Strong winter surface melt on an Antarctic ice shelf

Transcription

1 Strong winter surface melt on an Antarctic ice shelf P. Kuipers Munneke 1, A. J. Luckman 2, S. L. Bevan 2, E. Gilbert 3, C. J. P. P. Smeets 1, M. R. van den Broeke 1, W. Wang 4, C. Zender 4, B. Hubbard 5, A. Orr 3 and J. C. King 3 1 Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, The Netherlands 2 Glaciology Group, Department of Geography, Swansea University, Swansea SA2 8PP, UK 3 Britisth Antarctic Survey, National Environmental Research Council, Cambridge CB3 0ET, UK 4 Department of Earth System Science, University of California, Irvine, California, USA 5 Centre for Glaciology, Department of Geography and Earth Sciences, Aberystwyth University, Aberystwyth SY23 3DB, UK Increased surface melt, driven by atmospheric warming, plays a central role in the observed sudden and rapid collapse of ice shelves surrounding the Antarctice Peninsula 1 4. Ice-shelf demise is projected to intensify across Antarctica under ongoing warming 5, 6, with the potential to raise global sea levels significantly 7 following large-scale thinning and acceleration of ice-shelf tributary glaciers Surface melt has hitherto been associated with the austral summer season, due to higher temperatures and strong insolation. Here, we use in-situ and satellite data, and weather model results, to show that significant and strong surface melt events on Larsen C Ice Shelf frequently occur in the middle of winter. In the absence of solar radiation, these multi-day melt events are driven by outbreaks of warm and dry föhn winds descending from the Antarctic Peninsula mountain range, resulting in turbulent fluxes of sensible heat that drive surface melt fluxes in excess of 200 W m 2. In 2015 and 2016, 1

2 23% of the annual melt flux was produced in winter, and we suggest that parts of Larsen C Ice Shelf are turning into a melt-induced blue-ice area 11 in current conditions. The observed formation of winter melt ponds preconditions the ice shelf for more meltwater-driven instability, and faster flow by warming and softening the ice 12. Future southward migration of föhn-driven wintertime melt provides additional mechanisms for ice-shelf instability around the continent. The current mass imbalance of the Antarctic Ice Sheet is made up almost entirely of enhanced basal melt and calving 13. Of the 90 Gt (1 Gt = kg) of meltwater produced annually, almost none runs o directly into the ocean 14, and instead is refrozen in underlying snow and firn layers. The indirect impact of meltwater is profound, however: collapse of coastal ice shelves by fracturing of meltwater-filled crevasses has led to a manifold increase in ice flow of tributary glaciers 8 10, explaining part of the increased dynamical ice loss witnessed in the Antarctic Peninsula in recent decades 15. Sustained high rates of meltwater refreezing removes the layer of snow and ice that would otherwise be able to keep meltwater from englacial cryohydrologic warming and softening of the ice 12. Model- and satellite-derived melt fluxes range from mm w.e. y 1 on ice shelves in Dronning Maud Land and the Amundsen Sea sector, to 220 mm w.e. y 1 on average over Larsen C Ice Shelf 16, with regions of Larsen C peaking at mm w.e. y 1 on average. The bulk of the energy for snowmelt in Antarctica is provided by solar radiation, aided by turbulent fluxes of sensible and latent heat directed towards the surface 17. At most locations, the sensible heat flux 2

