M2 Amplitude Research Articles

Is there a quasi 60-year oscillation in global tides?

We analyze long-term tide gauge records to examine whether there is a quasi 60-year oscillation in global tides. We find that there is a significant 55-year cycle at more than half of the tide gauges analyzed. For M2 tide, the amplitudes of the 55-year cycle range from 1.82 ± 0.80 mm (Los Angles) to 26.34 ± 6.75 mm (Cuxhaven), while for S2 tide, the amplitudes of the 55-year cycle range from 1.02 ± 0.48 mm (San Francisco) to 8.61 ± 2.25 mm (Cuxhaven). At Cuxhaven, shallow water constituents M4, MS4 and M6 tides which obtain energy from M2 and S2 tides also show significant 55-year cycles. It is found that the 55-year cycle significantly influences the estimation of the long-term trend of tidal amplitudes at Newlyn, Cuxhaven, Eastport and Victoria. The influence of the 55-year cycle on long-term trends of tidal amplitudes at Cuxhaven is the strongest among these stations. At Cuxhaven, the long-term trend of M2 (S2) amplitudes sharply increases from 0.68 ± 0.10 (0.22 ± 0.03) mm/yr to 0.93 ± 0.06 (0.31 ± 0.03) mm/yr when the 55-year cycle is removed. It is also found that the 55-year cycle in tides can partly explain the rapid change in M2 and S2 tides since 1980s in the North Atlantic. Quasi-60 year variations in sea levels and sea water temperature may be responsible for the 55-year cycle in tides while the detailed mechanisms still need further research.

Identification of M2 tidal velocity and energy in the eastern waters of Aceh based on numerical simulation

The influence of the M2 harmonic component in the waters of East Aceh is more significant than the other components. It is due to the fluctuation in M2 amplitude, which affects the tides. This study aims to compare the model currents velocity simulation results with data from the Geospatial Information Agency and TPXO7.2 and find the tidal energy extraction value of M2. The modeling method used is the non-hydrostatic method. The research location is in Aceh’s eastern waters by taking two research stations, namely Lhokseumawe waters and Langsa waters. The resulting tidal flow velocity in Lhokseumawe waters is 0.8344 m/s, while in Langsa waters, it is 0.8485 m/s. The tidal current strength in Lhokseumawe Waters is 1.2223 x 104 kW and Langsa Waters 1.2854 x 104 kW. Based on these results, Langsa waters have a tidal current speed higher than Lhokseumawe waters, which is 0.0241 m/s, with a tidal current strength of 6.31 x 102 kW. Due to differences in the depth layer or friction movement of the bottom of the waters and the characteristics of Lhokseumawe waters, which are wider than Langsa waters.

Large-scale changes of the semidiurnal tide along North Atlantic coasts from 1846 to 2018

Abstract. We investigated the long-term changes of the principal tidal component M2 along North Atlantic coasts, from 1846 to 2018. We analysed 18 tide gauges with time series starting no later than 1940. The longest is Brest with 165 years of observations. We carefully processed the data, particularly to remove the 18.6-year nodal modulation. We found that M2 variations are consistent at all the stations in the North-East Atlantic (Cuxhaven, Delfzijl, Hoek van Holland, Newlyn, Brest), whereas some discrepancies appear in the North-West Atlantic. The changes started long before the 20th century and are not linear. The secular trends in M2 amplitude vary from one station to another; most of them are positive, up to 2.5 mm/yr at Wilmington since 1910. Since 1990, the trends switch from positive to negative values in the North-East Atlantic. Concerning the possible causes of the observed changes, the similarity between the North Atlantic Oscillation and M2 variations in the North-East Atlantic suggests a possible influence of the large-scale atmospheric circulation on the tide. Our statistical analysis confirms large correlations at all the stations in the North-East Atlantic. We discuss a possible underlying mechanism. A different spatial distribution of mean sea level (corresponding to water depth) from one year to another, depending on the low-frequency sea-level pressure patterns, could impact the propagation of the tide in the North Atlantic basin. However, the hypothesis is at present unproven.

