Ice-albedo feedback in the Arctic Ocean shifting to seasonal ice zone
Decrease of sea ice concentration (Increase of open water fraction)
Enhanced solar heating at the upper ocean☀

Low albedo(~0.07)

Lateral melting
Bottom melting

Accelerated ice melting
High albedo(~0.7)

Sea iceOcean
Sea ice area of Arctic in September
1979 – 2012 [unit：10**6km2]

2000’s average

2012, September
2013, September

Sea ice Iceberg
The origins are same or different ?
Sea ice ＝ frozen sea watercontain some salt

1－3m

Iceberg ＝ calving from glacier or ice shelf
originating from snow
contain no salt
200-300m

Iceberg

Sea iceExisting amount of ice in the earth• Ice sheet, Glacier
Antarctic
Greenlandthe others• Sea ice• Permafrost

89%
9%
1%
0.1%
1%

Antarctica
Ice sheet thckness
３ｋｍ単位ｍNewtonArctic Ocean
Sea ice thckness
(m)
321

太平洋側

３ｍ

大西洋側

MarchSeptember

GlobalOcean
distribution
Arcticof multi-year
sea ice fromicesatellite
zone
remote sensing→seasonal ice zone?
Antarctic, Okhotsk Sea
seasonal ice zone

Satellite Remote Sensing• Active sensor; Radar : strong power（emit the electromagnetic wave & measure its reflection）
SAR (Synthetic Aperture Radar)
Microwave scatterometer
Microwave altimeter• Passive sensor; Radiometer : relatively weak power（measure the electromagnetic waves from objects）
Microwave radiometer （SSM/I, AMSR）
Visible, infrared radiometer (MODIS, GMS)
(Geostationary Meteorological Satellite)

Visible light
Reflection ofSun light

Infrared light
Earth radiation

Absorption band

Electromagnetic wave

Microwaves（penetrate cloud）

How many year has passed since global sea ice
distribution can be known from satellite?
１．80 yr
２．60 yr
３．40 yr
４．20 yt

We can monitor the whole earth surface
only for 40-50 years
Drastic change in the earth has likely
occurred for recent 40-50 years

Satellite remote sensing: Life line of sea ice study
AMSR2

1972No satellite data for ice-covered area
1972

ESMR
The first global observation of sea icefrom satellite microwaves

1978

SMMR

1987

SSM/I
resolution of 25㎞

2002

AMSR, AMSR-E
Japanese sensor
resolution of 6.3－12.5km

2012

AMSR2
Successor of AMSR2 ?

Sea ice areain the NorthernHemisphere1978

２０１２1979-2000

2012

Trends of sea ice area from satellite microwaves

Antarctic

1979Arctic

2007Calculation of sea ice drift
from satellite image

Maximum cross-correlation method
Resolution： 37.5×37.5km
Daily ice drift data are obtained globally

Sea ice thickness measured by satellite
ICESat (Ice, Cloud, and Land Elevation Satellite)

ICESat (2003-2009）
Laser altimeter
Assumption of Isostasy
Density of ice and snow
Thickness of snow
snow

Sea ice

CryoSat-2(Radar altimeter)(2010～）

Spring (April) Ice thickness http://www.bbc.co.uk/news/science-environment-23964372

http://www.esa.int/Our_Activities/Observing_the_Earth/Rapid reduction of Arctic sea ice in summer
第一期水循環変動観測衛星「しずく」（GCOMW1）

From 1979 to 2012
Arctic sea ice area[10**6km2]
50万km2/decade

150万km2/decade

700万km2

Orange：2000’ s average
2012, 16 Sept 349万km2

Purple：2012, Sept
white：2013年, Sept

Summer ice has nearly halved!
AMSR2
JAXA
Japanese sensor

http://www.ijis.iarc.uaf.edu/cgi-bin/seaice-monitor.cgi?lang=eSummer Arctic sea ice is declining more rapidly than prediction
September Arctic sea ice area（1900-2100）

Observations

Prediction by IPCC
After NSIDC

海氷の厚さ

IPCC: Intergovernmental Panel on Climate Change
Ice area decreases by
10% per 10 years
Arctic Research Center has been
established in Hokkaido Univ. last year.

