SIO 210: Natural climate variability (decadal modes and longer time scales) L. Talley 2014 DRAFT...
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SIO 210: Natural climate variability (decadal modes and longer time scales) L. Talley 2014 DRAFT (2010 lecture with few edits) Climate equilibria, forcing,
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 1Talley SIO210 (2014)
Slide 2
Elements of the climate system Atmosphere Ocean Land surface
Biological and chemical cycles 2Talley SIO210 (2014)
Slide 3
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.)
3Talley SIO210 (2014)
Slide 4
Climate forcing External forcing for earths 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 4Talley SIO210
(2014)
Slide 5
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 5Talley SIO210 (2014)
Slide 6
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) 6Talley
SIO210 (2014)
Slide 7
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 Feedback? None since air temperature does not change
incidence of volcanoes Volcano eruption Temperature decrease
Negative coupling Reduction in sunlight 7Talley SIO210 (2014)
Slide 8
Positive feedbacks 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 Reflection increase Positive coupling
Temperature decrease Negative coupling Albedo = reflectivity, scale
of 0-1 with 0 = no reflection, 1 = all reflected 8Talley SIO210
(2014)
Slide 9
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. 9Talley SIO210
(2014)
Slide 10
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 Rahmstorf, Nature, 2002
10Talley SIO210 (2014)
Slide 11
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) 11Talley SIO210 (2014)
Slide 12
Salt oscillator (Stommel 1961): example of hysteresis Cooling,
freshening Warming, evaporation Model: (1) increase freshwater at
high latitudes. Starts to reduce overturn and reduce high latitude
SST slightly. Then overturn shuts off, SST drops abruptly. (2)
Reduce freshwater at high latitudes. Takes a long time to restore
overturn - overshoot (hysteresis) DPO section 7.10.4 NADW formation
rate 12Talley SIO210 (2014)
Slide 13
North Atlantic thermohaline circulation variations - millenial
time scales and abrupt climate change Rahmstorf, Nature, 2002
13Talley SIO210 (2014)
Slide 14
Can the N. Atlantic conveyor change? Cartoon of conveyor and
measurement arrays in place from Quadfasel (Nature, 2005) 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 14Talley SIO210 (2014)
Slide 15
North Atlantic salinity variations Can these changes in surface
salinity create changes in circulation? Curry (WHOI) 15Talley
SIO210 (2014)
Slide 16
Observed changes: Freshening of the Atlantic and Nordic Seas
(Dickson et al, Phil Trans Roy Soc 2003) 16Talley SIO210
(2014)
Slide 17
Labrador Sea Water variations (Dickson et al., and I.
Yashayaev) 17Talley SIO210 (2014)
Slide 18
Changes in Atlantic water mass salinity (Curry et al, 2003)
18Talley SIO210 (2014)
Slide 19
Is the N. Atlantic conveyor changing, possibly in response?
Bryden et al. (Nature, 2005) measurements at 25N suggested a
slowdown. They have since withdrawn this conclusion their results
were probably aliased by the large seasonal cycle. Cartoon of
conveyor and measurement arrays in place from Quadfasel (Nature,
2005) 19Talley SIO210 (2014)
Slide 20
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 20Talley SIO210 (2014)
Slide 21
The PDO versus ENSO 20-30 year time scale 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.
ENSO pattern (sort of an EOF): mostly tropical Pacific Decadal
Oscillation pattern (sort of EOF): tropics and Aleutian Low
21Talley SIO210 (2014)
Slide 22
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: dont know what you have until you are
many years into them. 22Talley SIO210 (2014)
Slide 23
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 23Talley SIO210 (2014)
Slide 24
SST changes due to changing winds and circulation Adapted from
Miller, Chai, Chiba, Moisan and Neilson (J. Oceanogr., 2004)
COOLING WARMING When the Aleutian Low is strong, get: westerlies
Stronger Ocean currents weaker 24Talley SIO210 (2014)
Slide 25
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
25Talley SIO210 (2014)
Slide 26
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 26Talley SIO210
(2014)
Where are we in the NAO? High or neutral
http://www.cgd.ucar.edu/cas/jhurrell/indices.html 28Talley SIO210
(2014)
Slide 29
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) 29Talley SIO210 (2014)
Slide 30
N. Atlantic oxygen changes: ascribed to high NAO since about
1989, reduced ventilation in the NE Atlantic (Gruber, 2004)
30Talley SIO210 (2014)
Slide 31
Arctic Oscillation surface temperature variations (land)
(Wallace) High AO: Cold high latitudes (Canada, Labrador Sea) Warm
Siberia and continental US, warm subtropics 31Talley SIO210
(2014)
Slide 32
Southern Annular Mode NAM Circumpolar mode; variation in
surface pressure and hence in westerly and polar easterly wind
strength 32Talley SIO210 (2014)
Slide 33
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) 33Talley SIO210 (2014)
Slide 34
Observed global ocean changes that might be anthropogenic
(Levitus et al 2005) 0.037C warming (0- 3000 m) 34Talley SIO210
(2014)
Slide 35
Is there anthropogenic climate change? Yes (IPCC TAR) Levitus
et al (2000) heat storage changes in the North Pacific, Pacific,
World 35Talley SIO210 (2014)
Slide 36
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. 36Talley SIO210 (2014)
Slide 37
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.
37Talley SIO210 (2014)
Slide 38
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 38Talley SIO210 (2014)
Slide 39
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 Saltier Fresher Saltier Fresher Saltier
39Talley SIO210 (2014)
Slide 40
Extra slides 40Talley SIO210 (2014)
Slide 41
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 Grass Clouds Concrete Water
http://gingerbooth.com/courseware/daisy.html Reasonable initial T
High initial T Negative feedback Positive feedback 41Talley SIO210
(2014)
Slide 42
Pacific Decadal Oscillation SST and SLP patterns: note
similarity of SST to ENSO pattern and also large amplitude in the
central N. Pacific 42Talley SIO210 (2014)
Slide 43
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 North Atlantic Oscillation pattern correlated with
sea level pressure 43Talley SIO210 (2014)
Slide 44
Decadal variations in ENSO associated with ocean subtropical
changes (McPhaden and Zhang) 44Talley SIO210 (2014)
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 46Talley SIO210
(2014)
Slide 47
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 Upper
ocean Deep ocean 47Talley SIO210 (2014)
Slide 48
Is the N. Atlantic conveyor changing? This week in Nature (vol.
438, 1 December 2005) Bryden et al (2005) 48Talley SIO210
(2014)
Slide 49
Variations in central Pacific continued Warming from surface to
about 1000 m (27.2 sigma0) Salinification to about 800 m (27.0
sigma0) 1000 T S density T S 27.0 49Talley SIO210 (2014)
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 51Talley SIO210
(2014)
Slide 52
All the pacific indices ncep_ncar 52Talley SIO210 (2014)
Slide 53
All the pacific indices ncep_ncar 53Talley SIO210 (2014)