Atmosphere-ocean interactions Exchange of energy between oceans & atmosphere affects character of each In oceans –Atmospheric processes alter salinity & temperature of sea water –Temperature & salinity changes lead to significant differences in water density –Density changes lead to vertical circulation of sea water –Vertical circulation affects the distribution of nutrients & oxygen in sea water, thereby controlling the distribution of life in sea

Ocean-atmosphere interactions In atmosphere –Ocean-atmosphere interactions affect temperature & humidity of air –Temperature & humidity changes lead to significant differences in local air density –Density differences drive circulation in atmosphere, generating a pattern of prevailing winds Feedback mechanisms are important –Prevailing winds drive ocean surface currents, which affect humidity & temperature of atmosphere, which in turn alter the atmospheric circulation pattern, etc.

Heat budget is immensely important Heat budget controls sea water temperature & salinity directly & indirectly. We need to know energy inputs & outputs in order to understand system, & need to know how heat is exchanged.

Heat exchange mechanisms Conduction - molecules or atoms exchange kinetic energy of directly through collisions Convection - heat transfer via 3-step process: conduction, advection, & conduction Radiation - objects loose or gain energy by emitting or absorbing electro-magnetic (EM) radiation –Objects above 0°K emit EM radiation –Energy content of radiation proportional to frequency & inversely proportional to wavelength –Emitting & absorbing objects behave like black-bodies

Heat exchange mechanisms Conduction - molecules or atoms exchange kinetic energy of directly (through collisions) with other molecules or atoms Convection - heat transfer a three step process of conduction, advection, & conduction –Conduction warms part of an object –The warmed part moves to a cooler location –Heat conducts from the transported warm part to its cooler surroundings Radiation - objects loose or gain energy by emitting or absorbing electro-magnetic (EM) radiation –Objects at any temperature above 0°K emit EM radiation –Energy content of EM radiation proportional to frequency & inversely proportional to wavelength –Emitting & absorbing objects behave like black-bodies

Structure of earth’s atmosphere Temperature (T) In troposphere (0 to km), T decreases uniformly; troposphere heated by IR radiation from Earth’s surface Temperature minimum occurs at tropopause In stratosphere (10-17 km to ~ 50 km), T increases with height due to absorption of UV radiation from space (by ozone) In mesosphere (50 to 100 km), T decreases with height In thermosphere (at heights >100 km), T increases with height, again due to absorption of UV radiation

Structure of earth’s atmosphere Pressure (P) At 5 km height, P is half of sea level pressure At 16 km height, P is one tenth of sea level pressure –90% of atmosphere’s mass is within 16 km of earth, i.e. in the troposphere At 30 km height, pressure is 1/100th of sea level pressure –99% of atmosphere’s mass is within 30 km of surface, i.e. in the troposphere & the lower third of the stratosphere

Chemical composition of atmosphere (in weight percent)

Distribution of constituents in dry air Turbulent mixing means that most constituents of dry air are uniformly distributed Two exceptions –Ozone (O 3 ) - O 3 generated by reaction in upper atmosphere; concentration highest 15 to 80 km above surface –Methane (CH 4 ) - CH 4 generated by biological processes at Earth’s surface; concentration is highest at surface & falls to very small values >10 km above surface

Variations in atmospheric composition Concentration of H 2 O is highly variable [H 2 O] controlled by tropospheric processes –Evaporation from sea surface –Condensation leading to precipitation Concentration of CO 2 is varying rapidly [CO 2 ] affected by many natural processes; human activity led to significant, rapid change in [CO 2 ] –Prior to industrialization & significant human population growth, [CO 2 ]  260 ppm –In 1958, [CO 2 ] = 315 ppm –In 1980, [CO 2 ] = 338 ppm –In 1999, [CO 2 ] = 367 ppm

Importance of variations in atmospheric composition H 2 O, CO 2, O 3, & CH 4 are important in the heat budget of the atmosphere-ocean system All are greenhouse gasses,i.e. absorb IR radiation emitted at surface –H 2 O is the most significant greenhouse gas –O 3 controls stratospheric heat content –CO 2 has low concentration but absorbs IR radiation that would otherwise pass through an ‘IR window’ –CH 4 also low concentration but absorbs IR radiation that would otherwise pass through an ‘IR window

