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Ch. 2 The Earth System
The Ocean and cryosphere
•  Text, Ch. 2.1.1, Ch. 2.1.2
•  Supplementary reading (required for graduate
students, optional for undergraduate students):
–  Cravatte et al. 2009
–  Saenger et al. 2009
–  Shepherd and Wingham 2007
Scales and climate components:
climate
Glacier-interglacer: 104-5 yrs
Tectonic, Atmosphere,
ocean, land, biosphere,
cryosphere
millennium: 103 yrs
centennial: 100 yrs
decadal: 10-50 yrs
Atmosphere, ocean, land,
biosphere, cryosphere
Interannual : 2-7 yrs
seasonal: 1 yr
Smaller scales influence
large scales
geological: ≥106 yrs
Synoptic: days-a week
Diurnal scale: day
Mesoscale: a few hours
Microscales: a few minutes
Atmosphere, surface
conditions
Large-scale
influence
smaller
scales
weather
intraseasonal: 20-90 days
2.1.1: The Oceans
Why?
•  The oceans covers 72% of the earth’s surface, mass
~ 250 times of the atmosphere; regulate the earth’s
temperature
•  Main source of water vapor for the atmosphere
•  Main source of climate variability on interannual,
decadal, multi-decadal, centennial and millennial time
scales; e.g., ENSO, AMO,
•  Delayed climate response to change of the forcing;
•  Key part of carbon cycle
Current state of the ocean:
• 
• 
• 
Sea water density ~ 1.02-1.03×103 kg/m3 depends on its temperature,
salinity and pressure (T, S, P).
To focus on the small change that is matter for ocean circulation, that is
the depart from 1×103 kg/m3, we use, σ, which represents the departure
from 103 kg/m3, in unit of g/kg, to describe sea water density.
For example, σ=35 means that sea water density is 35 g/kg higher than
1.002X103 kg/m3
Density distribution in Atlantic
Lynn and Reid (1968)
source:http://oceanworld.tamu.edu/resources/
ocng_textbook/chapter06/chapter06_05.htm
Potential density:
•  Potential density σθ is the density a
parcel of water would have if it were
raised adiabatically to the surface
without change in salinity.
σθ = σ( s, θ, 0)
σθ is especially useful because it is a
conserved thermodynamic property
Change of potential density, T and S as
a function of depth from a sounding in
the subtropical Atlantic Ocean.
Numbers along the curve indicate
depth in unit of 100 m.
How do T and S influence density?
•  Ocean temperature, T, ranges from -2°C-32°C globally.
–  σê with Té for sea water monotonically. But for fresh water, its density
increase with T from 0°-4°C.
–  Question: Why both sea and lake ice float at top of the water?
•  Salinity, S, of the sea water ranges from 34-36 g/kg of water (or
°/°°).
–  Question: Where do you expect S be highest and lowest, respectively, on
earth?
•  Salinity has stronger influence
on σ near the freezing point. Its
influence decreases rapidly from
0-5C, reduces to 1/3 to ¼ when
T>5C.
•  Question: Is salinity change
more or less important in
tropics vs. sub-polar region?
ΔT equivalent to 1g/kg of ΔS in terms of
its influence on σ at sea level.
Pycnocline, thermocline, halocline:
•  Pycnocline is referred to a layer with strong density
gradient.
•  Thermocline: a layer with strong temperature
gradient.
•  Halocline: layers with fresh water above and salt
water below (stable).
•  In tropics: pycnocline follows thermocline because
vertical T gradient prohibit mixing of water with
different densities.
•  In high latitudes: pycnocline is often parallel to
halocline
Global distributions of T and S:
Global SST distribution:
Fig. 2.11 Annual mean sea surface temperature. Top panel: the total
field. Bottom panel: the departure of the local sea surface
temperature at each location from the zonally averaged field. [Based
on data from the Comprehensive Ocean-Atmosphere Data Set.
Courtesy of Todd P. Mitchell.]
Why?
Global salinity distribution:
Vertical and seasonal changes of T and S:
• 
• 
T and S decrease with depth of the ocean. Why?
Greatest T and S changes occur near ocean surface. Why?
Vertical profiles of T and S and
potential density (contours) from a
sounding in subtropical Atlantic Ocean.
