The role of a changing Southern Annular Mode in warming the Larsen Ice Shelf region

The role of a changing Southern Annular Mode in warming the Larsen Ice Shelf regionG.J. Marshall1, A. Orr2,N.P.M. van Lipzig3 and J.C. King1

1. British Antarctic Survey2. European Centre for Medium range Weather Forecasts3. K.U. Leuven Geo-Instituut

International Workshop on Antarctic Peninsula Climate Variability: observations, models and plans for IPY research, Boulder, USA, 14-16th May 2006

G.J. Marshall1, A. Orr2,N.P.M. van Lipzig3 and J.C. King1

1. British Antarctic Survey2. European Centre for Medium range Weather Forecasts3. K.U. Leuven Geo-Instituut

International Workshop on Antarctic Peninsula Climate Variability: observations, models and plans for IPY research, Boulder, USA, 14-16th May 2006

The Larsen B collapseThe Larsen B collapse

Suggested primary mechanism for collapse is crevasse propagation by meltwater filling during summer (e.g. Scambos et al. 2000).

For this mechanism to be valid an increase in regional summer temperatures is necessary in order to produce sufficient meltwater (van den Broeke 2005; Vaughan 2006).

Suggested primary mechanism for collapse is crevasse propagation by meltwater filling during summer (e.g. Scambos et al. 2000).

For this mechanism to be valid an increase in regional summer temperatures is necessary in order to produce sufficient meltwater (van den Broeke 2005; Vaughan 2006).

Recent changes in Antarctic Peninsulanear-surface temperatures

Recent changes in Antarctic Peninsulanear-surface temperatures

Trends in annual and seasonal near-surface temperatures calculated over 1965-2000, except for Bellingshausen (1968-2000) and Marambio (1970-2000).

Units are C per decade.

Significance values are shown if the trend is at the <1%, <5% or <10% level.

Trends in annual and seasonal near-surface temperatures calculated over 1965-2000, except for Bellingshausen (1968-2000) and Marambio (1970-2000).

Units are C per decade.

Significance values are shown if the trend is at the <1%, <5% or <10% level.

Contemporaneous changes in the summer SAM

Contemporaneous changes in the summer SAM

Several modelling studies have suggested that recent trends in the SAM are primarily a response to anthropogenic forcing: ozone depletion and/or greenhouse gas increases

(e.g. Thompson & Solomon, 2002; Gillet & Thompson, 2003; Marshall et al., 2004;Schindell & Schmidt., 2004)

Several modelling studies have suggested that recent trends in the SAM are primarily a response to anthropogenic forcing: ozone depletion and/or greenhouse gas increases

(e.g. Thompson & Solomon, 2002; Gillet & Thompson, 2003; Marshall et al., 2004;Schindell & Schmidt., 2004)

Summer SAM values from an index derived from observationsSummer SAM values from an index derived from observations

Summer SAM-temperature relationships in the PeninsulaSummer SAM-temperature

relationships in the Peninsula

STATION

Bellingshausen 0.12 (0.03)

Esperanza 0.47 (0.21)

Faraday 0.12 (0.04)

Marambio 0.36 (0.15)

O’Higgins 0.25 (0.07)

Orcadas 0.29 (0.04)

Punta Arenas 0.16 (0.05)

Summer correlation and (in parantheses)the absolute value of the regression coefficients between the detrended SAMand detrended Peninsula surface temperatures.

Summer correlation and (in parantheses)the absolute value of the regression coefficients between the detrended SAMand detrended Peninsula surface temperatures.

The change in summer temperatures at Esperanza (full line) and the variability explained due to the SAM.

The change in summer temperatures at Esperanza (full line) and the variability explained due to the SAM.

Changes in near-surface winds associated with SAM variabilityChanges in near-surface winds associated with SAM variability

Difference in ERA-40 10 m winds between strongly positive (1981, 1982, 2000) and strongly negative (1985, 1992) summer SAM.

Difference in ERA-40 10 m winds between strongly positive (1981, 1982, 2000) and strongly negative (1985, 1992) summer SAM.

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1. Vectors indicate flow overthe northern Peninsula

2. Area of lee cyclogenesis3. Poleward delection of

winds due to conservationof potential vorticity

1. Vectors indicate flow overthe northern Peninsula

2. Area of lee cyclogenesis3. Poleward delection of

winds due to conservationof potential vorticity

SAM-related changes in MSLP and near-surface temperature

SAM-related changes in MSLP and near-surface temperature

Difference in ERA-40 (a) MSLP and (b) 2 m temperature between strongly positive and negative summer SAM. Note the ‘bulls-eye’ of positive temperature anomaly >1°C in the north-east Peninsula.

