Sea level measurements for Inferring ground deformations in Rabaul Caldera

Rabaul tide gauge records from 1968 through 1985 give the amount of vertical movement in the northern part of Rabaul Caldera. Monthly mean sea level data were compared with other regional tide gauge stations to remove large scale oceanographic effects. No large vertical movements (> 0.3 cm/yr) were noted in this portion of the caldera. The results of sea level measurements at other points around the caldera, from 1981 through 1983 are consistent with the 1 to 10 cm of uplift observed on Matupit Island from optical leveling surveys. There was relatively little vertical movement ( 0.1 cm/yr) in the Vulcan area.     

Vertical Land Motion in the Mediterranean Sea from Altimetry and Tide Gauge Stations

We have computed estimates of the rate of vertical land motion in the Mediterranean Sea from differences of sea level heights measured by the TOPEX/Poseidon radar altimeter and by a set of tide gauge stations. The comparison of data at 16 tide gauges, using both hourly data from local datasets and monthly data from the PSMSL dataset, shows a general agreement, significant differences are found at only one location. Differences of near-simultaneous, monthly and deseasoned monthly sea level height time-series have been considered in order to reduce the error in the estimated linear-term. In a subset of 23 tide gauge stations the mean accuracy of the estimated vertical rates is 2.3 ± 0.8 mm/yr. Results for various stations are in agreement with estimates of vertical land motion from geodetic methods. A comparison with vertical motion estimated by GPS at four locations shows a mean difference of ?0.04 ± 1.8 mm/yr, however the length of the GPS time-series and the number of locations are too small to draw general conclusions.     

Vertical Land Motion in the Mediterranean Sea from Altimetry and Tide Gauge Stations

We have computed estimates of the rate of vertical land motion in the Mediterranean Sea from differences of sea level heights measured by the TOPEX/Poseidon radar altimeter and by a set of tide gauge stations. The comparison of data at 16 tide gauges, using both hourly data from local datasets and monthly data from the PSMSL dataset, shows a general agreement, significant differences are found at only one location. Differences of near-simultaneous, monthly and deseasoned monthly sea level height time-series have been considered in order to reduce the error in the estimated linear-term. In a subset of 23 tide gauge stations the mean accuracy of the estimated vertical rates is 2.3 ± 0.8 mm/yr. Results for various stations are in agreement with estimates of vertical land motion from geodetic methods. A comparison with vertical motion estimated by GPS at four locations shows a mean difference of -0.04 ± 1.8 mm/yr, however the length of the GPS time-series and the number of locations are too small to draw general conclusions.     

中国沿海验潮站及其邻近地区陆地垂直运动分析

利用中国沿岸验潮站GNSS和邻近地区陆态网络GNSS基准站观测数据,结合卫星高度计和验潮站海平面观测数据分析了中国沿海验潮站及其邻近地区陆地垂直运动特征。中国沿海海平面观测以及验潮站和陆态网GNSS基准站观测结果显示,中国沿海省区市及沿海验潮站陆地垂直运动总体表现为：辽宁至江苏沿海上升、上海至福建泉州沿海沉降、福建厦门至广西沿海升降交替的格局,局部滨海平原地区如华北平原天津南部、河北平原的沧县则表现出显著的沉降特征。验潮站陆地的抬升与沉降是沿海相对海平面变化的重要组成部分,准确掌握验潮站及其邻近区域的陆地垂直运动特征,可为沿海相对海平面变化分析、海平面变化影响评估以及未来海平面上升预测提供依据。     

Absolute sea level variability of Arctic Ocean in 1993–2018 from satellite altimetry and tide gauge observations

Arctic absolute sea level variations were analyzed based on multi-mission satellite altimetry data and tide gauge observations for the period of 1993–2018. The range of linear absolute sea level trends were found ?2.00 mm/a to 6.88 mm/a excluding the central Arctic, positive trend rates were predominantly located in shallow water and coastal areas, and negative rates were located in high-latitude areas and Baffin Bay. Satellite-derived results show that the average secular absolute sea level trend was (2.53±0.42) mm/a in the Arctic region. Large differences were presented between satellite-derived and tide gauge results, which are mainly due to low satellite data coverage, uncertainties in tidal height processing and vertical land movement (VLM). The VLM rates at 11 global navigation satellite system stations around the Arctic Ocean were analyzed, among which 6 stations were tide gauge co-located, the results indicate that the absolute sea level trends after VLM corrected were of the same magnitude as satellite altimetry results. Accurately calculating VLM is the primary uncertainty in interpreting tide gauge measurements such that differences between tide gauge and satellite altimetry data are attributable generally to VLM.     

