A. Bonaduce, N. Pinardi Mediterranean Sea level recostruction during the last century A. Bonaduce (1), N. Pinardi (2) (1) Centro Euro-Mediterraneo per i Cambiamenti Climatici (2) University of Bologna, Environmental Science

General Introduction: Global and Regional sea-level change Why reconstructing sea-level field ? Data: tide-gauge and satellite altimery data-sets Method Comparing in-situ and satellite altimetry data Reduced space optimal intepolation (Kaplan,1997) EOFs selection (Calafat, 2011) Results Sea-level reconstruction validation at basin and local scales Reconstructed sea-level trend spatial and temporal varibility Conclusions and future work Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Outline

SL is an indicator of climate variability and change (EEA, 2008) and ocean heat content (IPCC, 2009) At global (Church, 2010) and regional scale (Cazenave, 2001) SL is studied using different observational instruments (Tsimplis, 2005,Cazenave, 2009) OGCM (Mellor, 1995) and statistical tecqniques (Church, 2004, Calafat, 2009, Klein and Lichter, 2009) One of the biggest sources of uncertainty related to sea-level change is that we still do not know what the sea-level response to climate change scenarios is (Church et al., 2010). (Bindoff et al., 2007) aims at providing the changes over the coming century, which are affected by great uncertainties. The best possible strategy to minimize this future uncertainty is to look at the past and try to understand in details all sea-level variability components. In Med. Sea observational data available with different spatial and temporal resolution Reconstruction allow to “merge” different sea-level signals both at Global (Church, 2004) and regional (Calafat, 2010) scales Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Introduction

Reconstruct the sea-level signal in the Meditteranean sea during the last century using satellite altimetry and tide-gauge data Validate reconstruction results with altrimetry (basin scale) and tide-gauge data (locally) Estimate sea-level trends over a centennial time period Investigate the sea-level trend spatial and temporal variability in the Med. Sea during last century Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Objectives

Mediterranean Sea level reconstruction A. Bonaduce, N. Pinardi Sea Level (SL) from observations Data: Permanent Service for Mean Sea Level (PSMSL) in-situ Tide Gauge ( ongoing) PSMSL Revised Local Reference (RLR) dataset (Woodworth & Player, 2003) RLR: tide-gauge datum history and a common denominator of 7000 mm below the average sea level (Klein & Lichter, 2009) 45 stations in Med during the period ongoing (satellite era)

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Sea Level (SL) from observations Data: remote-sensing Satellite Altimetry (Oct 1992 – ongoing) AVISO Delayed Time (M)Sea Level Anomaly (MSLA) (Pujol & Larnicol, 2005) merged product: 6 satellites Topex/Poseidon, Jason-1, Jason-2, ERS-1, ERS-2, Envisat ± 1 week; long wave correction, inverse barometer correction applied (and others..) satellite tracks cutted at 25 km from the coast to avoid reflection (AVSIO, 2011) Spatially homogeneus: regular grid 1/8° (-5 W E; 30 N - 46 N) (Ducet et al., 2000) *ECMWF : European Centre for Medium-Range Weather Forecast

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Sea Level (SL) from observations Methods: remote-sensing and in-situ SL signal decomposition = steric component (Mellor and Ezer, 1995) Satellite Altimetry signal Tide Gauge signal Inverse barometer effect (Dorandeu and Le Traon, 1999) (ECMWF analysis) Altimetry What we compare Tide gauge ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 )

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Sea Level (SL) from observations Methods: Satellite steric signal GRACE: Satellite Gravimetry -----> Sea Level Equivalent (Don Chumbers, 2006) (2002 – ongoing; 1° horiz. Resolution !! ) Steric Signal: Satellite Altimetry - GRACE (Garcia, 2006, Garcia, 2010) SATELLITE - GRACE [Garcia, 2010 time window]

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Sea Level (SL) from observations Part I Results: low frequency SL variability 1993 – 2010 monthly data [cm] West Med: Marseille Central Med: Valletta Adriatic Sea: Trieste East Med: Hadera

