Transcript powerpoint presentation

SIO 210: Natural climate variability (decadal
modes and longer time scales)
L. Talley 2014
DRAFT (2010 lecture with few edits)
•
•
•
•
Climate equilibria, forcing, feedbacks, hysteresis
(ENSO - use notes from previous lecture)
Pacific Decadal Oscillation - ENSO modulation
North Atlantic Oscillation, Arctic Oscillation & Northern
Annular Mode
• Southern Annular Mode
• North Atlantic meridional overturning and climate
change
• Impacts of anthropogenic forcing
• Reading: In DPO Chapter S15
Talley SIO210 (2014)
1
Elements of the climate system
Atmosphere
Ocean
Land surface
Biological and chemical cycles
Talley SIO210 (2014)
2
Climate variability vs. climate change
• Current common usage and in, e.g., the
Intergovernmental Panel on Climate Change
(IPCC) usage. (But not used by all climate
scientists.)
• “Climate variability” = natural variability
– Natural “modes” of variability
• “Climate change” = anthropogenic forcing
– (due to man-made changes in greenhouse gases,
land surfaces, species distributions, etc.)
Talley SIO210 (2014)
3
Climate forcing
External forcing for earth’s climate includes
earth orbit parameters (solar distance factors)
solar luminosity
moon orbit
volcanoes and other geothermal sources
tectonics (plate motion)
greenhouse gases (to the extent that they are not part of the
climate system itself)
land surface (likewise with respect to the climate system)
Internal forcing: looking at each element of the climate system and
how it is forced by another element
(e.g. winds forcing ocean, change in ice extent forcing
atmosphere or ocean, etc)
Interactions – sometimes include feedbacks
Talley SIO210 (2014)
4
Natural climate modes with interannual to
millenial time scales that involve the ocean
ENSO: interannual time scale (> 1 year, < 10 years)
Pacific Decadal Oscillation: decadal time scale
North Atlantic Oscillation or Arctic Oscillation or Northern
Annular Mode: decadal time scale
Southern Annular Mode: decadal time scale
Atlantic overturning mode: centennial time scale
(centennial and longer time scales - VERY sparse
data sets, require more modeling to isolate processes)
What sets the time scales?
decadal to centennial suggests longer processes than
just atmosphere - for instance ocean circulation or changes
in land surface
Talley SIO210 (2014)
5
Stability and equilibria
Asymptotically stable: force
system away from initial
condition and the system
returns to initial state
Stable or Neutral: force away
and system stays where it was
pushed to (not illustrated
here).
Unstable: force away and
system moves to a different
state. This usually implies
multiple possible stable
equilibria, with forcing that is
strong enough to push into a
different equilibrium state.
Kump, Kasting and Crane (2003)
Talley SIO210 (2014)
6
Forcing (coupling) with no feedback
• Cause and effect: example of negative coupling (increase in
one parameter causes a decrease in the other)
• Volcano causes aerosols
• Causes cooling and decrease in temperature
Negative coupling
Volcano
eruption
Reduction in
sunlight
Temperature
decrease
Feedback? None since air temperature does not change incidence
of volcanoes
Talley SIO210 (2014)
7
Positive feedbacks
Albedo = reflectivity, scale of 0-1
with 0 = no reflection, 1 = all
reflected
• Example: ice-albedo feedback
– Increased ice and snow cover increases albedo
• (Positive coupling, denoted by arrow)
– Increased albedo decreases temperature of atmos.
• (negative coupling, denoted by circle)
– Decreased temperature of atmos. Causes ice increase
• (negative coupling, denoted by circle)
– Two negatives cancel to make positive; net is positive
feedback (“runaway”, unstable)
Ice
increase
Positive coupling
Negative coupling
Reflection
increase
Negative coupling
Temperature
decrease
Talley SIO210 (2014)
8
How might the ocean feed back on climate modes and
create decadal to centennial to millenial time scales?
Note that advection time scales are similar to these climate
modes:
ocean gyres - decades
ocean basins - centuries
global ocean - ~1000 years
(1) Advection of heat and salinity anomalies: from surface
forcing regions, subducted, and then returning to surface where
they change the forcing for the atmosphere, or change the ice
extent.