3 towards the surface increases in winter due to higher wind speeds, but insu ciently to compensate for the net thermal cooling to space, and the absence of solar radiation in the dark, polar night. As a consequence, the occurrence of surface melt has so far been observed and reported in summer only, when solar radiation is abundant. Here, we use a surface energy balance model (Methods section) to report strong melt fluxes in the austral winter, derived from measurements of an automatic weather station (AWS) located in Cabinet Inlet, on Larsen C Ice Shelf in the Antarctic Peninsula. Two years of AWS observations (November 2014 November 2016) reveal a cumulative melt of 0.89 m w.e. (water equivalent) in Cabinet Inlet (Figure 1). We find that 71% of this melt (0.65 m w.e.) occurs in austral summer, which we define here to last from 1 November to 31 March. This summer melt mostly occurs in prolonged episodes of days to weeks (average X days), with peak daily melt fluxes of W m 2. The melt energy in these conditions is supplied mostly by absorbed solar radiation, and to a lesser extent by turbulent fluxes of sensible heat delivering heat from the atmospheric boundary layer to the surface 14. However, 29% of the surface melt in the period under consideration (0.24 m w.e.) is generated in the austral winter, here defined from 1 April to 31 October. Most of the wintertime melt occurred in the austral winter of 2016 (0.19 m w.e.), and a smaller fraction in 2015 (0.05 m w.e.). Wintertime surface melt took place in all months except July. As opposed to summer melt, the winter melt episodes are usually shorter (at most a few days, X days on average) and more intense, with daily-mean melt fluxes ranging from 25 to over 130 W m 2. During the strongest wintertime melt episode in the record (25 30 May 2016, Figure 2), the 3

4 combination of high wind speed (5 13 m s 1 ) and warm air (5 13 C at 2 m above the surface) results in a large turbulent flux of sensible heat transporting heat from the boundary layer to the surface. A negative flux of longwave radiation (longwave cooling) o sets some of the sensible heat flux. Still, the resulting melt flux is dominated by the sensible heat flux, and frequently reaches up to 200 W m 2 with extremes to over 300 W m 2. During melt, the strong winds advect dry air with relative humidity between 40 and 65%. Interestingly, the turbulent flux of latent heat is small, and often directed toward the surface, indicative of condensation of moisture onto the surface. While relative humidity of the near-surface air is low ( 50%), its temperature is so high that the specific humidity is higher than at the vapour-saturated surface. Thus, the moisture gradient is directed toward the surface. The combination of strong wind, high temperature, and low relative humidity is common to all wintertime melt events. These are fingerprints for föhn, an adiabatically warming wind descending from the Antarctic Peninsula mountain range after having lost latent heat due to precipitation on the windward side of the mountains 18, 19. Additional heating can occur due to the drawdown of potentially warm and dry air from aloft when the flow is blocked at lower levels on the windward side, and due to entrainment of potentially warm and dry air from upper levels into the flow over the mountains 20. The vertical cross-section (Figure 2D) over the Antarctic mountain range through Cabinet Inlet from a high-resolution numerical weather prediction model (Methods) confirms the occurrence of föhn during May 2016, with moist air rising on the windward side of the mountains, and relatively dry, adiabatically warmed air descending on its leeside. The numerical model further indicates that melt conditions during this event were not restricted to Cabinet Inlet, 4

5 but occurred over most of the ice shelf (see map in Figure 2C). There is extra scope possible here for a Sentinel InSAR image of melt ponds in the inlets in the NW of Larsen C during the May 2016 föhn event. Peter and Adrian to discuss. Radar scatterometry is an active remote sensing technique which is sensitive to the presence of liquid water in snow or firn. We use QuikSCAT ( ) and ASCAT ( ) sensors to estimate the number of meltdays over the Antarctic Peninsula for each winter season. In some years, little to no melt occurs during winter, whereas in other years, the number of meltdays over Larsen C exceeds 5. The winter of 2016 stands out as one of the most intense winter melt seasons since No trend is evident from the 17-year record presented above. Implications and conclusions to follow in a discussion that puts these findings in a broader context. Peter to coordinate. All welcome to provide input. 1. Scambos, T. A., Hulbe, C., Fahnestock, M. & Bohlander, J. The link between climate warming and break-up of ice shelves in the Antarctic Peninsula. J. Glaciol. 46, (2000). 2. Vaughan, D. G. et al. Recent rapid regional climate warming on the Antarctic Peninsula. Clim. Change (2003). 3. Cook, A. J. & Vaughan, D. G. Overview of areal changes of the ice shelves on the Antarctic Peninsula over the past 50 years. The Cryosphere 4, (2010). 4. Banwell, A. F., MacAyeal, D. R. & Sergienko, O. V. Breakup of the Larsen B Ice Shelf 5

