1,720,983 research outputs found
Reconstruction of Mediterranean sea level fields for the period 1945–2000
No full textThe distribution of sea level in the Mediterranean Sea is recovered for the period 1945–2000 by using a reduced space optimal interpolation analysis. The method involves estimating empirical orthogonal functions from satellite altimeter data spanning the period 1993–2005 that are then combined with tide gauge data to recover sea level fields over the period 1945–2000. The reconstruction technique is discussed and its robustness is checked through different tests. For the altimetric period (1993–2000) the prediction skill is quantified over the whole domain by comparing the reconstructed fields with satellite altimeter observations. For past times the skill can only be tested locally, by validating the reconstruction against independent tide gauge records. The reconstructed distribution of sea level trends for the period 1945–2000 shows a positive peak in the Ionian Sea (up to 1.5 mm yr? 1) and a negative peak of ? 0.5 mm yr? 1 in a small area to the south-east of Crete. Positive trends are found nearly everywhere, being larger in the western Mediterranean (between 0.5 and 1 mm yr? 1) than in the eastern Mediterranean (between 0 and 0.5 mm yr? 1). The estimated rate of mean sea level rise for the period 1945–2000 is 0.7 ± 0.2 mm yr? 1, i.e. about a half of the rate estimated for global mean sea level. These overall results do not appear to be very sensitive to the distribution of tide gauges. The poorest results are obtained in open-sea regions with intense mesoscale variability not correlated with any tide gauge station, such as the Algerian Basin
Quantifying recent acceleration in sea level unrelated to internal climate variability
Full text linkSea level observations suggest that the rate of sea level rise has accelerated during the last 20?years. However, the presence of considerable decadal-scale variability, especially on a regional scale, makes it difficult to assess whether the observed changes are due to natural or anthropogenic causes. Here we use a regression model with atmospheric pressure, wind, and climate indices as independent variables to quantify the contribution of internal climate variability to the sea level at nine tide gauges from around the world for the period 1920–2011. Removing this contribution reveals a statistically significant acceleration (0.022?±?0.015?mm/yr2) between 1952 and 2011, which is unique over the whole period. Furthermore, we have found that the acceleration is increasing over time. This acceleration appears to be the result of increasing greenhouse gas concentrations, along with changes in volcanic forcing and tropospheric aerosol loading
Comparison of Mediterranean sea level fields for the period 1961–2000 as given by a data reconstruction and a 3D model
No full textTwo Mediterranean sea level distributions spanning the last decades are examined. The first one is a reconstruction of sea level obtained by a reduced-space optimal interpolation applied to tide gauge and altimetry data. The second distribution is obtained from a 3D (baroclinic) regional circulation model. None of the two representations includes the mechanical atmospheric forcing. Results are presented for two different periods: 1993–2000 (for which altimetry data are available) and 1961–2000 (the longest period common to both distributions).The first period is examined as a test period for the model, since the reconstruction is very similar to altimetry observations. The modelled sea level is in fair agreement with the reconstruction in the Western Mediterranean and in the Aegean Sea (except in the early nineties), but in the Ionian Sea the model departs from observations. For the whole period 1961–2000 the main feature is a marked positive trend in the Ionian Sea (up to 1.8 mm yr? 1), observed both in the reconstruction and in the model. Also the distribution of positive trends in the Western Mediterranean (mean value of 1.1 mm yr? 1) and the smaller trends in the Aegean Sea (0.5 mm yr? 1) are similar in the reconstruction and in the model, despite the first implicitly accounts for sea level variations due to remote sources such as ice melting and the second does not. The interannual sea level variability associated with key regional events such as the Eastern Mediterranean Transient is apparently captured by the reconstruction but not by the model (at least in its present configuration). Hence, the reconstruction can be envisaged as a useful tool to validate further long-term numerical simulations in the region
A Mediterranean sea level reconstruction (1950–2008) with error budget estimates
No full textReconstructed sea level fields are commonly obtained by using techniques that combine long-term records from coastal and island tide gauges with spatial covariance structures determined from recent altimetric observations. In this paper we estimate the error budget of the Mediterranean sea level reconstructions based on a reduced space optimal interpolation. In particular, we characterize the baseline error of the methodology, which is linked to the capacity of tide gauges to capture open sea processes and to the representativity of the selected EOFs. Also, we analyze the impact of the non-stationarity of the EOFs and the uneven tide gauge spatial distribution. Results suggest that the baseline error is the dominant contribution in most areas of the Mediterranean (average value of 2.7 cm). In particular, the error due to the truncation of the EOFs is the largest contribution to the baseline error. The other error sources have a more localized impact, which can be important in certain areas with atypical mesoscale activity. The skills of the reconstruction are more dependent on the length of the period than on the particular years used to compute the EOFs. Redundant tide gauges improve the reconstruction only slightly while a single tide gauge at a critical location improves it significantly. In addition we estimate the total error linked to all sources of uncertainty. Finally, we also present an updated sea level reconstruction which includes several improvements with respect to previous reconstructions. The comparison with independent data shows that this new reconstruction provides better results with respect to previous products