利用验潮和GPS观测技术监测海平面绝对变化

GPS技术可以确定验潮站水准点的地壳垂直形变,结合验潮数据获得的海平面相对变化,可以确定海平面的绝对变化。采用我国3个沿海验潮站两期GPS观测数据,计算了这些点位的地壳垂直运动速率。提出要监测验潮站的地壳垂直运动,最好采用多年连续GPS观测数据。     

Interannual Sea Level Variations in the South China Sea Over 1950–2009

Spatial patterns of interannual sea level variations in the South China Sea (SCS) are investigated by analyzing an EOF-based 2-dimensional past sea level reconstruction from 1950 to 2009 and satellite altimetry data from 1993 to 2009. Long-term tide gauge records from 14 selected stations in this region are also used to assess the quality of reconstructed sea levels and determine the rate of sea level along the coastal area. We found that the rising rate of sea levels derived from merged satellite altimetry data during 1993–2009 and past sea level reconstruction over 1950–2009 is about 3.9 ± 0.6 mm/yr and 1.7 ± 0.1 mm/yr, respectively. For the longer period, this rate is not significantly different from the global mean rate (of 1.8 ± 0.3 mm/yr). The interannual mean sea level of the SCS region appears highly correlated with Niño 4 indices (a proxy of El Niño-Southern Oscillation/ENSO), suggesting that the interannual sea level variations over the SCS region is driven by ENSO events. Interpolation of the reconstructed sea level data for 1950–2009 at sites where tide gauge records are of poor quality (either short or gapped) show that sea level along the Chinese coastal area is rising faster than the global mean rate of 1.8 mm/yr. At some sites, the rate is up to 2.5 mm/yr.     

Steric sea level changes estimated from historical ocean subsurface temperature and salinity analyses

An historical objective analysis of subsurface temperature and salinity was carried out on a monthly basis from 1945 to 2003 using the latest observational databases and a sea surface temperature analysis. In addition, steric sea level changes were mainly examined using outputs of the objective analyses. The objective analysis is a revised version of Ishii et al. and is available at 16 levels in the upper 700 m depth. Artificial errors in the previous analysis during the 1990s have been worked out in the present analysis. The steric sea level computed from the temperature analysis has been verified with tide gauge observations and TOPEX/Poseidon sea surface height data. A correction for crustal movement is applied for tide gauge data along the Japanese coast. The new analysis is suitable for the discussion of global warming. Validation against the tide gauge reveals that the amplitude of thermosteric sea level becomes larger and the agreement improves in comparison with the previous analysis. A substantial part of local sea level rise along the Japanese coast appears to be explained by the thermosteric effect. The thermal expansion averaged in all longitudes from 60°S to 60°N explains at most half of recent sea level rise detected by satellite observation during the last decade. Considerable uncertainties remain in steric sea level, particularly over the southern oceans. Temperature changes within MLD make no effective contribution to steric sea level changes along the Antarctic Circumpolar Current. According to statistics using only reliable profiles of the temperature and salinity analyses, salinity variations are intrinsically important to steric sea level changes in high latitudes and in the Atlantic Ocean. Although data sparseness is severe even in the latest decade, linear trends of global mean thermosteric and halosteric sea level for 1955 to 2003 are estimated to be 0.31 ± 0.07 mm/yr and 0.04 ± 0.01 mm/yr, respectively. These estimates are comparable to those of the former studies.     

Evaluation of vertical crustal movements and sea level changes around Greenland from GPS and tide gauge observations

To better monitor the vertical crustal movements and sea level changes around Greenland, multiple data sources were used in this paper, including global positioning system(GPS), tide gauge, satellite gravimetry, satellite altimetry, glacial isostatic adjustment(GIA). First, the observations of more than 50 GPS stations from the international GNSS service(IGS) and Greenland network(GNET) in 2007–2018 were processed and the common mode error(CME) was eliminated with using the principal component analysis(PCA). The results show that all GPS stations show an uplift trend and the stations in southern Greenland have a higher vertical speed. Second, by deducting the influence of GIA, the impact of current Gr IS mass changes on GPS stations was analysed, and the GIA-corrected vertical velocity of the GPS is in good agreement with the vertical velocity obtained by gravity recovery and climate experiment(GRACE). Third, the absolute sea level change around Greenland at 4 gauge stations was obtained by combining relative sea level derived from tide gauge observations and crustal uplift rates derived from GPS observations, and was validated by sea level products of satellite altimetry. The results show that although the mass loss of Gr IS can cause considerable global sea level rise, eustatic movements along the coasts of Greenland are quite complex under different mechanisms of sea level changes.     

