Transcript powerpoint presentation

SIO 210: Natural climate variability (decadal
modes and longer time scales)
L. Talley 2016
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Climate equilibria, forcing, feedbacks, hysteresis
(ENSO - 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
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Climate variability vs. climate change
• Usage 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.)
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Elements of the climate system
Sun
Atmosphere
Ocean
Cryosphere (ice, snow)
Land surface
Biological and chemical cycles
(Moon, in its effect on tidal cycles and hence
mixing)
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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/couplings: sometimes include feedbacks
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Natural climate modes with interannual to
millenial time scales that involve the ocean
Interannual time scale (> 1 year, < 10 years)
ENSO
Decadal time scale (10 to multiple decades)
Pacific Decadal Oscillation (or Pacific-North America
Pattern)
North Atlantic Oscillation (or Arctic Oscillation or NAM)
Southern Annular Mode
Centennial time scale
Atlantic Overturning mode
What sets the time scales?
decadal to centennial suggests longer processes than just
atmosphere - for instance ocean circulation, ocean’s planetary
wave propagation, or changes in land surface
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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)
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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
Moderate
– Grass
initial T
– Clouds
– Concrete
– Water
High initial T
Negative
feedback
Positive
feedback
• http://gingerbooth.com/courseware/daisy.html
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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
Feedback? None since air temperature does not change incidence
of volcanoes
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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 cover
Positive coupling
(Ice increase ->
reflectivity increase)
Negative coupling
(Temperature increase ->
ice cover decrease)
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Reflectivity
(albedo)
Negative coupling
Temperature
(Reflectivity/albedo increases ->
temperature decreases)
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How might the ocean be coupled into climate modes and
create decadal to centennial to millenial time scales?
Note that advection time scales are similar to these climate
modes:
tropical ocean – years
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.4/6/2017
Talley
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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.
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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.
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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
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NAO SST pattern
High NAO:
After DPO 6th Fig. S15.2
Warm subtropical N. Atlantic, warm subtropical N. Pacific
Cool subpolar N. Atlantic, cool subpolar N. Pacific
i.e. also associated with weak Aleutian Low
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Arctic Oscillation sea level pressure
pattern
DPO 6th Fig. S15.10
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Where are we in the
NAO?
High: exceptionally cold Labrador
Sea Water, warm/non-existent
Eighteen Degree Water
Current status:
http://www.cpc.nce
p.noaa.gov/products
/precip/CWlink/pna
/nao.shtml
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NAO-related N. Atlantic ventilation
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)
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N. Atlantic oxygen changes: ascribed to high NAO since
about 1989, reduced ventilation in the NE Atlantic
(Gruber, 2004)
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Southern Annular Mode
NAM
Circumpolar mode; variation in surface pressure and hence
in westerly and polar easterly wind strength
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Stability and equilibria for the ocean: can the
Atlantic Meridional Overturning Circulation turn
on and off?
Cooling, freshening
Warming, evaporation
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Rahmstorf, Nature,
2002
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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 – see next slide)
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North Atlantic thermohaline circulation variations millenial time scales and abrupt climate change
Rahmstorf, Nature, 2002
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Salt oscillator (Stommel 1961): example of
hysteresis
Cooling, freshening
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 (through
circulation of high salinity into
high latitude box - overshoot
(hysteresis)
DPO section 7.10.4
NADW formation rate
Starts to reduce overturn and
reduce high latitude SST slightly.
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North Atlantic salinity variations
Can these changes in surface salinity create changes in circulation?
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Curry4/6/2017
(WHOI)
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Observed changes:
Freshening of the Atlantic and Nordic Seas
(Dickson et al, Phil Trans Roy Soc 2003)
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Labrador Sea
Water variations
(I. Yashayaev various
publications)
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Changes in Atlantic water mass salinity
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(Curry et al, 2003)
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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, but
more recent
papers begin to
suggest a
slowdown as well.
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Cartoon of
“conveyor” and
measurement
arrays in place from
Quadfasel (Nature,
2005) 4/6/2017
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