Sea ice thickness for three decades

What degree has surface air temperature
increased in a global averagefor recent 50 years？
１．5.2 ℃
２．2.6 ℃
３．1.3 ℃
４．0.65℃
５．0.32℃

Warming by 0.65 ℃
for recent 50 years
Warming by 0.74 ℃
for recent 100 years

Global warming

Polar amplification

Air temperature anomalies for recent 10 years

Prediction of global warming by climate model with Earth Simulator(2071～2100 Ta average)－(1971～2000 average)
Amplification in the Arctic positive feedback
Decline of Arctic sea ice→decrease of albedo→increased absorption of solar radiation→further decrease of sea ice→reduction of heat insulation by sea ice→enhanced oceanic heat flow→warming of air

Change in the Arctic sea ice associated with recent global warming
・Reduction in summer sea ice extent particularly after the 2000s. [e.g. Comiso et al., 2008]
・Thinning of sea ice. [e.g. Rothrock et al., 2008]
・Reduction of multi-year sea ice. (shift to the seasonal ice zone) [Comiso, 2012]

Sea Ice Extent (x106 km2)

Drastic Ice reduction is explained by the combined effect of various factors.
(prolonged melting season, increased sea ice export, increased heat inflow, etc.)

2012
2000s
1980s
http://www.ijis.iarc.uaf.edu/jp/seaice/extent.htm

Ice-ocean albedo feedback becomes 9月の海氷域
increasingly important
for the Arctic Ocean shifting to the seasonal sea ice coverChange in the summer Arctic
ice extent (accelerated after 2000)

Stroeve et al. (2012)

Kwok and Rothrock (2009)

Mean sea ice thickness obtained from submarine and
ICESat observations （comparison for three time periods）

Change in sea ice age in the Arctic Ocean (decrease in MYI)

≥5th-year
4th-year
3rd-year
2nd-year
1st-year

Stroeve et al. (2012)

What is the (ice–ocean) albedo feedback effect?
●Albedo is the ratio of reflection to the solar radiation.
●Sea ice (commonly covered with the white snow) reflects 60–70 % of the solar radiation,
while the black open water fraction reflects only less than 10 %.
●Once the ice concentration has reduced, the heat input into the upper ocean is intensified
because of its lower albedo. Then, the accelerated ice melt by warmed ocean results more
open water fraction and more sea ice melt.
●Such positive feedback is assumed as a key factor of the “polar amplification” of global
waring. (Note that the quantitative evaluation is still insufficient)
Decrease in sea ice concentration (increase in open water fraction)
Enhanced solar heating through OW
Low albedo(~0.07)

Accelerated ice melting by warmed upper ocean
High albedo(~0.7)

Sea ice

Lateral melting
Basal melting

Ocean

・In seasonal sea ice zones (Antarctic Ocean and Sea of Okhotsk)
Ice-ocean albedo feedback triggered by offshore-ward wind (divergent ice motion)
controls the seasonal/interannual variation in sea ice melt.

☀Decrease in sea ice concentration (Increase in open water fraction)

Enhanced solar heating at the upper ocean
Low albedo(~0.07)

Lateral melting
Bottom melting

Accelerated ice melting
High albedo(~0.7)

Sea iceOceanIn the Arctic Ocean,・Is the necessary condition for the feedback satisfied?
・What is the physical process triggering and translating the feedback?
Focusing on the Pacific Arctic where the summer ice extent is drastically reduced.

Data (analysis period: 36 years from 1979 to 2014)
・ Satellite Microwave Radiometer (SMMR, SSM/I) Heat Budget, Ice Melt Volume, Ice
Divergence
Sea ice concentration (Bootstrap algorithm)
Melt onset date（Markus et al., 2009）

Spatial resolution: 25km×25km polar-stereo graphic gridTemporal resolution: 1day (2days for SMMR)・NSIDC Ice Motion Vector Ice Melt Volume, Ice Divergence(based on satellite/buoy observations and meteorological reanalysis)

Spatial resolution: 25km×25km EASE gridTemporal resolution: 1day
・NSIDC Sea Ice Age Heat Budget (estimation of albedo/pond fraction)
Spatial resolution: 12.5km×12.5km EASE gridTemporal resolution: 1week
・ICESat Sea Ice Thickness (2003–2008) Ice Melt Volume
Spatial resolution: 10m
Temporal resolution: 1season (from Feb to May)
・Meteorological Reanalysis (ERA-interim) Heat Budget
Spatial resolution: 1.5o×1.5o
Temporal resolution: 1/4dayOverview of the heat budget analysis
・Heat input into the upper ocean through OW

Qw = (∑n[(1 − Cn)Fwn Sgn])/Se
C: Ice concentration
Fw: Net heat flux at the water surface
Sg: Unit grid cell area (≈ 25km×25km)
Se: Sea ice extent (= ∑n Sgn)

・Ice Melt Volume (converted to the equivalent heat )Qm = (ρi Lf hi dSa/dt)/Se

September ice conc.

