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

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

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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

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

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

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

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

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?

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

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 2008- 31 January 2014 Analysis time period: Jan 1, 2009- 31 Dec 2013

The ANTARES Observatory location Central Lat, Lon: 42°48'N - 6°10'E Depth of bottom: 2478 m Toulon tide gauge station is 40 km away Time frequency of raw data acquisition: 2 minutes Toulon Antares

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

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)

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

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

Reconstruction STEP 3: gap filling averaged DATA filled DATA: RTS L3 L1 L8 An example of gap filling: 27-29 May 2009

Reconstructed Time Series (RTS) Gaps > 1 hour 2009 134 2010 91 2011 58 2012 50 2013 23 1 January 2009-31 December 2013 Data point frequency 30 minutes

Filtering STEP 4: detiding with Doodson Filter RTS RTS detided

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

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

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

Comparison between sea level at Toulon and bottom pressure Antares equivalent Sea level July 2 July 11 Correlation coefficient = 0.7 (1 January 2009-15 October 2013)

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)

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: http://marine.copernicus.eu/

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)

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

Bottom Pressure Sensor (BPS) Anchored at 2478 m depth Made by GENISEA Titanium container Pressure sensor DRUCK piezo-electric range 0-300 bars Precision <0.01 bars Sensitivity 0.01 db