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PhD Defense


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Fernando Paolo's doctoral defense on Sep 2, 2015 at the University of California, San Diego.

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PhD Defense

  1. 1. Interannual and decadal variations of ice shelves using multi-mission satellite radar altimetry, and links with oceanic and atmospheric forcings Fernando S. Paolo Scripps Oceanography Committee PhD defense Sep 2, 2015 University of California, San Diego Helen A. Fricker, Chair Sarah Gille Falko Kuester Jean-Bernard Minster Laurie Padman David T. Sandwell
  2. 2. Dissertation structure Ch1. Introduction Ch3. Trend analysis Ch4. Variability analysis Ch2. Time-series construction In revision for Remote Sens. Environ. Published in Science 2015 In preparation for publication
  3. 3. The Antarctic ice sheet interacts with the ocean through the ice shelves Modified from NASA
  4. 4. The ice in Antarctica flows just like rivers do on continents NASA, Rignot et al. 2011 East West
  5. 5. Circumpolar Deep Water melts the ice shelves from below
  6. 6. Ice shelves restrain the flow from the ice sheet interior to the ocean Modified from Hughes 2011 gravitational driving stress Plan view of an ice shelf highs
  7. 7. Large portions of the Antarctic ice sheet are prone to instability Vaughan and Arthern 2007, Schoof 2007 Grounded below sea level (prone to instability) q H, grounding-line thickness ∝ ax ax-δ q q+Δ
  8. 8. Future ice-sheet contribution to sea-level rise is (highly) uncertain IPCC AR5, Velicogna andWahr 2013 Total SLR Thermal expansion Mass increase Greenland Antarctica
  9. 9. Summary The ice sheets are currently loosing mass at an accelerated rate Large portions of the Antarctic ice sheet are prone to instability Ice-shelf loss ➔ increased ice discharge ➔ sea-level rise
  10. 10. Scientific questions How have ice shelves varied over time? What are the spatial patterns? What are the links to climate variability? How widespread are the changes?
  11. 11. Three satellite radar altimetry missions: 18 years of continuous data (1994-2012) = − ( − ) ERS-1 ERS-2 Envisat
  12. 12. Detecting changes in the vertical component is challenging over floating ice ∂ ∂ = ∂ ∆ − ∂ ρ− + ∂ ρ− + (ρ− − ρ− ) ( ˙ + ˙ + ∇ · v) changes in ocean height changes in ocean density changes in firn density changes in surface mass changes in basal mass changes in velocity field ice-ocean density contrast Shepherd et al. 2004; Padman et al. 2012
  13. 13. We averaged height measurements over 30-km grid cells and 3-month bins Paolo et al. 2015b We stack several time series to smooth out incoherent signal
  14. 14. We estimated trends using polynomial regularized regression ˆ( ) = β + β + β + β Minimizing Subject to Fit (bias) (variance) |β | ( − ˆ ) Lasso regularization: Tibshirani 1996 Cross validation:
  15. 15. Efron andTibshirani 1993 We estimated uncertainties by bootstrapping the residuals of the fit ∗ ( ) = ˆ( ) + ε∗ ( ) ε( ) = ( ) − ˆ( )Residual of the fit Bootstrap sample We performed a total of 1,330,000 sets of calculations Combined error σ = (σ∗) + (σ ) Trend fit Derivative 95% CI of the fit
  16. 16. Paolo et al. 2015b Over 1600 time series We constructed time series and maps of ice-shelf height change and acceleration
  17. 17. Paolo et al. 2015a 18 years of changes show a clear spatial pattern: West ice shelves are thinning fast East ice shelves not so much 18% volume loss in less than 2 decades
  18. 18. Time There is large variability in the ice-shelf system Paolo et al. 2015a
  19. 19. Paolo et al. 2015a, 2015b West Antarctic ice shelves: Volume-loss rate increased by ~70% from the 1990s to 2000s East Antarctic ice shelves: Earlier increase in volume ceased in the 2000s All Antarctic ice shelves: Volume-loss rate accelerated -25 km3/yr ➔ -310 km3/yr
