The global ionospheric maps (GIMs) produced by JPL are used to investigate the longitudinal structure of the low latitude ionosphere. As a proxy of the ionization parameter at low latitudes, the ...latitudinally integrated total electron content (ITEC) is first extracted from low latitude GIMs and then Fourier filtered to obtain the wavenumber‐4 components. We then study in detail the diurnal, seasonal and solar cycle variations of the wave patterns. It is found that the wavenumber‐4 patterns are intense and well developed in boreal summer and early boreal autumn, but quite weak in boreal winter. This seasonal variation is consistent with that of the zonal wind of the non‐migrating tide mode DE3. We also found that the wavenumber‐4 patterns shift eastward with a shifting speed that is smaller in daytime than at night. This is attributed to the contribution of both the eastward propagation of DE3 in E‐region and the zonal E ×B ion drifts in F‐region. Our results support the suggestion that the longitudinal wavenumber‐4 structure of the low latitude ionosphere should be originated from the non‐migrating tide mode DE3.
Coastal flooding has become a major issue for low‐lying coastal cities in China, and a lot of research has focused on assessing flood risk from storms and associated extreme sea levels. High‐tide ...flooding (HTF), however, which leads to minor inundation and occurs more frequently as sea level continues to rise has not been assessed comprehensively. Here, we analyze HTF along the China coastline using tide gauge records. We show that the frequency of HTF has increased (Xiamen doubled HTF frequency every 11.5–37.4 years, with the median being 17.5 years). As a result, the cumulative loss ratios of HTF are higher than those of major (or extreme) flooding events in several locations. To gain insights into the processes driving HTF changes, we decompose still water levels that occurred during HTF into five components, including non‐linear trend, interannual to decadal mean sea level (MSL) variability, seasonal MSL cycle, tidal anomaly, and nontidal residuals. It is evident that due to sea‐level rise (SLR) fewer components need to combine to raise the water levels above HTF thresholds. We show that the South China Sea coast already experiences HTF purely driven by high spring tides, and will also see the fastest future increase in the number of tide‐only HTF events. In general, China will experience more HTF days under all warming scenarios as SLRs. This long‐term trend will be modulated by the nodal cycle of ocean tides leading to more rapid increases in HTF in the 2030s and 2050s.
Plain Language Summary
High‐tide flooding (HTF), which usually occurs during high tides in coastal areas, is one of the most apparent consequences of sea‐level rise (SLR). Analyses of HTF in the past and future have been carried out for the United States, Australia, and Europe, but no comprehensive analysis exists for the coast of China. Here, we analyze HTF along the China coast based on available sea level records. We find, for example, that the frequency of HTF has increased and doubled every 17.7 years. High tidal levels alone will lead to more HTF days due to SLR along the South China Sea coast. In the future, the China coast will experience more HTF days under different warming scenarios, with more rapid increases in the 2030s and 2050s due to long‐term cycles in ocean tides. The analysis presented here advances our understanding of HTF along the China coastline, providing a basis for future research and adaptation planning.
Key Points
The frequency of High‐tide flooding (HTF) along China coastline has increased, with a doubling time of 11.5–37.4 years in Xiamen
Cumulative loss ratios of HTF are higher than those of extreme flooding at several locations
China will experience more HTF days under all warming scenarios, with more rapid increases in the 2030s and 2050s
The atmospheric tide at ionospheric heights is composed of those locally generated and those propagated from below. The role of the latter in producing the variability of the daytime ionosphere is ...examined using the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model. The impact of upward‐propagating tides is evaluated by running simulations with and without tidal forcing at the lower boundary (approximately 96 km), which imitates the effect of tides from below. When migrating diurnal and semidiurnal tides at the lower boundary is switched on, the intensity of E region currents and the upward velocity of the equatorial F region vertical plasma drift rapidly increase. The low‐latitude ionospheric total electron content (TEC) first increases, then gradually decreases to below the initial level. The initial increase in the low‐latitude TEC is caused by an enhanced equatorial plasma fountain while the subsequent decrease is due to changes in the neutral composition, which are characterized by a global‐scale reduction in the mass mixing ratio of atomic oxygen O1. The results of further numerical experiments indicate that the mean meridional circulation induced by dissipating tides in the lower thermosphere is mainly responsible for the O1 reduction; it acts like an additional turbulent eddy and produces a “mixing effect” that enhances net downward transport and loss of O1. It is stressed that both electrodynamic effects and mixing effects of upward‐propagating tides can be important in producing the variability of ionospheric plasma density. Since the two mechanisms act in different ways on different time scales, the response of the actual ionosphere to highly variable upward‐propagating tides is expected to be complex.
