1. Infrasound Technology WS – Bermuda, November 6 th, 2008 1 Microbarom signals recorded in Antarctica - a measure for sudden stratospheric warming? L. Ceranna, A. Le Pichon, E. Blanc & L. Evers BGR / B3.11, Hannover, Germany KNMI, DeBilt, The Netherlands CEA / DASE, Bruyères-le-Châtel, France

2. Infrasound Technology WS – Bermuda, November 6 th, 2008 2 Content   The Antarctic Infrasound Array I27DE  mb-Signal Detections at I27DE  Causes and Effects  Conclusions siting and design microbarom signals recorded from Feb-2003 to Oct-2008 background noise level and prevailing winds variability of the stratospheric wave duct sudden stratospheric warming (SSW) and its signature

3. Infrasound Technology WS – Bermuda, November 6 th, 2008 3 Siting and Design of the Infrasound Station I27DE 5°W 10°W 71°S 72°S satellite image of the Ekström Ice Shelf at the German Neumayer Research Base, Antarctica nine element infrasound array 2 km aperture optimum detection conditions for mb- signals (T dom =5 s) noise reduction by snow coverage continuous operation since Feb-2003

4. Infrasound Technology WS – Bermuda, November 6 th, 2008 4 Track of the Ocean Swell around Antarctica averaged 990 hPa isobar from Jun-1979 to Feb-2000 [Simmonds et al. 2003] considered as path of the clockwise moving ocean swell around Antartica predicted direction of continuous mb-signals throughout the years HWM-93 radial strato- spheric wind speed from source to I27DE [Hedin et al. 1996] Mar - Nov Dec - Feb

5. Infrasound Technology WS – Bermuda, November 6 th, 2008 5 Detections of mb-Signals at I27DE, Feb-2003 – Oct-2008 T dur > 100 s Σ130,500 ~63 / day station azimuth follows to first order well the variation predicted by HWM-93 trace velocities (V T ) of detections are in accordance with typical range for Is phases Austral winter 2006 is an exception - strong decrease in number of detections - strong increase in V T

6. Infrasound Technology WS – Bermuda, November 6 th, 2008 6 Causes and Effects for anomalous mb-Signal Detection although no comparable observations at I55US (Windless Bight, Antarctica) not an artifact, since array configuration remained unchanged - anomaly is only limited to Austral winter 2006 absence of a stratospheric duct, i.e. It instead of the usually prevailing Is phases - It shows higher trace velocities compared to Is - It is subjected to strong attenuation at 5 s period increased background noise level due to increased prevailing wind speeds because wind speeds above 10 m/s strongly affect the detection capability - wind speed < 10 m/s (72 %); wind speed < 15 m/s (84 %) sudden stratospheric warming (SSW) events are known as a potential source for reduced detection capability of infrasound stations [Evers & Haak, 2005] - more frequently in northern hemisphere (minor & major events) - SSW are associated with a slowing down of the polar vortex leading to a decrease and sometimes to a reversal in zonal wind speed [Hoffmann et al., 2007] - in 2002 a major SSW event observed in Antarctica [Varotsos, 2003]

7. Infrasound Technology WS – Bermuda, November 6 th, 2008 7 mb-Signal Detection in the Periods from 01-Mar to 31-Oct Σ 20,000 > 10 m/s (29 %) no wind: ~28,000 Σ 6,000 > 10 m/s (38 %) no wind: ~10,000 Σ 18,000 > 10 m/s (32 %) no wind: ~26,000 high wind speed (increased background noise level) explains gaps in signal detection, however, it is NOT causing the reduced number of detections during Austral winter 2006

8. Infrasound Technology WS – Bermuda, November 6 th, 2008 8 ECMWF Temperature and Wind Speed Profiles ECMWF profiles (61/91 levels) ~ [0 70] km between 75°S and 55°S every 1° between 80°W and 55°E every 1° from 01-Mar to 31-Oct every 24 h path length: 2700 → 1000 → 1800 km sound speed radial wind speed source I27DE

9. Infrasound Technology WS – Bermuda, November 6 th, 2008 9 Effective Sound Speed Ratios: ECMWF, HWM-93 ECMWF averaged along path HWM-93 averaged along path V eff =sound speed + radial wind speed V eff -ratio=max{V eff strat }/V eff surf stratospheric duct always exists, even in 2006: excluding It observations note: high V eff ratio is essential precondition for measuring high V T duct no duct duct no duct

10. Infrasound Technology WS – Bermuda, November 6 th, 2008 10 Effective Sound Speed Ratios: ECMWF, 61/91 levels ECMWF at the source ECMWF at the station V eff =sound speed + radial wind speed V eff -ratio=max{V eff strat }/max{V eff tropo }

11. Infrasound Technology WS – Bermuda, November 6 th, 2008 11 Effective Sound Speed Profiles, averaged along Path MSISE/HWM-93 smooth gradients ? ? ? ? ? ? ? station azimuth > 0° during Austral winter are in contradiction to HWM-93 prediction, therefore, no stratospheric duct SSW occur along with reversal in stratospheric wind direction observed in 2005, 2006, and 2007, however, most pronounced in 2006

12. Infrasound Technology WS – Bermuda, November 6 th, 2008 12 Effective Sound Speed Profiles, averaged along Path (cont.) ECMWF clear events showing increase of V eff in upper tropopause / lower stratosphere high V eff values in the upper stratosphere, however, in 2006 lower than in 2005 and 2007 being in accordance with stratospheric cooling at that time during Austral winter 2005 to 2007 SSW events occurred and and can be correlated to observations Veff ratio between troposphere and stratosphere instead of ground to stratosphere

13. Infrasound Technology WS – Bermuda, November 6 th, 2008 13 Examples of Wave Propagation in 2006 I27DE 11-Jul-2006 β=-54.5° V T =370 m/s 11-Jul-2006 β=-38.8° V T =361 m/s 15-Sep-2006 β=-86.1° V T =366 m/s Is IwIs

14. Infrasound Technology WS – Bermuda, November 6 th, 2008 14 Examples of Wave Propagation in 2006 (cont.) 06-Mar-2006 β= -68.7° V T =NaN 08-Mar-2006 β= -65.5° V T =NaN - no rays - V eff in troposphere ≈ V eff in stratosphere V eff in troposphere {source} > V eff in stratosphere {receiver} I27DE

15. Infrasound Technology WS – Bermuda, November 6 th, 2008 15  I27DE’s siting and design are well suited for measuring mb-signals. Moreover, I27DE has been operated for more than 5 years without any major problems.  Generally, detection capability is reduced due to increased background noise level caused by prevailing wind speeds above 10 m/s.  During Austral winter 2006 an anomaly was observed for mb-signals, the number of detections was strongly reduced while trace velocities shows unusual (for Is) high values. - The high trace velocities V T might be caused by high effective sound speeds V eff in the troposphere along with relatively low values in the stratosphere (in 2006 stratospheric cooling was observed). - The reduced number might be explained by SSW events. Although such events were also observed in 2005 and 2007 (β > 0°), they were most prominent in 2006.  Detection of mb-signals is a measure for SSW events. Conclusions