•  What are the sources of influences on
ocean surface T and s?
•  Net heat flux at the surface:
–  Solar, infrared, sensible and latent fluxes
•  Mixing of cooler water from deeper ocean
•  Heat advection by ocean current
•  Net fresh water at the surface
•  Mixing with deeper water from below
•  Advect water with a different S
evap
precip
adv
IR, SH, LH solar
adv
Mixing of deeper water
Mixing of cooler water
Example: Determine the influence of weather on
S and T in the ocean surface layer:
•  A heavy tropical storm dumps 20 cm of rainfall in a
region of ocean where S=35.00 g/kg and the water is
well mixed in the top 50 m and this layer is referred to
as the ocean surface layer. How would this storm
influence the salinity of the ocean surface layer? The
density of pure water is 1.004X103 kg/m3.
•  After the storm, persistent wind causes evaporation
at the rate of 10 cm/month. How long it would take to
increase salinity back to 35 g/kg assuming no rainfall
during this period?
How change of rainfall influences ocean surface
and circulation?
S. Cravatte et al.: Observed freshening and warming of the western Pacific Warm Pool
Table 2 Expansions of the surface area (inside the three boxes drawn
in Fig. 1a) covered by waters warmer than fixed thresholds and
deduced from linear regressions
575
shows the time/longitude diagrams of the 4!S–4!N averaged SST and SSS. The SSS front clearly shifted eastward
during the past few decades (Fig. 7b). A linear fit to the
position of isohalines from 1955 to 2003 (red line) indicates an eastward displacement of 17! ± 3! of longitude
per 50 years, whereas a linear fit from 1978 to 2003 indicates a much smaller displacement. A linear fit to the
position of isohalines from 1955 to 1995 indicates an
eastward displacement of 26! ± 1! of longitude per
50 years. Therefore, it seems that the equatorial salinity
front is subject to gradual eastward movement and to
decadal displacements of about the same amplitude. The
westward retreat of the eastern edge of the Fresh Pool at the
S. Cravatte et al.: Observed freshening and warming of the western Pacific Warm Pool
end of the 1990s, corresponding to a possible shift to a
negative PDO phase (Peterson and Schwing 2003), coun(a)movement and may explain in
(c)
teracts the eastward gradual
part this slowdown of the eastward expansion.
The same calculations are made for 29!C isotherm. Its
eastward shift is similar for the 1955–2003 period, between
15 and 20! of longitude per 50 years, depending on the
SST products and the meridional average (0!, 2!S–2!N,
4!S–4!N). From 1978 to 2003, the zonal shift is weaker. It
is negligible for the HadiSST product, but still significant
Cravatte et al. 2009: change of rainfall and evaporation have caused a
freshening and warming of the Pacific warm pool since 1955.
Why is change of S correlated with change of T?
Water
HADISST
1955–
2003
ERSST
1978–
2003
1955–
2003
1978–
2003
Warmer than 28!C
4.2
6.9
4.3
6.6
Warmer than
28.5!C
6.0
8.7
5.5
8.5
Warmer than 29!C
8.9
10.7
7.6
11.5
Warmer than
29.5!C
6.2
8.5
6.7
14.3
Warmer than 30!C
0.8
1.6
1.0
2.8
Estimates are given for the HADISST (see Fig. 6a) and the ERSST
products. Units are 106 km2 per 50 years
atmosphere interactions. It is known that it migrates zonally at ENSO timescales over thousands of kilometers
(Picaut et al. 2001 for a review), but its displacements at
decadal or lower-frequency timescales have not been
documented yet. As reported in Sect. II-3, accessible
proxies for this eastern edge are the position of the 34.6 and
34.8 isohalines, and the position of the 29!C. Figure 7
Fig. 7 Longitude-time
diagrams of 4!S-4!N averaged
SST (a) and SSS (b). Contour
intervals are 0.25!C for SST
warmer than 28!C and 0.2 for
SSS fresher than 35 pss. Linear
fits to the position of 29!C
isotherm and 34.8 isohaline are
shown in red. The time series on
the extreme right panel shows
the number of full bins in the
area (4!S–4!N/140!E–175!W),
as expressed in percentage of
the total number of bins inside
the surface area. SST and SSS
data have been filtered with a
25-month Hanning filter to filter
out variations shorter or equal to
one year
(a)
571
(b)
(b)
(d)
Fig. 3 Linear trends in SST (a) and SSS (b). Units are !C/50 years
and pss/50 years. Positions of the 28.5!C isotherms (c) and of the 34.8
isohalines (d), averaged during 1956–1965 (black), 1966–1975 (red),
1976–1985 (green), 1986–1995 (blue) and 1996–2003 (light blue).
