Examining a downslope warming wind event over the Antarctic Peninsula through modeling and aircraft observations: can mountain waves cause surface melting on the Larsen Ice Shelf?

Daniel Grosvenor, Thomas Choularton, Martin Gallagher (University of Manchester, UK);Thomas Lachlan Cope and John King (British Antarctic Survey).

•ContentsContents•The Antarctica Peninsula region – recent rapid regional The Antarctica Peninsula region – recent rapid regional warming and the collapse of Larsen Bwarming and the collapse of Larsen B•The Föhn (downslope windstorm) effect The Föhn (downslope windstorm) effect •Aircraft observations and modelling of such an event over Aircraft observations and modelling of such an event over Larsen C ice shelf:-Larsen C ice shelf:-

•What did the event look like and how well does the What did the event look like and how well does the model capture it?model capture it?•What caused the event?What caused the event?•How likely are such events and could they play a role How likely are such events and could they play a role in warming trends on the east side of the peninsula?in warming trends on the east side of the peninsula?•What impact could such events have on the Larsen C What impact could such events have on the Larsen C ice shelf?ice shelf?

The Antarctic Peninsula region

Larsen B

Larsen C

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Topography

Scale comparisonScale comparison

•Warmer oceanic air is generally deflected along the mountain barrier

•Therefore, east side is usually under the influence of cold air from the continent

•Annual mean temperatures on east are 5-10oC cooler than on the west at comparable latitudes

•However, when westerlies impact on the peninsula in the right conditions the warmer air can cross the mountain barrier making east side warmer

•Extra warming is also possible from adiabatic descent of air down the mountain slope and/or latent heat release – Föhn effect

WARMER OCEANIC AIR

COLD CONTINENTAL AIR

The Antarctic Peninsula warming trend

Warming trend oC per decade, 1965-2000, except where notedMarshall et al. (2006), Journal of Climate

Possible causes of amplified warming:-

-Southern Annular Mode (SAM) index has been increasing, especially in summer – leads to stronger westerlies-Linked to ozone loss (mainly) and greenhouse gases (less so)-May lead to more Föhn events.

1968-2000

1970-2000

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•Peninsula has shown a major warming – Faraday station showed 0.56 oC per decade (1951-2000) compared with mean global warming of 0.6 oC in whole of 20th century.

•The warming in summer on the east side has been greater than for the west side

Warming of westerly flow can be enhanced by latent heat release on the upslope side and/or adiabatic descent of air from above, on the downslope side:-

Warming of westerly flow can be enhanced by latent heat release on the upslope side and/or adiabatic descent of air from above, on the downslope side:-

Moist air rising in saturated adiabat – gains heat from latent heat of condensation

Some of the moisture precipitates out

Possible dry adiabatic descent leading to warm temperatures at the surface

Dry adiabatic descent from aloft leads to warmer temperatures at the surface and gravitational acceleration produces strong winds.

Wave breaking aloft allows air to descend on upslope of terrain and keep descending on downslope

Type 1 Type 2

The Föhn/downslope flow

•Strengthening of westerly winds due to increasing trend in summer SAM index could be causing increased frequencies of Föhn events and thus might account for the enhanced warming trend observed on the east side.

Collapse of the Larsen B ice shelf

Larsen C

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MODIS satellite images 31st Jan, 2002

Larsen B

Scale comparisonScale comparison

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Collapse of the Larsen B ice shelf

MODIS satellite images 17th Feb, 2002

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Larsen C

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Collapse of the Larsen B ice shelf

MODIS satellite images 23rd Feb, 2002

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Larsen C

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Collapse of the Larsen B ice shelf

MODIS satellite images 5th Mar, 2002

Sediment data below the ice shelf suggests that it had been around for the last ~11,700 years.Evidence for glacier speed up – northern AP glaciers contribute ~0.16 mm/year to global sea levels

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Larsen C

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3200 km2 lost

Larsen B

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Collapse of the Larsen B ice shelf

Larsen C

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Larsen B

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• Modelled using a special polar version of WRF (Weather Research and Forecasting) mesoscale model

• Initialised and boundaries driven by ECMWF 0.5 degree global model

• 3 nests - 30, 7.5 and 1.875 km resolution

The 6th January, 2006 Föhn flow case

• A British Antarctic Survey (BAS) flight over the Peninsula made observations in a Föhn flow

The synoptic situation

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Pressure at 2.3 km for 06 Jan 06:00 UTC (hPa) for ecmwf-ml-0.5-nudging

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Wind direction directed to the west where it rotates and impacts onto west side of the peninsula

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Pressure (hPa) at 2.3 km from ECMWF analysis

Flight track and model results

Colours are wind speedArrows give wind speed and direction

Wind speed(m/s)

Rothera - BAS research station

Aircraft observed jet windspeed maxima here

4th vertical model level; ~293 m above surface

18 UTC, 5th Jan

Wind speed(m/s)

4th vertical model level; ~293 m above surface

Wind speed(m/s)

