The Ocean Heat Budget

1
Chapter 5

• The Ocean Heat Budget
• Physical oceanography
• Instructor Dr. Cheng-Chien Liu
• Department of Earth Sciences
• National Cheng Kung University
• Last updated 16 October 2003

2
Introduction

• The sunlight reaching Earth
• 1/2 ? oceans and land
• 1/5 ? atmosphere
• The heat stored by the ocean
• Evaporation and infrared radiation ? atmosphere
• Transported by current ? ameliorate Earths
  climate ? development of ice age

3
The oceanic heat budget

• Heat budget
• The sum of the changes in heat fluxes into or out
  of a volume of water
• QT QSW QLW QS QL QV
• The resultant heat gain or loss QT wm-2
• Insolation QSW, the flux of sunlight into the sea
• Net Infrared Radiation QLW, net flux of infrared
  radiation from the sea
• Sensible Heat Flux QS, the flux of heat out of
  the sea due to conduction
• Latent Heat Flux QL, the flux of heat carried by
  evaporated water
• Advection QV, heat carried away by currents
• Change in energy
• DE CpmDT
• Cp ? 4.0 ? 103 J kg-1 0C-1

4
The oceanic heat budget (cont.)

• Importance of the ocean
• During an annual cycle
• Cp(Rock) ? 800 J kg-1 0C-1 ? 0.2Cp(water)
• Exchange heat depth
• Water 100m
• Land 1m
• Exchange heat mass
• Water 100?1000 100,000 kg
• Land 1?3000 3,000 kg
• Typical change in temperature
• Water 100C
• Land 200C
• Ratio of seasonal heat storage DEoceans / DEland
  ? 100

5
Heat budget terms

• QSW
• Factors influencing QSW
• qs fn(latitude, season, time of day)
• Length of day fn(latitude, season)
• The cross-sectional area
• The surface absorbing sunlight fn(qs)
• Attenuation
• k fn(clouds, path length, gas molecules,
  aerosol, dust)
• Reflectivity
• fn(qs, surface roughness)
• Fig 5.2 surface solar insolation
• Average annual range (Fig 5.3)
• 30 lt QSW lt 260 Wm-2

6
Heat budget terms (cont.)

• QLW
• Fig 5.4 Atmospheric transmittance
• Greenhouse effect
• Greenhouse gasses
• Factors influencing QLW
• The clarity of the atmospheric window
• fn(clouds thickness, cloud height, atmospheric
  water-vapor content)
• Changes in water vapor and clouds are more
  important than changes in Tsurface
• Water Temperature
• Ice and snow cover
• Average annual range
• -60 lt QLW lt -30 Wm-2

7
Heat budget terms (cont.)

• QL
• Factors influencing QL
• Vwind
• Relative humidity
• Average annual range
• -130 lt QL lt -10 Wm-2
• QS
• Factors influencing QS
• Vwind
• Air-sea temperature difference
• Average annual range
• -42 lt QS lt -2 Wm-2

8
Direct calculation of fluxes

• Gust-Probe Measurements of Turbulent Fluxes ? the
  only method
• Characteristics
• On low-flying aircraft or offshore platforms
• Usually at 30m height
• Need fast-response instruments
• Measure u, v, humidity, T
• Expensive
• Measurements ? large space or longer time
• Only for calibration
• Calculation (Table 5.1 Notation Describing
  Fluxes)
• T ltru'w'gt rltu'w'gt ? ru2
• QS Cpltrw't'gt rCpltw't'gt
• QL LE ltw'q'gt

9
Indirect calculation of fluxes Bulk formulas

• Bulk formulas
• The observed correlations between fluxes and
  variables
• T rCDU102
• QS rCpCSU10 (Ts - Ta)
• QL rLECLU10 (qs - qa)
• Ta ? thermometers on ships
• Ts ? thermometers on ships or AVHRR
• qa ? relative humidity made from ships
• qs ? Ts (assuming saturated air on surface)
• CD, CS, CL ? correlating gust-probe measurements
  with the variables in the bulk formulas (Table
  5.1 suggested values)

10
Indirect calculation of fluxes Bulk formulas
(cont.)

• Calculations of each variable
• Wind stress and speed
• See chapter 4
• Sources of error
• Sampling error (insufficient measurements in time
  and space)
• CD
• Insolation
• QSW S(1-A) C
• S 1365 W m-2
• A albedo
• C constant including absorption by ozone, other
  gasses and cloud droplets
• Sources of error
• Angular distribution of sunlight reflected from
  clouds and surface
• Daily variability of QSW

