Annual Sea Level Variations Research Articles

Annual sea level variations off Atlantic Canada from satellite altimetry

Annual cycle of sea level off Atlantic Canada has been investigated based on a merged satellite altimetry dataset and a monthly temperature and salinity dataset. The altimetric results were compared with coastal tide-gauge data and steric height calculated from the temperature and salinity dataset. There was a general north-south variation in the amplitude of the altimetric annual cycle, increasing from 4 cm in the Labrador Sea to 15 cm in the Gulf Stream and the North Atlantic Current Region. The annual cycle in the deep ocean can approximately be accounted for by the steric height variability relative to 700 m, in which the thermosteric effect was the dominant contributor. The halosteric effect over the continental slope, especially over the northern Labrador Slope was also important. While the thermosteric effect occurred dominantly at the top 100 m water column, there was substantial halosteric variation in the 100–300 m water column. The annual sea level cycle along the Canadian Atlantic coast showed a complicated pattern in amplitude, but the phase was highly coherent with the highest sea level in fall. The steric height accounts for a substantial portion of the coastal annual cycle, but other factors such as wind forcing may be equally important.

GNSS-IR Measurements of Inter Annual Sea Level Variations in Thule, Greenland from 2008–2019

Studies of global sea level often exclude Tide Gauges (TGs) in glaciated regions due to vertical land movement. Recent studies show that geodetic GNSS stations can be used to estimate sea level by taking advantage of the reflections from the ocean surface using GNSS Interferometric Reflectometry (GNSS-IR). This method has the immediate benefit that one can directly correct for bedrock movements as measured by the GNSS station. Here we test whether GNSS-IR can be used for measurements of inter annual sea level variations in Thule, Greenland, which is affected by sea ice and icebergs during much of the year. We do this by comparing annual average sea level variations using the two methods from 2008–2019. Comparing the individual sea level measurements over short timescales we find a root mean square deviation (RMSD) of 13 cm, which is similar to other studies using spectral methods. The RMSD for the annual average sea level variations between TG and GNSS-IR is large (18 mm) compared to the estimated uncertainties concerning the measurements. We expect that this is in part due to the TG not being datum controlled. We find sea level trends from GNSS-IR and TG of −4 and −7 mm/year, respectively. The negative trend can be partly explained by a gravimetric decrease in sea level as a result of ice mass changes. We model the gravimetric sea level from 2008–2017 and find a trend of −3 mm/year.

Annual sea level variations in the Red Sea observed using GNSS

SUMMARY Annual sea level variations in the Red Sea have amplitudes of 15–20 cm as observed using various techniques such as tide gauges, satellite altimetry and recently Gravity Recovery and Climate Experiment (GRACE) satellite data. In this study, we demonstrate that Global Navigation Satellite System (GNSS) observations can also be used to measure the effect of these sea level variations. The extra water mass presses on the seafloor, which causes horizontal and vertical deformations. Using time-series from 10 coastal GNSS stations, we observe annual horizontal and vertical loading displacements with amplitudes of 2–5 mm. When correcting for atmospheric, hydrological and surface water loading and a residual geocentre motion, significant annual signals of approximately 0.5 and 2 mm are still observed for the horizontal and vertical components, respectively. In the northern Red Sea, the observed annual signals and predicted annual sea level loading show good agreement. This confirms that the signal is mostly a result of the variations in water mass and thermal expansion. Furthermore, we conclude that the uncertainties in the hydrological model over Ethiopia and Eritrea influence the loading over the southern Red Sea, which was underestimated in previous studies using GRACE data.

Global and regional factors contributing to the past and future sea level rise in the Northern Adriatic Sea

