The seasonal cycle and variability of sea level in the South China Sea

The spatial and temporal characteristics of the seasonal sea level cycle in the South China Sea (SCS) and its forcing mechanisms are investigated using tide gauge records and satellite altimetry observations along with steric and meteorological data. The coastal mean annual amplitude of the seasonal cycle varies between zero and 24 cm, reaching a maximum between July and January. The maximum mean semiannual amplitude is 7 cm, peaking between March and June. Along the coast, the seasonal cycle accounts for up to 92% of the mean monthly sea level variability. Atmospheric pressure explains a significant portion of the seasonal cycle with dominant annual signals in the northern SCS, the Gulf of Thailand and the north-western Philippines Sea. The wind forcing is dominant on the shelf areas of the SCS and the Gulf of Thailand where a simple barotropic model forced by the local wind shows annual amplitudes of up to 27 cm. In the deep basin of the SCS, the Philippines Sea and the shallow Malacca Strait, the steric component is the major contributor with the maximum annual amplitudes reaching 15 cm. Significant variability in the seasonal cycle is found on a year-to-year basis. The annual and semiannual amplitudes vary by up to 63% and 45% of the maximum values, 15 cm and 11 cm, respectively. On average, stepwise regression analysis of contribution of different forcing factors accounts for 66% of the temporal variability of the annual cycle. The zonal wind was found to exert considerable influence in the Malacca Strait

The sources of sea‐level changes in the Mediterranean Sea since 1960

Past sea-level changes in the Mediterranean Sea are highly non-uniform and can deviate significantly from both the global average sea-level rise and changes in the nearby Atlantic. Understanding the causes of this spatial non-uniformity is crucial to the success of coastal adaptation strategies. This, however, remains a challenge owing to the lack of long sea-level records in the Mediterranean. Previous studies have addressed this challenge by reconstructing past sea levels through objective analysis of sea-level observations. Such reconstructions have enabled significant progress toward quantifying sea-level changes, however, they have difficulty capturing long-term changes and provide little insight into the causes of the changes. Here, we combine data from tide gauges and altimetry with sea-level fingerprints of contemporary land-mass changes using spatial Bayesian methods to estimate the sources of sea-level changes in the Mediterranean Sea since 1960. We find that, between 1960 and 1989, sea level in the Mediterranean fell at an average rate of −0.3 ± 0.5 mm yr−1, due to an increase in atmospheric pressure over the basin and opposing sterodynamic and land-mass contributions. After 1989, Mediterranean sea level started accelerating rapidly, driven by both sterodynamic changes and land-ice loss, reaching an average rate of 3.6 ± 0.3 mm yr−1 in the period 2000–2018. The rate of sea-level rise shows considerable spatial variation in the Mediterranean Sea, primarily reflecting changes in the large-scale circulation of the basin. Since 2000, sea level has been rising faster in the Adriatic, Aegean, and Levantine Seas than anywhere else in the Mediterranean Sea

The ability of a barotropic model to simulate sea level extremes of meteorological origin in the Mediterranean Sea, including those caused by explosive cyclones

Storm surges are responsible for great damage to coastal property and loss of life every year. Coastal management and adaptation practices are essential to reduce such damage. Numerical models provide a useful tool for informing these practices as they simulate sea level with high spatial resolution. Here we investigate the ability of a barotropic version of the HAMSOM model to simulate sea level extremes of meteorological origin in the Mediterranean Sea, including those caused by explosive cyclones. For this purpose, the output of the model is compared to hourly sea level observations from six tide gauge records (Valencia, Barcelona, Marseille, Civitavecchia, Trieste, and Antalya). It is found that the model underestimates the positive extremes significantly at all stations, in some cases by up to 65%. At Trieste, the model can also sometimes overestimate the extremes significantly. The differences between the model and the residuals are not constant for extremes of a given height, which limits the applicability of the numerical model for storm surge forecasting because calibration is difficult. The 50 and 10 year return levels are reasonably well captured by the model at all stations except Barcelona and Marseille, where they are underestimated by over 30%. The number of exceedances of the 99.9th and 99.95% percentiles over a period of 25 years is severely underestimated by the model at all stations. The skill of the model for predicting the timing and value of the storm surges seems to be higher for the events associated with explosive cyclones at all stations

A Mediterranean sea level reconstruction (1950–2008) with error budget estimates

Reconstructed 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

Reconstruction of Mediterranean sea level fields for the period 1945–2000

The 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

Mass contribution to Mediterranean Sea level variability for the period 1948–2000

The 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 fields for the period 1961–2000 as given by a data reconstruction and a 3D model

Two 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

Comparison of Mediterranean sea level variability as given by three baroclinic models

We 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