Xiangbo Feng

Sea level extremes at the coasts of China

Hourly sea level records from 1954 to 2012 at 20 tide gauges at and adjacent to the Chinese coasts are used to analyze extremes in sea level and in tidal residual. Tides and tropical cyclones determine the spatial distribution of sea level maxima. Tidal residual maxima are predominantly determined by tropical cyclones. The 50 year return level is found to be sensitive to the number of extreme events used in the estimation. This is caused by the small number of tropical cyclone events happening each year which lead to other local storm events included thus significantly affecting the estimates. Significant increase in sea level extremes is found with trends in the range between 2.0 and 14.1 mm yr−1. The trends are primarily driven by changes in median sea level but also linked with increases in tidal amplitudes at three stations. Tropical cyclones cause significant interannual variations in the extremes. The interannual variability in the sea level extremes is also influenced by the changes in median sea level at the north and by the 18.6 year nodal cycle at the South China Sea. Neither of PDO and ENSO is found to be an indicator of changes in the size of extremes, but ENSO appears to regulate the number of tropical cyclones that reach the Chinese coasts. Global mean atmospheric temperature appears to be a good descriptor of the interannual variability of tidal residual extremes induced by tropical cyclones but the trend in global temperature is inconsistent with the lack of trend in the residuals.

Nodal variations and long‐term changes in the main tides on the coasts of China

The long-term changes in the main tidal constituents (O1, K1, M2, N2, and S2) along the coasts of China and in adjacent seas are investigated based on 17 tide-gauge records covering the period 1954–2012. The observed 18.61 year nodal modulations of the diurnal constituents O1 and K1 are in agreement with the equilibrium tidal theory, except in the South China Sea. The observed modulations of the M2 and N2 amplitudes are smaller than theoretically predicted at the northern stations and larger at the southern stations. The discrepancies between the theoretically predicted nodal variations and the observations are discussed. The 8.85 year perigean cycle is identifiable in the N2 parameters at most stations, except those in the South China Sea. The radiational component of S2 contributes on average 16% of the observed S2 except in the Gulf of Tonkin, on the south coast, where it accounts for up to 65%. We confirmed the existence of nodal modulation in S2, which is stronger on the north coast. The semidiurnal tidal parameters show significant secular trends in the Bohai and Yellow Seas, on the north coast, and in the Taiwan Strait. The largest increase is found for M2 for which the amplitude increases by 4–7 mm/yr in the Yellow Sea. The potential causes for the linear trends in tidal constants are discussed.

Spatial and temporal variations of the seasonal sea level cycle in the northwest Pacific

The seasonal sea level variations observed from tide gauges over 1900–2013 and gridded satellite altimeter product AVISO over 1993–2013 in the northwest Pacific have been explored. The seasonal cycle is able to explain 60–90% of monthly sea level variance in the marginal seas, while it explains less than 20% of variance in the eddy-rich regions. The maximum annual and semiannual sea level cycles (30 and 6 cm) are observed in the north of the East China Sea and the west of the South China Sea, respectively. AVISO was found to underestimate the annual amplitude by 25% compared to tide gauge estimates along the coasts of China and Russia. The forcing for the seasonal sea level cycle was identified. The atmospheric pressure and the steric height produce 8–12 cm of the annual cycle in the middle continental shelf and in the Kuroshio Current regions separately. The removal of the two attributors from total sea level permits to identify the sea level residuals that still show significant seasonality in the marginal seas. Both nearby wind stress and surface currents can explain well the long-term variability of the seasonal sea level cycle in the marginal seas and the tropics because of their influence on the sea level residuals. Interestingly, the surface currents are a better descriptor in the areas where the ocean currents are known to be strong. Here, they explain 50–90% of interannual variability due to the strong links between the steric height and the large-scale ocean currents.

Application of Two-Dimensional Wavelet Transform in Near-Shore X-Band Radar Images

Among existing remote sensing applications, land-based X-band radar is an effective technique to monitor the wave fields, and spatial wave information could be obtained from the radar images. Two-dimensional Fourier Transform (2-D FT) is the common algorithm to derive the spectra of radar images. However, the wave field in the nearshore area is highly non-homogeneous due to wave refraction, shoaling, and other coastal mechanisms. When applied in nearshore radar images, 2-D FT would lead to ambiguity of wave characteristics in wave number domain. In this article, we introduce two-dimensional Wavelet Transform (2-D WT) to capture the non-homogeneity of wave fields from nearshore radar images. The results show that wave number spectra by 2-D WT at six parallel space locations in the given image clearly present the shoaling of nearshore waves. Wave number of the peak wave energy is increasing along the inshore direction, and dominant direction of the spectra changes from South South West (SSW) to West South West (WSW). To verify the results of 2-D WT, wave shoaling in radar images is calculated based on dispersion relation. The theoretical calculation results agree with the results of 2-D WT on the whole. The encouraging performance of 2-D WT indicates its strong capability of revealing the non-homogeneity of wave fields in nearshore X-band radar images.