Evaluation of new CryoSat-2 products over the ocean
Full text linkThe CryoSat-2 satellite, primarily dedicated to precise monitoring of the Cryosphere, is demonstrating its capability to provide valuable altimetric data also over the ocean. Here we present the results of a global assessment and validation of the new Geophysical Ocean Product (GOP) distributed by the European Space Agency (ESA) since April 2014, focusing on the sea surface height anomaly (SSHA), the significant wave height (SWH), and the wind speed. Our assessment involves only Low Resolution Mode (LRM) and Pseudo LRM (PLRM) data, since full SAR processing is not already operationally implemented in the GOP. The global assessment is conducted on the basis of measurement noise and along-track spectral and crossover analysis, whereas the validation is performed against a variety of in situ observations such as tide gauges, buoys and Argo floats as well as data from the WaveWatch III (WWIII) model. The performance of the GOP is compared to that of Jason-2 and CryoSat-2 data from the Radar Altimeter Database System (RADS). The mean value of the 20-Hz SSHA noise at 2 m SWH is 6.3 cm for LRM and 10.2 cm for PLRM, and the standard deviation of the crossovers is ~ 5.4 cm. The mean 20-HZ SWH noise over the global oceans is 49.4 cm and 69.8 cm, for LRM and PLRM respectively. CryoSat-2 and Jason-2 show almost identical performance when SSHAs are validated against tide gauges, with a median correlation and root mean square difference (RMSD) of 0.78 and 7.1 cm for the GOP, 0.76 and 7.3 cm for Jason-2, and 0.79 and 7.8 cm for CryoSat-2 from RADS. The median correlation with Argo-derived steric heights is 0.68 for the GOP, 0.74 for Jason-2, and 0.67 for CryoSat-2 from RADS. However, the correlation shows a strong latitudinal dependence, with higher values at low latitudes (median value larger than 0.80 in the 10°S-10°N band). The median RMSD between the SSHAs and steric heights is 5.3 cm for the GOP, 4.6 cm for Jason-2, and 5.1 cm for CryoSat-2 from RADS. The GOP and Jason-2 show also identical performance when SWHs are compared to buoy data, with a slope and RMS error of 0.98 and 15 cm for GOP, 0.97 and 16 cm for Jason-2, and 1.05 and 17 cm for CryoSat-2 from RADS. On the other hand, the GOP wind speed exhibits a bias of about 2 m/s relative to both Jason-2 and to buoy data. Differences between the GOP and WWIII SWH are smaller than 20% of the SWH almost everywhere. In summary the GOP products are fit for oceanographic applications
On the ability of global sea level reconstructions to determine trends and variability
Full text linkWe investigate how well methods based on empirical orthogonal functions (EOFs) can reconstruct global mean sea level (GMSL). We first explore the analytical solution of the method and then perform a series of numerical experiments using modelled data. In addition, we present a new GMSL reconstruction for the period 1900-2011 computed both with and without a spatially uniform EOF (EOF0). The method without the EOF0 uses global information, which leads to a better reconstruction of the variability, though with some underestimation. The trend, however, is not captured, which motivates the use of the EOF0. When the EOF0 is used the method reduces to the generalized weighted mean with regularization of altimetry records at tide gauge locations, and thus it uses no global information. This results in a poor reconstruction of the variability. Although the trend is better captured (biases smaller than ±25%) with the EOF0, using the covariance matrix of deseasonalized monthly time series as the basis for determining the contribution of each tide gauge to the trend is dubious because it assumes that the inter-annual variability and the trend are driven by the same mechanisms. A significant fraction of the inter-annual to decadal variability (~4mm peak-to-peak and ~2mm standard error) in the new GMSL reconstruction without the EOF0 is consistent with land hydrology changes associated with the El Niño-Southern Oscillation (ENSO). When the EOF0 is used, we find no correlation with either the ENSO or land hydrology changes, and decadal fluctuations are ~5 times greater
Mechanisms of decadal sea level variability in the eastern North Atlantic and the Mediterranean Sea