Steric height trends at Ocean Station Papa in the Northeast Pacific Ocean

Abstract

We estimate secular changes in steric sea level in the northeast Pacific Ocean using the 27‐year time series of monthly hydrographic observations for Station PAPA (50°N, 145°W). Linear trends based on the entire data record suggest that steric heights relative to 1000 db are increasing at a rate of 0.93 mm/yr and that 67% of this increase is due to thermosteric changes at depths below 100 m; the smaller halosteric contribution to the steric trend appears to be confined to the upper 100 m. A trend of 0(1 mm/yr) is consistent with estimates of sea level rise based on coastal tide gauge records. However, a critical examination of the results indicates that sea level changes of such small magnitude would be masked by the large (1–10 cm) interannual variability of open ocean steric height. This is verified by recalculation of trends using abridged versions of the data set. We conclude that our trend estimates are still open to question and that the present 27‐year time series is too short to permit accurate resolution of possible climate‐induced changes in global sea level.     

Inferring the global mean sea level from a global tide gauge network

An attempt is made to infer the global mean sea level(GMSL) from a global tide gauge network and frame the problem in terms of the limitations of the network. The network,owing to its limited number of gauges and poor geographical distribution complicated further by unknown vertical land movements,is ill suited for measuring the GMSL. Yet it remains the only available source for deciphering the sea level rise over the last 100 a. The poor sampling characteristics of the tide gauge network have necessitated the usage of statistical inference. A linear optimal estimator based on the Gauss-Markov theorem seems well suited for the job. This still leaves a great deal of freedom in choosing the estimator. GMSL is poorly correlated with tide gauge measurements because the small uniform rise and fall of sea level are masked by the far larger regional signals. On the other hand,a regional mean sea level(RMSL) is much better correlated with the corresponding regional tide gauge measurements. Since the GMSL is simply the sum of RMSLs,the problem is transformed to one of estimating the RMSLs from regional tide gauge measurements. Specifically for the annual heating and cooling cycle,we separate the global ocean into 10-latitude bands and compute for each 10-latitude band the estimator that predicts its RMSL from tide gauges within. In the future,the statistical correlations are to be computed using satellite altimetry. However,as a first attempt,we have used numerical model outputs instead to isolate the problem so as not to get distracted by altimetry or tide gauge errors. That is,model outputs for sea level at tide gauge locations of the GLOSS network are taken as tide gauge measurements,and the RMSLs are computed from the model outputs. The results show an estimation error of approximately 2 mm versus an error of 2.7 cm if we simply average the tide gauge measurements to estimate the GMSL,caused by the much larger regional seasonal cycle and mesoscale variation plaguing the individual tide gauges. The numerical model,Los Alamos POP model Run 11 lasting 3 1/4 a,is one of the best eddy-resolving models and does a good job simulating the annual heating and cooling cycle,but it has no global or regional trend. Thus it has basically succeeded in estimating the seasonal cycle of the GMSL. This is still going to be the case even if we use the altimetry data because the RMSLs are dominated by the seasonal cycle in relatively short periods. For estimating the GMSL trend,longer records and low-pass filtering to isolate the statistical relations that are of interest. Here we have managed to avoid the much larger regional seasonal cycle plaguing individual tide gauges to get a fairly accurate estimate of the much smaller seasonal cycle in the GMSL so as to enhance the prospect of an accurate estimate of GMSL trend in short periods. One should reasonably expect to be able to do the same for longer periods during which tide gauges are plagued by much larger regional interannual(e. g.,ENSO events) and decadal sea level variations. In the future,with the availability of the satellite altimeter data,we could use the same approach adopted here to estimate the seasonal variations of GMSL and RMSL accurately and remove these seasonal variations accordingly so as to get a more accurate statistical inference between the tide gauge data and the RMSLs(therefore the GMSL) at periods longer than 1 a,i. e.,the long-term trend.     