Key Points
Upward‐propagating tides provide electrodynamic effects and mixing effects
Electrodynamic effects can enhance the ionospheric plasma density
Tidal mixing effects can reduce the ionospheric plasma density
Tidal strain time series recorded at the Vyhne Tidal Station are used to test thirteen different ocean tide loading models. Ten models have been calculated using SPOTL: EOT11a, HAMTIDE11a, ...OSU.TPXO72atlas, OSU.TPXO72, TPX070, DTU10, CSR4.0, FES2004, FES95.2.1, SCHW1 and three other models were chosen from the Free Ocean Tide Loading Provider (FOTLP) created by Scherneck and Bos: FES2012, FES2014b, GOT00.2. Hourly sampled strain data, corrected for temperature, were subjected to correction for ocean tide loading. The test of models was focused on the diurnal and semi-diurnal tidal harmonic constituents O1, P1, K1 and M2. A negligible difference between the individual global ocean tide loading models was only found mainly due to using different Earth models and Green functions. The amplitude factors for O1, P1, K1 and M2 derived from the measurements are 1.019, 1.226, 0.842 and 1.131, respectively. The average amplitude factors for these tidal components were obtained after ocean load correction using SPOTL routines: 1.121, 1.332, 0.916, 1.283, and in the case of the three models using FOTLP: 1.046, 1.486, 1.067, 1.317. The corrected amplitude factor became closer to the theoretical value only for K1.
Almost 9 years of sea surface height observations from the TOPEX/Poseidon (T/P) satellite altimetry mission are used to observe the geocentric pole tide deformations of the sea surface. If the oceans ...are assumed to have an equilibrium response, then satellite altimeters effectively observe the equipotential surface that is associated with the solid Earth and ocean pole tide deformations. The long‐wavelength component of the geocentric pole tide deformations at the Chandler wobble period is observed from T/P altimetry to be consistent with the theoretical self‐consistent equilibrium response of the ocean pole tide. The geocentric pole tide explains 70% of the variance in the degree 2 order 1 spherical harmonic component of the residual sea surface heights that are observed by T/P, after removing the seasonal, inverse barometer, and lunisolar tidal effects. If the long‐wavelength component of the ocean pole tide is assumed to have an equilibrium response at the Chandler wobble period, then satellite altimetry proves to be another geodetic technique that can be used to estimate the Love number k2 at that period.
Since the launch of Seasat, TIROS-N, and Nimbus-7 satellites equipped with ocean observation sensors in 1978, a new era of ocean remote sensing has opened. Today, remotely sensed data have been ...widely used in oceanographic studies. This reprint collects various advanced ocean remote sensing technologies and their applications, including the use of artificial intelligence techniques to explore ocean information and bibliometric analysis to assess researchers and trends in this scientific field. The observations of various sensors enrich the application of ocean environment monitoring and ocean dynamical analysis. If you are interested in understanding the application of ocean remote sensing, this monograph should be very helpful.
Land and sea vertical datums have traditionally been separate reference levels, due to the different methodologies and observations used in their derivations. However, the need for a seamless ...connection of these datums has become important due to a wide range of applications in the coastal zone. This study evaluates existing methods, oceanographic and geodetic models and observations for the development of an operational model of coastal heights called AUSHYDROID. We use a complex study area in north west Australia to gain a new insight into how accurately the separation between the lowest astronomical tide (LAT) and a reference ellipsoid can be estimated from existing models, which could form the basis of a national AUSHYDROID model for Australia. Our results are the first attempt to use this combination of existing data in this location, suggesting that ellipsoidal heights of LAT can be estimated to an accuracy of ∼0.2 m at the coast, with the combination of DTU18MSS and FES2014b having the lowest RMS of 0.125 m. However, in some complex coastline areas such as bays and estuaries, the differences increase to >0.5 m so that additional tide gauge observations with GNSS levelling connections and improved models are needed in these regions.