ρi: Ice concentration
Lf: Latent heat of sea ice for fusion
hi: Mean ice thickness (assume a constant thickness of 1m)
Sa: Sea ice area (= ∑n Cn Sgn)
dSa/dt: Decrease of ice area (ice export is removed)

September ice motion◇Parameterization of ice surface albedo

Ice Surface Albedo

Date ofmelt onset

Date offreezeup

Melt onset date 2007

Markus et al. (2009)

Multi-year ice
First-year ice
Perovich et al. (2012)@Melting snow

@Pond evolution

Ice thickness (hi) from the ICESat observation2007 Spring

・Mean ice thickness in the analysis area is 1.5m.
However, in fact, approx. 1m-thick sea ice is dominant.
・Assuming that a reduction in ice extent through melt is
confined to FYI only.

54→Here the value of 1m is used as the typical ice thickness
for calculation of ice melt volume.

Probability Density [1/m]

0.1
1.0

3
2
10 [m]

2003 2004 2005
2006 2007 2008

0.95±0.23 m

0.08
0.8

0.06
0.6

0.04
0.4

0.02
0.2

0.00
0
0.0

0.5
0.5

1
1.0

1.5
1.5

Ice Thickness [ｍ]

2
2.0

2.5
2.5

33.0

・Ice retreat is mainly explained by the melting,and the impact of ice export is much smaller.
・The effect of melt ponds is up to ~20 % ofthe heat input through OW.

→Melting caused by heat input through
OW (mainly due to the solar radiation)
controls sea ice retreat.
⇒Necessary condition for the ice-ocean
albedo feedback is satisfied!

Annual Heat Input [x102 MJ m-2]

・ Heat input through the open water
fraction corresponds well with the ice melt
volume.

Ice Melt Vol. (latent heat) [MJ/m2]☆Results of heat and ice budget analysis in the ice covered area (ice conc. ≥ 30%）
6

1979–20144▲ May◀ Jun▼ Jul▶ Aug20640

24Heat Input through OW [MJ/m2]

R = 0.88（trend removed）
Heat input through OW
Ice melt volume (latent heat)Ice export volume (latent heat)201980

1990

2000

2010

6

・To explore the specific trigger of the feedback effect,sea ice divergence (Div) is determined as the ice area export from the boundary(B) and ice edge (E) divided by ice extent (Se).

Div = (∫B C uo dl + ∫B C uo dl)/Se

Early Summer Div (mid-May to early-Jun) vs. Ice Melt Volume
・Div in the early summer corresponds well with the 1-2 months lagged ice melt
volume and thus the annual value of Qm.

R=0.68 (0.54@detrended)

Ice Melt Volume [x102 MJ m-2]

Early Summer Div [% mo-1]

The albedo feedback enhances the subsequent ice melt within a couple of months.
※An increasing trend is apparent after the 2000s.

Representation of the feedback using a simplified model・Y-axis–Time evolution of ice retreat is reproduced by a simplified
ice-ocean coupled model.

→Quantifying the enhancement of ice melt by divergent
ice motion through the feedback effect.
※Based on the model used by Ohshima and Nihashi (2005),
MYI distribution is introduced to represent the Arctic Ocean.
C1−CCMYOhshima and Nihashi (2005)

Modified for theArctic Ocean

CFY

・The effect of ice motion is introduced to the model as,

dC/dt = −a0 CFY (T − Tf)/hi − U ∂C/∂y + AH ∂2C/ ∂y2 + ψC
Melting of ice

Ice motion in May

Advection of ice

Horizontal
diffusion

Ice resistance(internal stress)

90
80
70
90

Climatology (after 2000)

Latitude at 180 oN

80
70
90

・Heat input through OW is the
only heat source for ice melt.
(2-dimensional ice retreat model)

Lower Divergence807090・Effect of Div is introduced byusing ice drift data obtained
from satellite observations.

Higher Divergence
8070

Satellite Observations

Results of the modelDiv:
Ice conc. reduction by ice drift

Div has been doubled

Adv: Ice export volume

Qw has increased 1.5 times
Qw:
Heat input through OW
Qm: Ice melt volume

Qm has increased 2.3 times

Drastic reduction of summer Arctic sea ice

Summary○Ice-ocean albedo feedback has received increasing attention in the context of drastic
sea ice reduction in the Arctic Ocean shifting to a seasonal ice cover.
○Heat budget analysis shows that the ice retreat is mainly explained by ice melting,and it corresponds well with the heat input through the open water fraction.
→Heat input through OW is the primary driver of seasonal and interannual
variations in Arctic ice retreat.
○Divergent ice motion in the early melt season indicates a significant correlation withice retreat lagged by 1-2 months and also the annual value of ice melt volume.
○A simplified model indicates that divergent ice motion accelerates the ice retreat
through the enhanced ice melting.
→Divergent ice motion can be a trigger of ice retreat through the ice-ocean albedo
feedback.
○From the comparison between before and after 2000, increased heat input throughOW (through a doubled ice divergence) explains roughly 70 % of the observed 2.3fold increase in sea ice melt volume.