海平面上升情景下辽东湾海岸侵蚀及影响评估

基于海洋站潮位观测和中国沿海海平面变化影响调查等数据,分析了辽东湾沿岸海平面变化及海岸侵蚀状况,并定量评估了未来海平面上升情景下,辽东湾两岸典型沙质海岸侵蚀影响和沙滩养护投入。分析预测和评估结果表明:1980-2017年,辽东湾沿海海平面上升速率为3.0 mm/a,其中辽东湾东岸沿海海平面上升速率明显高于西岸。2009-2017年,辽宁营口白沙湾、绥中网户、绥中南山港和绥中团山气象观测场岸段后退和下蚀较为严重,部分岸段滩肩蚀退达2～3 m/a。预计2100年,辽东湾沿海海平面上升幅度在20～48 cm之间,由海平面上升引发的辽东湾海岸侵蚀土地损失为23.1 km2,土地经济损失为1410万元。为减缓海岸侵蚀,旅游沙滩和一般沙滩养护总投入分别为11亿元和46亿元,全岸段养护成本较高,应选取旅游沙滩等重点岸段进行养护。     

利用GPS拖体在万山区域测量平均海面技术研究

Wanshan area has been chosen to be the specified field to calibrate and validate(Cal/Val) the HY-2 altimeter and its follow-on satellites. In March 2018, an experiment has been conducted to determine the sea surface height(SSH) under the HY-2 A ground track(Pass No. 203). A GPS towing-body(GPS-TB) was designed to measure the SSH covering an area of about 6 km×28 km wide centered on the HY-2 A altimeter satellite ground track. Three GPS reference stations, one tide gauge and a GPS buoy were placed in the research area, in order to process and resolve the kinematic solution and check the precision of the GPS-TB respectively. All the GPS data were calculated by the GAMIT/GLOBK software and TRACK module. The sea surface was determined by the GPS-TB solution and the tide gauge placed on Zhiwan Island. Then the sea surface of this area was interpolated by Arc GIS10.2 with ordinary Kriging method. The results showed that the precision of the GPS-TB is about 1.10 cm compared with the tide gauge placed nearby, which has an equivalent precision with the GPS buoy. The interpolated sea surface has a bias of –1.5–4.0 cm with standard deviation of 0.2–2.4 cm compared with the checking line. The gradient of the measured sea surface is about 1.62 cm/km along the HY-2 orbit which shows a good agreement compared with the CLS11 mean sea surface(MSS). In the Cal/Val of satellites, the sea surface between the tide gauge/GPS buoy and the footprint of altimeter can be improved by this work.     

GNSS-Based Sea Level Monitoring

This paper attempts to assess the use of Global Navigation Satellite System (GNSS) as an accurate, reliable, and easy tool for sea level measurement. The GNSS technique was incorporated into a float based tide gauge system. A prototype of such an instrument was developed based on principles of conventional tide gauges, where high frequency noise is reduced mechanically. The ability of the GNSS based tide gauge (GTG) to monitor sea levels was tested in several experiments. The performance of the GTG was compared to that of a traditional tide gauge. The method of data analysis and data comparison between the GPS measurements and the tide gauge data is presented. The results show that the GTG is equal in performance to the traditional float operated tide gauge. It seems that the GTG is capable of delivering the same level of accuracy (1 cm), and its results are as reliable as its competitor, the traditional float tide gauge. The suggested instrument can be easily integrated into the array of permanent GNSS stations and assist in absolute measurements of sea level changes, caused by global warming and the greenhouse effect, for example.     

GNSS-Based Sea Level Monitoring

This paper attempts to assess the use of Global Navigation Satellite System (GNSS) as an accurate, reliable, and easy tool for sea level measurement. The GNSS technique was incorporated into a float based tide gauge system. A prototype of such an instrument was developed based on principles of conventional tide gauges, where high frequency noise is reduced mechanically. The ability of the GNSS based tide gauge (GTG) to monitor sea levels was tested in several experiments. The performance of the GTG was compared to that of a traditional tide gauge. The method of data analysis and data comparison between the GPS measurements and the tide gauge data is presented. The results show that the GTG is equal in performance to the traditional float operated tide gauge. It seems that the GTG is capable of delivering the same level of accuracy (1 cm), and its results are as reliable as its competitor, the traditional float tide gauge. The suggested instrument can be easily integrated into the array of permanent GNSS stations and assist in absolute measurements of sea level changes, caused by global warming and the greenhouse effect, for example.     

多系统GNSS浮标潮位提取精度研究

为实现多频多模GNSS浮标在远距离海洋潮汐测量中的应用,基于精密单点定位(precision pointing positioning,PPP)数据处理策略获取潮位信息,以压力验潮仪为参考,对GNSS浮标测量海面高进行经验模态分解(empirical mode decomposition,EMD),滤去高频波浪和噪声,获取潮位进行精度分析。结果表明:多系统可以提高PPP解算潮位精度。GPS/GLONASS双系统和GPS/GLONASS/Bei Dou三系统PPP提取潮位与验潮仪潮位差值的最大误差均小于18cm,RMSE小于6. 5cm。因此,多系统PPP解算GNSS浮标海面高可以实现远离海岸的潮位获取与监测,能够提高海上潮位测量的效率。     

