Download Climate change and its impact on ocean variability

9/18/2009
Climate change and its
impact on ocean
variability
• The ocean circulation - a global system of surface and
deep currents - is powered by two different 'engines'.
• Movement in the top few hundred to a thousand
metres is driven mainly by the prevailing winds.
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9/18/2009
Depth
1000m
The Shallow, Swift Wind-driven Circulation
4000m
Temperature along a section in the mid-Pacific (152W)
• The ocean circulation - a global system of surface and
deep currents - is powered by two different 'engines'.
• Movement in the top few hundred to a thousand
metres is driven mainly by the prevailing winds.
• Vertical circulation is driven by cold, salty water
sinking at high latitudes, returning towards the equator
at depth and being replaced by warm water moving
towards the poles at the surface.
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Depth
1000m
The Slow, Deep Thermohaline Circulation
4000m
Temperature along a section in the mid-Pacific (152W)
• The ocean circulation - a global system of surface and
deep currents - is powered by two different 'engines'.
• Movement in the top few hundred to a thousand
metres is driven mainly by the prevailing winds.
• Vertical circulation is driven by cold, salty water
sinking at high latitudes, returning towards the equator
at depth and being replaced by warm water moving
towards the poles at the surface.
• This is known as the thermohaline circulation from the
combination of temperature (~thermo) and saltiness
(~haline) that controls high-latitude sinking.
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Nomenclature
• Meridional Overturning Circulation (MOC): Total
northward/southward flow, over latitude and depth
• Thermohaline Circulation (THC): Part of MOC
driven by heat & water exchange with atmosphere
• MOC is observable quantity; THC an interpretation
• Often used synonymously, but wind-driven MOC
part should be considered separately
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Why does it matter?
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Why do we care if the oceans change?
Why do we care if the oceans change?
• Potential for positive feedbacks
influencing global climate
– Changing cryosphere
– Carbon uptake
– Thermohaline circulation
• Sea-level rise
• Impact of acidification on ecosystems
• Impact of climate change on
ecosystems (warming, freshening,
∆mixed layer, ∆light, ∆circulation, ∆sea
ice, ∆winds)
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Why do we care if the oceans change?
• Potential for positive feedbacks
influencing global climate
– Changing cryosphere
– Carbon uptake
– Thermohaline circulation
• Sea-level rise
• Impact of acidification on ecosystems
• Impact of climate change on
ecosystems (warming, freshening,
∆mixed layer, ∆light, ∆circulation, ∆sea
ice, ∆winds)
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General pattern of the surface currents – a balance of heat
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• The ocean’s heat capacity is about 1,000
times larger than that of the atmosphere.
• Heat flux related to ocean regions.
• The oceans net heat uptake since 1960 is
around 20 times greater than that of the
atmosphere (Levitus et al., 2005).
Heat Flux across the Ocean/Atmosphere (Watts/m2)
Da Silva
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v
Western boundary currents = negative heat flux into the atmosphere
v
Eastern boundary currents = postive heat flux from the atmosphere
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Meridional heat transport
T Anomaly (N-S sections)
Atlantic
Pacific
Indian
The heat anomaly is due to upwelling of anomalously warm
deep layers in the Southern Ocean....role of eddies?
Results have shown that the majority
of transport between ocean basins occurs within
the Southern Ocean
=?
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However….
“the meridional heat flux required to completely balance this
transport must include another form of mechanism other than
the mean flow …. Such as areas of high variability that result in
the generation of eddies”
Morrow et al., 2004
The meridional heat flux
required to balance the
0.3–0.7 PW (1PW=1015W) of heat
lost by the ocean to the
atmosphere at high southern
latitudes must come from eddy
transport
Eddy kinetic energy (Ke) from 5 years Topex/Poseidon
(Dec 1992 to Dec 1997).
Source: Stammer
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Projecting the behaviour of the
Southern Ocean, including the
carbon sink, in greenhouse scenarios
will thus require models that capture
realistically the effect of the ACC
eddy variability.
Boning et al., Nature 2008
So its important!!
• Zonally averaged NCEP SST data suggest
increasing temperatures..
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Energy content
changes 1961–
2003 (blue) and
1993–2003
(burgundy).
Depth
1000m
The Slow, Deep Thermohaline Circulation
4000m
Temperature along a section in the mid-Pacific (152W)
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So.. Why does this happen?
What are the mechanics driving the GTH?
What is a stable water column?
•
•
Salinity increases with depth, temperature decreases with depth
A stable water column is layered or stratified, like a three layered cake
Now let’s look at an unstable water column
•
•
•
Salinity uniform with depth, temperature uniform with depth
An unstable water column is not stratified, it is well mixed
Dense water continually sinking
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•
Where is coldest surface water?
