Transcript pptx
OEAS 604: Introduction to Physical Oceanography • Global Heat Balance • Chapter 2,3 – Knauss • Chapter 5 – Talley et al. Outline • • • • • Heat balance and budget – general properties Shortwave and Longwave radiation Latent and sensible heat flux Total heat budget – terms that contribute Atmospheric and oceanic redistribution of heat Heat in the Ocean – General Comments • • • Nearly all heat entering the ocean occurs at the air-sea interface (oceansediment interface inputs ~ 0.1 Watts m-2) To a first approximation, the mean temperature of the ocean does not change on an annual basis (steady state, the flux in equals the flux out) Steady state balance only applies at annual time scales and in integrative manner - heat content of the ocean changes on daily to seasonal time scales Flux out Flux in Heat is exchange between the ocean and the atmosphere occurs as a result of 1. 2. 3. 4. Short-wave radiation (insolation) received from the sun [Qs] Longwave radiation (net infrared radiation) [Qb] Latent heat flux (evaporation) [Qe] Sensible heat flux (air-sea temperature difference) [Qh] Change in Heat content with time Flux in Flux out QT = QS - (Qb + Qe + Qh ) + QV Advectio n At annual time scales and average over the ocean, the heat entering the ocean is balanced by the heat leaving, so: Qs = Qb + Qe + Qh Electromagnetic Radiation • Concentration of energy is not the same at all wavelengths but has a peak at a wavelength that is given by Wien’s Law • Stefan-Boltzmann Law – all bodies radiate energy at a rate proportional to the fourth power of their absolute temperature K4 The Solar Constant A flat plate just beyond the earth’s atmosphere, perpendicular to the rays of the sun receives about 1368 Watts m-2 (the solar constant). Earth is not a flat disk (area = πR2), but a sphere with surface area of 4πR2, so the energy is spread over larger area. On average, the top of the atmosphere receives 342 Watts m-2. This varies with at any given spot on Earth due to declination of the sun. Short-wave radiation (insolation) received from the sun [Qs] The sun radiates energy as a blackbody with a temperature of 5800°K Surface ocean radiates energy as blackbody with temperature of 17°C (290°K) Wien’s Law: The wavelength of maximum transmission is inversely proportional to the absolute temperature lmax b = K b=2,897,768.5 nm·K. • • • About 49% is in the visible spectrum (400-700 nm) About 9% is in the UV (~100-400 nm) Remainder in IR (~7002500 nm) Less irradiance reaches surface of Earth because of absorption by molecules, clouds or aerosols. Average loss by absorption is roughly 19% Factors Affecting Qs • Height of sun above horizon (function of latitude, season, and time of day) • The length of day (function of latitude and season) • Reflectivity - albedo • Attenuation – Clouds (absorb and scatter radiation) – Path through the atmosphere (function of height above horizon) – Gas molecules (H2O, O3, CO2) – Aerosols (volcanic, terrestrial and marine) – Dust Albedo: the ratio of the amount of radiation reflected by an object to the energy incident upon it Albedo varies considerably Planetary average is about 30% increase decrease increase decrease Talley et al. (2011) Increased ice cover leads to cooling of ocean Satellite-derived Aerosol Map MODIS Image/NASA Optical Depth Red – small particulates and smoke (Africa) and pollution (Europe, North America) Green – dust from Africa or sea salt in areas with high winds Cloud cover Red – shallow clouds Green – deep convective clouds Blue – mixed clouds Longwave radiation (infrared) or Back Radiation [Qb] All bodies with a temperature above absolute zero radiate heat energy The amount is proportional to the fourth power of the absolute temperature - Stefan-Boltzmann law: Qb = c sK 4 where cs = 5.67 × 10-8 W m-2K-4 ( Stefan-Boltzmann constant) and K is degrees Kelvin Using average temperature of the oceans of 18°C (~ 291 Kelvin), the back radiation of the oceans is Qb = 5.67 ´10-8 ( 2914 ) » 400 W / m2 Ocean absorbs about 50% of the incoming 342 Watts m-2 How can the ocean radiate more than twice the amount it absorbs? Greenhouse Effect Short wave length incoming radiation (mostly visible) passes through the atmosphere. Longer wave length back radiation (infrared) is more effectively absorbed and reflected back to earth by the gases in the atmosphere. Because atmosphere effectively traps the longer wave infrared wave lengths, the effective back radiation is roughly 50 to 75 W m-2 Net longwave (infrared) flux depends on: • Cloud thickness—Thicker clouds allow less heat to escape • Cloud height—clouds radiate heat towards Earth as black body (~T4) and high clouds are colder than low clouds • Water vapor content—more humid atmosphere lets less heat escape • Water temperature—hot water radiates more heat (~T4) • Ice and snow cover--water is warmer than ice and radiates back more heat • Greenhouse gas concentration—(CO2, water vapor, methane, ozone, etc…) Cloud fraction (monthly average for August, 2010) from MODIS on NASA’s Terra satellite Gray scale ranges from black (no clouds) to white (totally cloudy) From NASA Earth Observatory (2010) Talley et al. (2011) Cloud cover measured in oktas proportion in eighths of sky covered by clouds as seen in plan view 0=clear sky; 8=covered sky Used to reduce solar radiation from clear sky estimates Outgoing Longwave Radiation (OLR) for Sept. 15–Dec. 13, 2010 NOAA ESRL (2010); Talley et al. (2011) Latent heat flux (evaporation) [Qe] The energy that is added to break the hydrogen bonds to allow evaporation is removed by the water vapor. Evaporation removes heat. difficult to measure; biggest loss term in heat budget; estimated as about 100 W m-2 Sensible heat flux [Qh] • • • Heat energy transferred between the ocean surface and air The flux is proportional to the temperature gradient (temperature difference) Flux is also dependent on the turbulence in the ocean and atmosphere Controlled by wind speed and air-sea temperature difference Fog – atmosphere warmer Sea smoke – ocean warmer Distribution of 100 units of incoming shortwave radiation from the sun to Earth’s atmosphere and surface: long-term world averages. (Talley et al. 2011) Global Heat Budget This balance is approximate and does not hold for any given location in the ocean or over short time periods. Significant heat is transported by atmosphere and ocean Averaged over an entire year this balance holds, but there are seasonal changes in oceanic heat content. Heat input through the sea surface (1 PW = 1015 W) (world ocean) for 1º latitude bands for all heat flux components From the NOCS climatology (Grist and Josey, 2003); Talley et al. (2011) From: National Oceanography Centre, Southampton (NOCS) climatology (Grist and Josey, 2003) Talley et al. ( 2011) Advective Heat Term [Qv] • Rate of heat loss/gain by a water body due to currents usually in a horizontal direction • Measured through a vertical area of one square meter • Small but critical component of heat budget Poleward heat transport (W) for world’s ocean Annual mean Summary of various direct estimates (points with error bars, based on temperature and velocity) and indirect estimates. The range of estimates illustrates the overall uncertainty of heat transport calculations. From Ganachaud and Wunsch (2003), reprinted inTalley et al.(2011) Numbers/arrows are the meridional heat transports (PW) calculated from ocean velocities and temperatures (Bryden and Imawaki (2001); Talley 2003). Positive transports are northward. Data from NOCS climatology (Grist and Josey, 2003) Next Class • Continue heat budget • Chapter 2,3 – Knauss • Chapter 5 – Talley et al.