Sea ice near an ice shelf

Pat Langhorne

Department of Physics

University of Otago

EC-PORS, Wellington, 27 Feb 2014

2003

Jono Leitch (field assistant)

Greg Leonard (post-doc)

Craig Purdie (PhD student)

2009

Brian Staite (field assistant)

Andy Mahoney (post-doc)

Alex Gough (PhD student)

Erebus Bay: 15 July 2009• Callaghan Innovation

– Tim Haskell• University of Otago

– Inga Smith, Ken Hughes, Alex Gough, David Dempsey, Pat Wongpan, Andy Mahoney, Craig Purdie, Greg Leonard

• NIWA – Mike Williams, Craig Stevens, Natalie Robinson, Stefan Jendersie

• University of Canterbury – Wolfgang Rack, Daniel Price

• Victoria University of Wellington – Joe Trodahl, Malcolm Ingham, Mark McGuinness

• Rest of the World– Christian Haas (Canada)

What is the contribution of the ocean, conditioned by processes at the base of an Antarctic ice shelf, to the winter growth and thickness of the coastal sea ice?

Overarching research question

EC-PORS, Wellington, 27 Feb 2014

West Antarctica

East Antarctica

WeddellSea

RossSea

Amundsen Sea

Bellinghausen Sea

Indian Ocean

Pacific Ocean

Filchner

Fimbulisen

Ronne

Amery

Larsen

Ross

Antarctica: more than half of coastline is ice shelf (Bindschadler et al, 2011)

Ice shelves and sea ice

Interaction between an ice shelf and the ocean causes cooling of the water below its surface freezing point. Ice crystals form and float up under the ice shelf and under sea ice.

Figure from Hughes (MSc, unpublished)

What we will show…• Key features of sea ice-ocean interaction near an ice

shelf as illustrated by 2003 and 2009 winter campaigns. (Leonard et al, 2006; 2011: Purdie et al, 2006: Mahoney et al, 2011: Gough et al, 2012a,b: Gough et al, 2013)

• Ask how pervasive is the influence of ice shelves on sea ice thickness around Antarctica.

• Examine whether these conditions have changed in the past century.

West Antarctica

East Antarctica

WeddellSea

RossSea

Amundsen Sea

Bellinghausen Sea

Indian Ocean

Pacific Ocean

Filchner

Fimbulisen

Ronne

Amery

Larsen

Ross

McMurdoSound

McMurdo Sound extensively researched, beginning about 100 years ago

Observations from winters 2003 and 2009

Observation 1:

Seasonal evolution of much of the water column to temperatures below the surface freezing point.

Signature of interaction with ice shelf

Members of 2009 winter team setting up an oceanographic mooring through the sea ice cover

In the ocean (2008/09)

temperature

Rapid warming in summer, more gradual cooling through autumn

salinity

Rapid freshening as sea ice melts, gradually more saline as sea ice freezes.

spring summer autumn winter spring

From Mahoney et al (2011)

How have ocean conditions changed in McMurdo Sound?

Ocean surface temperature seems unchanged since 1900s.

Salinity has probably decreased.

Ocean surface temperatures held just below freezing point in winter - regulating influence of basal melting deep in the ice shelf cavity?

Figure compiled by Ken Hughes, Andy Mahoney & Mike Williams

Observation 2:

Effect of supercooled water on ice growth in the ocean and at the ice-water interface.

Observations from winters 2003 and 2009

Ice crystals in the water column:Underwater snow storms do happen

Paul Cziko, University of Oregon, McMurdo Sound, Oct 2012

Ice crystals form at depth in the ocean, rise and become large

close to the ice-water interface where the water is supercooled

in situ.

Photo: Brett Grant, NIWA

…. and they cling to everything, providing buoyancy.

31 August 2009

The crystals accumlate and grow beneath the sea ice, forming a porous layer:

the sub-ice platelet layer

Photo: 2009 winter team

5 cm

Observation 3:

Effect of supercooled water on the structure and thickness of the sea ice cover.

Observations from winters 2003 and 2009

Timeline of sea ice-ocean interaction

From Mahoney et al (2011) J. Geophys. Res.

“regular” ice

sub-ice platelet layer

platelet ice

no sensible thermal energy remaining

in near-surface ocean

Timeline of sea ice-ocean interaction

From Mahoney et al (2011) J. Geophys. Res.

“regular” ice

sub-ice platelet layer

platelet ice

persistent presence of ice shelf water

Difference of ocean temperature at 50 m depth from the freezing point temperature at the surface.

How much sea ice is formed due to heat flux to the ocean?

Time period Ice growth /

m

% ice growth due to heat

flux to ocean

Mar-Sep 2003

1.81 14%

Feb-Oct 2009

2.10 12%

Taken from Purdie et al (2006) & Gough et al (2012)

Sites within 10 km of the ice shelf edge

How pervasive is the effect of ice shelves?

We have observed that ice shelves contribute to sea ice thickness, but …..

- is this effect important to the climate system?

- is this sea ice a measure of the “health” of an ice shelf?

Figure from Hughes et al (submitted, JGR)

Observations of frazil and platelet ice around Antarctica

Figure compiled by Ken Hughes & Alex Gough

McMurdo Sound

McMurdo Ice Shelf

Ross Island

Sub-ice platelet layer in McMurdo Sound

Related to mean late-winter heat flux to the ocean.

Data principally from 1986, 2009 & 2011, 2013.

Figure from Gough (PhD thesis), Price et al (submitted) and modified and extended by Ken Hughes.

McMurdo Sound

McMurdo Ice Shelf

Ross Island

Shown that it is possible to detect sub-ice platelet layer using helicopter-borne electromagnetic induction, but extending to satellite altimeter will be very difficult.

Interrogating sea ice

Timeline of platelet ice observationsMcMurdo Sound: 1902-present

Figure compiled by Ken Hughes

• Observations and implications

– Sea ice close to an ice shelf is 15% thicker than the case where only atmospheric heat flux. Effect may extend for 100s km

– Platelet ice can be used as a proxy for integrated oceanic heat flux.

– Thus we can use 100 year record of sea ice and ocean observations in McMurdo Sound to estimate time variability of this heat flux.

– Presence of subice platelet layer will influence the sea ice freeboard and therefore satellite measurements of sea ice thickness.

Where to from here?

Antarctic sea ice is a complex beast: a big challenge to measure sea ice thickness on regional scale.

AcknowledmentsAntarctica New Zealand,

2003 & 2009 Scott Base winter teams,

University of Otago, Canterbury, VUW & Antarctica New Zealand scholarships,

Foundation for Research Science and Technology,

Marsden Fund.

EC-PORS, Wellington, 27 Feb 2014

Satellite proxy measurements

Sea ice freeboard derived from ICESat (2003-2009). Black = first year ice; coloured = multi-year sea ice in McMurdo Sound

From Price et al (2013) JGR.

Freeboard maximum is aligned with platelet ice maximum

167oE

1m ice 1m platelets

164oE 30' 165oE 30' 166oE 30' 78oS

54'

48'

42'

36'

30'

Ross Island

McMurdo Ice Shelf

1m ice1m platelets