Source of heat (energy) in ocean- atmosphere system Primary source is insolation = radiation from sun Geothermal heat  1/5000th of energy reaching Earth from sun Insolation - mix of wavelengths (  ) of EM radiation –Corresponds to radiation from black-body with T  6000°C –Maximum intensity of radiation has  0.5 x m = 5,000 Å, corresponding to visible red light –Visible light + near infrared radiation + near ultraviolet radiation together account for more than 90% of energy from sun Call this ‘short wave radiation’

Source of heat (energy) in ocean- atmosphere system Primary source is radiation from sun, called insolation Next most important source, geothermal heat, contributes about 1/5000th of the amount of energy reaching Earth from the sun Insolation is a mix of wavelengths & frequencies of EM radiation, corresponding to radiation from a black-body with a temperature of ~6000°C –Maximum intensity of radiation has a wavelength of 0.5 micrometers (0.5 x m = 5,000 Å), corresponding to visible red light –Visible light + near infrared radiation + near ultraviolet radiation together account for more than 90% of energy from sun Use the term ‘short wave radiation’ to refer to the mix of EM radiation that makes up insolation because this radiation has wavelengths on the order thousands of angstroms

What happens to the shortwave radiation from the sun? 6% scattered off the top of the atmosphere 20% reflected off the top of clouds 4% travels through the atmosphere as shortwave radiation & reflects off land or sea surface 30% of incident insolation returned to space as short wave radiation; it does not affect earth’s heat budget in any way The remaining 70% of insolation is absorbed & utilized by atmosphere for some time

What happens to the shortwave radiation from the sun? 70% of incident radiation is absorbed by components in the earth system 19% absorbed by CO 2, water vapor, & dust in the atmosphere 51% travels through the atmosphere as short wave radiation & is absorbed by continents & oceans Oceans are more significant sink due to greater surface area & due to high heat capacity of water

Atmosphere-ocean system is not heating up at a rate commensurate with the amount added daily by insolation How does Earth system lose heat?

Heat exchange methods Surface exchanges with atmosphere by conduction (i.e. sensible heat exchange) & through phase changes (i.e. latent heat exchange) Surface & atmosphere radiate energy back into space –Earth's surface is about 300°K = 30°C, so the re- radiated EM radiation has lower energy content –Radiation from Earth is long-wave radiation, with a mean  10 x m = 100,000 Å –This radiation is invisible infra-red radiation

Heat exchange methods Continents & oceans exchange with atmosphere by conduction (i.e. sensible heat exchange) & through phase changes (i.e. latent heat exchange) Continents, oceans, & atmosphere radiate energy back into space as black bodies –Earth's surface is cooler than surface of the sun, so the re- radiated EM radiation has lower energy content –Corresponds to radiation from a black body with a temperature of about 300°K = 30°C –Radiation from Earth is long-wave radiation, with a mean wavelength of 10 micrometers (10 x m = 100,000 Å) –This radiation is invisible to us, but we sense it as infra-red radiation

Greenhouse effect 21% of energy received from sun is re- emitted as IR radiation by oceans & continents –About 1/3 of that IR radiation, 6% of the original 100% received from the sun, escapes directly into space –Atmosphere absorbs the remaining 2/3 of the IR radiation emitted at surface; 15% of the original 100% received from the sun is absorbed & stored by the atmosphere in a process that we call the greenhouse effect

Atmosphere’s heat balance Atmosphere is warmed by absorbing –Short wave radiation from the sun –Sensible & latent heat from Earth’s surface –Long-wave (IR) radiation from Earth’s surface Atmosphere itself emits long-wave (IR) radiation into space –An amount = 38% of the original 100% received from sun emitted by CO 2 & H 2 O –An amount = 26% of the original 100% received from sun emitted by clouds

Global average heat budget Earth system in rough balance Amount of energy received from sun as shortwave radiation = amount of energy radiated by Earth as long-wave (IR) radiation Temperature at which this balance is struck depends on how opaque is the atmosphere to long-wave (IR) radiation Opacity of atmosphere depends on its composition, primarily on the relative proportions of different greenhouse gasses