This study aims at discussing evolution of Sea Level (SL) in the Northern Adriatic Sea for the 20th and 21st century. A Linear Regression Model (LRM) which aims at describing the effect of regional processes, is built and validated. This LRM computes the North Adriatic mean SL variations using three predictors: the Mean Sea Level Pressure (MSLP) in the Gulf of Venice, the mean Sea Temperature (ST) of the water column in the South Adriatic and the Upper Level Salinity (ULS) in the central part of the basin. SL data are provided by monthly values recorded at 7 tide gauges distributed along the Italian and Croatian coasts (available at the PSMSL, Permanent Service of Mean Sea Level). MSLP data are provided by the EMULATE data set. Mediterranean ST and ULS data are extracted from the MEDATLAS/2002 database. The study shows that annual SL variations at Northern Adriatic stations are very coherent, so that the Northern Adriatic SL can be reconstructed since 1905 on the basis of only two stations: Venice and Trieste. The LRM is found to be robust, very successful at explaining interannual SL variations and consistent with the physical mechanisms responsible for SL evolution. Results show that observed SL in the 20th century has a large trend, which cannot be explained by this LRM, and it is interpreted as the superposition of land movement and a remote cause (such as polar ice melting). When the LRM is used with the MSLP, ST and ULS from climate model projections for the end of the 21st century (A1B scenario), it produces an SL rise in the range from 2.3 to 14.1cm, with a best estimate of 8.9cm. However, results show that the behavior of the remotely forced SL rise is the main source of future SL uncertainty and extrapolating its present trend to the future would expand the range of SL uncertainty from 14 to 49cm.

Multimission empirical ocean tide modeling for shallow waters and polar seas

A new global ocean tide model named DTU10 (developed at Technical University of Denmark) representing all major diurnal and semidiurnal tidal constituents is proposed based on an empirical correction to the global tide model FES2004 (Finite Element Solutions), with residual tides determined using the response method. The improvements are achieved by introducing 4 years of TOPEX–Jason 1 interleaved mission into existing 18 years (1993–2010) of primary joint TOPEX, Jason 1, and Jason 2 mission time series. Hereby the spatial distribution of observations are doubled and satellite altimetry should be able to recover twice the spatial variations of the tidal signal which is particularly important in shallow waters where the spatial scale of the tidal signal is scaled down. Outside the ±66° parallel combined Envisat, GEOSAT Follow‐On, and ERS‐2, data sets have been included to solve for the tides up to the ±82° parallel. A new approach to removing the annual sea level variations prior to estimating the residual tides significantly improved tidal determination of diurnal constituents from the Sun‐synchronous satellites (e.g., ERS‐2 and Envisat) in the polar seas. Extensive evaluations with six tide gauge sets show that the new tide model fits the tide gauge measurements favorably to other state of the art global ocean tide models in both the deep and shallow waters, especially in the Arctic Ocean and the Southern Ocean. One example is a comparison with 207 tide gauge data in the East Asian marginal seas where the root‐mean‐square agreement improved by 35.12%, 22.61%, 27.07%, and 22.65% (M2, S2, K1, and O1) for the DTU10 tide model compared with the FES2004 tide model. A similar comparison in the Arctic Ocean with 151 gauge data improved by 9.93%, 0.34%, 7.46%, and 9.52% for the M2, S2, K1, and O1 constituents, respectively.

On steric and mass-induced contributions to the annual sea-level variations in the South China Sea

The steric sea level ( SLA steric), water-mass change ( SLA mass) and their contributions to the sea level anomaly ( SLA) in the South China Sea (SCS) are studied by using altimetry data, Ishii data and GRACE data. Annual harmonic results and correlation analysis indicate that the SLA steric has a significant contribution to the SLA over deep basin and that its phase leads SLA 1–3 months in different parts of the SCS, whereas the SLA mass explains the SLA to a great extent over shallow water areas. The area-averaged SLA in the western SCS has an amplitude of ∼ 7.6 cm and a phase of ∼ 360°, which is ∼ 3.8 cm higher and peaks ∼ 108° later than SLA steric. The SLA mass has an amplitude of ∼ 6.3 cm and a phase of ∼ 337°. The simultaneous correlation coefficient between SLA and SLA mass is 0.78, which indicates that mass variation dominates the sea-level variation in the western SCS. Unlike the western SCS, the area-averaged SLA in the central SCS peaks at 254° with a smaller amplitude of ∼ 3.3 cm, which is ∼ 1.8 cm lower and peaks ∼ 15 days later than corresponding SLA steric. The simultaneous correlation coefficient between SLA and SLA steric is 0.81; therefore, the contribution of steric variation to the sea-level variation is dominant in the central SCS.

Spatial variability in annual sea level variations around the Korean peninsula

Analyses of annual sea level variations (Sa) around the Korean peninsula reveal systematic spatial patterns in the phase lags and amplitudes. Sa amplitudes in this region vary between 10 and 20 cm, with atmospheric pressure accounting for only 7.7 to 9.4 cm. These patterns are dominated by annual changes in thermosteric sea levels in the Yellow Sea (YS) and East/Japan Sea (EJS), with only minor contributions from seasonal wind‐driven set up.