(2) Or similar advection that changes the upper ocean
stratification, hence changing the mixed layer depths heated
and cooled by the same air-sea fluxes, thus changing surface
temperature
(3) Or propagation of anomalies via Rossby or Kelvin waves,
which then reset the temperature in remote locations.
Talley SIO210 (2014)
9
Stability and equilibria for the ocean: can the N.
Atlantic “conveyor” turn on and off and what
would be the result for climate?
Cooling, freshening
Warming, evaporation
Talley SIO210 (2014)
Rahmstorf, Nature, 2002
10
North Atlantic thermohaline circulation variations millenial time scales and abrupt climate change
(1) If, say, fresh water is dumped on the northern North
Atlantic through excessive melting or runoff, how will the N.
Atlantic overturning circulation change? Will it:
Absorb the freshwater and return to nearly the initial
condition (asymptotically stable)? (stay in the initial
equilibrium state)
Shift to a slightly different state and remain there?
(neutrally stable) (stay in essentially the same equilibrium
state)
Jump into a completely different state of overturn
(unstable)? (new equilibrium state)
(2) If the freshwater forcing is continuously changing
(increasing and decreasing), what is the response?
(“hysteresis” predicted)
Talley SIO210 (2014)
11
Cooling, freshening
Salt oscillator (Stommel 1961): example of
hysteresis
Warming, evaporation
Model:
(1) increase freshwater at high
latitudes.
Then overturn shuts off, SST
drops abruptly.
(2) Reduce freshwater at high
latitudes. Takes a long time to
restore overturn - overshoot
(hysteresis)
Talley SIO210 (2014)
DPO section 7.10.4
NADW formation rate
Starts to reduce overturn and
reduce high latitude SST slightly.
12
North Atlantic thermohaline circulation variations millenial time scales and abrupt climate change
Rahmstorf, Nature, 2002
Talley SIO210 (2014)
13
Can the N. Atlantic “conveyor” change?
Interest in change
since it would have
some consequences
for subpolar SST
and for the storm
tracks that might
respond to location
and strength of
ocean fronts
Cartoon of
“conveyor” and
measurement
arrays in place from
Quadfasel (Nature,
2005)
Talley SIO210 (2014)
14
North Atlantic salinity variations
Can these changes in surface salinity create changes in circulation?
Talley SIO210 (2014)
Curry (WHOI)
15
Observed changes:
Freshening of the Atlantic and Nordic Seas
(Dickson et al, Phil Trans Roy Soc 2003)
Talley SIO210 (2014)
16
Labrador Sea
Water variations
(Dickson et al.,
and I.
Yashayaev)
Talley SIO210 (2014)
17
Changes in Atlantic water mass salinity
Talley SIO210 (2014)
(Curry et al, 2003)
18
Is the N. Atlantic “conveyor” changing,
possibly in response?
Bryden et al.
(Nature, 2005)
measurements at
25°N suggested
a slowdown.
They have since
withdrawn this
conclusion – their
results were
probably aliased
by the large
seasonal cycle.
Talley SIO210 (2014)
Cartoon of
“conveyor” and
measurement
arrays in place from
Quadfasel (Nature,
2005)
19
Natural climate modes with decadal time scales
that involve the ocean
ENSO: interannual time scale (earlier lecture)
Pacific Decadal Oscillation: decadal time scale
North Atlantic Oscillation or Arctic Oscillation or Northern
Annular Mode: decadal time scale
Southern Annular Mode: decadal time scale
Atlantic overturning mode: centennial time scale
(centennial and longer time scales - sparse data
sets, require more modeling to isolate processes)
What sets the time scales?
decadal to centennial suggests longer processes than
just atmosphere - for instance ocean circulation or changes
in land surface
Talley SIO210 (2014)
20
The PDO versus ENSO
Pacific Decadal Oscillation
pattern (sort of EOF):
tropics and Aleutian Low
20-30 year time scale
ENSO pattern (sort of an EOF):
mostly tropical
3-7 year time scale
Similar patterns, but ENSO is very peaked in the tropics, and the
PDO is spread out to higher latitudes, particularly N. Pacific.