Enhanced nonmigrating tides SW1 (SWx represents semidiurnal westward mode with zonal wave number x) and SW3 during sudden stratospheric warming (SSW) were traditionally attributed to nonlinear ...interactions of quasi‐stationary planetary waves with the migrating tide SW2. Recent studies specified hypothetically that responsible for the interactions is the 16‐day wave, instead of the broadly accepted quasi‐stationary planetary waves. It is suspected that the 16‐day‐wave‐triggered secondary waves, at periods ∼12.4 and ∼11.6 hr, were detected at low‐frequency resolutions and misinterpreted as SW1 and SW3, respectively. While He et al. (2018, https://doi.org/10.1002/2018JD028400) associated the 11.6‐hr oscillation conclusively to the SW3‐like signature during SSW 2013 by diagnosing its wave number, the SW1‐like 12.4hr wave, however, has never been explicitly resolved, given its proximity to the period of an active lunar tide. Here, using the coherency in the mesospheric wind between two longitudinal sectors during SSW 2009, we identify a 12.4‐hr oscillation dominated by wave number 1 and therefore associate it to the SW1‐like signature.
Plain Language Summary
The winter upper atmosphere is populated by global‐scale oscillations among which the most radical one appears nearby the period of 12 hr. The existing studies suggest that the near‐12‐hr oscillation consists of at least six waves, namely, the migrating lunar and solar tides, two nonmigrating tides, and two secondary waves of the nonlinear interactions between the migrating solar tide and traveling planetary waves. The current work, together with a counterpart work (He et al., 2018), disentangles the near‐12‐hr oscillation into high‐frequency‐resolved isolated spectral peaks and investigates the longitudinal variation in the phase of the isolated peaks through jointly studying the mesospheric wind collected by longitudinally separated radars in early 2009 and early 2013. Results suggest that the nonmigrating tides did not enhance in both cases and demonstrate that the occurrence of secondary waves might have been misinterpreted as the nonmigrating tides in studies at low‐frequency resolutions.
Key Points
We report an upper atmospheric pseudo‐SW1, a 12.4‐hr oscillation characterized by westward traveling wave number 1 structure
The nonlinear interaction between the 16‐day wave and the semidiurnal migrating tide SW2 triggers the pseudo‐SW1
The broadly reported SW1‐like enhancements during sudden stratospheric warming events are conceptually not tides
Understanding drivers of tidal change is a key challenge in predicting coastal floods in the next century. Whilst interactions between tides and atmospheric surges have been studied, the effects of ...wind and pressure on tides on an annual scale over the Northwestern European shelf have not been investigated. Here, a modelling approach using the shallow water MARS model is carried out to understand and quantify meteorological effects on tidal characteristics. The model setup is validated against the GESLA 3 tide gauge database. Combined and relative influences of wind and pressure are investigated using four modelling scenarios: tide only; tide, wind, and pressure; tide and wind; and tide and pressure. Influences are investigated using a single year of tidal forcing, and across multiple years of meteorological data to examine the sensitivity to temporally changing meteorological conditions. It is found that meteorology influences tidal constituent amplitudes by +/−1 cm, yielding changes that may locally reach 15 cm in the predicted highest tide. Analysis of the shallow water equations show three non-linear interaction terms between tide, wind, and pressure (advective effects, quadratic parameterization of bottom friction, and shallow water effect). Part of the observed changes is shown to arise from meteorologically induced mean sea-level changes.
Radar interferograms provide a map of the phase difference between the reference and repeat acquisitions modulo <inline-formula> <tex-math notation="LaTeX">{2\pi } </tex-math></inline-formula>. Under ...ideal conditions, the phase can be unwrapped to provide an absolute phase connection across the map, although there is always an unknown integer phase ambiguity (i.e., <inline-formula> <tex-math notation="LaTeX">{N2\pi } </tex-math></inline-formula>) for the entire map. Here, we demonstrate a practical time series method to solve for these integer ambiguities in order to recover the absolute phase change between the first and last SAR images. An important first step is to correct the phase of each SAR image for the well-known solid earth tide, which typically produces a line of sight offset ±150 mm, as well as, trends along and across each image of ~20 mm. This tide correction significantly reduces the noise in the InSAR time series, especially at the L-band. These tidally corrected interferograms are then unwrapped and used to solve for a set of integer ambiguities that achieves phase closure when summing around loops in the stack. There is an infinite number of ambiguity combinations that achieve loop closure; thus, regularization is required. In contrast to previous studies that use a least-squares approach to find the ambiguities, we adopt an <inline-formula> <tex-math notation="LaTeX">{L}_{1} </tex-math></inline-formula>-norm approach to find the minimum number of ambiguity corrections needed to achieve loop closure. We note that the split-spectrum ionospheric correction can introduce <inline-formula> <tex-math notation="LaTeX">{N\pi } </tex-math></inline-formula> ambiguities and suggest two approaches for correcting both <inline-formula> <tex-math notation="LaTeX">{N2\pi } </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">{N\pi } </tex-math></inline-formula> ambiguities.
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