Comment on “On the steric and mass‐induced contributions to the annual sea level variations in the Mediterranean Sea” by David García et al.

[1] Garcia et al. [2006] (hereafter referred to as GCDVG) present an analysis of mass-induced sea level variation (SLVmass) of the Mediterranean Sea based on satellite altimetry, ocean modeling and Gravity Recovery and Climate Experiment (GRACE) gravity measurements. In particular, GCDVG find consistency between altimeter sea level variations corrected for steric effects and changes in oceanic mass derived from Gravity Recovery and Climate Experiment (GRACE). GCDVG show that the annual cycle of the sea level variability (SLVtotal) is dominated by changes in its steric part (SLVsteric) and that the basin average of mass (SLVmass) is maximum in February. They conclude that, when the sea level is rising (falling) the Mediterranean Sea is actually losing (gaining) mass. [2] GCDVG appear to use an unrealistically large estimate of steric sea level variability (SLVsteric) and neglect changes in continental water storage that can significantly affect GRACE measurements over the Mediterranean Sea. These issues are examined in turn below by reviewing relevant individual basin averages of the Mediterranean Sea in comparison to results of other studies. In particular, while also finding consistency between altimetry and GRACE measurements, Fenoglio-Marc et al. [2006] (hereafter referred to as FKB), in a similar study of the Mediterranean Sea, find steric effects only accounting for about 60% of the total annual sea level variability and the annual cycle of SLVmass having a maximum in November.

Reply to comment by L. Fenoglio‐Marc et al. on “On the steric and mass‐induced contributions to the annual sea level variations in the Mediterranean Sea”

This work is supported by the Spanish Science and Technology Ministry Project ESP2006-11357.

On the steric and mass‐induced contributions to the annual sea level variations in the Mediterranean Sea

The sea level variation (SLVtotal) is the sum of two major contributions: steric and mass‐induced. The steric SLVsteric is that resulting from the thermal and salinity changes in a given water column. It only involves volume change, hence has no gravitational effect. The mass‐induced SLVmass, on the other hand, arises from adding or subtracting water mass to or from the water column and has direct gravitational signature. We examine the closure of the seasonal SLV budget and estimate the relative importance of the two contributions in the Mediterranean Sea as a function of time. We use ocean altimetry data (from TOPEX/Poseidon, Jason 1, ERS, and ENVISAT missions) to estimate SLVtotal, temperature, and salinity data (from the Estimating the Circulation and Climate of the Ocean ocean model) to estimate SLVsteric, and time variable gravity data (from Gravity Recovery and Climate Experiment (GRACE) Project, April 2002 to July 2004) to estimate SLVmass. We find that the annual cycle of SLVtotal in the Mediterranean is mainly driven by SLVsteric but moderately offset by SLVmass. The agreement between the seasonal SLVmass estimations from SLVtotal – SLVsteric and from GRACE is quite remarkable; the annual cycle reaches the maximum value in mid‐February, almost half a cycle later than SLVtotal or SLVsteric, which peak by mid‐October and mid‐September, respectively. Thus, when sea level is rising (falling), the Mediterranean Sea is actually losing (gaining) mass. Furthermore, as SLVmass is balanced by vertical (precipitation minus evaporation, P–E) and horizontal (exchange of water with the Atlantic, Black Sea, and river runoff) mass fluxes, we compared it with the P–E determined from meteorological data to estimate the annual cycle of the horizontal flux.

Annual cycles of sea level and sea surface temperature in the western Mediterranean Sea

In the western Mediterranean Sea, sea level variations are the result of seasonal steric effect, atmospheric forcing and mesoscale circulation activity, particularly in the Alboran and Algerian basins, as well as water mass budget variations including flux variations through the Straits of Gibraltar and Sicily. However, the partition of the magnitude of the sea level variations associated with these different components remains a challenging problem. A better determination of the steric effect component will allow a better knowledge of the other annual components. The aim of this study is to improve the estimation and understanding of the annual sea level variations in the different basins of the western Mediterranean Sea, using sea level anomalies (SLA) derived from TOPEX/POSEIDON and ERS‐1/2 altimeters and sea surface temperatures (SST) derived from NOAA/AVHRR infrared imagery, obtained between 1992 and 1998. The first mode of empirical orthogonal function (EOF) analysis on SLA is annual and represents 63% of the total variance. The first mode of EOF analysis on SST anomalies is also annual and represents 98% of the total variance. The lag between the annual variations of SLA and SST is about 40 days. From these EOF results and a parameterisation of the mixed layer, the seasonal dynamic height and the steric effect are estimated and removed from the SLA. The remaining signal is used to examine the annual cycles of the sea level due to water mass budget variations, mesoscale oceanic variability and atmospheric forcing.