The regions where the linear trends are not significant at the 90%
confidence level are hatched in black
available, and in the western Coral Sea including the
eastern coast of Australia. It is also worth noting that the
freshening observed in the southeastern tropical Pacific is
mainly due to a rather sudden and strong freshening of
about one pss observed at the end of the 1990s, linked to
123shift referred to earlier.
the mid- to late-1990s climate
From 1955 or from 1978 to the mid-1990s, this region
exhibits a significant increase in salinity (not shown),
coverage is sufficient) during boreal autumn, in both cases
during the salinity seasonal minimum. Figure 4 illustrates
this feature along 175!E. Around 10!S, under the SPCZ,
the freshening is maximum in March–April, just before and
during seasonal SSS minimum. From 5!S to 7!N, it is
maximum in August–September, also contributing to the
decrease of the SSS seasonal minimum. Conversely,
southward of 25!S, in the region of high salinity, the
Ocean circulation:
Surface currents:
•  What similarity do you see
between the ocean surface
winds and currents?
•  Ocean surface currents mainly
follow winds with about 45ο to
the right (left) in N.H (S.H.).
Ekman effect (due to earth’s
rotation and friction).
winds
currents
http://oceanmotion.org/html/background/patterns-of-circulation.htm
Global ocean circulation:
–  Driven by both buoyancy and winds
–  Transport heat, nutrients and chemical tracers between
surface and deep ocean layers.
Role in carbon cycle:
•  Deep ocean stores far
more carbon than the
terrestrial ecosystem
including soil.
•  ~1/4 of the anthropogenic
emission of CO2 is
absorbed by marine
ecosystem; As ocean
temperature increases,
less would be absorbed.
•  On time scale 100-1000
yrs, ocean carbon cycle
becomes more important
than terrestrial carbon
cycle.
Distribution of marine biosphere and its
connection to circulation:
•  What cause an increase of
carbon and decrease of
oxygen with depth in the
ocean surface layer?
•  What cause ocean desert in
the subtropical gyro regions?
•  Why is PP higher along
coastal regions, northern highlatitudinal oceans and
equatorial eastern Pacific and
Atlantic oceans?
Summary:
•  How does ocean influence climate?
–  Exchange energy, water and carbon with the atmosphere
–  Interannual variability of the ocean dominates interannual to
millennial climate variabilities.
•  What drive ocean surface currents and deep circulation?
–  Winds and density/buoyancy change
•  What control sea water density change? How do these
controlling factors vary geographically and why?
–  Primarily T and S
–  The influence of S increase as T decreases, from tropics to high latitudes
•  What control changes of temperature and salinity of the ocean?
–  Surface heat and water fluxes, up- and down-welling, ocean currents
•  What control primary productivity of the ocean?
–  Mixing with water from deep oceans forced by storms, coastal and
equatorial upwelling.
2.1.2 Cryosphere
•  What is the roles of cryosphere in climate?
•  What is the distribution of different components of cryosphere?
•  How has cryosphere changed in recent decades? How would it
affect climate in the 21st century?
•  What is cryosphere?
–  Cryo (frozen), a component of the
earth’s climate system
comprised of water in its solid
state. It consists of
• 
•  glaciers & ice sheets,
•  snow,
•  permafrost (continuous and
discontinuous)
•  sea ice (perennial and seasonal).
Largest fresh water reservoir on earth
Cryospheric component
!
Area (% of earth surface)
Antarctic ice sheet
"
"
"2.7
"
Greenland ice sheet "
"
"0.35
"
Alpine glaciers
"
"
"0.01
"
Sea-ice (in season of maximal extent)
"7
"
Seasonal snow cover "
"
"9
"
Permafrost
"
"
"5
"
!Mass (103 kg/m2)!
" 53 "
"5"
"0.2"
"0.01"
"<0.01"
"1"
Cryospheric component
!
