulation systems along the equator meet

    • ITCZ is located farther north in July and farther south in January

      • ITCZ is not the same as the equator because of the unequal distribution of land and water between the hemispheres

• Local weather will be affected by air pressure

  • In a high-pressure area it will be sunny and dry

  • In a low pressure area, it will be cloudy and rainy

    

• India is in the Northern Hemisphere

  • Figure a represents summer (July) and figure b represents winter (January)

    • Note that these images represent surface winds

• In July, the difference in heat capacity between lans and water causes more heating of land than water, which causes a low pressure with rising air over land

  • The rising air over the continent pulls in moist air from the ocean

  • The ocean air is warm (close to equator) and carries a lot of moisture because warm air can carry more water vapor than cold air

  • Moist air rises in the low-pressure over land, resulting in heavy rains that are called monsoon rains

• In January, the ocean is warmer than land, forming a low-pressure area with rising air over the ocean

  • Air is pulled from the continent towards the ocean, and there are dry conditions over land

• Moist ocean air, with high water vapor content, blows towards land

• Land has a nearshore mountain range that forces the moist air upwards

• The upper atmosphere is cold and will condensate the water vapor into rain

  • Ocean side of the mountain chain gets a lot of rain, while the landward side of mountain chain is dry

• Ocean surface T > 27 C => low-pressure system can form a tropical depression

  • Hurricanes form only when the ocean-surface water temperature is high enough to cause evaporation

    • High evaporation only happens when the ocean surface is very warm - the cut off temperature is 37 C or 80 F

  • Winds around system clockwise in SH, counter-clockwise in NH

• High wind speed close to LP => air picks up water vapor

• Vapor condenses as it rises => heat is released => fuels the hurricane

• Typhoon, cyclone = hurricane formed in western Pacific

• Hurricanes have a LP air system in their center

  • Winds around LP systems spin in opposite directions in the northern and southern hemisphere

    • Hurricanes cannot cross the equator - the change in spin direction would stall and cancel it

• A hurricane cannot form at the equator because the Coriolis effect is not large enough

  • Coriolis effect occurs because of the difference in rotational speeds between latitudes

    • Difference in rotational speed is very small close to equator

• The South Pacific normally has a high-pressure area close to Chile and the prevailing surface winds are southeasterly

  • Air in this circulation cell will rise when it encounters Indonesia, which has high humidity

    • Cell is completed by air at high altitude flowing back towards South America

• Southeasterly surface wind pushes surface water from South America towards Indonesia, which causes upwelling along the South American coast

  • Upwelling adds nutrients to surface waters along the South American coast and is the reason for the very rich fisheries

• Top graph shows normal conditions

• Mid graph shows transition into El Nino conditions

• Bottom graph shows El Nino conditions

• Years with El Nino conditions tend to have higher average global T

  • When El Nino, conditions occur, the warm surface water is close to the west coast of South America and forms a “lid” preventing upwelling of the nutrient-rich deep water

    • As a result, the fish population declines rapidly and the effects on the economy of nations depending on these fisheries (e.g. Chile) can be severe

• Years with El Nino conditions tend to have higher average global T, but the more detailed effects of El Nino can be seen in this figure

  • The take-home message is that El Nino effects are not only local-they affect the whole planet

• We are in the first year of an El Nino

  • Will probably last 2 years total

    • Makes the globe warmer

    • May increase hurricane frequency

    • May make the winter warmer