LAUSR

Tilt of the mean dynamic topography (MDT) is to first-order balanced by the Coriolis force in the deep ocean. The geostrophy, however, breaks down at the coast due to the no normal flow condition, which leads to fundamentally different dynamics governing the alongshore MDT. There have been a growing number of studies in estimating the tilt of coastal MDT and their governing dynamics in recent years (e.g., Higginson et al., 2015; Huang, 2017; Kuroishi, 2013; Lin et al., 2015; Woodworth et al., 2012). Two approaches are often used in these studies to estimate the coastal MDT. The geodetic approach incorporates tidal gauge and Global Positioning System (GPS) measurements to obtain the mean sea surface referenced to the reference ellipsoid and then to the geoid (i.e., MDT) with the aid of a geoid model. The ocean approach directly uses output from an ocean model of which the sea level, to a good approximation, has already been referenced to the geoid. Close agreement between the two independent approaches in the estimation of coastal MDT has been found in many previous studies (e.g., Higginson et al., 2015; Lin et al., 2015; Woodworth et al., 2012), thereby allowing investigators to use model output to diagnose the dynamics. Abstract Tilt of coastal mean dynamic topography (MDT) has recently been investigated in many regions, but few studies have examined the seasonality of the MDT along the coast of the Chinese mainland. The China seas are relatively unique as they experience prominent seasonal changes in monsoons and the China Coastal Currents, which are supposed to affect the coastal MDT significantly. This study investigates the Chinese coastal MDT tilt and its seasonality based on the geodetic and ocean approaches. The two independent approaches show a good agreement in their respective estimates. The Chinese coastal MDT presents an overall northward drop and undergoes an evident seasonality with steeper slopes in winter and flatter ones in summer. A dynamical analysis based on alongshore momentum equation suggests that the alongshore MDT along the coast of the Chinese mainland is a counter balance of contributions from the alongshore wind and the coastal current-induced bottom friction, with both of them having prominent seasonality. The overall northward drop is in fact induced primarily by the strong alongshore wind stress in winter and by coastal currents in summer, in particular for the segment between Xiamen and Zhapo. The wintertime coastal MDT is well predicted by the arrested topographic wave model, which however could not reproduce the summertime tilt. The summertime MDT is more likely affected by the open-ocean forcing (i.e., the latitudinal difference of the Kuroshio strength) and a return flow caused by flattening the coastal tilt piled up by the previous winter monsoon.