Clima en España: Pasado, presente y futuro
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Clima en España: Pasado, presente y futuro Madrid, Spain, 11 – 13 February. Long-term sea level variability: what observations tell us F. M. Calafat 1 , M. Marcos 1 , M. N. Tsimplis 2 , D. Gomis 1 , A. Pascual 1 , S. Montserrat 1. 1 IMEDEA (UIB - CSIC), Mallorca, SPAIN.

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Clima en España: Pasado, presente y futuro Madrid, Spain, 11 – 13 February

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Clima en espa a pasado presente y futuro madrid spain 11 13 february

Clima en España: Pasado, presente y futuroMadrid, Spain, 11 – 13 February

Long-term sea level variability: what observations tell usF. M. Calafat 1, M. Marcos 1, M. N. Tsimplis 2, D. Gomis 1, A. Pascual1, S. Montserrat 1

1 IMEDEA (UIB - CSIC), Mallorca, SPAIN.

2 National Oceanography Centre, Southampton, UK.


Clima en espa a pasado presente y futuro madrid spain 11 13 february

OVERVIEW

  • What do tide gauges tell us?

    • Variations of the seasonal cycle in the Mediterranean Sea and the Atlantic Iberian coast.

    • Mediterranean Sea level trends estimated from the longest stations.

  • What does altimetry offer?

  • Reconstruction of sea level fields by combining satellite altimetry and tide gauge data.

    • Reconstruction for the period 1993-2000.

    • Reconstruction for the period 1945-2000.

  • Summary and conclusions


Clima en espa a pasado presente y futuro madrid spain 11 13 february

What do tide gauges tell us?

Variations of the seasonal cycle in the Mediterranean

  • Interdecadal and climatic changes are likely to affect the seasonal cycles

  • We investigate the spatial and temporal variability of the seasonal sea level cycle.

  • We identify the changes in the forcing responsible for the variability of the seasonal sea level cycle.

THE DATASET

  • Tide gauge data are mean monthly sea level from PSMSL and ESEAS-RI with a minimum length of around 40 years.

  • The atmospheric contribution was obtained from HIPOCAS data (Alvarez-Fanjul et al., 1997).


Clima en espa a pasado presente y futuro madrid spain 11 13 february

What do tide gauges tell us?

METHODOLOGY

The mean annual and semi-annual cycle have been estimated for each station by a least square fit of the equation:


Clima en espa a pasado presente y futuro madrid spain 11 13 february

What do tide gauges tell us?

Mean seasonal sea level cycle: 1962-1989

ANNUAL

SEMI-ANNUAL


Clima en espa a pasado presente y futuro madrid spain 11 13 february

What do tide gauges tell us?

Temporal variability: observed sea level annual cycle


Clima en espa a pasado presente y futuro madrid spain 11 13 february

What do tide gauges tell us?

Temporal variability: Atmospherically-induced annual cycle

  • The atmospherically-induced seasonal sea level cycle reach 4 cm . Maximum amplitudes are reached in both cases in the eastern basin.

  • Annual amplitude changes are very consistent in all areas, except in the eastern Mediterranean and the Cantabric stations.


Clima en espa a pasado presente y futuro madrid spain 11 13 february

What do tide gauges tell us?

Temporal variability: residual sea level annual cycle

  • After removing the atmospheric pressure and wind effects there is still significant temporal variability in the seasonal cycle.

  • In the Atlantic the temporal variability of the annual signal of the residuals appears more consistent between stations than that of the observations


Clima en espa a pasado presente y futuro madrid spain 11 13 february

What do tide gauges tell us?

Sea level trends

  • Available data sets:

    Tide gauge records:

partial spatial coverage and …

they do not measure absolute sea level, but sea level relative to land marks… and land is not static

• Model corrections (GIA)

- The post glacial rebound have been corrected by means of the GIA model ICE-5G (VM2, Peltier, 2004)

  • Small values in the Med Sea (up to +0.3 mm/yr in the Adriatic and Aegean)


Clima en espa a pasado presente y futuro madrid spain 11 13 february

What do tide gauges tell us?

Sea level trends in Southern Europe for the period 1960-2000

  • Sea level trends obtained from the 21 longest records (>35 yr) are smaller in the Mediterranean (0.3 ± 0.4 to −0.7 ± 0.3 mm/yr) than in the neighbouring Atlantic sites (1.6 ± 0.5 to –1.9 ± 0.5 mm/yr) for the period 1960–2000.

  • The atmospheric contribution accounts for 20–50 per cent of the observed yearly sea level variability and introduces negative trends of –0.2 to –0.9 mm/yr.


Clima en espa a pasado presente y futuro madrid spain 11 13 february

What does altimetry offer?