Area (% of earth surface)
Antarctic ice sheet
"
"
"2.7
"
Greenland ice sheet "
"
"0.35
"
Alpine glaciers
"
"
"0.01
"
Sea-ice (in season of maximal extent)
"7
"
Seasonal snow cover "
"
"9
"
Permafrost
"
"
"5
"
!Mass (103 kg/m2)!
" 53 "
"5"
"0.2"
"0.01"
"<0.01"
"1"
What can we infer from ice mass listed above?
–  Surface surface area: 5.1X1014 m2, total land area: 1.45X1014 m2!
–  103 kg/m2: equivalent to depth of liquid water in meter per unit
area.!
•  If Antarctic ice sheet melted, it would create 53 m
deep water layer over entire earth.!
•  How much would sea-level rise?! 76M = 53mX5.1/(5.1-1.45)
Role in climate system:
•  Largest fresh water storage:
–  Influence sea-level rise
–  Water resources
–  Influence ocean circulation
•  Regular earth’s albedo change
•  Regular regional-global climate
How do we estimate water in snow and ice?
•  Snow (ice) equivalent depth: snow is porous
and its porosity depends on temperature and
age of the snow. A measure of liquid water
contained in snow is water equivalent depth,
h m:
hm=ρs/ρw·hs
ρs,ρw: density of the snow and water, respectively.
hs: depth of the snow/ice layer
hm: The depth of water that will resulted from
complete melt of snow/ice.
Snow relative density, ρs/ρw ranges from 0.15-0.4
Snow/ice albedo (whiteness):
•  Albedo: ratio of the reflected vs. incident
radiative flux. It is a function of wavelength.
Surface
Typical Albedo
Fresh asphalt
Conifer forest (Summer)
Worn asphalt
Deciduous trees
Bare soil
Green grass
Desert sand
New concrete
Fresh snow
0.04
0.08, 0.09 to 0.15
0.12
0.15 to 0.18
0.17
0.25
0.40
0.55
0.80–0.90
Ice sheets:
• 
• 
• 
• 
The largest storage of global surface fresh water
The greatest contributor to global sea level rise
A critical process for abrupt climate change
Highly reflective for solar radiation
The largest ice sheets: Antarctic and Greenland
Reservoirs of water
!
!
!Mass spread over the Earth surface !
!(equivalent to water equivalent depth)"
Atmosphere
"
Fresh water (lakes and rivers)
Fresh water (underground)
Greenland ice sheet "
Antarctic ice sheet
"
Alpine glaciers
"
"
"0.03 (103 Kg/m2 or m in depth)"
"0.6 "
"15 "
"5 "
"53
""
"0.2
""
•  If the Greenland ice sheet were completely melted, how high
would global sea-level rise? How much heat it would absorb
(expressed as how many days of sunlight received by the
earth).
Reservoirs of water
!
!
!Mass spread over the
!
!Earth surface"
Greenland ice sheet "
"5 (103 Kg/m2 or m in depth)"
Antarctic ice sheet
"
"53 "
"
Global surface area: 5.1X1014 m2; Global oceanic area: 3.65X1014 m2"
Latent heat of fusion: 3.34X105 J/kg
""
Earth receives ~ 1.8X1017 W at the top of the atmosphere, assuming the
earth’s albedo is 0.3
•  If the Greenland ice sheet were completely melted, how
much heat it would absorb and how high would global sealevel rise?
Complete melt of Greenland ice sheet absorbs:
Q=ViceXL=5Kg/m2X5.1X1017m2 X 3.34X105J/kg
=8.5X1023 J of heat
It takes ~ 77.7 days of total solar radiation received by the earth system (assuming
30% of earth’s albedo):
t=Q/S=8.5X1023 J/(1.8X1017J/s)=4.7X106s/(2.4X3.6X104s/day)=77.7 days
meters of sea level rise for complete melt of Greenland ice: 5 mX100%/72%~7 m
•  ~ 8.5X1023 J, or 78 days of total solar radiation received by the earth system.
7m
Antarctic Ice Sheet:
•  Creep rate: near zero at the
divides of the ice sheet, and
>10 m/yr at the periphery;
Why?
•  Creep rate is especially high
in the W. Antarctic.