Global tide simulations with ICON-O: testing the model performance on highly irregular meshes

The global tide is simulated with the global ocean general circulation model ICON-O using a newly developed tidal module, which computes the full tidal potential. The simulated coastal M2 amplitudes, derived by a discrete Fourier transformation of the output sea level time series, are compared with the according values derived from satellite altimetry (TPXO-8 atlas). The experiments are repeated with four uniform and sixteen irregular triangular grids. The results show that the quality of the coastal tide simulation depends primarily on the coastal resolution and that the ocean interior can be resolved up to twenty times lower without causing considerable reductions in quality. The mesh transition zones between areas of different resolutions are formed by cell bisection and subsequent local spring optimisation tolerating a triangular cell’s maximum angle up to 84°. Numerical problems with these high-grade non-equiangular cells were not encountered. The results emphasise the numerical feasibility and potential efficiency of highly irregular computational meshes used by ICON-O.

Response of sea level to tide, atmospheric pressure, wind forcing and river discharge in the Kitimat Fjord System

The response of sea level to tide, atmospheric pressure, wind forcing, and river discharge in the Kitimat Fjord System (KFS) is investigated by using observations from tide gauges and results from a high-resolution ocean circulation model. Tidal harmonic analysis of the observed hourly sea level at the head and mouth of Douglas Channel shows that the largest semidiurnal tide (M2) alone accounts for 79% of the total sea level variance, and the top four major tidal constituents (M2, S2, K1, and O1) account for 94% of the total variance. In addition, the amplitude of M2 increases slightly from 1.586 to 1.647 m between the mouth and the head. For the observed subtidal sea level, the local inverse barometer response due to the variation of atmospheric pressure accounts for 59% of the variance. The variation of subtidal sea level difference between the mouth and the head of Douglas Channel is observable and can be largely attributed to the along-channel wind (54%) with an additional 13% of the variation due to the daily-averaged and seasonal-cumulative river discharges.A fjord-shelf three-dimensional ocean circulation model for KFS is developed to simulate the sea level variability. The model performance in simulating the sea level is assessed by comparing model results with observed harmonic constituents of ~70 tidal stations inside the model domain and time series recorded at the two tide gauges of KFS. The modelled tides are in good agreement with observations with a domain-averaged root-mean-square error estimate of ~0.04 m for M2. For the subtidal sea level, the model is able to capture the observed synoptic variations. Further analysis suggests that the underestimates of subtidal sea level in the ocean circulation model are mainly due to the weaker-than-observed wind forcing taken from the atmospheric model especially during the peak of strong down-fjord katabatic wind events.

Increased occurrence of red-tides of fish-killing dinoflagellate Karenia mikimotoi and related environmental conditions in Imari Bay, Japan

Blooms of the dinoflagellate Karenia mikimotoi cause mass mortality of aquaculture fishes and have become a worldwide environmental problem in coastal waters, especially in Asia. One area is Imari Bay, Japan, where blooms have recurrently formed in recent years. This study examines the meteorological and oceanographical conditions causing the recent increase in occurrence. The recent frequency of occurrence (2012–2018) is nearly four-fold previous years (1999–2011). In recent years, total nitrogen increased in the eastern area where the bloom initially develops, and The Tsushima warm current (TWC) have intensified during spring. Spring total nitrogen has increased as a result of climate change involving increasing rainfall and weakening monsoon rather than eutrophication because of the asynchrony between increasing and total nitrogen in rivers. Intensity of TWC changed in synchrony with M2 amplitude with lunar nodal cycle. Potential methodology for the medium term prediction of blooms is suggested in Imari Bay using environmental parameters.

A Regional Operational Model for the North East Atlantic: Model Configuration and Validation

An operational model for an area of the northeast Atlantic that encompasses all of Ireland’s territorial waters has been developed. The model is an implementation of the Regional Ocean Modelling System (ROMS) and uses operationally available atmospheric and boundary forcing, and a global tide solution for tidal forcing. River forcing is provided by climatological daily discharge rates for 29 rivers across Ireland, west Britain, and west France. It is run in an operational framework to produce 7-day hindcasts once a week, and daily 3-day forecasts which are published in a number of formats. We evaluated the model skill by comparing with measured data and calculating statistics such as mean error, root mean square error (RMSE), and correlation coefficient. The observations consist of satellite Sea Surface Temperature (SST), total surface velocity fields from satellite, water level time series from around the Irish coast, and temperature and salinity data from Array for Real-Time Geostrophic Oceanography (ARGO) and Conductivity Temperature Depth (CTD) profiles. The validation period is from 1 January 2016 until 31 December 2019. The correlation coefficient between the model and satellite SST is 0.97 and recorded in March and April 2018. The model error is about 5% of the total M2 amplitude in the Celtic Sea recorded at Dunmore East tide gauge station. The maximum RMSE between the model and the CTD temperature profiles is 0.8 °C while it is 0.17 PSU for salinity. The model correctly defines the shelf water masses around Ireland. In 2019 the Irish Coastal Current (ICC) was very strong and well defined along most of the western Irish coast. The model results have well reproduced the ICC front for the whole simulation period.