6 triggered by chain reaction drainage of supraglacial lakes. Geophys. Res. Lett. 40, (2013). 5. Kuipers Munneke, P., Ligtenberg, S. R. M., van den Broeke, M. R., van Angelen, J. H. & Forster, R. R. Explaining the presence of perennial liquid water bodies in the firn of the Greenland Ice Sheet. Geophys. Res. Lett. (2014). 6. Trusel, L. D. et al. Divergent trajectories of Antarctic surface melt under two twenty-firstcentury climate scenarios. Nat. Geosci. 8, (2015). 7. DeConto, R. M. & Pollard, D. Contribution of Antarctic to past and future sea-level rise. Nature 531, (2016). 8. De Angelis, H. & Skvarça, P. Glacier surge after ice shelf collapse. Science 299, (2003). 9. Rott, H., Müller, F., Nagler, T. & Floricioiu, D. The imbalance of glaciers after disintegration of Larsen-B ice shelf, Antarctic Peninsula. The Cryosphere 5, (2011). 10. Wuite, J. et al. Evolution of surface velocities and ice discharge of Larsen B outlet glaciers from 1995 to The Cryosphere 9, (2015). 11. Winther, J.-G., Jespersen, M. N. & Liston, G. E. Blue-ice areas in Antarctica derived from NOAA AVHRR satellite data. J. Glaciol. 47, (2001). 12. Phillips, T., Rajaram, H. & Ste en, K. Cryo-hydrologic warming: A potential mechanism for rapid thermal response of ice sheets. Geophys. Res. Lett. 37 (2010). 6

7 13. Depoorter, M. A. et al. Calving fluxes and basal melt rates of Antarctic ice shelves. Nature 502, (2013). 14. Kuipers Munneke, P., Picard, G., van den Broeke, M. R., Lenaerts, J. T. M. & van Meijgaard, E. Insignificant change in Antarctic snowmelt volume since Geophys. Res. Lett. 39 (2012). 15. Harig, C. & Simons, F. J. Accelerated West Antarctic ice mass loss continues to outpace East Antarctic gains. Earth Planet. Sci. Lett. 415, (2015). 16. Trusel, L. D., Frey, K. E., Das, S. B., Kuipers Munneke, P. & van den Broeke, M. R. Satellitebased estimates of Antarctic surface meltwater fluxes. Geophys. Res. Lett. (2013). 17. Van den Broeke, M. R., Reijmer, C. H., van As, D., van de Wal, R. S. W. & Oerlemans, J. Seasonal cycles of Antarctic surface energy balance from automatic weather stations. Ann. Glaciol. 41, (2005). 18. Kuipers Munneke, P., van den Broeke, M. R., King, J. C., Gray, T. & Reijmer, C. H. Nearsurface climate and surface energy budget of larsen c ice shelf, antarctic peninsula. The Cryosphere 6, (2012). 19. Luckman, A. et al. Surface melt and ponding on Larsen C Ice Shelf and the impact of föhn winds. Antarct. Sci. 26, (2014). 20. Elvidge, A. D. et al. Föhn jets over the Larsen C Ice Shelf, Antarctica. Quart. J. Roy. Meteorol. Soc. 141, (2014). 7