  20. 20. Schoof et al. 2010, Paolo et al. 2015a We observe faster ice-shelf melt rates near the grounding lines CDW
  21. 21. Summary At current rates some ice shelves may disappear within this century Ice shelves are decaying fast, leading to Antarctic mass-loss increase Enhanced inflow of warm CDW is melting West Antarctica
  22. 22. Short observational records with different scales in time with large errors (noisy) and many simultaneous time series Motivation Given Can we distinguish between regular deterministic behavior (cycles) and irregular behavior (noise)? Question
  23. 23. Multivariate Singular Spectrum Analysis identifies common oscillatory modes Vautard et al. 1992, Golyandina et al. 2001, Ghil et al. 2002, Time Multivariatedataset Time Reconstructedcomponent Window Rank EigenvectorEigenvalue Signal Noise
  24. 24. Paolo et al. in prep. 140 time series There is statistically significant energy at the interannual band in AS
  25. 25. f = 0.22 ➞ T ≈ 4.5 years Paolo et al. in prep. Time window 9 years Time span 18 years We identified an interannual oscillation in Amundsen Sea ice-shelf height
  26. 26. NOAA ? ENSO is the strongest natural climate fluctuation at interannual time scales Southern Oscillation Index (SOI)
  27. 27. Paolo et al. in prep. Time window 6 years Time span 18 years Low and hight frequency modes of ENSO are identified in the SOI series T ≈ 4.5 years T ≈ 2.5 years f = 0.22, 0.40
  28. 28. Paolo et al. in prep. Low-freq mode of ENSO Ice-shelf height variability El Nino events Interannual ice-shelf height in Amundsen is strongly correlated with ENSO
  29. 29. Ok, but does this make sense? ρ 0 (+∆SST) ρ 0 (−∆SST) SOI−SST Correlation (ρ) Riffenburgh 2007, Kwok and Comiso 2002, Cullather et al. 1996 Higher moisture convergence Higher snowfall along the coast Higher cyclonic activity During an El Nino event: Lower temperature along the coast
  30. 30. Summary There is statistically significant interannual variability in AS height This variability is strongly correlated with El Nino-Southern Oscillation First direct observational evidence of the ENSO-AIS teleconnection?
  31. 31. Thank you.
  32. 32. Circumpolar Deep Water melts the ice shelves from below Jenkins et al. 2010, Jacobs et al. 2011 Temperature SalinityPine Island Ice Shelf
  33. 33. As the ice shelves thin, so does the adjacent grounded ice Pritchard et al. 2012 Grounded-ice thinning Ice-shelf thinning
  34. 34. As the ice shelves thin, so does the adjacent grounded ice Pritchard et al. 2012 Grounded-ice thinning Ice-shelf thinning
  35. 35. As the ice shelf is removed the glaciers behind speed up Rignot et al. 2004, Scambos et al. 2004 Before collapse After collapse Horizontal velocity (InSAR) Horizontal velocity (InSAR)Flow rate (Landsat)
  36. 36. The geometry of the bed constrains the stability of a marine ice sheet Vaughan and Arthern 2007, Joughin and Alley 2011, Schoof 2007 = Stable Unstable Retrograde bed slope q ax Grounding-line thickness H (steady state) (at the GL)∝
  37. 37. time 1 time 2 H1 H2 ∆ = ( − ρ /ρ ) ∆ ≈ ∆ ρ ρ ∆
  38. 38. Paolo et al. 2015a x is time (1994 to 2012) y is thickness change (m) rates are in (m/decade) Short records do not capture the trend
  39. 39. Morris andVaughan 2003, Paolo et al. 2015a −9℃ isotherm moving southward Limit of ice-shelf viability appears to be moving southward?
  40. 40. Paolo et al. 2015a CDW CDW
  41. 41. Paolo et al. in prep. There is statistically significant energy at the interannual band The interannual component explains a larger portion of the total variance Interannual Annual
  42. 42. Riffenburgh 2007