Impacts of Potential Sea-Level Rise on Tidal Dynamics in Khor Abdullah and Khor Al-Zubair, Northwest of Arabian Gulf

According to recent studies, sea-level rise (SLR) not only increases the coastal flooding probability but also may change the characteristics of tidal systems in coastal waterbodies. To investigate such changes in Khor Abdulla and Khor Al-Zubair, a two-dimensional depth-averaged model was applied. The calibration and validation were performed through the comparison of model results with measurements of water level and flow velocities in particular sites within model domain. SLR scenarios of 0.25, 0.5, 0.75, and 1 m were simulated. The simulation results indicate that the response of the principal tidal constituents was spatially nonuniform and exhibited a nonlinearity with SLR scenarios. However, the M2 amplitude decreases in most locations with SLR, with the most reduction occur in the upper reaches of the study area, while S2 and N2 amplitudes display a slight increase in a most sites along KA and KZ. Similarly, the amplitudes of K1 and O1 experience to a notable decrease in most locations when sea level rises. Additionally, there is a reduction in constituents’ phases with SLR scenarios. The decrement in constituents’ phases indicates that these waves will arrive earlier when sea level rises due to the nature of tidal wave propagation and their dependency on the water depth. Furthermore, the magnitude of the tidal currents also displays a spatially irregular response to SLR with a noticeable shift in the time of peak flood and ebb current velocities in many locations, while the direction of the currents remains unchanged at most sites of the study area.

Numerical modelling of the Caspian Sea tides

Abstract. The Caspian Sea is the largest enclosed basin on Earth and a unique subject for the analysis of tidal dynamics. Tides in the basin are produced directly by the tide-generating forces. Using the Princeton Ocean Model (POM), we examine details of the spatial and temporal features of the tidal dynamics in the Caspian Sea. We present tidal charts of the amplitudes and phase lags of the major tidal constituents, together with maps of the form factor, tidal range, and tidal current speed. Semi-diurnal tides in the Caspian Sea are determined by a Taylor amphidromic system with anticlockwise rotation. The largest M2 amplitude is 6 cm and is located in Türkmen Aylagy (called Turkmen Bay hereafter). For the diurnal constituents, the Absheron Peninsula separates two individual amphidromes with anticlockwise rotation in the north and in the south. The maximum K1 amplitudes (up to 0.7–0.8 cm) are located in (1) the south-eastern part of the basin, (2) Türkmenbaşy Gulf, (3) Mangyshlak Bay; and (4) Kizlyar Bay. As a result, the semi-diurnal tides prevail over diurnal tides in the Caspian Sea. The maximum tidal range, of up to 21 cm, has been found in Turkmen Bay. The strongest tidal currents have been located in the straits to the north and south of Ogurja Ada, where speeds reach 22 and 19 cm s−1, respectively. Numerical simulations of the tides using different mean sea levels (within a range of 5 m) indicate that spatial features of the Caspian Sea tides are strongly sensitive to changes in mean sea level.

The significance of coastal bathymetry representation for modelling the tidal response to mean sea level rise in the German Bight

Abstract. Due to climate change an accelerated mean sea level rise is expected. One key question for the development of adaptation measures is how mean sea level rise affects tidal dynamics in shelf seas such as the North Sea. Owing to its low-lying coastal areas, the German Bight (located in the southeast of the North Sea) will be especially affected. Numerical hydrodynamic models help to understand how mean sea level rise changes tidal dynamics. Models cannot adequately represent all processes in overall detail. One limiting factor is the resolution of the model grid. In this study we investigate which role the representation of the coastal bathymetry plays when analysing the response of tidal dynamics to mean sea level rise. Using a shelf model including the whole North Sea and a high-resolution hydrodynamic model of the German Bight we investigate the changes in M2 amplitude due to a mean sea level rise of 0.8 and 10 m. The shelf model and the German Bight Model react in different ways. In the simulations with a mean sea level rise of 0.8 m the M2 amplitude in the shelf model generally increases in the region of the German Bight. In contrast, the M2 amplitude in the German Bight Model increases only in some coastal areas and decreases in the northern part of the German Bight. In the simulations with a mean sea level rise of 10 m the M2 amplitude increases in both models with largely similar spatial patterns. In two case studies we adjust the German Bight Model in order to more closely resemble the shelf model. We find that a different resolution of the bathymetry results in different energy dissipation changes in response to mean sea level rise. Our results show that the resolution of the bathymetry especially in flat intertidal areas plays a crucial role for modelling the impact of mean sea level rise.

