OEAS 604: Introduction to
Physical Oceanography
• Global Heat Balance
• Chapter 2,3 – Knauss
• Chapter 5 – Talley et al.
Outline
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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
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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.
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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]
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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