No full textDecadal sea level variations from tide gauge records along the western European coast and in the Mediterranean Sea commencing in the late 19th and early 20th centuries are examined relative to large-scale atmospheric forcing. Recent studies have provided evidence for a link between sea level in the eastern North Atlantic and atmospheric forcing, however the nature of this relationship is still unclear. Here the outputs of a regional barotropic model and a nearly global baroclinic model are used in conjunction with wind stress and heat flux data to explore the physical mechanisms responsible for the observed sea level variability. All tide gauge records show significant decadal variability (up to 15 cm) and are highly correlated with the NAO and among themselves at decadal periods. There is a coherent sea level signal that affects the eastern boundary of the North Atlantic northward of 25°N and is limited to a narrow band of the order of a few hundred kilometers along the coast. This band tends to become narrower towards higher latitudes. We find that longshore wind and wave propagation along the boundary are the major contributors to coastal sea level variability but no significant contribution from mass redistribution linked to changes in the strength of the subtropical gyre is observed. The mass component dominates sea level in the Mediterranean and is mainly driven by mass exchanges with the Atlantic, which explains the correlation between both regions. Southward of 25°N, sea level changes are mainly driven by heat advection through Ekman fluxes
Inter-annual to decadal sea-level variability in the coastal zones of the Norwegian and Siberian Seas: The role of atmospheric forcing
Full text linkInter-annual to decadal sea-level variations from tide gauge records in the coastal zones of the Norwegian and Siberian Seas are examined for the period 1950–2010 using a combination of hydrographic observations, wind data, and theory. We identify two large areas of highly coherent sea-level variability: one that includes the Norwegian, Barents, and Kara Seas, and another one that includes the Laptev, East Siberian, and Chukchi Seas. We provide evidence of a new contribution to the sea-level variability along the Norwegian coast associated with the poleward propagation of sea-level fluctuations along the eastern boundary of the North Atlantic. When this propagating signal is combined with the local wind, we are able to explain over 70% of the variance along the Norwegian coast. The steric component explains ~61% of the sea-level (corrected for the inverse barometer) variability along the Norwegian coast. The high coherency between the sea level along the Norwegian coast and that in the Barents and Kara Seas suggests that part of the Norwegian signal propagates further north into these regions. We introduce an atmospheric vorticity index that explains much of the sea-level variability in the Laptev, East Siberian, and Chukchi Seas with correlations ranging from 0.73 to 0.81. In the East Siberian Sea, we identify a sea-level increase of ~22 cm between 2000 and 2003, which is partly explained by the vorticity index, and a decline of ~15 cm after 2003, which we relate to the strengthening of the Beaufort Gyre
Mass contribution to Mediterranean Sea level variability for the period 1948–2000
No full textThe mass contribution to Mediterranean Sea level variability is estimated from steric-corrected altimetry and from GRACE observations for the period August 2002 to December 2006. The two signals are highly correlated (0.8) and display coherent trends, provided that a proper spatial averaging kernel is used to extract the gravity signal from GRACE coefficients (the same filter is applied to all fields in order to obtain consistent and comparable signals). The good agreement between GRACE observations and steric-corrected altimetry supports the quantification of the long-term mass contribution in terms of non-steric sea level in the Mediterranean. For the past decades, total sea level fields are reconstructed using a reduced-space optimal interpolation of altimetry and tide gauge data. The steric component is evaluated from hydrographic observations available for the same period for the upper 700 m. The errors associated with total sea level and the steric component are evaluated in order to obtain the uncertainty of non-steric sea level. Results indicate that the mass content of the Mediterranean basin has increased at a rate of 0.8 ± 0.1 mm/yr for the period 1948–2000. When the effect of the atmospheric pressure is removed, the trend of the mass component increases up to 1.2 ± 0.2 mm/yr
Comparison of Mediterranean sea level variability as given by three baroclinic models
No full textWe compare the results of three baroclinic models with the aim of evaluating their skills in reproducing Mediterranean long-term sea level variability. The models are an ocean-ice coupled forced global model (ORCA), a regional forced ocean model (OM8) and a regional coupled atmosphere-ocean model (MITgcm). Model results are compared for the period 1961–2000 against hydrographic observations for water mass properties and steric sea level, and against satellite altimetry data and a reconstruction for sea level. All models represent the temperature variability of the upper layers reasonably well, but exhibit a considerable positive drift in the temperature of the deep layers due to an imbalance between the surface heat flux and the heat flux through Gibraltar. OM8 and MITgcm simulate the process of dense water formation better than ORCA thanks to their higher resolution in the model grid and in the atmospheric forcings. Concerning sea level variability, MITgcm is the only model that simulates well the inter-annual sea level variability associated with the Eastern Mediterranean Transient. However, none of the models is able to reproduce other features that have clear signatures on sea level. The inter-annual variability of Mediterranean mean sea level is better reproduced by the ORCA model because it is the only one considering the mass contribution from the Atlantic. The lack of that component in the regional models is a major shortcoming to reproduce Mediterranean sea level variability. Finally, mean sea level trends are overestimated by all models due to the spurious warming drift in the deep layers
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