~
at the poles
•
A component of salt – through brine rejection..
•
At the poles, the water column is unstable and is well
mixed because of sinking cold and salty water
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The instability of the higher latitudes gives rise to deep water
formation
1.
South Pole off coast of Antarctica
•
Antarctic Bottom Water (AABW)
~ 1°C, 34.7 ppt
~ densest water in the ocean
NADW and AABW form
at surface, sink and then
spread out in horizontal
direction at the bottom of
the ocean
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Causes of fresher shelf water
• Increased glacial
ice melt?
• More precipitation?
• Less sea ice
formation?
• Change in winds
and ocean
circulation?
Davis et al., Vaughan; Science, 2005
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The instability of the higher latitudes gives rise to deep water
formation
2.
North Pole off coast of Greenland
•
North Atlantic Deep Water (NADW)
~ 3°C, 34.9 ppt
~ very dense but not as dense as AABW
NADW and AABW form
at surface, sink and then
spread out in horizontal
direction at the bottom of
the ocean
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But what is the Importance of the
Thermohaline circulation?
Lets say the “heat pump” can be turned ON or OFF ….what will happen?
On – today!
•
•
There is vigorous mixing at the poles
~
~
dense surface water sinks at the poles
thermohaline circulation is initiated
~
this is the ‘switch’ that turns the conveyor belt on
As the global conveyor belt returns water to the poles through surface
currents, the oceans give off the heat picked up at the lower
latitudes to the land masses at the higher latitudes (i.e. northern Europe)
~
~
oceans acting as a ‘heat pump’ to warm the land masses
Hence UK has a mild climate, Norway ice free ports..
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BUT WHAT IF WE TURN IT OFF?
•
There is no vigorous mixing at the poles – water column becomes stable
~
there is no dense water sinking at the poles
(surface waters warmed, polar ice caps melt)
~
~
•
thermohaline circulation slows down
the global conveyor belt ‘switch’ is turned off
There is no heat pump to warm the land masses
~
much colder in northern Europe
How could the THC slow down?
• Increased rainfall, melting of the cryosphere are all possible
consequences of higher temperatures, and could reduce North
Atlantic surface salinity sufficiently to slow down the formation
of deep water. If this happens, the THC may shut down. Once
stopped, the heat conveyor may take time to recover, and the
consequences would be a cooling of northwest Europe.
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Effect on Europe if the THC slows down
HADCM3 simulation where large amounts of fresh water
was added to the North Atlantic at year 2050.
London 1683 ‘little ice age’
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The Importance of the Global Conveyer Belt (cont’d)
What do we think happened to cause the ‘Younger Dryas’?
•
As earth warmed during warming 2000 year warming period
~
~
~
~
~
~
~
the surface waters also warmed
polar ice caps melted (surface waters less salty)
northern Atlantic surface water less dense
no vigorous mixing, interruption in thermohaline circulation
global conveyor belt turned off
no heat transfer to northern Europe
ice sheets, no forests grow
Another example of when the global conveyor belt is turned off:
•
We also have records of a prolonged period of cold in northern
Europe from 1650-1850
~
~
known as the “Little Ice Age”
could have been caused by an interruption or slow-down in
thermohaline circulation, conveyor belt slowed down (sluggish)
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Salinity a key indicator!
• Ocean salinity changes are a sensitive
indicator for detecting changes in
precipitation, evaporation, river runoff and
ice melt.
• Estimates of changes in the freshwater
content of the global ocean suggest that
the global ocean is freshening.
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Salinity increase due to an increase
in evaporation
Salinity decrease due to an increase in precipitation
Ice melt, changes in the GTH/MOC
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Is there evidence for salinity
change?
• Yes – in the Southern Ocean – formation
site of the AABW
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Freshening of AABW
115E, 61S to 63.3S
1995
2005
0.1
0
THETA
-0.1
-0.2
.27
28
-0.3
-0.4
35
28 .
.30
28
-0.5
0.017 psu
34.655 34.66 34.665 34.67 34.675 34.68 34.685 34.69 34.695
Rintoul 2006
SALINITY
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Freshening of the Mode Waters: global warming?
(Wang et al. 1999)….temperature shows a similar trend
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• Warming in the Southern Ocean has been
attributed to a southward shift and
increased intensity of the Southern
Hemisphere westerlies, which would shift
the ACC slightly southward and intensify
the subtropical gyres (e.g., Cai, 2006).
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Growing evidence of a slow down?
Growing evidence of a slow down?
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Growing evidence of a slow down?
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Transport data from moorings
deployed across the tropical Atlantic
suggest that since 1957 the volume
transport has slowed by 30%
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• Climate models show that the Earth’s climate system responds to
changes in the MOC and suggest that this overturning might
gradually decrease in transport in the 21st century as a
consequence of anthropogenic warming and additional freshening in
the North Atlantic.
