The Ice/Ocean Interface During Summer: Implications for Ice-Albedo Feedback

Miles McPhee

McPhee Research Company

SEARCH OSM

28 Oct 2003

•Underice melt ponds and false bottoms

•Storage and sequestration of heat in the upper ocean

• False bottoms/underice meltponds may significantly impact IAF by:

1) Shielding thin ice from oceanic heat flux and bottom melting, and

2) Decreasing the aggregate ocean-to-ice heat flux by acting as a source of heat at the ice/water interface rather than the usual latent heat sink.

• The first direct estimates of the interface heat and salt exchange coefficients (from WARPS 2003) indicate T ~ 12.5x10-3 with T/ S ~ 50

• Heat storage in the summer mixed layer has an appreciable direct impact on IAF by absorbing solar radiation (that would otherwise go to melting) during the time of maximum solar angle.

• A less direct effect on IAF is sequestering heat below the midsummer meltwater cap. Evidence suggests that in the Canada Basin, the ice/mixed layer system is a net source of heat for the upper pycnocline.

Underice melt ponds and false bottoms

Citation: Notz, D., M. G. McPhee, M. G. Worster, G. A. Maykut, K. H. Schluenzen, and H. Eicken, Impact of underwater-ice evolution on Arctic summer sea ice, J. Geophys. Res., 108(C7), 3223, doi:10.1029/2001JC001173, 2003.

During the 1975 AIDJEX Project in the Beaufort Gyre, Arne Hansonmaintained an array of depth gauges at the main station Big Bear. Hereare examples showing a decrease in ice thickness for thick ice, but an increase at several gauges in initially thin ice.

Thick ice (BB-4 – BB-6) ablated 30-40 cm by the end of melt season. “Falsebottom” gauges showed very little overall ablation during the summer. The box indicates a 10-day period beginning in late July, when false bottoms apparently formed at several sites.

Thick ice gauges

False Bottom Gauges

00*0

00*0

''

''

SSuSw

TTuTw

wS

wh

rh

pTms

z hturbTsm

khu

K

dzu

uTw

TzT

)/()/(1

'

/''

)(

0*

00*

0*0

0

8.0

///

/1

32

r

kk rShTmsSmsSh

Tslh

70/35 Sh

.

Fresh W a ter Layer

Tu p =0 o C

Tw = -1.7 o C

h

Sea water ~ slightly abo ve freezing

Multiyear Ice

False Bottom

T 0 S 0 w T 0 w S 0 u *0

h

TT

c

K

c

H up

p

i

p

ice 0

Assuming a linear temperature gradient in the thin false bottom:

If the upper layer is fresh, temperature 0oC:

h

mST

c

K

c

H up

p

i

p

ice 0

ice2up1wH

21

hLL

h0*p1

iceL2wLHice0iceL2LH

2

0

)1(

/

thicknessice is where

provided

0)()(

SmTTT

mA

QT

h

hucK

SQSTTSSASQTTAS

S

i

This modifies the heat equation slightly from one in which the conductive heat flux in the ice is specified directly, but nevertheless leads to a quadratic for S0

false bottom “true” bottomupward heat flux

down

“water table”

Winter ARctic Polynya Study – Mar-Apr, 2003

Special thanks to Anders Sirevaag, Ilker Fer and Ursula Schauer

)(''

)(''

00*0

00*0

SSuSw

TTuTw

wS

wT

ice000

00

00

/''

''

''

SwSwS

Q

qTww

qwQTw

L

L

)( 00 STT f Measure these quantities with a turbulence cluster 1 m below the ice.

Estimate from the temperature gradient in the ice

psu 4

mK 7.21/

s mK 1054.6''

s mpsu 1073.1''

psu 34.430 C -0.962

1-

1-5

1

1-5

1

11

ice

ice

m

m

mo

m

S

dzdT

Tw

Sw

ST

50

1046.2

0124.0

C -1.491

psu 27.39

s m 1040.7

S

4S

0

0

-170

T

T

oT

S

w

Summer Heat Storage in the Upper Ocean

1976

A modest anticyclonic surface motion field can induce the downwelling velocity needed to drive the trapped summer heat downward. During AIDJEX this was about 20 MJ m-2 (equivalent to about 8 cm ice ablation).

Conclusions

• Heat storage in the summer mixed layer has an appreciable direct impact on IAF by absorbing solar radiation (that would otherwise go to melting) during the time of maximum solar angle.

• A less direct effect on IAF is sequestering heat below the midsummer meltwater cap. Evidence suggests that in the Canada Basin, the ice/mixed layer system is a net source of heat for the upper pycnocline

• The first turbulence measurements providing direct estimates of the interface heat and salt exchange coefficients indicate T ~ 12.5x10-3 with T/ S ~ 50

• False bottoms/underice meltponds may significantly impact IAF by:

1) Shielding thin ice from oceanic heat flux and bottom melting, and

2) Decreasing the aggregate ocean-to-ice heat flux by acting as a source of heat at the ice/water interface rather than the usual latent heat sink.