Altimetry data:

short time coverage (from 1993)

MSTEP website


Clima en espa a pasado presente y futuro madrid spain 11 13 february

The reconstruction of sea level fields

F.M. Calafat and D. Gomis, 2009 in GPC

  • Tide gauge data

  • From the data archive of the PSMSL. All data used are Revised Local Reference data.

  • All tide gauges were corrected using the GIA values from the ICE-4G VM2 (Peltier, 2001)

  • The period of reconstruction 1945-2000 was broken into 6 smaller periods.

  • The seasonal cycle was removed from tide gauges by means of a harmonic analysis.

  • The satellite altimetry dataset

  • Gridded Sea Level Anomaly were collected at CLS by combining several altimeter missions.

  • The resolution of altimetry fields is 1/4º , resulting in a total of 5022 grid points covering the Mediterranean basin for the period 1993-2005.

  • The seasonal cycle was removed from altimetry data by means of a harmonic analysis.


Clima en espa a pasado presente y futuro madrid spain 11 13 february

The reconstruction of sea level fields

Methodology

: Spatial EOFs from altimetry data

: tide gauge data

  • Reduced-space optimal interpolation

  • We minimize for changes in sea level. That avoids the problem of locating all station in a single, constant vertical frame.

  • A spatially-constant EOF (named EOF-0) is added to account for spatially uniform sea level changes.

  • The method gives theoretical estimate for the error.

  • Optimal interpolation accounts for non-homogeneous distributions giving less weight to highly correlated stations.

Reconstruction of Mediterranean Sea level fields for the period 1945-2000


Clima en espa a pasado presente y futuro madrid spain 11 13 february

The reconstruction of sea level fields

Distribution of sea level trends for the period 1993-2000

  • Sea level trends for the period 1993-2000

  • The reconstructed and observed trends are very similar, showing positive values above 10 mm/yr in the eastern Mediterranean and smaller rates in the western Mediterranean.

  • The reconstruction also reproduces the marked negative trend of more than -15 mm/yr observed in the Ionian Sea.

  • The reconstructed mean sea level rise for the period 1993-2000 is 4.0 0.7 mm/yr and 3.9 0.6 mm/yr for the altimetry .


Clima en espa a pasado presente y futuro madrid spain 11 13 february

The reconstruction of sea level fields

Comparison between the reconstruction and tide gauge observations

  • We used tide gauge records that did not enter the computations.

  • Here we show a comparison with the tide gauge located at Alexandria.

  • The correlation between the two time series is 0.7.


Clima en espa a pasado presente y futuro madrid spain 11 13 february

The reconstruction of sea level fields

Distribution of sea level trends for the period 1945-2000

  • Maximum positive rate in the Ionian Sea and nearly zero trends in the Aegean Sea.

  • Small positive trends in the western Mediterranean and even smaller trends in the Eastern Mediterranean.

  • Reconstruction reproduces the EMT even for the pre-altimetric period.


Clima en espa a pasado presente y futuro madrid spain 11 13 february

The reconstruction of sea level fields

Mean Sea level for the period 1945-2000

  • Therate of mean sea levelrisefortheperiod 1945-2000

  • Forthewholeperiodthe mean sea levelriseis 0.7 0.2 mm/yr.

  • Global mean sea level has beenrising at a rate of 1.5 0.4 mm/yr.

  • TheatmosphericcontributiontoMediterranean sea level has beenestimatedtobe -0.7 0.1 mm/yrfortheperiod 1961-2000.

  • Theatmosphericcontributionexplainsthedifferencebetween global and Mediterraneantrends.


Clima en espa a pasado presente y futuro madrid spain 11 13 february

Summary and conclusions

  • The mean seasonal sea level cycle estimated from tide-gauge records has amplitudes ranging between 3-7 cm for the annual signal and 1-3 cm for the semi-annual signal. The annual cycle reaches its maximum values mainly between September and November except in the eastern basin, where it peaks in July. The semi-annual cycle peaks mainly in January/February (in March in the eastern Mediterranean)

  • The temporal evolution of the observed seasonal sea level cycle has revealed a large range of variations. During the period 1950-2000, decadal changes in annual and semi-annual signals are consistent.

  • After removing the atmospheric pressure and wind effects there is still significant temporal variability in the seasonal cycle.

  • The reconstruction gives, for the first time in the Mediterranean, the spatial distribution of trends spanning more than five decades.

  • The distribution of sea level trends for the period 1945-2000 has as a main feature the maximum positive trend in the Ionian Sea (up to 1.2 mm/yr). Minimum values are obtained in the Aegean Sea, where trends are nearly zero.

  • The rate of mean sea level rise for the period 1945-2000 is 0.7 0.2 mm/yr.

  • The rate of mean sea level rise is in good agreement with global estimates if we take into account the negative trends caused by the atmospheric contribution.


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