Difference in ERA-40 (a) MSLP and (b) 2 m temperature between strongly positive and negative summer SAM. Note the ‘bulls-eye’ of positive temperature anomaly >1°C in the north-east Peninsula.

Patterns of ChangeMechanisms for the stronger SAM-related warming on the east coastMechanisms for the stronger SAM-related warming on the east coast

1. Climatological temperature gradient: the Peninsula barrier separates relatively warm maritime air in the west from cooler continental air to the east.

1. Climatological temperature gradient: the Peninsula barrier separates relatively warm maritime air in the west from cooler continental air to the east.

Contours of interpolated mean annual temperature(Morris & Vaughan, 2003).Contours of interpolated mean annual temperature(Morris & Vaughan, 2003).

2. Föhn effect: stably stratified air masses originating west of the barrier will warm through adiabatic compression as they descend on the lee side.

2. Föhn effect: stably stratified air masses originating west of the barrier will warm through adiabatic compression as they descend on the lee side.

Regional climate model resultsRegional climate model results

The Peninsula orography is highly smoothed (too wide and shallow) in ERA-40. Therefore air flow in the region is poorly constrained.

Check results using regional model integrations at higher spatial resolution (e.g. RACMO at 14 km). Results indicate enhanced warming in north-east Peninsula only, associated with a positive SAM.

The Peninsula orography is highly smoothed (too wide and shallow) in ERA-40. Therefore air flow in the region is poorly constrained.

Check results using regional model integrations at higher spatial resolution (e.g. RACMO at 14 km). Results indicate enhanced warming in north-east Peninsula only, associated with a positive SAM.

Difference between positive and negative summer SAM near-surface temperatures from a high-resolution (14 km) regional model.

Difference between positive and negative summer SAM near-surface temperatures from a high-resolution (14 km) regional model.

Wind roses when near-surface summer temperatures at Esperanza are (a) greater than 1.1°C (long-term upper-quartile temperature); (b) less than or equal to 1.1°C. Period of analysis is December 1988 to February 2004.

Wind roses when near-surface summer temperatures at Esperanza are (a) greater than 1.1°C (long-term upper-quartile temperature); (b) less than or equal to 1.1°C. Period of analysis is December 1988 to February 2004.

Observational support for proposed mechanisms

Observational support for proposed mechanisms

ConclusionsConclusions• The recent summer trend in the Southern Annular Mode (SAM) has resulted in

20% stronger circumpolar westerly winds.

• The reduced blocking effect of the Antarctic Peninsula allows greater frequency of advection of relatively warm maritime air across the northern Peninsula from west to east.

• A climatological temperature gradient across the barrier and formation of a föhn wind on the lee side cause a summer temperature sensitivity to the SAM three times greater east of the Peninsula than to the west.

• High-resolution model data, with accurate orography, and available observations support the proposed mechanisms for the enhanced eastern warming.

• The greatest summer warming associated with a more positive SAM is observed in the region where the northern Larsen Ice Shelf has collapsed.

• Model studies have revealed that the trends in the summer SAM are principally due to anthropogenic factors.

• Thus, we demonstrate a process that provides regional amplification of a hemispheric-scale signal, which in turn is primarily a consequence of anthropogenically related climate change.

• Human activity has contributed to the break-up of the Larsen Ice Shelf.

• The recent summer trend in the Southern Annular Mode (SAM) has resulted in 20% stronger circumpolar westerly winds.

• The reduced blocking effect of the Antarctic Peninsula allows greater frequency of advection of relatively warm maritime air across the northern Peninsula from west to east.

• A climatological temperature gradient across the barrier and formation of a föhn wind on the lee side cause a summer temperature sensitivity to the SAM three times greater east of the Peninsula than to the west.

• High-resolution model data, with accurate orography, and available observations support the proposed mechanisms for the enhanced eastern warming.

• The greatest summer warming associated with a more positive SAM is observed in the region where the northern Larsen Ice Shelf has collapsed.

• Model studies have revealed that the trends in the summer SAM are principally due to anthropogenic factors.

• Thus, we demonstrate a process that provides regional amplification of a hemispheric-scale signal, which in turn is primarily a consequence of anthropogenically related climate change.

• Human activity has contributed to the break-up of the Larsen Ice Shelf.