Observational study of super typhoon Meranti (2016) using satellite, surface drifter, Argo float and reanalysis data

The present work describes the basic features of super typhoon Meranti(2016)by multiple data sources.We mainly focus on the upper ocean response to Meranti using multiplatform satellites,in situ surface drifter and Argo floats,and compare the results with the widely used idealized wind vortex model and reanalysis datasets.The pre-existing meso-scale eddy provided a favor underlying surface boundary condition and also modulated the upper ocean response to Meranti.Results show that the maximum sea surface cooling was 2.0℃after Meranti.The satellite surface wind failed to capture the core structure of Meranti as the idealized wind vortex model deduced.According to the observation of sea surface drifters,the near-inertial currents were significantly enhanced during the passage of Meranti.The temperature and salinity profiles from Argo floats revealed both the mixed-layer extension and subsurface upwelling induced by Meranti.The comparison results show that the sea surface temperature and surface wind in the reanalysis datasets differs from those in remote sensing system.Sea surface cooling is similar in both satellite and in situ observation,and sea surface salinity response has a lower correlation with the precipitation rate.

Two-step harmonic analysis for capturing seasonally-varying amplitudes and phase lags of the predominant tidal constituents

Recent studies have revealed that the predominant tidal constituents have seasonal variations at some locations.However,how to accurately obtain these variations remains a problem for the traditional harmonic analysis(HA)due to the tradeoff between length of time window and resolution of constituents.Therefore,a method named as"two-step HA"is developed in this study,which consists of both long-and short-time-window HA.Through a series of ideal experiments,practical application at two tidal gauges and comparison with the traditional HA,the feasibility and accuracy of the two-step HA are verified:The two-step HA performs better than the traditional HA in estimating monthly amplitudes and phases for the predominant constituents,whether they have seasonal variability or not.In addition to capturing variations of the predominant constituents at tidal gauges,the two-step HA would be useful in investigation of the coherence and incoherence of internal tides.

A nonlinear Schrodinger equation for gravity waves slowly modulated by linear shear flow

Assume that a fluid is inviscid, incompressible, and irrotational. A nonlinear Schr?dinger equation(NLSE) describing the evolution of gravity waves in finite water depth is derived using the multiple-scale analysis method. The gravity waves are influenced by a linear shear flow, which is composed of a uniform flow and a shear flow with constant vorticity. The modulational instability(MI) of the NLSE is analyzed, and the region of the MI for gravity waves(the necessary condition for existence of freak waves) is identified. In this work, the uniform background flows along or against wave propagation are referred to as down-flow and up-flow, respectively. Uniform up-flow enhances the MI, whereas uniform down-flow reduces it. Positive vorticity enhances the MI, while negative vorticity reduces it. Hence, the influence of positive(negative)vorticity on MI can be balanced out by that of uniform down(up) flow. Furthermore, the Peregrine breather solution of the NLSE is applied to freak waves. Uniform up-flow increases the steepness of the free surface elevation, while uniform down-flow decreases it. Positive vorticity increases the steepness of the free surface elevation, whereas negative vorticity decreases it.