A Simple Model for Arctic Sea Ice: Variability and Trends in September Minimum Ice Extent Kyle Armou - PowerPoint PPT Presentation

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A Simple Model for Arctic Sea Ice: Variability and Trends in September Minimum Ice Extent Kyle Armou

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All ice that exists on September 15th each year is promoted to the MY ice category. Ice that grows in open water after September 15th is considered FY ice ... – PowerPoint PPT presentation

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Title: A Simple Model for Arctic Sea Ice: Variability and Trends in September Minimum Ice Extent Kyle Armou


1
A Simple Model for Arctic Sea Ice Variability
and Trends in September Minimum Ice ExtentKyle
ArmourUniversity of WashingtonDepartment of
PhysicsCecilia BitzUniversity of
WashingtonDepartment of Atmospheric
SciencesElizabeth HunkeLos Alamos National
Laboratory
2
FY First-Year Ice MY Multi-Year Ice
W Winter maximum ice area S Summer minimum
ice area
All variables are for northern hemisphere
3
FY First-Year Ice MY Multi-Year Ice
W Winter maximum ice area S Summer minimum
ice area f m f area of FY ice at summer
minimum m area of MY ice at summer minimum
4
FY First-Year Ice MY Multi-Year Ice
  • W Winter maximum ice area
  • S Summer minimum ice area f m
  • f area of FY ice at summer minimum
  • m area of MY ice at summer minimum
  • ? fraction of FY ice that survives summer melt
    season
  • ? fraction of MY ice that survives summer melt
    season

5
FY First-Year Ice MY Multi-Year Ice
Simple model for summer minimum ice area in year
n
6
FY First-Year Ice MY Multi-Year Ice
Simple model for summer minimum ice area in year
n
7
mean state and trends (equilibrium and
slowly-varying component)
high-frequency variability (interannual
timescales)
8
Equilibrium summer minimum ice area
If all parameters remain at their equilibrium
values, then
?
9
(No Transcript)
10
where are we??
11
where are we??
12
where are we??
13
Equilibrium FY and MY summer minimum ice areas
summer minimum FY ice area
summer minimum MY ice area
14
Equilibrium FY and MY summer minimum ice areas
summer minimum FY ice area
summer minimum MY ice area
15
Equilibrium FY and MY summer minimum ice areas
summer minimum FY ice area
summer minimum MY ice area
16
Sea Ice Simulation
Latest CICE ModelV4 from LANL (developed by Hunke
and Lipscomb) No Atmosphere - NCEP reanalysis
(hindcast) Mixed Layer Ocean - monthly
varying OHFC from CCSM run, annually
repeating First-Year and Multi-Year ice
categories FY ice area tracer (distinct from
the ice age tracer of Hunke and Bitz) All ice
that exists on September 15th each year is
promoted to the MY ice category Ice that
grows in open water after September 15th is
considered FY ice
17
1979-2006 March and September average ice extent
CICE hindcast - reproduces trend and
variability in September ice extent - has no
trend in March ice extent
Observations of ice extent (obs) from NSIDC
18
March Exchange of FY and MY ice area
FY
area
MY
Anticorrelated FY has increased while MY has
decreased Little variability or trend in total
ice area
MY
R -0.7
106 km2
106 km2
FY
19
September Independent FY and MY ice area
MY - m
area
FY - f
No correlation Only MY ice area has
declined Variability about the same, equally
important for variability in total ice area in
Summer
m
R -0.04
106 km2
106 km2
f
20
September Independent FY and MY ice area
MY - m
area
FY - f
MY ice survival rate
?
?
FY ice survival rate
21
FY and MY ice survival rates
MY - ?????????
FY - ?? 0.23
Not significantly correlated Trends in ? and ??
are comparable ( -0.02/decade)
?
R 0.3
?
22
Larger trend in MY area than FY area
(f summer minimum FY ice area)
(m summer minimum MY ice area)
23
Larger trend in MY area than FY area
MY - ?
MY - m
area
FY - ?
FY - f
24
Sea Ice Age Model of Maslanik et al
25
mean state and trends (equilibrium and
slowly-varying component)
high-frequency variability (interannual
timescales)
26
Perturbation Analysis
27
Perturbation Analysis
not significantly correlated winter ice area has
no significant influence on summer ice area
106 km2
R 0.10
106 km2
28
Perturbation Analysis
not significantly correlated winter ice area has
no significant influence on summer ice area
106 km2
R 0.10
106 km2
29
Perturbation Analysis
variance explained
6
106 km2
R 0.24
106 km2
30
Perturbation Analysis
variance explained
6
69
37
106 km2
106 km2
R 0.83
R 0.61
31
Perturbation Analysis
variance explained
69
37
6
- Summer minimum ice area depends almost entirely
on FY and MY ice survival rates (which are
functions of that years conditions - weather,
etc.) - Little year-to-year memory in summer
minimum area (similar to observations) - ?
is slightly anticorrelated with previous years
summer minimum ice area (R -0.21)
32
What is the underlying trend?
Could take various fits to declining ice extent
September minimum ice extent - NSIDC
ice extent
33
What is the underlying trend?
Could take various fits to declining ice extent
September minimum ice extent - NSIDC
ice extent
34
What is the underlying trend?
Could take various fits to declining ice extent
September minimum ice extent - NSIDC
ice extent
But the trajectory of declining sea ice area is
inherently a nonlinear process that depends on
the survival rates
?
35
What is the underlying trend?
Using trends in survival rates instead
survival rates from CICE hindcast
MY - ?
FY - ?
?
36
What is the underlying trend?
Using trends in survival rates instead
survival rates from CICE hindcast
MY - ?
FY - ?
obs - NSIDC
CICE projection
ice extent
37
IPCC AR4 Models 2007
Fig from RealClimate.org (2006)
38
Summary
  • Trend analysis
  • sensitivity of September ice area to a changing
    climate depends critically on the difference
    between MY and FY ice survival rates
  • Perturbation analysis
  • summer minimum area depends upon ice survival
    rates in that year there is very little memory
    of the previous years area
  • winter ice area does not significantly influence
    summer area
  • About 20 of FY ice and 60 of MY ice survives
    in CICE hindcast.
  • is this consistent with other models or
    observations?