Seasonal variation of the principal tidal constituents in the Bohai Sea

Abstract. The seasonal variation of tides plays a significant role in water level changes in coastal regions. In this study, seasonal variations of four principal tidal constituents, M2, S2, K1, and O1, in the Bohai Sea, China, were studied by applying an enhanced harmonic analysis method to two time series: 1-year sea level observations at a mooring station (named E2) located in the western Bohai Sea and 17-year sea level observations at Dalian. At E2, the M2 amplitude and phase lag have annual frequencies, with large values in summer and small values in winter, while the frequencies of S2 and K1 amplitudes are also nearly annual. In contrast, the O1 amplitude increases constantly from winter to autumn. The maxima of phase lags appear twice in 1 year for S2, K1, and O1, taking place near winter and summer. The seasonal variation trends estimated by the enhanced harmonic analysis at Dalian are different from those at E2, except for the M2 phase lag. The M2 and S2 amplitudes show semi-annual and annual cycles, respectively, which are relatively significant at Dalian. The results of numerical experiments indicate that the seasonality of vertical eddy viscosity induces seasonal variations of the principal tidal constituents at E2. However, the tested mechanisms, including seasonally varying stratification, vertical eddy viscosity, and mean sea level, do not adequately explain the observed seasonal variations of tidal constituents at Dalian.

Numerical study of M2 tide on the Gulf of Thailand

According to the previous study, M2 from SCS amplified around the coastal region but almost low in the Gulf of Thailand (hereafter GoT). To detect the M2 at the shallow part of GoT, we used the 2D numerical model simulation. The model was driven by M2 amplitude and phase from a high-resolution spatial data provided by the Geospatial Indonesia Agency. M2 model was well verified with the recent studies and assimilation data. We showed the amphridomic point around southeastern of GoT around 103.5 E and 8.5-8.6 N. The amphridomic point has clockwise direction. We also showed a virtual amphridomic point at the east of GoT (100 E and 11.5-12 N). The M2 amplitude was highest in the Bangkok Bay. The study showed an important of the high-resolution model simulation on the GoT tides.

Nineteenth‐Century Tides in the Gulf of Maine and Implications for Secular Trends

AbstractSince the early twentieth century, the amplitudes of tidal constituents in the Gulf of Maine and Bay of Fundy display clear secular trends that are among the largest anywhere observed for a regional body of water. The M2 amplitude at Eastport, Maine, increased at a rate of 14.1 ± 1.2 cm per century until it temporarily dropped during 1980–1990, apparently in response to changes in the wider North Atlantic. Annual tidal analyses indicate M2 reached an all‐time high amplitude last year (2018). Here we report new estimates of tides derived from nineteenth century water‐level measurements found in the U.S. National Archives. Results from Eastport, Portland, and Pulpit Harbor (tied to Bar Harbor) do not follow the twentieth century trends and indicate that the Gulf of Maine tide changes commenced sometime in the late nineteenth or early twentieth centuries, coincident with a transition to modern rates of sea‐level rise as observed at Boston and Portland. General agreement is that sea level rise alone is insufficient to cause the twentieth‐century tide changes. A role for ocean stratification is suggested by the long‐term warming of Gulf of Maine waters; archival water temperatures at Boston, Portland, and Eastport show increases of ∼2 °C since the 1880s. In addition, a changing seasonal dependence in M2 amplitudes is reflected in a changing seasonal dependence in water temperatures. The observations suggest that models seeking to reproduce Gulf of Maine tides must consider both sea level rise and long‐term changes in stratification.