Mediterranean Sea level reconstruction A. Bonaduce, N. Pinardi Merging different sea-level signals

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Sea Level (SL) Reconstruction technique Methods 1 st step: SVD of satellite altimetry data detrended (take the spatial info. from lead EOFs) 2 nd step: optimal interpolation (Kaplan, 2000) of tide gauge data over the satellite grid (take the temporal info. from tide gauge data) pre-processed as described before; GIA corrected using ICE-5G model (Peltier, 2004). We consider only the stations comparable with satellite altimetry data during the satellite era. Result : SL with satellite spatial resolution during the tide gauge time window (> 100 yr) In Med 4 stations with > 100 year of data: Marseille, Trieste, Venezia, Genova

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi EOFs selection (Calafat, 2011) Methods Training period: computing EOFs Validation period: reconstruction period where EOFs are not computed X-axis = n° EOFs; Y-axes = RMSE [cm] Several reconstructions considering altimetry data in the tide-gauges positions and increasing n° EOFs considered.

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Tide-gauge stations used 8 sub-periods choosing the most Complete time series Up to 12 months gaps: splines > 12 months gaps: linear fitting

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Results Reconstruction validation during the satellite era: Cor. patt. and RMSE Correlation Map RMSE [cm]

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Results Reconstruction validation during the satellite era: Trend spatial variability Reconstructed sea-level trend 1993 – 2010 Trend diffeences Rec - Altimetry 2.

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Results Reconstruction validation

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Results Sea-level Reconstruction 1993 – 2010 (monthly) 1885 – 2010 (annual)

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Results Sea-level Reconstruction – 2010 (annual) DJFM NAO index (Hurrel, 2009)

Sea level climate variability in the Mediterranean seaA. Bonaduce, N. Pinardi Sea level reconstruction Results: Trend spatial variability over a centennial time period ( )

Sea level climate variability in the Mediterranean seaA. Bonaduce, N. Pinardi Sea level reconstruction Results: Trend variations as function of period considered (Leibman et al., 2010) Overturning (25 years); Stable Positive (> 90 years) No signigicant sea-level trend (0.1 +/- 0.1 mm yr-1) x n° years Change in Annual Sea level

Mediterranean Sea level reconstruction A. Bonaduce, N. Pinardi Conclusions Sea-level reconstructions are a powerfull technique to merge the sea-level signal recorded by different instruments Comparing with satellite altimetry: reconstruction skill result very high along the coast and in the shallow water areas, while low correlation patterns have been found in deep water areas and where the mesoscale activity is high (i.e. Iera-Petra gyre) or present particular dynamics (i.e. Ionian sea). Low number of EOFs considered affect the reconstruction skill Locally reconstruction results are comparable with tide-gauge data during the past During the satellite era altimetry and reconstructed sea-level trends, return the same leading patterns, with the most marked differences in the western part of the basin (i.e. Balearic Islands) and in the Ionian Sea. The basin mean reconsrt. sea-level trend is 2.1 +/- 0.7 mm yr-1 Over a centennial time period is not possible to identify a significant trend (0.1 +/- 0.1 mm yr-1). Over a decadal time scale it is possible to observe an overturning of sea-level trend phases (positive and negative), while a stable positive trend is observable in the Med. Sea considering more than 90 years of data.

Mediterranean Sea level reconstructionA. Bonaduce, N. Pinardi Future work Unormalized and unrotated EOFs have been used to reconstruct the sea-level signal. Looking at the differences between altimetry and reconstruction, further analysis will be carried out using rotated EOFs (Richman, 1985) in order to account for the domain shape dependency and sub-domain instability. Other data “merging” methods will be analysed (i.e. OceanVar; Dobricic, 2008) in order to be able to consider an higher number of leading EOFs to reconstruct the Sea-level signal. Perform sea-level reconstruction using both satellite altimetry and Mediterranean Sea ocean re-analysis data (Adani et al., 2010)