Lecture The Oceans

Transcription

1 Lecture The Oceans Moving Heat (Reprise) Sound Heat Budget of the Earth Basic budget Latitude variations Surface temperatures Sea ice Reminders - 3 ways to move heat Conduction -- Must be touching (Ocean Surface <--> atmosphere ) Convection -- Warm buoyant material rises, cool denser material sinks Radiation -- All materials give off electro-magnetic radiation with a maximum wavelength determined by the temperature Convection Heat transfer due to density-driven currents Convection very important in the atmosphere and the oceans Atmosphere - Heated at bottom, cooled at top Cooling Convection in the Oceans- Cooling of surface water in far north, far south Warming of surface water at low latitude North Atlantic Cooling Equator Heating Heating Sound Propagated over very long distances -- not "attenuated Marine animals -- communication & detection Scientists and the Navy -- depth & detection Velocity varies with depth depends on T, S, and pressure Max. V near surface: refraction (bending) out of that zone "shadow zone" 1

2 Sound Velocity varies with depth SOFAR Channels depends on T, S, and pressure Often, Max. V near surface: refraction (bending) out of that zone "shadow zone" Min. V. at ~ 1,000 m -- SOFAR channel refraction into that channel global-scale transmission Possible to measure changes in ocean temperatures by measuring speed of sound in SOFAR channels over huge distances "HEAT BUDGET" Where does incoming solar radiation go? Incoming Solar Energy can be: Reflected Absorbed Absorbed and re-radiated out to space The balance or imbalance of these: Controls earth surface temperatures Drives ocean and atmospheric circulation THE GLOBAL HEAT BUDGET Of incoming solar radiation how much is... Reflected back out to space? 35% (31% by the atmosphere, 4% by land and sea) Absorbed? 65% (17.5% in atm.; 47.5% by land & sea) ALMOST HALF OF THE INCOMING ENERGY IS ABSORBED BY THE LAND AND SEA 2

3 FIG. 6.2 Of the energy absorbed by land & sea (47.5%), how much is... 1) Re-radiated (infrared) directly to space? % 2) Transferred to the atmosphere?... 42% MOST OF THE SOLAR RADIATION ABSORBED BY LAND AND SEA IS TRANSFERRED BACK TO THE ATMOSPHERE AT SOME POINT Transfer of heat from land and sea to the atmosphere happens in two ways: 1) Evaporation and condensation of water vapor % (out of the total 42%) heat uptake by evaporation -- cools surface heat released in condensation -- warms atm. NOTE THE IMPORTANCE OF WATER VAPOR!!!! 2) Conduction and radiation = 12.5% Most of the infrared energy radiated by the earth s surface is absorbed in the atmosphere by "greenhouse gases" In absence of this greenhouse effect, the earth would be -10 C! Part of this is the greenhouse effect Does Energy Input Balance Energy Output? This is a Global Average, but... 3

4 The heat budget varies with latitude Solar energy influx decreases with increasing latitude max. at Equator min. at poles The heat budget varies with latitude Solar energy influx decreases with increasing latitude max. at Equator min. at poles...and at any given latitude, heat input output low latitudes: input > output high latitudes: input < output BUT...Global heat budget must be balanced: solar rad. = reflection + re-radiation If heat input output then temperature should increase or drop constantly, without end. A thermal catastrophe!?...no! What maintains the balance? Heat is transferred from lower latitudes to higher latitudes by circulation of oceans and atm. Warm water and air currents: Equator --> poles Cool water and air currents: poles --> Equator The latitude imbalance in solar-energy gains and losses is the driving mechanism for the circulation of the oceans and atmosphere. SURFACE TEMPERATURES Latitude "belts" of roughly constant T -- Altered in/ near oceans by surface currents Seasonal temperature variations: Continents -- large T range Oceans -- small T range Reason for this difference? Heat capacities: land vs. water Oceans stabilize surface T of Earth 4

5 Fig. 6.5 Fig. 6.4 Note two different scales!!! Fig. 6.6 SEA ICE Sea icebergs Extent of sea-ice coverage... 5

6 SEA ICE Sea ice icebergs Thickness of sea-ice limited... Ice is a thermal insulator Max. thickness (single season) -- ~ 2 meters Sea-ice formation: salinity (+density) increase Exclusion of salt ions from ice. Remaining high salinity water sinks. This forms the densest and deepest water masses in the oceans. 6