A. Bonaduce, N. Pinardi Mediterranean Sea level reconstruction during the last century A. Bonaduce...

A. Bonaduce, N. Pinardi

Mediterranean Sea level reconstruction during the last century

A. Bonaduce (1), N. Pinardi (2)(1) Centro Euro-Mediterraneo per i Cambiamenti Climatici (Bologna, Italy)

(2) University of Bologna, Environmental Science (Ravenna, Italy)

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 reconstruction A. Bonaduce, N. Pinardi

Objectives


Sea Level (SL) from observations Data: Permanent Service for Mean Sea Level (PSMSL) in-situ

Tide-gauge (1885 – ongoing; 93 stations)

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 1992 - ongoing (satellite era)


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 - 36.875 E; 30 N - 46 N) (Ducet et al., 2000)

T/P J1 J2 E1 E2 En

Data period 1992-2005 2002-on 2009-on 1992 - 2005 1999-2003 2003-on

Inverse Bar. ECMWF ECMWF ECMWF ECMWF ECMWF ECMWF*ECMWF : European Centre for Medium-Range Weather Forecast


Sea Level (SL) from observations Methods: remote-sensing and in-situ SL signal decomposition

sla sat x,y,t =η sat x,y,t −η satmean x,y

η sat x,y,t = η satst t η sat

mean x,y η satvar x,y,t

⟨η sat x,y,t ⟩ = η satst t = steric component (Mellor and Ezer, 1995)

Satellite Altimetry signal

Tide Gauge signal

η ins x i ,y i ,t = η satst t η ins

mean x i ,y i ηinsIB x i ,y i ,t η ins

var x i ,y i ,t C

η insIB x i ,y i ,t =

1ρg P − P ref Inverse barometer effect (Dorandeu and Le Traon, 1999)

(ECMWF analysis)η ins x i ,y i ,t = ηins x i ,y i ,t − ηins

IB

[ [ η ins] ] = ηinsmean [[ η sat

st ] ] C = β

⟨ ⟩ = areaaverage; [ [ ] ] = temporalmean

Altimetry What we compare

sla sat' = sla sat − η sat

st t η ins' = η ins − η sat t − βTide gauge

C = Local datumcor .

( 1 )

( 2 )

( 3 )

( 4 )

( 5 )

( 6 )

( 7 )


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

ηsatst t

[Garcia, 2010 time window]


Sea Level (SL) from observations Results: low frequency SL variability 1993 – 2010 monthly data [cm]

West Med: Marseille Central Med: Valletta

Adriatic Sea: Trieste East Med: Hadera


Sea Level (SL) Reconstruction Methods

1st step: SVD of satellite altimetry data detrended (take the spatial info. from lead EOFs) GIA correction ICE-5G (Stocchi and Spada, 2009) 2nd 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 ICE-5G model (Spada and Galassi, 2012, accept.)

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


Sea Level (SL) Reconstruction Methods : 1st step “feature extraction”

Z = data matrix n x m; where n = data point, and m = time steps

U = left singular vectors (eigenvectors of cov. Matrix , EOFs )

L = singular values; V = right singular vectors

SVD on altimetry data

Considering only EOFs leading modes M (space reduction):

Space dependent Time dependent EOFs(M) temporalamplitudes

We want estimate instead

Estimated from tide-gauge dataTARGET


Sea Level (SL) Reconstruction Methods : 2nd step optimal interpolation

Z0 = tide-gauge data; H = transfer operator (0 and 1); Λ = eigenvalues

R = error covariance matrix (consider sampling error and cov. of truncated modes)

Minimizing the cost function:

P = estimate for error covariance in the solution

Using this solution in the previous expression (TARGET) we can reconstruct sea-level field with altimetry spatial domain over tide-gauge time window


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-gauge positions and increasing n° EOFs considered.

Variance (%)


Tide-gauge stations used (1985 - 2010)

8 sub-periods(15 years/each) choosing the mostComplete time series

Up to 6 months gaps: splines

> 6 months gaps: linear fitting

> 12 months gaps: discarded

Red dots = discarded Green dots = used


Results Reconstruction validation during the satellite era: Cor. patt. and RMSE

Correlation Map

RMSE [cm]

Sea level climate variability in the Mediterranean sea A. Bonaduce, Tutor: N. Pinardi

Sea Level (SL) from observations Results: Satellite altimetry trend spatial variability

SLA Trend 1993 – 2010 [mm yr-1] : 2.1 +/- 0.7 (seasonal signal removed; GIA correction applied )


Results Reconstruction validation during the satellite era: Trend spatial variability

Reconstructed sea-level trend 1993 – 2010

2.5 +/- 0.5 mm yr-1

Trend difference

Altimetry – Reconstr.

(difference < 2 mm/yea are not shown )


Results Reconstruction validation


Results Sea-level Reconstruction

1993 – 2010 (monthly)

1885 – 2010 (annual)


Results Sea-level Reconstruction 1885 - 2010

1885 – 2010 (annual)

DJFM NAO index(Hurrel, 2009)


Sea level reconstruction Results: Trend spatial variability over a centennial time period (1885 - 2010)


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 1885 - 2010 (0.1 +/- 0.1 mm yr-1)

x n° years Change in Annual Sea level


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 reversal).

Low number of EOFs considered affect 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 leading patterns The basin mean reconsrt. sea-level trend is 2.5 +/- 0.5 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.


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)

A. Bonaduce, N. Pinardi Mediterranean Sea level reconstruction during the last century A. Bonaduce...

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