1. Understanding ocean bottom pressure variability with Antares data
Nadia Pinardi (1), Sara Zanella(1*),
Annarita Margiotta (1,2), Stefano Cecchini(2)
Marco Zavatarelli (1), Luca Giacomelli (1)
(1) Department of Physics and Astronomy
University of Bologna
(2) Istituto Nazionale di Fisica Nucleare
Bologna
(*) Master degree recipient

2. OUTLINE Statement of the problem
The bottom pressure sensors at the Antares Observatory
The reconstruction of a long time series of BP without gaps: methodology and assumptions
Analysis of the BP for high and low frequency components
Conclusions

3. The Global Ocean Observing System (IOC-Unesco and WMO programs)
Visit the new site:

4. What does Antares Bottom Pressure Sensors (BPS) network add to this?
High frequency data, 2 minutes acquisition rate resolving ocean high frequency processes
Unique method to measure open ocean tides at high and low frequencies (pole tide and Chandler wobble)
The BPS can measure changes in the mass of the ocean with high accuracy also because of the vicinity of a tide gauge station providing the tidal measurements

5. How did we start? From IL07 Red: Floor 6 pressure (2193 m)
Blue: tide gauge
The sea level observations of Toulon
are available from REFMAR (refmar.shom.fr)
Deep water mass
formation event, 60 cm!
Relative displacement
20 cm
March-April 2009

6. The physical nature of the bottom pressure signal
The bottom pressure is largely determined by the hydrostatic balance
where is the sea level and
is the atmospheric pressure

7. The physical nature of the bottom pressure signal
The bottom pressure is largely determined by the hydrostatic balance
The last term is proportional to the
weight of the ocean or the ocean mass

8. The physical nature of the bottom pressure signal
Since
Monitoring the mass of the ocean at Antares means indirectly to say something about
how temperature and salinity properties change

9. The questions
How similar are the barotropic tides at Antares and Toulon tide gauge station?
What is the dominant variability period for the ocean mass term?
Can we associate circulation structures to the mass changes?

10. The Bottom Pressure Sensors (BPS) at Antares
Deep water circulation and density
Bottom pressure sensors at
Antares
Antares

11. The bottom Pressure Sensors (BPS) at Antares
For our analysis only L1, L3, L8 are analyzed since others do not add information
Data collection period: 22 May January 2014
Analysis time period: Jan 1, Dec 2013

12. The ANTARES Observatory location
Central Lat, Lon: 42°48'N - 6°10'E
Depth of bottom: m
Toulon tide gauge station is 40 km away
Time frequency of raw data acquisition: 2 minutes
Toulon
Antares

13. The bottom pressure data: gaps
Pressure Time series
IL07 L1 L3 L8
IL07 (red)
and bottom
pressure sensors (green)
2190 m
2480 m
2480 m
2480 m

14. Reconstruction of a long bottom pressure time series: methodology of work
Reconstruction STEP 1: despiking
Reconstruction STEP 2: averaging on 30 minutes
Reconstruction STEP 3: gap filling
Filtering STEP 4: de-tiding
Filtering STEP 5: detrending for instrument drift
Output: 5 years almost continuous BP Recontsructed Time Series (RTS)

15. Reconstruction STEP 1: despiking
RAW DATA
DESPIKED DATA
BPS (L1)

16. Reconstruction STEP 2: averaging on 30 minutes
averaged DATA
DESPIKED DATA
An example of averaging for November 19, 2009: tidal signal is similar within 1 mm between three stations

17. Reconstruction STEP 3: gap filling
averaged DATA
filled DATA: RTS L3 L1 L8
An example of gap filling: May 2009

18. Reconstructed Time Series (RTS)
Gaps > 1 hour
2009
134
2010 91
2011 58
2012 50
2013 23
1 January December 2013
Data point frequency 30 minutes

19. Filtering STEP 4: detiding with Doodson Filter
RTS
RTS detided

20. Fitering STEP 5: detrending
RTS detided
RTS detrended
((from Watts and Kontoyiannis, 1989)

21. The final RTS for bottom pressure at Antares
Seasonal steric, multi-month and mesoscale signals
2010: Anomalous winter

22. The BPS in the Pacific ocean and the Mediterranean Sea
35 cm
Phase change, minimum later in the year
55 cm cm
Hughes et al., 2012

23. Comparison between sea level at Toulon and bottom pressure
Antares equivalent
Sea level
July 2
July 11
Correlation coefficient = 0.7 (1 January October 2013)

24. large scale circulation
Analysis of low frequency variability in RTS: power spectra with tides removed
PERIOD (days)
PHENOMENA I 51
mesoscales II 57
III
107
Steric effect/
large scale circulation IV
156
Steric effectl/ V
172 VI
191
VII
286
VIII
343 IX
429
Chandler wobble (433)

25. Copernicus currents: http://marine.copernicus.eu/
Bottom pressure and circulation structures: mesoscales and steric effects
Jul-Aug-Sep 2012
Toulon
Toulon
Antares
Antares
Copernicus currents:

26. Preliminary conclusions
We developed a methodology to analyze the BPS data at Antares, producing a 5 year Reconstructed Time Series-RTS, the first of its type, quality controlled and filtered from instrument malfunctioning
The tidal signal was analyzed in the open ocean ocean for the first time in the Mediterranean, showing consistency with Toulon sea level records
The mass contribution to bottom pressure includes mesoscales and large scale circulation steric effects and the Chandler Wobble frequency (433 days)

27. Preliminary conclusions
Antares could offer the possibility to evaluate the mass changes in the ocean by also having a maintenance service that would change the sensors after few years, this reducing the effects of trends on the accuracy of the recovered signal.
Being deep but close to the tide gauge in Toulon, tides could be subtracted by using the observed data thus offering a more precise reconstruction of BP signals
The BPS should be coupled to T,S sensors and may be other bottom measurements to unravel the correlation between mass and heat and salt changes

28. Bottom Pressure Sensor (BPS)
Anchored at 2478 m depth
Made by GENISEA
Titanium container
Pressure sensor DRUCK piezo-electric range bars
Precision <0.01 bars
Sensitivity 0.01 db