Contribution of equatorial Pacific winds to southern tropical Indian Ocean Rossby waves

Westward propagating features, identified as Rossby waves, have been observed and modeled in the southern tropical Indian Ocean (STIO) between 10° and 30°S. These STIO Rossby waves, which have broad zonal and meridional extents, could interact with the westward flowing South Equatorial Current (SEC) as well as coastal currents on the south shore of Java (the South Java Current) and along the western shore of Australia (the Leeuwin Current). Previous work has attributed these waves to variations in wind stress along the west coast of Australia, Ekman pumping in the STIO interior, and a combination of both. This study investigates the importance of a third factor: remotely forced coastal Kelvin waves. Observations show that changes in wind stress curl in the eastern equatorial Pacific create annual upwelling and downwelling Rossby waves. Numerical model results confirm previous studies that demonstrate these waves, upon reaching the western boundary of the Pacific, create poleward propagating coastal Kelvin waves along the western shore of the Irian Jaya/Australia land mass. Direct observations of annual sea level variations along the northwest coast of Australia show a phase lag from the northernmost station to the southernmost that is not explained by direct wind forcing, suggesting that this signal is propagating from the Indonesian seas. It is shown in this work that when these waves reach the Indian Ocean, they are in phase with the local Ekman forcing and enhance the STIO Rossby waves. In the model the signal from the equatorial Pacific accounts for almost 80% of the energy of the STIO Rossby wave near the coast of Australia and 10% of the energy offshore.

Annual sea level variations in the southern tropical Indian Ocean from Geosat and shallow‐water simulations

Sea level variations in the Indian Ocean north of 20°S are analyzed from Geosat satellite altimeter data in 1987–1988. These observed variations are compared with numerical simulations from a reduced‐gravity model forced by observed winds over the same period. The first complex empirical orthogonal function of observed and simulated variations is an annual signal. For this signal, observations and simulations are highly correlated in both time and space. Off‐equatorial sea level variations propagate westward and poleward as Rossby waves. The strongest annual variations occur in the southeastern tropical Indian Ocean. The maximum amplitude (∼12 cm) is located at about 90°E, 12°S, although the wind stress curl is weak there and east of it. The signal propagates from the eastern boundary to the southwest across almost all the basin.

An Analysis of Annual Sea Level Variations in European Waters

Summary Regression analysis has been applied to all long series of annual mean sea level heights for European stations in order to estimate the components due to secular variation, air pressure variations and the nodal tide. The multiple correlation coefficients are in general found to lie between 0.8 and 1.0. Secular variations (rise or fall in sea level relative to the land) are well defined; they range from almost 90 cm/century fall at the head of the Gulf of Bothnia to a rise of 20 cm/century in the southern North Sea, English Channel and Mediterranean. The variations are consistent enough to enable isopleths to be constructed. The results are used to predict the probable value of mean sea level at certain stations in 1970. Expressions are obtained for annual mean sea level anomalies, arising from variations in the air pressure distribution, which enable the computation of ‘normal’ mean sea level (under normal atmospheric conditions) and ‘isobaric’ mean sea level (on the assumption of an isobaric atmosphere). The isobaric estimates represent an approximate to the geoid in European waters, though clear indications exist of a slope due to the spatial distribution of sea water density; the principal isobaric surface slopes are found to be more than 20 cm upwards from the North Sea into the Baltic, 8 cm downward to the north along the Norwegian coast, and 10 cm downward from the English Channel into the Mediterranean. The permanent deformation of the sea surface due to prevailing winds and air pressure gradients is well marked, rising more than 20 cm from the Mediterranean to the Gulf of Bothnia. Estimates of the nodal tide (period 18.6 years) suggest its existence in the equilibrium form, but the degree of scatter in the results is too great to permit a more definite conclusion.