Lunar Tide in the F Region Ionosphere

AbstractIn this paper the magnetic latitude (±50°) and longitude, local solar time (LST), and month‐to‐month variability of the M2 lunar semidiurnal tide in the ionosphere is revealed through analysis of topside F region electron density measurements by the Planar Langmuir Probe instrument on the CHAMP satellite during the two 5‐year periods 2001–2005 and 2005–2009. The local time precession rate of CHAMP is such that 5 years is required to obtain full LST coverage for every month of the year at each latitude of interest. Thus, each 5‐year period is compressed into one equivalent year. M2 amplitudes, expressed in terms of percent residuals from a background value, range between about 3% and 20% and vary considerably with LST and magnetic longitude. Longitude variations in principle arise from zonal asymmetries in lunar tidal forcing due to Earth and ocean tides, as well as magnitude of Earth's magnetic field. The O1 diurnal tide is also discussed in the context of CHAMP and other measurements of the F region ionosphere.

Summer Seasonal Sea-level Rise and Associated Potential Storm Surge Vulnerability of the West Coast of Korea

Kim, H.-J. and Suh, S.-W., 2019. Summer seasonal sea-level rise and associated potential storm surge vulnerability of the West Coast of Korea. In: Lee, J.L.; Yoon, J.-S.; Cho, W.C.; Muin, M., and Lee, J. (eds.), The 3rd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 91, pp. 21-25. Coconut Creek (Florida), ISSN 0749-0208.Typhoon-induced storm surge usually occurs during the summer season. Thus, it is important to account for seasonal sea-level (SL) rise in coastal hazard mitigation. In this study, we analyzed one hour intervals of tidal records for Incheon (INC), Kunsan (KUN), and Mokpo (MOK) stations, located on the West Coast of Korea (WCK), as well as atmospheric pressure, to determine the correlation of SL fluctuations as they relate to the inverse barometer effect. Results exhibited annual (summer) sea-level rise (SLR) of 0.35 (0.32), 0.39 (0.39), and 0.55 (0.53) cm/year at the three stations, respectively. However, the mean SL (MSL) during the summer season was approximately 20 cm higher than the annual MSL. It was found that seasonal atmospheric pressure highly affected SL, with 1.58–1.73 cm per hPa owing to the inverse barometer effect; however, there were phase lags in the summer of one month or more. Although the nodal factor accounted for ± 3.7% of the M2 tidal amplitude over 18.6-year return period, the resulting M2 amplitude was maximized in 2090 at 2.87 m, 2.14 m, and 1.62 m at the three reference stations, owing to the macro-tidal environment of the WCK than in 2100. Thus, the target year associated with global warming and SLR, which is normally 2100, should be shifted one decade prior to ensure effective coastal storm surge vulnerability hazards analyses.

Mangrove density impacts on tidal dynamic in Segara Anakan Lagoon, Indonesia

The present study aims to understand the tidal dynamics in a tropical coastal lagoon, Segara Anakan, Java, Indonesia. It is a shallow lagoon, with complex tidal channels and connected to the Indian Ocean by two channel outlets. It covers an overall area of about 12.000 ha. Three–quarters of the lagoons area have mangroves and the remaining quarter is covered by water. The Delft3D model has been implemented and validated using observation data. Scenario with uniform and spatially bottom drag coefficient were created in order to investigate the influence of the mangrove density on tidal dynamics. Based on tidal harmonic analysis, the M2 amplitude attenuates from both the western and the eastern lagoon inlets to the interior of the lagoon as the tidal wave is constricted by the narrow lagoon inlet and shallowness of the lagoon. For the case of uniform mangrove density, tidal harmonic analysis reveals M2 amplitude decreasing of 0.5 to 0.25 m from lagoon inlets toward central lagoon due to bottom friction effect. The tidal propagation into lagoon show increasing M2 tidal phase with maximum delay of 2.5 hours at the central lagoon. For the case of spatially mangrove density, the dampening of the tidal wave is stronger of 18% and the phase delay is longer 0.8 hours compare to the case of uniform mangrove density.

Temporal changes in the response of the nodal modulation of the M2 tide in the Gulf of Maine

Accurate estimation of the changes of tides is important for sea level prediction and fundamental for local coastal planning. Here we use a novel method, enhanced harmonic analysis (EHA), to extract temporal changes in the response of the nodal modulation of the M2 tide in the Gulf of Maine. The results show that the temporal variations of nodal amplitudes at four stations in the Gulf of Maine are similar from 1970s to 2013. They all increase in the 1970s to peak values in the 1985s and in the 2000s they decrease to minimum values. The North Atlantic Oscillation (NAO), which is one of the major climate modes in the North Atlantic Ocean, may be responsible for temporal changes in the response of the nodal modulation of the M2 tide from 1970s to 2013. Also, the results obtained by the new method indicate that the traditional method of harmonic analysis slightly underestimates the trend of M2 amplitude in the Gulf of Maine.