• However, observations of changes in the MOC strength and
variability are fragmentary.
• Observed changes in MOC transport, water properties and water
mass formation are inconclusive. This is partially due to decadal
variability and partially due to inadequate long-term observations.
• From repeated hydrographic sections in the subtropics, Bryden et al.
(2005) concluded that the MOC transport at 25°N had decreased by
30% between 1957 and 2004, but the presence of significant
unsampled variability in time and the lack of supporting direct
current measurements may reduce confidence in this estimate.
What Causes Sea Level to Change?
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The Bathtub Sea Level Model
Precipitation over Oceans
Runoff from Continents
+
Evaporation from Oceans
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Global mean surface temperatures have increased
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Regional variability
from historical tide gauges
New York
Brest
Honolulu
Buenos-Aires
Time
1900
Sea Level Budget (IPCC, mm/year)
1993-2003
2000
1961-2003
Thermal Expansion
1.6 ± 0.5
0.4 ± 0.1
+
Mountain Glaciers
0.8 ± 0.2
0.5 ± 0.2
+
Greenland Ice Melt
0.2 ± 0.1
0.1 ± 0.1
+
Antarctic Ice Melt
0.2 ± 0.3
0.1 ± 0.4
Land Water Storage
= Total of Observed Contributions
Observed Sea Level Change
?
2.8 ± 0.7
3.1 ± 0.7
?
1.1 ± 0.5
1.8 ± 0.5
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What does the IPCC say?
• The oceans are warming. Over the period 1961 to 2003, global
ocean temperature has risen by 0.10°C from the surf ace to a
depth of 700 m.
• Global ocean heat content (0–3,000 m) has increased at a rate
of 0.21 ± 0.04 W m–2 globally.
• Global ocean heat content observations show considerable
interannual and inter-decadal variability.
Measurements of Sea Level Change
GRAVITY
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Tide Gauges with Greater Than 10 Years of Measurements
> 50 Years of Measurements
Tide Gauges with Greater Than 10 Years of Measurements
> 50 Years of Measurements
Of course still poorly sampled…
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Tide Gauge Observations
150
3.2 mm/year
100
2.0 mm/year
∆MSL (mm)
50
0.8 mm/year
0
-50
Average Rate ~ 1.8 mm/year
-100
1880
1900
1920
1940
Year
1960
1980
2000
[Church and White, 2006]
Satellite Altimeters
TOPEX/Poseidon
1992-2005
Jason-1
2001 - ?
OSTM/Jason-2
2008
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Geographical distribution of sea level trends (19932005)
Global Mean Sea Level from Satellite Altimetry
Average Rate = 3.5 mm/year
(1993-2006)
1997-1998
El Nino
[Mitchum and Nerem, 2007]
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Thermal Expansion: Contribution to Sea Level
20
Rate = 0.4 mm/year (1955-2004)
15
Rate = 1.2 - 1.6 mm/year (1993-2004)
∆MSL (mm)
10
5
0
-5
-10
-15
1960
1970
1980
1990
2000
[LevitusYear
et al., 2005; Antonov et al., 2005]
Greenland Melt Extent
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Melting of the Greenland Icesheet
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Alaska Glacier Mass Changes from GRACE
Sea Level Contribution of 0.3 mm/year over 2002-2004
[Tamisiea et al., 2005]
Arctic Sea-ice melting
1990
2000
~10% decrease in sea-ice per
decade
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Antarctic Ice Mass Flux from
InSAR
SLR 0.4 to 0.6 mm/yr
-4 km3/yr
-2 km3/yr
-2 km3/yr
+5 km3/yr
-37±20 km3/yr
-2
km3/yr
-3 km3/yr
-49±20 km3/yr
+48 km3/yr
-38
km3/yr
-4 km3/yr
-114 km3/yr
-22 km3/yr
-56 km3/yr
+33 km3/yr
-33 km3/yr
+21 km3/yr
+5 km3/yr
[Rignot, 2005]
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Is Sea Level Rise Accelerating?
• Short answer: probably
• The satellite sea level record is too
short (~14 years) to rule out that
the recent rise is due to natural
decadal variability.
• This is only likely to be resolved by
having a longer satellite data
record (~30 years).
• The decline in satellite programs in
recent years has put this in
jeopardy.
NEAR FUTURE PERSPECTIVES
ALTIMETRY
for measuring
sea level
ARGO
GRACE
Swath altimetry
Long sea level time series
Thermal expansion + salinity
Land waters (climate + human activities)
Ice sheets mass balance
Ocean mass change + thermal expansion
Surface Waters monitoring
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Summary
•
•
•
•
•
•
Observations of sea level change are consistent with how we expect sea level to
respond in a warming climate.