11
Indirect calculation of fluxes Bulk formulas
(cont.)

• Calculations of each variable (cont.)
• Rainfall (water flux)
• Difficulties of ship measurements
• Rain falls horizontally and its path is distorted
  by the ships superstructure
• Most rain at sea is drizzle ? difficult to detect
  or measure
• TRMM (Tropical rain measurement mission 1997 )
  (Fig 5.5)
• Infrared observations ? height of cloud tops
• Microwave radiometer
• Re-analyses of the output from numerical weather
  forecast models
• Ship observations
• Combinations
• Sources of error
• Rain rate ? cumulative rain fall (Sampling error)
• Miss storm

12
Indirect calculation of fluxes Bulk formulas
(cont.)

• Calculations of each variable (cont.)
• Net long-wave radiation
• F ltegt (Fd ST4)
• ltegt average emissivity of the surface
• Fd downward flux (from satellite, microwave
  radiometer data or numerical models)
• S Stefan-Boltzmann constant
• F tends to be constant over space and time ? not
  necessary to improve
• Latent heat flux
• QL rLECLU10 (qs - qa)
• Difficult to measure from satellite (not
  sensitive to qs)
• Two indirect ways to use satellite measurements
• Monthly averages of surface humidity ? water
  vapor in the air column
• SST from AVHRR water vapor and wind from SSM/I

13
Indirect calculation of fluxes Bulk formulas
(cont.)

• Calculations of each variable (cont.)
• Sensible heat flux
• Ship observations of air-sea temperature
  difference and wind speed
• Numerical models output
• Almost small everywhere

14
Global data sets for fluxes

• COADS ? NOAA
• Two releases
• The first COADS release 70 million reports (1854
  1979)
• The second COADS release (1980 1986)
• 28 elements
• Weather, position,
• Summaries
• 14 statistics for each of eight observed
  variables
• Ta, Ts, Vwind, Psurface, q, cloudiness
• 11 derived variables
• Three principal resolutions
• Individual reports
• Year-month summaries in 20 by 20 boxes
• Decade-month summaries

15
Global data sets for fluxes (cont.)

• Satellite data (Table 5.3 levels)
• Operational meteorological satellites
• NOAA series of polar-orbiting, meteorological
  satellites
• SSM / I
• Geostationary meteorological satellites
• NOAA (GOES), Japan (GMS) and ESA (METEOSTATS)
• Experimental satellites
• Nimbus-7, Earth Radiation Budget Instruments
• Earth Radiation Budget Satellite, Earth Radiation
  Budget Experiment
• The European Space Agency's ERS-1 2
• The Japanese Advanced Earth Observing System
  (ADEOS)
• Quicksat
• The Earth-Observing System satellites Terra,
  Aqua, and Envisat
• Topex/Poseidon and its replacement Jason-1.

16
Global data sets for fluxes (cont.)

• International Satellite Cloud Climatology Project
• Collect observations of clouds
• by dozens of meteorological satellites (1985
  1995)
• Using visible-light instruments on polar-orbiting
  and geostationary satellites
• Goals
• Calibrate the the satellite data
• Calculate cloud cover using carefully verified
  techniques
• Calculate surface insolation

17
Global data sets for fluxes (cont.)

• Global Precipitation Climatology Project
• Three sources of data ? rain rate
• Infrared observations (GOES) ? the height of
  cumulus clouds
• The basic idea the more rain produced by cumulus
  clouds, the higher the cloud top, and the colder
  the top appears in the infrared. Thus rain rate
  at the base of the clouds is related to infrared
  temperature
• Measurements by rain gauges on islands and land.
• Radio-frequency emissions (SSM/I) from
  atmospheric water droplets
• Accuracy 1 mm/day
• Data available
• 2.50 ? 2.50 grid from July 1987 to December 1995
  (NASA GSFC)
• Xie and Arkin (1997)
• A 17-year data set based on seven types of
  satellite and rain-gauge data combined with the
  output from the NCEP/NCAR reanalyzed data from
  numerical weather models

18
Global data sets for fluxes (cont.)

• Reanalyzed Data From Numerical Weather Models
• Calculated from weather data using numerical
  weather models by various reanalysis projects
• Recent suggestions on the data
• Biased fluxes, The time-mean model outputs ? ship
  observations
• More accurate in the northern hemisphere
• Zonal means differences gt 40 Wm-2 between model
  and COADS data
• ECMWF data set averaged over 15 years gives a net
  flux of 8 Wm-2 into the ocean
• Summary
• Reanalyzed fluxes ? forcing ocean, GCM
• COADS data ? calculating time-mean fluxes

19
Geographic distribution of terms in the heat
budget

• Figure 5.6
• The mean annual radiation and heat balance
• Top of the atmosphere Insolation infrared
  radiation
• 342 107 235
• At the surface, latent heat flux net infrared
  radiation insolation
• 168 324 390 24 78
• Sensible heat flux is small
• The sunlight reaching Earth
• 1/2 (168 / 342) ? oceans and land
• 1/5 (67 / 342) ? atmosphere
• Driving of the atmospheric circulation
• Thunderstorms are large heat engines converting
  the energy of latent heat into kinetic energy of
  winds