A computer simulation study on the influences of loss and damage of primary muscle spindle afferents on soleus muscle stretch reflex.

Sensory loss is detrimental to sensorimotor control. Several studies have reported that loss and/or damage of primary (Ia) muscle spindle afferents significantly influence the stretch reflex responses of leg and foot muscles. However, a systematic experimental evaluation on how the impairment of Ia muscle spindle afferents affects the stretch reflex is difficult due to technical and ethical issues. In the present study, the aim was to use computer simulations of a multiscale neuromusculoskeletal model to investigate how changes in: i) the number of Ia afferents, ii) the synaptic conductance between Ia sensory fibers and spinal motor neurons (MNs), and iii) the conduction velocities (CVs) of Ia afferents, would influence the stretch reflex of a leg muscle (soleus). Simulation results showed that both anatomical and functional loss of Ia afferents exerted an influence on the amplitude of short-latency stretch reflex response (M1) and the late phase of medium-latency response (M2). Additionally, changes in CVs of Ia afferents mainly influenced the latency of M1 and the amplitude of M2. Our findings provide conceptual evidence that a combination of anatomical and functional loss, as well as changes in CVs of Ia afferents due to demyelination, can explain the stretch reflex responses observed in peripheral neuropathies.

A modelling study of ice effect on tidal damping in the Bohai Sea

Ocean tides have been reported to be significantly damped by sea ice due to the fact that the bottom of sea ice is rough and able to dissipate tidal energy. To examine this damping effect of sea ice on the astronomical tides in the Bohai Sea, where sea ice is common in winter, this study has developed a three-dimensional ice-ocean coupled model with and without sea ice to investigate potential changes in astronomical tides based on the Finite Volume Community Ocean Model (FVCOM). This newly developed model was first evaluated against field observations and remote sensing images, including water elevations and sea ice cover, and showed good agreement between the modelling results and the observed data. In using the air forcing data (2009–2010) and the default sea ice drag coefficient of 0.0075, the model results show that in the presence of sea ice, the major tidal constituent M2 is reduced by 1 cm at the top of the three bays (from north to south: Liaodong Bay, Bohai Bay and Laizhou Bay) in the Bohai Sea, but the tidal phase is almost not affected. A sensitivity run shows that the amplitude of M2 is decreased by 2 cm when increasing the drag coefficient from 0.005 to 0.01. Moreover, two extreme combinations of air temperatures and winds are simulated according to the standard deviation of wind speed and temperature over the past ten years, and the damping effects of sea ice on tides are almost negligible as well. Based on the water mass analysis, it is found that both the decreases of average volumetric flow rate for inflow and outflow due to ice through the three bays are relatively small compared to the average volumetric flow rate without ice. The convergence of the cross sectional area increases the tidal amplitude of M2, which compensates the damping effect caused by sea ice.

Evaluation of sea level rise and associated responses in Hangzhou Bay from 1978 to 2017

The mean sea level, extreme sea level, and astronomical tide in Hangzhou Bay were analyzed using the tide gage data from 1978 to 2017 in Tanxu station, and the effects of rising sea levels on floods were estimated. The mean sea level in Hangzhou Bay showed a significant rising trend of 4.6 mm per year in 1978–2017. This rate was much higher than the mean sea level of the China seas, and the extreme sea level in Hangzhou Bay increased at the rate of 0.011 m per year. During 1978–2017, the mean tide range increased at the rate of 1.30 cm per year. The amplitude of M2 significantly increased at the rate of 0.57 cm per year, whereas the phase lag decreased at the rate of −0.21° per year. The amplitude and phase lag of the K1 tidal component slightly decreased at the rate of −0.03 cm per year and −0.07° per year, respectively. The changes in extreme sea level in Hangzhou Bay were mainly caused by variations in mean sea level. The coastal areas of Hangzhou Bay at risk from flooding due to the 100-year sea level return will increase by about 400 km2 by 2050.