Talley SIO210 (2014)
21
Pacific Decadal Oscillation time series (Mantua and Hare)
Great website:
http://tao.atmos.washingto
n.edu/pdo/
1976 “regime shift”
to warm phase PDO,
strong Aleutian Low
The PDO was high after about 1976 (“regime shift”) and stayed pretty high until the late
1990s. It looked like it was entering a low phase, but we are back in high.
Lesson for decadal modes: don’t know what you have until you are many years into them.
Talley SIO210 (2014)
22
Aleutian Low, ocean circulation and
decadal change?
Stronger A.L. strengthens subpolar gyre and weakens
subtropical gyre - result is ocean warming along North
America
Subpolar gyre
Subtropical gyre
Talley SIO210 (2014)
23
SST changes due to changing winds and
circulation
When the Aleutian Low is strong, get:
Stronger
westerlies
COOLING
Ocean
currents
weaker
Adapted from Miller, Chai, Chiba, Moisan and Neilson (J. Oceanogr., 2004)
Talley SIO210 (2014)
24
The Arctic Oscillation (or North Atlantic Oscillation
or Northern Annular Mode)
“High” and “Low” refer to the anomaly of atmospheric
pressure difference between the Azores and Iceland
Talley SIO210 (2014)
25
NAO SST pattern
High NAO:
Warm subtropical N. Atlantic, warm subtropical N. Pacific
Cool subpolar N. Atlantic, cool subpolar N. Pacific
i.e. also associated with weak Aleutian Low
Talley SIO210 (2014)
26
Arctic Oscillation sea level pressure
pattern
Talley SIO210 (2014)
27
Where are we in the
NAO?
High or neutral
Talley SIO210 (2014)
http://www.cgd.ucar.edu/cas/jhurrell/indices.html
28
N. Atlantic changes: decrease in
oxygen at base of the surface layer ->
reduction in upper ocean ventilation
(concomitant increase in Labrador Sea
ventilation)
(Gruber, 2004; Johnson, 2004;Feely et al 2005)
Talley SIO210 (2014)
29
N. Atlantic oxygen changes: ascribed to high NAO since
about 1989, reduced ventilation in the NE Atlantic
(Gruber, 2004)
Talley SIO210 (2014)
30
Arctic Oscillation
surface
temperature
variations (land)
(Wallace)
High AO:
Cold high latitudes (Canada, Labrador
Sea)
Warm Siberia and continental US,
warm subtropics
Talley SIO210 (2014)
31
Southern Annular Mode
NAM
Circumpolar mode; variation in surface pressure and hence
in westerly and polar easterly wind strength
Talley SIO210 (2014)
32
Anthropogenic climate change
The ocean is an excellent integrator of change since its
heat capacity is large, and it is an enormous reservoir for
freshwater (compared with the atmosphere).
Long-term trends in heat content, salinity and oxygen are
observed.
Necessary to integrate over large areas to see this signal
separate from the decadal natural modes.
Patterns of A.C.C. might well resemble the natural climate
modes since these are, after all, the natural modes that
would be forced into a particular state.
Relation to anthropogenic climate change is made through
relation to atmosphere trends that are footprints of A.C.C.
(night vs. day temperature, troposphere vs. stratosphere
heating/cooling, low vs. high latitude warming)
Talley SIO210 (2014)
33
Observed global ocean changes that might be anthropogenic
(Levitus et al 2005)
0.037°C
warming (03000 m)
Talley SIO210 (2014)
34
Is there anthropogenic climate change?
• Yes (IPCC TAR)
Levitus et al (2000)
heat storage changes
in the North Pacific,
Pacific, World
Talley SIO210 (2014)
35
Observed changes: basin-scale temperature
Mostly warming but some cooling (presented by H.
Garcia). Especially note cooling in high latitude Atlantic
and Pacific, tropical Pacific and Indian. Not just noise.
