Ingo Sasgen 1 , Henryk Dobslaw 1 , Zdenek Martinec 2 , and Maik Thomas 1

1. GRACE-basedAntarcticsnowaccumulationrelatedto ENSO Ingo Sasgen1, Henryk Dobslaw1, Zdenek Martinec2, and Maik Thomas1 (1) GeoForschungsZentrum Potsdam, Department 1: Geodesy and Remote Sensing Section 1.3: Earth System Modelling, 14473 Potsdam, Germany. (2) School of Theoretical Physics, Dublin Institute for Advanced Studies, Dublin, Ireland.

2. Region of interest in Antarctica Satellite-laser altimetry (ICESat) Rate of surface-elevation change (2003 – 2007) [2] Mean annual accumulationPrecipitation minus evaporation ECMWF ERA40 (1958 – 2001) [1] m/a surface-elevation change [1] Genthon & Cosme, 2003; [2] Pritchard et al. 2009

3. Antarctic Peninsula and Amundsen Sea Sector • Antarctic Peninsula (198 × 103 km2): • warming faster than global average [1] • ongoing ice shelf disintegration and glacier acceleration [2] Amundsen Sea Sector (432 × 103 km2): • ice velocities and ice discharge extreme compared to the rest of the continent [3] Both regions receive about 20 % of Antarctic precipitation P-E > 400 mm/a defines study area (dark grey areas in left figure) [1] Vaughan et al. 2001; [2] Scambos et al., 2004; [3] Rignot et al., 2008

4. Antarctic Peninsula and Amundsen Sea Sector Can inter-annualmassanomaliesfrom GRACE beexplainedwith SMB estimatesbased on ECMWF data? Are inter-annualvariationsrelatedto large-scalechanges in theatmosphericcirculation? P-E > 400 mm/a defines study area (dark grey areas in left figure)

5. GRACE processing GRACE data: • 80 monthly GRACE gravity fields (GFZ RL04 , unconstrained) [1] • Stokes potential coefficients up to degree and order 120 • August 2002 to August 2009 Filtering: • Wiener optimal filtering [2] • Isotropic • Adaptive to signal-to-noise ratio • Resulting spatial resolution ~ 450 km Gravimetric inversion: • Forward modelling approach • Simulation of geoid-height anomaly and adjustment to GRACE data • Robust w.r.t biases Potential disturbance  Mass change [1] Flechtner et al. 2007; [2] Sasgen et al. 2006

6. ECMWF surface mass balance ECMWF operational data: • Model resolution T799L91 (i.e., 25 km) • 6 hourly forecasts (precipitation, evaporation) • 6 hourly analyses (surface pressure, spec. humidity, wind) Surface mass balance: 0.5° regular grid, 6 hours Moisture transports: 1° at 25 pressure levels

7. Seasonal variability and trends Area 198 x 103 km2 Trend 28.8 +/- 3.3 Gt / a Annual 9.3 +/- 7.6 Gt Area 432 x 103 km2 Trend 81.4 +/- 5.8 Gt / a Annual 24.1 +/- 7.8 Gt Offset, trendsandannualharmonicsareremoved in thefollowing.

8. Antarctic Peninsula GRACE

9. Antarctic Peninsula GRACE GRACE (11 months) • temporal averaging required to remove high-frequency noise

10. ECMWF Antarctic Peninsula Rms 16.4 Gt Correlation: 0.82 (filtered) 0.37 (unfiltered) Rms 13.2 Gt GRACE GRACE (11 months)

11. Amundsen Sea Sector GRACE

12. Amundsen Sea Sector GRACE GRACE (5 months)

13. Amundsen Sea Sector Rms 28.6 Gt Correlation: 0.70 (filtered) 0.67 (unfiltered) Rms 20.2 Gt GRACE GRACE (5 months) ECMWF

14. Anti-correlation between AP and AS Antarctic Peninsula Correlation: GRACE -0.40 ECMWF -0.38 Amundsen Sea Sector

15. ENSO signals in Antarctic snow accumulation Amundsen Sea Low isstrengthenedunder La Nina conditions, causingdecreasedPrecipitation in the Amundsen SeaSectorandincreasedprecipitation in theAntarcticPeninsula. Cullather et al. (1996), Bromwich et al. (1999) La Nina L Can this be confirmed by GRACE?

16. 2. ENSO signals in Antarctic snow accumulation Amundsen Sea Low isstrengthenedunder La Nina conditions, causingdecreasedPrecipitation in the Amundsen SeaSectorandincreasedprecipitation in theAntarcticPeninsula. Cullather et al. (1996), Bromwich et al. (1999) El Nino L Can this be confirmed by GRACE?

17. 2. ENSO signals in Antarctic snow accumulation La Nina = increasedprecipitation in theAntarcticPeninsula El Nino = increasedprecipitation in the Amundsen SeaSector (AS) accumulated SOI (5 months) • Amundsen Sea Low Pressure anomaly strong • Direction of moist air towards Peninsula • Dry wind from Antarctic interior in AS

18. 2. ENSO signals in Antarctic snow accumulation La Nina = increasedprecipitation in theAntarcticPeninsula El Nino = increasedprecipitation in the Amundsen SeaSector (AS) Averaged La Nina months from ECMWF operational data • Direction of moist air towards Peninsula • Dry wind from Antarctic interior in AS • Amundsen Sea Low Pressure anomaly strong

19. 2. ENSO signals in Antarctic snow accumulation La Nina = increasedprecipitation in theAntarcticPeninsula El Nino = increasedprecipitation in the Amundsen SeaSector Averaged El Nino months from ECMWF operational data • Amundsen Sea Low Pressure anomaly weak • Direction of moist air directly towards Amundsen Sea Sector

20. 2. ENSO signals in Antarctic snow accumulation La Nina = increasedprecipitation in theAntarcticPeninsula El Nino = increasedprecipitation in the Amundsen SeaSector Correlations -0.26 (unfiltered ) -0.33 (filtered) Amundsen Sea Sector

21. 2. ENSO signals in Antarctic snow accumulation La Nina = increasedprecipitation in theAntarcticPeninsula El Nino = increasedprecipitation in the Amundsen SeaSector Correlations -0.30 (unfiltered ) -0.39 (filtered) Antarctic Peninsula

22. Conclusions • Inter-annual variability from GRACE can be explained with SMB estimates based on ECMWF data • Anti-correlation between Antarctic Peninsula and Amundsen Sea Sector is -0.38 (GRACE filtered) • Correlation with accumulated SOI lagged by 10 months amounts to 0.39 (Antarctic Peninsula) and -0.33 (Amundsen Sea Sector) Sasgen, I., Dobslaw, H., Martinec, Z., Thomas, M. (2010), Satellite gravimetry observation of Antarctic snow accumulation related to ENSO, Earth Planet. Sci. Lett,, doi:10.1016/j.epsl.2010.09.015, in press.