8 21. Wang, W., Zender, C. S., van As, D., Smeets, C. J. P. P. & van den Broeke, M. R. A Retrospective, Iterative, Geometry-Based (RIGB) tilt-correction method for radiation observed by automatic weather stations on snow-covered surfaces: application to Greenland. The Cryosphere 10, (2016). Methods 1 Automatic Weather Station An automatic weather station (AWS) was erected in Cabinet Inlet (coordinates) in November Shortwave radiation was tilt-corrected using a MODIS satellite-guided procedure 21. Air temperature observations were unventilated, leading to overestimation during calm, sunny days. A correction function was derived from concurrent thermocouple observations during November 2014 through January Observations of relative humidity were corrected... (Paul). 2 Surface energy balance model The surface energy balance was computed using a model that forces the budget to close by looking for a surface temperature for which all the terms balance. If that temperature is above the freezing point, all excess energy is assumed to go into melting. Details of the model are in Kuipers Munneke et al., The model is validated by comparing computed surface temperature with observed values (computed from the outgoing longwave radiation with Stefan Boltzmann s law). The di erence between these is 0.60 K on average (RMS = 1.90 K). Further, the timing of melt is corroborated with surface height lowering observed by a sonic height ranger attached to the AWS 8

9 mast. 3 Numerical weather prediction model To study the atmospheric state during cases of föhn, we use output from the high-resolution MetOffice Unified Model (MetUM or UM), in a regional, nested domain at 1.5 km horizontal resolution. MetUM is based on... by Ella, Andrew, and John to complete, with references. 4 Radar scatterometry To compute the number of meltdays per winter, we use radar scatterometry from QuikSCAT ( ) and ASCAT ( ). Adrian, Suzanne to complete. Supplementary Information is linked to the online version of the paper at Acknowledgements We would like to thank... Author Contributions P.K.M. led the analysis and writing of the manuscript. A. L. and S. B. provided remote sensing data, E. G., A. O., and J. K. carried out the meteorological model analysis. C. J. P. P. S. and M. R. v. d. B. prepared automatic weather station data. W. W. and C. Z. performed a tilt correction on the radiation observations. Author Information The authors declare that they have no competing financial interests. Correspondence and requests for materials should be addressed to P.K.M. ( p.kuipersmunneke@uu.nl). 9

10 1 0 SW toa (W m -2 ) m 0.05 m 0.29 m 0.19 m 150 Cumulative melt (m w.e.) Melt flux (W m -2 ) 0 Nov Feb May Aug Nov Feb May Aug 0 Nov

11 T( o C), v (m s -1 ) wind T air melt RH A B Relative humidity (%) Energy flux (W m -2 ) sensible net longwave latent May 24-May 26-May 28-May 30-May C D 11

12 12

13 Figure 1 Daily mean melt flux (W m 2, red) and cumulative melt (m w.e., grey) for Nov 2014 Nov 2016, computed from Automatic Weather Station data. Background gradient shows daily mean top-of-atmosphere incoming solar radiation: black is 0, white is 514 W m 2. Text labels in the top of the panel denote cumulative melt (m w.e.) for austral summer (1 Nov 31 Mar) and winter (1 Apr 31 Oct) seasons. Figure 2 Hourly values of meteorological conditions and surface energy balance in Cabinet Inlet, May A: Air temperature (at 2 m above the surface, C, black), wind speed (at 10 m above the surface, m s 1, red), relative humidity (at 2 m above the surface, %, blue). B: net longwave radiation (grey), turbulent fluxes of sensible (red) and latent (blue) heat, and melt flux (orange). All fluxes in W m 2. C: Map showing potential temperature (in K) at 500 m above sea level over Larsen C, on 10 May 2016 at 1200 UTC. The dashed line indicates the cross-section in panel D, and black arrows indicate wind direction and strength. This needs a version at the maximum of the foehn event late May!. D: Vertical cross-section over the Antarctic Peninsula mountains into Cabinet Inlet from MetUM forecast for 10 May UTC. Color scale indicates potential temperature (K), and black arrows indicate vertical wind speed and direction. Grey bar in panels A and B corresponds to the time of the model forecast in C. Figures to be made publication-ready: Ella, Andrew... Figure 3 Annual maps over the Antarctic Peninsula showing number of meltdays per winter (1 Apr 31 Oct) between 2000 and 2016, observed by QuikSCAT (blue-background maps) and ASCAT (gray background). 13