Talley SIO210 (2014)
36
Observed changes: Southern Ocean
(Gille, Science 2002)
Broad warming in
southern ocean at
about 800 meters
Also note cooling
to the north of the
warm band
Accompanied by
cooling in central
Antarctica
This looks like the Southern Annular Mode pattern. Natural
climate modes might also be forced by anthropogenic change.
Talley SIO210 (2014)
37
Variations in central N. Pacific temperature, salinity and density
between 1985 and 2004
(Robbins, pers. comm. 2004)
Using CFCs measured concurrently, he is concluding that this is
at least partially anthropogenic
Talley SIO210 (2014)
38
Large-scale salinity changes: fresh areas freshening and salty
areas getting saltier. Suggests increase in atmospheric
hydrological cycle, which would be expected in a warmer world.
This can only be observed with ocean salinities rather than with
trends in evaporation-precipitation since the latter data sets are
very noisy.
Fresher, cooler
Fresher
Saltier
Saltier
Fresher
Talley SIO210 (2014)
Saltier
Fresher
39
Extra slides
Talley SIO210 (2014)
40
Daisyworld - simple model of feedbacks (Lovelock)
• Model designed to demonstrate
simple feedbacks that can affect
climate
• Albedo: fraction of light that is
reflected.
Totally reflected: albedo = 1
No reflection: albedo = 0
Albedo depends on the material
– Snow
– Ice
– Dirt
Reasonable
– Grass
initial T
– Clouds
– Concrete
– Water
High initial T
Negative
feedback
Positive
feedback
• http://gingerbooth.com/courseware/daisy.html
Talley SIO210 (2014)
41
Pacific Decadal Oscillation SST and SLP patterns:
note similarity of SST to ENSO pattern and also large
amplitude in the central N. Pacific
Talley SIO210 (2014)
42
Pattern for the PNA (like the NPI): this is an
intriguing pattern, suggesting a connection to the
Southern Ocean (Southern Annular Mode)
Pacific North American
pattern correlated with
sea level pressure
Talley SIO210 (2014)
North Atlantic Oscillation
pattern correlated with sea
level pressure
43
Decadal variations in ENSO associated with
ocean subtropical changes (McPhaden and
Zhang)
Talley SIO210 (2014)
44
Tropical Pacific and Southern ocean
connection (Yuan, 2004)
Talley SIO210 (2014)
45
What about changing N. Atlantic
meridional overturning?
• Freshening of subpolar N. Atlantic is intriguing
• Stommel model and many more sophisticated
models exploring effect of freshwater dump on
subpolar N. Atlantic
• Freshening would weaken the overturning
circulation
• Bryden et al. (2005) and other somewhat
recent papers reporting possible decrease in
overturning circulation at 24N and at the
Nordic Seas overflows
• But present consensus is probably that
ascribing variations to anthropogenic forcing is
difficult since natural variations (NAO or NAM)
are so large
Talley SIO210 (2014)
46
Is the N. Atlantic conveyor changing?
This week in Nature (vol. 438, 1 December 2005)
Bryden et al (2005)
Repeat hydrographic sections at 24N in the N. Atlantic
Transport per unit depth
Talley SIO210 (2014)
Upper ocean
Deep ocean
47
Is the N. Atlantic conveyor changing?
This week in Nature (vol. 438, 1 December 2005)
Bryden et al (2005)
Talley SIO210 (2014)
48
Variations in central Pacific continued
1000
27.0
T
S
density
Warming from surface to about
1000 m (27.2 sigma0)
T
S
Salinification to about 800 m
(27.0 sigma0)
Talley SIO210 (2014)
49
Arctic
Oscillation
precipitation
anomalies
(Wallace)
Talley SIO210 (2014)
50
Anthropogenic
warming signature
Wang and Schimel -->
IPCC TAR,
from Jones et al (2001)
The observed high latitude warming trend
is a signature of anthropogenic change
Talley SIO210 (2014)
51
All the pacific indices ncep_ncar
Talley SIO210 (2014)
52
All the pacific indices ncep_ncar
Talley SIO210 (2014)
53