39
Summary
  • Trend analysis
  • sensitivity of September ice area to a changing
    climate depends critically on the difference
    between MY and FY ice survival rates
  • Perturbation analysis
  • summer minimum area depends upon ice survival
    rates in that year there is very little memory
    of the previous years area
  • winter ice area does not significantly influence
    summer area
  • About 20 of FY ice and 60 of MY ice survives
    in CICE hindcast.
  • is this consistent with other models or
    observations?

Survival rate of MY ice in growth season
Survival rate of MY ice in melt season
40
(No Transcript)
41
Extra slides
42
Loss of MY ice during growth season
43
Latest CICE ModelV4 from LANL (developed by Hunke
and Lipscomb)
Tracer capability ice age, melt ponds, algae,
chemistry, etc Ice-thickness distribution Brine
pocket (mushy-layer) physics Elastic
viscous-plastic ice dynamics Incremental
remapping advection efficient for large numbers
of thickness categories and tracers, second-order
accurate in space, monotonicity preserving
State variables in each grid cell - 5 ice
thicknesses, 5 concentrations, 5 age classes, 5
snow depths, 25 temperatures, 25 salinities
(soon), and many other tracers No Atmosphere
(NCEP reanalysis), and Mixed Layer Ocean (OHF
from CCSM run, annually repeating)
44
1979-2006 March and September average ice extent
45
1979-2006 March and September average ice extent
obs
obs
106 km2
106 km2
R 0.60
R 0.83
cice
cice
106 km2
106 km2
46
Predictability of 2008 summer minimum
2007 was a 3.5? year (assuming linear trend in
ice extent)
obs
47
Predictability of 2008 summer minimum
2?S
2?S
95 confidence interval about linear trend
(assuming no memory of previous summers minimum
ice extent)
must have assumed incredibly low ice survival
rates!
obs
48
Predictability of 2008 summer minimum
2?S
2?S
Need to look at trends and variability of FY and
MY ice survival rates from observations
49
Larger trend in MY area than FY area
(f summer minimum FY ice area)
50
Larger fractional change in MY area than FY area
(f summer minimum FY ice area)
(m summer minimum MY ice area)
51
Larger fractional change in MY area than FY area
CICE hindcast
fractional FY area
fractional MY area
52
Projections of future ice decline
projected ice area from fits to survival rates
from CICE hindcast CCSM coupled GCM SRES A1B
ensemble mean
year
53
Stroeve et al (2007)
54
Uncertainty in the decline of September ice area
55
Uncertainty in the decline of September ice area
?
probability density
???? ?
56
Analogy to uncertainty in climate sensitivity
Temperature
?
Roe (2007)
57
Analogy to uncertainty in climate sensitivity
Temperature
Sea-ice area
?
?
Roe (2007)
58
Uncertainty in the decline of September ice area
probability density
probability density
59
September Ice Volume
Define a basin-average ice volume in terms of the
average thickness of ice within FY and MY
categories, and , respectively
Equilibrium Ice Volume
60
Larger fractional change in volume than area
Define a basin-average ice volume in terms of the
average thickness of ice within FY and MY
categories, and , respectively
Equilibrium Ice Volume
Equilibrium Ice Thickness
61
Larger fractional change in volume than area
Define a basin-average ice volume in terms of the
average thickness of ice within FY and MY
categories, and , respectively
Equilibrium Ice Volume
Equilibrium Ice Thickness
62
Larger fractional change in volume than area
CICE hindcast
63
Larger fractional change in volume than area
CICE hindcast
CCSM coupled GCM, SRES A1B ensemble mean
64
Perturbation Analysis
65
Perturbation Analysis
66
Perturbation Analysis
67
Perturbation Analysis
106 km2
106 km2
R -0.29
R 0.72
106 km2
106 km2
68
Perturbation Analysis
106 km2
106 km2
R 0.96
R 0.50
69
Perturbation Analysis
not significantly correlated winter ice area has
small influence on summer minimum FY ice area
106 km2
R -0.23
106 km2
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