Variability of Sand Mobility Surrounding Cylinder Object Freely Resting on the Seabed under the Action of Typhoon

In sandy sediments, scour and fill is the key process contributed to mine burial. The scour processes surrounding the cylinder mines freely resting on the sandy seabed under the 12-hr combined action of tidal currents and wind-generated waves, especially over typhoon events are numerically simulated using the DRAMBUIE model. The East China Sea is a good case study due to the dominant impact of summer typhoon events on sediment transport and scour. The numerical results show that the scour depth generally increases with time under the combined current and wave stresses exerted on the seabed, while the depth of the scour pit depends on infill once the currents subside. There is a positive relationship between the scour depth and the bottom orbital velocity after experiencing 12-hr wave action including storm waves, while the relation is not linear. The experimental results also display an elevated trend for scour depth with the increase of orbital velocity. The numerical results reveal a surprising phenomenon: the mobility of sand altering with the increasing bed shear stress larger than the certain threshold, which is also manifested as the curves of scour depth with the different grain size might cross each other. For laboratory experiments, the variability of sand mobility does not occur, likely because typhoon storm waves cannot be reproduced in the flume. More numerical tests indicate that the intersection will be triggered by the division of critical Shields parameter. The preliminary analysis suggests that the phenomenon never documented is likely generated from the error of empirical formulae.

Characteristics of oceanic mesoscale variabilities associated with the inverse kinetic energy cascade

Oceanic geostrophic turbulence theory predicts significant inverse kinetic energy (KE) cascades at scales larger than the energy injection wavelength. However,the characteristics of the mesoscale variabilities associated with the inverse KE cascade in the real oceans have not been clear enough up to now. To further examine this problem,we analyzed the spectral characteristics of the oceanic mesoscale motions over the scales of inverse KE cascades based on high-resolution gridded altimeter data. The applicability of the quasigeostrophic (QG) turbulence theory and the surface quasigeostrophic (SQG) turbulence theory in real oceans is further explored. The results show that the sea surface height (SSH) spectral slope is linearly related to the eddy-kinetic-energy (EKE) level with a high correlation coefficient value of 0.67. The findings also suggest that the QG turbulence theory is an appropriate dynamic framework at the edge of high-EKE regions and that the SQG theory is more suitable in tropical regions and low-EKE regions at mid-high latitudes. New anisotropic characteristics of the inverse KE cascade are also provided. These results indicate that the along-track spectrum used by previous studies cannot reveal the dynamics of the mesoscale variabilities well.

The near-inertial waves observed east of the Philippines

Based on mooring observations from Aug.1,2016 to Dec.14,2017,the characteristics and underlying mechanisms of near-inertial waves(NIWs)observed east of the Philippines were studied.Three strong NIW events were investigated in detail.The NIWs in EventⅠwere induced by typhoon Lan and had the strongest magnitudes of 0.35 m/s.The maximum near-inertial kinetic energy(NIKE)was shown at the ocean surface.The NIW in EventⅡwas stimulated by a moderate cyclonic wind with the extreme NIKE located at about 110-m depth.The existence of a cyclonic eddy during EventsⅠandⅡled to a blue shift of near-inertial frequencies.For EventⅢ,the surface near-inertial signals were also induced by local weak wind,whereas the real generation mechanisms for the subsurface NIWs remain unclear.In particular,during EventⅢ,there was a nonlinear wave-wave interaction between NIWs and semidiurnal(D)tides,which further induced strong D±f waves.Overall,the NIWs in the three events exhibited distinct vertical structures.The NIWs in EventsⅠandⅡwere dominated by lower mode s with elevated NIKE well confined to the upper 250 m and 270 m,respectively.In contrast,the NIW EventⅢwas dominated by higher modes and the NIWs penetrated downward beyond 360 m.Such deep penetration of NIWs could be attributed to the weak wind stress curl and positive sea level anomalies associated with an anticyclonic eddy.In addition,the three NIW events had e-folding timescales of less than 7 days.