4th vertical model level; ~293 m above surface

Wind speed(m/s)

4th vertical model level; ~293 m above surface

Wind speed(m/s)

4th vertical model level; ~293 m above surface

6 UTC, 6th Jan

Wind speed(m/s)

4th vertical model level; ~293 m above surface

Wind speed(m/s)

4th vertical model level; ~293 m above surface

Wind speed(m/s)

4th vertical model level; ~293 m above surface

Aircraft observed jet windspeed maxima here

15 UTC, 6th Jan

Wind speed(m/s)

4th vertical model level; ~293 m above surface

Wind speed(m/s)

4th vertical model level; ~293 m above surface

21 UTC, 6th Jan

Wind speed(m/s)

4th vertical model level; ~293 m above surface

Wind speed(m/s)

4th vertical model level; ~293 m above surface

Wind speed(m/s)

4th vertical model level; ~293 m above surface

Wind speed(m/s)

4th vertical model level; ~293 m above surface

9 UTC, 7th Jan

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Wind speed profile at 2006-01-06 15:00:00 for ecmwf-ml-0.5-nudging

Aircraft descentAircraft ascent2C -67 , -62.2

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Temperature (oC)

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Aircraft descentAircraft ascent2C -67 , -62.2

a)a) b)b) c)c)

Descent (20:40)Descent (20:40)

Ascent (22:01)Ascent (22:01)ModelModel

• Generally a good match for wind direction, temperature and the jet height.• However, the timing is bad:-

•Aircraft descent and ascent were at 20:40 and 22:01 UTC respectively.

• After 15 UTC the model jets die down.• Modelled wind jets are too weak at 15 UTC.• Due to meteorology change – wind direction no longer perpendicular to mountain

15 UTC 6th January (model)

Wind speed (m/s) Wind direction (degrees) Temperature (oC)

Model profiles at location C

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The changing synoptic situation that leads to the Föhn die down

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Pressure at 2.3 km for 06 Jan 06:00 UTC (hPa) for ecmwf-ml-0.5-nudging

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Wind direction directed more to the west where it rotates and impacts onto west side of the peninsula

Föhn situation6 UTC, 6th Jan

The changing synoptic situation that leads to the Föhn die down

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Pressure at 2.3 km for 07 Jan 09:00 UTC (hPa) for ecmwf-ml-0.5-nudging

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Non-Föhn situation

Wind direction coming from continent directed more along and to the east of the peninsula

Low pressure system has moved east

9 UTC, 7th Jan

Cross sections

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Potential temperature (K) cross section for 06 Jan 06:00 UTC for ecmwf-ml-0.5-nudging

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Component horizontal wind component (m s-1) cross section for 06 Jan 06:00 UTC for ecmwf-ml-0.5-nudging

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Well-mixed region caused by wave breaking

Leads to strong downslope winds

Approx. continuous stratification higher up

Well mixed upstream region

Blocked air below ~1 km

Wind directionWind direction

WestWest EastEast

Potential temperature (K) Wind speed (m/s)

•Model predicts that a “type 2” Föhn event took place with severe blocking on the upwind side

•Blocking may be necessary for downslope flow – does sea-ice play a role in the degree of blocking?

•The past literature has often assumed that Föhn warming would not occur over the Antarctic Peninsula when the wind speed is low enough, or the stability high enough to produce upstream blocking.•Indicates likely importance of upper level winds and direction rather than speed for these events.

6 UTC, 6th Jan

Melting of ice shelves over the 3 simulation days

•Production of melt water thought to have been the primary cause of the breakup of Larsen B through crevasse propagation.•Clear reduction in melting moving south down the peninsula ice shelves•Although northern sections of Larsen C experience similar melting to Larsen B•Most important energy source for melting is from the Sun rather than the warm Föhn air – but melting would not begin without the warm air above the ice surface (on/off switch)

Larsen B

Larsen C

Total melting (mm water equivalent)

Summary and conclusions

Summer warming on the east side of the peninsula has been greater than that for the west and likely led to the collapse of the

Larsen B ice shelf.

Observations show that a Föhn event occurred on 6th January. Model reproduces the observed jet structure and

direction well except for the duration/timing and wind speed under-prediction. This looks to be the result of large scale

analysis problems (meteorology)

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Aircraft descentAircraft ascent2C -67 , -62.2

Modelling suggests that a “type 2” Föhn event occurred - driven by mountain wave breaking and characterised by

upstream blocking

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Distance (km)

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Suggests Föhn events can occur at relatively low wind speeds in contrast to past literature for this region - may be more dependent on the stability and upper level wind direction – blocking looks to be a necessary condition –

sea-ice, katabatic flow dependence?

Warm air likely acts as an on/off switch for ice shelf surface melting rather than a significant energy source –

most of the energy comes from the Sun

Acknowledgements

• I would like to thank Dave Bromwich and his team for supplying me with the polar WRF modifications.

• I also thank ECMWF for the use of their high resolution analysis data.

• And NCAS for the time provided on the HECTOR supercomputer.