Sea level rose faster in the last decade than the 20th century average.
Whether the current rate of rise is accelerating can only be resolved with longer
satellite time series.
Presently, ocean warming, melting of mountain glaciers, and melting of the polar
ice caps are contributing in roughly equal amounts to the observed rise.
The largest uncertainty in future sea level rise projections is the contribution of
Greenland and Antarctica.
Many of the remaining questions about sea level rise can only be answered with
continued satellite measurements, which are in serious jeopardy.
Effects of Sea Level Rise
1 meter
2
meters
4
meters
8
meters
GFD
L
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• IPCC 2007 - 10-year rates of global sea level change from tide
gauge (black) satellite altimetry (green) and contributions from
thermal expansion (red)
• The ocean varies over a broad range of time scales, from seasonal
to decadal (e.g., circulation in the main subtropical gyres) to
centennial and longer (associated with the MOC).
• The main modes of climate variability are the El Niño-Southern
Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), the
Northern Annular Mode (NAM), which is related to the North Atlantic
Oscillation (NAO), and the Southern Annular Mode (SAM).
•
Forcing of the oceans is often related to these modes, which cause
changes in ocean circulation through changed patterns of winds and
changes in surface ocean density.
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March 1998 - El Niño
October 1988 - La Niña
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Changing Southern Hemisphere climate:
the Southern Annular Mode
Sen Gupta & England 2006
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Northern Annular Mode
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Oceans and CO2
• Passive ocean CO2 uptake
• Reduction in surface ocean pH ➾ ‘ocean
acidification’
• Passive uptake cannot keep up with
increased anthropogenic CO2 ➾ active
uptake (‘CO2 sequestration’) ➾ ocean
acidification
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“Anthropogenic C02 Invasion”
• Impacts of “invading CO2” on ocean
chemistry and biology
• Original methodological challenges related to
measuring the anthropogenic part of DIC
• Models of future DIC concentrations are key:
– 0.3-0.5 by 2030
– 0.8-1.4 by 2300
(Caldeira & Wickett, 2005)
Impacts on Ocean Biology
• Corals (ocean acidification induces a
decrease in C++; cumulative impacts: >T,
>pH)
• Reduced growth, calcification and survival
of many other shallow benthic species
• Affects fish physiology
• Invertebrates’ physiology also affected
(also cumulative impacts: >T, <dissolved
O 2)
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Impacts of Ocean Acidification
• Generally, ocean acidification affects ocean biology and
how the ocean functions as a system
• Passive uptake of CO2 will gradually invade the deep
ocean
Tropical (above) and subtropical pteropods;
stony cold water corals (right)
CO2 Sequestration in the Deep?
• Naturally, most deep organisms tend to avoid
natural pH variations (e.g. vent plumes)
• Liquid CO2 lake scenario on the deep ocean
floor: many factors affecting stability
• Judicious choices should be made (delivery
schemes, injection rates, droplet size, bottom
bathymetry, water column injection, etc.)
• Iron fertilization as a mitigation strategy? Side
effects include <dissolved 02, >atmospheric
N2O, <nutrients downstream from a
fertilization side)
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Are We in Trouble?
“Geo-engineering schemes are not
well understood.
Planet-sized geo-engineering
means planet-sized risks.“
Caldeira, K.
What Can We Do?
• IOCIOC-SCOR Ocean Acidification Symposium Series
• Policy side: Royal Society Policy Report
Recommendations
– There is a clear risk of significant adverse effects of ocean
acidification. This risk should be taken into account by policymakers and other relevant national and international bodies.
– Any targets set for CO2 emission reductions should take account
of the impact on ocean chemistry and acidification as well as
climate change.
– Ocean acidification and its impacts on the oceans needs to be
taken into account by the Intergovernmental Panel on Climate
Change and kept under review by international scientific bodies.
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Royal Society Policy Report
Recommendations (cont.)
•
The increased fragility and sensitivity of marine ecosystems due to ocean
acidification, climate change, deteriorating water quality, coastal
deforestation, fisheries and pollution needs to be taken into consideration
during the development of any policies that relate to their conservation,
sustainable use and exploitation, or effects on the communities that depend
on them.
•
Tackling ocean acidification cannot be done by any country alone. A major
internationally--coordinated research effort (including monitoring) into ocean
internationally
chemical changes should be launched, with additional investments.
•
International research collaboration should be enhanced, from laboratory,
mesocosm and field studies to global monitoring.
•
Action needs to be taken now to reduce global emissions of CO2 to the
atmosphere to avoid the risk of large and irreversible damage to the oceans.
We recommend that all possible approaches be considered to prevent CO2
reaching the atmosphere. No option that can make a significant contribution
should be dismissed.
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