An observation of the three-dimensional structure of a cross-shelf penetrating front off the Changjiang mouth

Satellite images and in situ hydrography and nutrient observations are used to study the three-dimensional structure of a penetrating front off the Changjiang mouth in the East China Sea. The satellite images of MODIS/Terra on October 16, 2006 show anomalously high concentrations of chlorophyll and total suspended sediments extending offshore from the mean coastal front off the Chiangjiang mouth. However, the sea-surface temperature image of the same satellite shows no signature of this extension at all. The in situ hydrography measured during the same time as the satellite images discloses that the surface layer of the front is occupied by low-salinity waters from the nearshore area, which suggests a cross-shelf jet covering a distance of about 50 km. A counter intrusion underneath the surface offshore jet is indicated by an increase of the subsurface temperature and salinity during the offshore excursion of the front, which leads to a significant temperature inversion at the head of the front. The vertical temperature and salinity distribution shows propensity of double diffusive processes, which may contribute to the ensuing evolution of the front. The nutrient distribution of the front is consistent with the cross-shelf jet derived from the hydrographic data, with surface and bottom intensified concentrations and minimum mid-depth concentrations, suggesting that the primary transport of the surface nutrients is in the horizontal cross-shelf direction. The potential dynamics of the frontal intrusion is discussed in light of wind forcing and frontal instability. The intrusion is found to be very important to cross-shelf and cross-front exchanges in the East China Sea.