Absolute Calibration of the Jason-1 Altimeter Using UK Tide Gauges

This article describes an “absolute” calibration of Jason-1 (J-1) altimeter sea surface height bias using a method developed for TOPEX/Poseidon (T/P) bias determination reported previously. The method makes use of U.K. tide gauges equipped with Global Positioning System (GPS) receivers to measure sea surface heights at the same time, and in the same geocentric reference frame, as Jason-1 altimetric heights recorded in the nearby ocean. The main time-dependent components of the observed altimeter-minus-gauge height-difference time series are due to the slightly different ocean tides at the gauge and in the ocean. The main harmonic coefficients of the tide differences are calculated from analysis of the copious TOPEX data set and then applied to the determination of T, P, and J-1 bias in turn. Datum connections between the tide gauge and altimetric sea surface heights are made by means of precise, local geoid differences from the EGG97 model. By these means, we have estimated Jason-1 altimeter bias determined from Geophysical Data Record (GDR) data for cycles 1–61 to be 12.9 cm, with an accuracy estimated to be approximately 3 cm on the basis of our earlier work. This J-1 bias value is in close agreement with those determined by other groups, which provides a further confirmation of the validity of our method and of its potential for application in other parts of the world where suitable tide gauge, GPS, and geoid information exist.     

Improving the Coastal Mean Dynamic Topography by Geodetic Combination of Tide Gauge and Satellite Altimetry

Abstract

The ocean mean dynamic topography (MDT) is the surface representation of the ocean circulation. The MDT may be determined by the ocean approach, which involves temporal averaging of numerical ocean circulation model information, or by the geodetic approach, wherein the MDT is derived using the ellipsoidal height of the mean sea surface (MSS), or mean sea level (MSL) minus the geoid as the geoid. The ellipsoidal height of the MSS might be estimated either by satellite or coastal tide gauges by connecting the tide gauge datum to the Earth-centred reference frame. In this article we present a novel approach to improve the coastal MDT, where the solution is based on both satellite altimetry and tide gauge data using new set of 302 tide gauges with ellipsoidal heights through the SONEL network. The approach was evaluated for the Northeast Atlantic coast where a dense network of GNSS-surveyed tide gauges is available. The typical misfit between tide gauge and satellite or oceanographic MDT was found to be around 9?cm. This misfit was found to be mainly due to small scale geoid errors. Similarly, we found, that a single tide gauge places only weak constraints on the coastal dynamic topography.     

Absolute Sea Level Surface Modeling for the Mediterranean from Satellite Altimeter and Tide Gauge Measurements

Changes in the height of the ocean can be described through the relative and absolute sea level changes depending on the geodetic reference the sea level records are related to. Satellite altimetry provides absolute sea level (ASL) measurements related to the global geodetic reference, whereas tide gauges provide relative sea level (RSL) measurements related to the adjacent land. This study aims at computing the ASL surfaces for different time epochs from combined satellite altimeter and tide gauge records. A method of sea level data fusion is proposed to enable modeling of the impact of present and future sea level changes on the coast. Sea surface modeling was investigated for ten different gridding methods commonly used for the interpolation of altimeter data over the open ocean and extrapolation over the coastal zones. The performance of gridding methods was assessed based on the comparison of the gridded altimeter data and corrected tide gauge measurements. Finally, the sea level surfaces related to the GRS80 global reference ellipsoid were computed for the Mediterranean Sea over the altimeter period. In addition, the current sea level trends were estimated from both sea level measurements.     

基于观测数据的2003?2014年北冰洋海平面时空特征

本文对比了3个不同机构提供的北冰洋月均高度计数据,发现英国极地观测与建模中心和丹麦科技大学空间中心两套数据比较一致且空间覆盖率高,适用于北冰洋海平面变化研究,而前者在数据分辨率、平滑性和与验潮站的符合程度方面均更优。对高度计和验潮站数据的分析表明,北冰洋海平面的气候态特征表现为加拿大海盆的高值和欧亚海盆的低值之间形成鲜明对比;海平面的变化以季节变化和北极涛动引起的低频变化为主,加拿大海盆的季节和年际振幅均较大,俄罗斯沿岸海平面季节变化显著。2003?2014年,北冰洋平均海平面呈上升趋势,其中加拿大海盆海平面上升最快,而俄罗斯沿岸海平面有微弱下降趋势。加拿大海盆和俄罗斯沿岸由于海冰变化显著,不同高度计产品以及高度计与验潮站数据之间差别较大,使用时需慎重。