20
Geographic distribution of terms in the heat
budget (cont.)

• Fig 5.7
• The zonal average of the oceanic heat-budget
  terms
• Zonal average an average along lines of constant
  latitude
• QSW max at EQ
• QS is small
• The areal-weighted integral ? 0 ? errors in the
  various terms in the heat budget
• Can be reduced by using additional information
  (constraint)
• Heat and fresh water transport
• Fig 5.8 annual-mean QSW and QLW
• Fig 5.9 annual-mean QL
• Fig 5.10 annual-mean QS

21
Meridional heat transport

• Meridional transport
• North-south transport
• Meridional heat transport
• ? the divergence of the zonal average of the heat
  budget measured at the top of the atmosphere
• ? satellite
• Assumption steady state

22
Meridional heat transport (cont.)

• Heat Budget at the top of the Atmosphere
• Measurement
• Insolation ? meteorological satellites and by
  special satellites (e.g. the Earth Radiation
  Budget Experiment Satellite)
• Back radiation ? infrared radiometers
• The net heat flux across the top of the
  atmosphere insolation - net infrared radiation
• Calculation
• Average the satellite observations zonally ?
  zonal average
• Calculate their meridional derivative ? the
  north-south flux divergence
• Divergence the heat transport by the atmosphere
  and the ocean
• Errors
• Calibration of instruments
• Inaccurate angular distribution of reflected and
  emitted radiation

23
Meridional heat transport (cont.)

• Oceanic Heat Transport
• Three methods
• Surface Flux Method
• Measurements wind, insolation, air, and sea
  temperature, and cloudiness
• Bulk formulas ? the heat flux through the sea
  surface
• Integration ? the zonal average of the heat flux
  (Figure 5.7)
• the meridional derivative ? the flux divergence
  heat transport in the ocean
• Direct Method
• Measurements current velocity and temperature
  from top to bottom along a zonal section spanning
  an ocean basin ? the heat transport
• The flux northward velocity ? heat content
• Residual Method
• Measurements atmospheric measurements or the
  output of numerical weather models ? the
  atmospheric heat transport
• The oceanic contribution the top-of-the-atmosphe
  re heat flux (satellite) - the atmospheric
  transport (Figure 5.11)

24
Meridional fresh water transport

• The Earth's water budget
• Dominated by evaporation (86) and precipitation
  (78) over the ocean
• A map of the net evaporation (Fig 5.12)
• Meridional transport of fresh water by the
  Atlantic (Fig 5.13)
• Same calculation as the heat transport
• Significance
• Understanding the global hydrological cycle,
  ocean dynamics, and global climate

25
Variation in solar constant

• Solar constant ? constant
• Sunspots and faculae (bright spots)
• ? the output varied by 0.2 over centuries
• ? the changes in global mean temperature of
  Earth's surface of 0.4C (Fig 5.14)
• A small 12yr, 22yr, and longer-period variations
  of SST
• measured by bathythermographs and ship-board
  thermometers over the past century
• Solar variability ? climate change ?

26
Important concepts

• Sunlight is absorbed primarily in the tropical
  ocean. The amount of sun-light changes with
  season, latitude, time of day, and cloud cover.
• Most of the heat absorbed by the oceans in the
  tropics is released as water vapor which heats
  the atmosphere when water is condenses as rain.
  Most of the rain falls in the tropical
  convergence zones, lesser amounts fall in
  mid-latitudes near the polar front.
• Heat released by rain and absorbed infrared
  radiation from the ocean are the primary drivers
  for the atmospheric circulation.

27
Important concepts (cont.)

• The net heat flux from the oceans is largest in
  mid-latitudes and offshore of Japan and New
  England.
• Heat fluxes can be measured directly using fast
  response instruments on low-flying aircraft, but
  this is not useful for measuring heat fluxes over
  oceanic areas.
• Heat fluxes through large regions of the sea
  surface can be calculated from bulk formula.
  Seasonal, regional, and global maps of fluxes are
  available based on ship and satellite
  observations.

28
Important concepts (cont.)

• The most widely used data sets for studying heat
  fluxes are the Comprehensive Ocean-Atmosphere
  Data Set and the reanalysis of meteorological
  data by numerical weather prediction models.
• The oceans transport about one-half of the heat
  needed to warm higher latitudes, the atmosphere
  transports the other half.
• Solar output is not constant, and the observed
  small variations in output of heat and light from
  the sun seem to produce the changes in global
  temperature observed over the past 400 years.