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 (ocean-

sediment 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 inFlux out

Heat is exchange between the ocean and the atmosphere occurs as a result of

1. Short-wave radiation (insolation) received from the sun [Qs]2. Longwave radiation (net infrared radiation) [Qb]3. Latent heat flux (evaporation) [Qe]4. Sensible heat flux (air-sea temperature difference) [Qh]

At annual time scales and average over the ocean, the heat entering the ocean is balanced by the heat leaving, so:

Change in Heat content

with timeFlux in Flux out

Advection

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

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.

The Solar Constant

Short-wave radiation (insolation) received from the sun [Qs]

The sun radiates energy as a blackbody with a temperature of 5800°KSurface ocean radiates energy as blackbody with temperature of 17°C (290°K)

• About 49% is in the visible spectrum (400-700 nm)

• About 9% is in the UV (~100-400 nm)

• Remainder in IR (~700-2500 nm)

Wien’s Law: The wavelength of maximum transmission is inversely proportional to the absolute temperature

b=2,897,768.5 nm·K.

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 considerablyPlanetary average is about 30%

Talley et al. (2011)

decrease

increase

increase

Increased ice cover leads to cooling of ocean

decrease

Satellite-derived Aerosol MapMODIS Image/NASAOptical 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 coverRed – shallow cloudsGreen – 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:

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

Ocean absorbs about 50% of the incoming 342 Watts m-2

How can the ocean radiate more than twice the amount it absorbs?

Because atmosphere effectively traps the longer wave infrared wave lengths, the effective back radiation is roughly 50 to 75 W m-2

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.

Greenhouse Effect

• 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…)

Net longwave (infrared) flux depends on:

Cloud fraction (monthly average for August, 2010) from MODIS on NASA’s Terra satelliteGray 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)

difficult to measure; biggest loss term in heat budget; estimated as about 100 W m-2

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.

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.

Averaged over an entire year this balance holds, but there are seasonal changes in oceanic heat content.

Significant heat is transported by atmosphere and ocean

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

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)

Poleward heat transport (W) for world’s oceanAnnual mean

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)

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