The EU-FP7 ERA-CLIM2 Project Contribution to Advancing Science and Production of Earth System Climate Reanalyses

ERA-CLIM2 is a European Union Seventh Framework Project started in January 2014. It aims to produce coupled reanalyses, which are physically consistent data sets describing the evolution of the global atmosphere, ocean, land-surface, cryosphere and the carbon cycle. ERA-CLIM2 has contributed to advancing the capacity for producing state-of-the-art climate reanalyses that extend back to the early 20th century. It has led to the generation of the first ensemble of coupled ocean, sea-ice, land and atmosphere reanalyses of the 20th century. The project has funded work to rescue and prepare observations, and to advance the data51 assimilation systems required to generate operational reanalyses, such as the ones planned by the European Union Copernicus Climate Change Service. This paper summarizes the main goals of the project, discusses some of its main areas of activities, and presents some of its key results.

Observing sea levels in the China seas from satellite altimetry

Due to the threat of global warming, extensive studies of the natural and anthropogenic causes of sea level change have been performed. The use of satellite altimetry contributes enormously to such studies, especially where in situ observations are rare. This chapter highlights the authors’ recent investigations of sea level measurements in the China seas made by satellite altimetry. Different sea level components are investigated. Progress is being made towards a better estimation of the ocean tides in the China seas using a comprehensive combination of satellite altimetry products. The seasonal sea level cycle, another crucial component of sea level in the China seas, is also systematically studied by using different analysis approaches. We finally explore the long-term trends and variability of mean sea level by analyzing the latest (1993–2016) satellite altimetry. The relationships between mean sea level and large-scale ocean circulation and climate variability are also examined.

Improved SST‐Precipitation Intraseasonal Relationships in the ECMWF Coupled Climate Reanalysis

The European Centre for Medium‐range Weather Forecasts (ECMWF) has produced the ocean‐atmosphere coupled reanalysis for the twentieth century, CERA‐20C, following on from the similar but atmosphere‐only reanalysis ERA‐20C. Here we demonstrate the capability of CERA‐20C in producing more physically consistent ocean and atmosphere boundary conditions, by focusing on sea surface temperature (SST)‐precipitation intraseasonal relationships. CERA‐20C reproduces well the observed SST‐precipitation correlations, while these relationships are poorly represented in ERA‐20C, with the greatest discrepancies in the early 1900s. The improved relationships in CERA‐20C are due to intraseasonal improvements in SST that are not present in the external HadISST2 product. In CERA‐20C, SST‐precipitation relationships are slightly weaker in the 1900s than in the 2000s, mainly due to differences in the assimilated observation density. We also find that the coupled model initialized from CERA‐20C in the 2000s realistically simulates these relationships, while relaxing SST toward HadISST2 tends to damp these relationships. CERA‐20C has improved mean and variance in precipitation over ERA‐20C, but these are mostly due to improvements in the atmospheric model and not due to coupled feedbacks.

Study on reverse calculation for unidirectional waves from shallow water

ABSTRACT Feng, X., Shaw, A.G.P. and Zhang, W., 2013. Study on reverse calculation for unidirectional waves from shallow water In the coastal region, some wave measurements are usually deployed in shallow water. However, the knowledge of deep-water waves is more required in some cases. Targeting the east coast of Taiwan with a steep slope, we experimentally detect the capability of the linear spectrum theory in reversely estimating unidirectional deep-water waves from the shallow-water waves. Tank experiments of irregular waves show that the nonlinearity of wave-wave interactions significantly contributes to the total energy transfer. The energy of incident waves around peak frequency is transferred to lower and higher frequency domains. Even though waves are travelling through a short distance under a steep slope, the nonlinear wave-wave interactions cannot be neglected. In the application of the linear spectrum theory, the reversely calculated deep-water spectrum is found to be overestimated in frequency over 2Hz and in total energy of spectrum, but underestimated around peak frequency. There is evidence to show that the ratio of H1/3/Hs is correlated with the spectral bandwidth and Kurtosis.