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

1. The role of a changing Southern Annular Mode in warming the Larsen Ice Shelf region G.J. Marshall1, A. Orr2, N.P.M. van Lipzig3 and J.C. King1 1. British Antarctic Survey 2. European Centre for Medium range Weather Forecasts 3. 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

2. The 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).

3. 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.

4. 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) Summer SAM values from an index derived from observations

5. Summer SAM-temperature relationships in the Peninsula Summer correlation and (in parantheses) the absolute value of the regression coefficients between the detrended SAM and detrended Peninsula surface temperatures. The change in summer temperatures at Esperanza (full line) and the variability explained due to the SAM.

6. 3 1 2 Changes in near-surface winds associated with SAM variability • Vectors indicate flow over • the northern Peninsula • 2. Area of lee cyclogenesis • Poleward delection of • winds due to conservation • of potential vorticity Difference in ERA-40 10 m winds between strongly positive (1981, 1982, 2000) and strongly negative (1985, 1992) summer SAM.

7. 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.

8. Mechanisms for the stronger SAM-related warming on the east coast Patterns of Change • Climatological temperature gradient: the Peninsula barrier separates relatively warm maritime air in the west from cooler continental air to the east. 2. Föhn effect: stably stratified air masses originating west of the barrier will warm through adiabatic compression as they descend on the lee side. Contours of interpolated mean annual temperature (Morris & Vaughan, 2003).

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

10. Observational support for proposed mechanisms 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.

11. Conclusions • 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.