P
Satellite image of the Antarctic ice
sheet and the rate of creep of the
ice (m/yr) on a logarithmic scale.
Greenland Ice Sheet:
•  Lower latitudes and smaller than the
Antarctic ice sheet,
•  S. Greenland is highly vulnerable to
climate change because summer
temperature reaches melting point (-5C).
•  However, the danger of complete melt of
Greenland is not as high as it was
believed during the 21st century,
although it is still highly vulnerable to
global warming in long-term.
IV
Sea Ice:
• 
• 
• 
Sea ice in arctic covers maximumly 3% of the
earth and in Antarctic covers maximumly 4%
of the earth’ surface, and about 1-3 m thick
(not much mass, 0.01 m)
Sea ice cover in Antarctic varies seasonally
from 2 to 14 X1012 m2, and in Arctic varies
from 4 - 11 X1012 m2.
Why does sea ice varies more in Antarctic
than in Arctic?
• 
• 
• 
Sea ice is a fractal field comprised of
ice floes.
A new pack of ice is formed by
freezing of water in newly formed
leads in region where wind drag
pack ice away from shore; after
reach 1 m thick, it is formed by
collisions of ice floes;
Sea ice moves with transpolar drift
stream.
Fridtjof Nansen
(1861-1930)
floes
leads
Floes streaming southward off the
east coast of Greenland
•  Important to North
Atlantic Deep Water
(NADW) formation:
–  Ice is formed by fresh
water and concentrated
salt water is left behind as
brine;
–  Brine mixed with sounding
water. This heavy saline
surface water sinks in
Arctic to form NADW.
•  Why is sea-ice only a few
meters thick?
Influence of ice sheet and sea-ice melting on the ocean
thermohaline circulation:
•  Melting of Greenland Ice Sheet and Arctic sea ice increase fresh water
discharge to the source region of North Atlantic Deep Water, reduce surface
water density and weaken the deep ocean convection;
•  Strong melting of the Greenland Ice Sheet in future could weaken the NADW
formation and thermohaline circulation.
Alpine glaciers:
•  Alpine glaciers, smaller ice
sheets, can exist at any
latitudes although their altitudes
increase from < 1 km in high
latitudes to 4-6 km in tropics;
Why?
•  Alpine glacier retreat has been
observed globally.
P
T to form
glacier for
given P
Air T
Snow:
Distribution and variations:
•  Seasonal snow covers
~9% of the global surface,
mainly in high latitudes
and high altitudes;
•  Snow cover varies strongly
(50%), seasonally, weekly,
interannually, decadally;
Impact on climate:
• 
In NH high latitudes, snow can
increase surface albedo from <0.2
to 0.5-0.8.
• 
Good isolator for surface:
Thermal conductivity k is a
measure of a material’s ability to
transfer heat. A high value of k is a
good conductor while a low value
of k is a good insulator. k has
units of Wm-1K-1
Fresh Snow
0.03
(better than fiberglass insulation!)
Old Snow
0.4
Ice
2.1
Permafrost:
•  Structure and distribution of the
permafrost:
Permafrost:
• 
• 
• 
The top few meters of soil thaws
during summer and freezes in winter.
Below a few meters, the soil
temperature remains constant around
0˚C. It would takes hundreds of years
for the permafrost to adjust to air
temperature;
Carbon locked up in the permafrost >
carbon stored in global vegetation.
Summary:
• 
What is cryosphere?
–  Cryo (frozen), a component of the earth’s climate system
comprised of water in its solid state. It consists of
• 
• 
• 
• 
• 
glaciers & ice sheets,
snow,
permafrost (continuous and discontinuous)
sea ice (perennial and seasonal).
What is the distribution of different components of cryosphere?
–  Largest mass in Antarctic and Greenland, 58 m deep of water globally if
they melt completely;
–  Sea ice and land snow cover 8-16% of the earth’s surface
–  Greenland and W. Antarctic ice sheet, Arctic sea ice and alpine glaciers
have retreated rapidly in recent decades.
• 
What is the roles of cryosphere in climate system?
–  Largest storage of global surface fresh water
–  Contribute to the thermal inertial of the earth’s climate
–  Contribute to albedo of the earth
–  Controls fresh water flux in the polar region, thus influence oceanic
thermohaline circulation;
–  Store more carbon than that by global vegetation