Sea surface salinity changes in the East China Sea during 1997–2001: Influence of the Yangtze River

Abstract

The role of the Yangtze River discharge in changing sea‐surface salinity (SSS) in the East China Sea during the 1997–2001 periods is investigated by using (1) in situ SSS measurements along a merchant ship track running from Taiwan strait to Korea strait, (2) ocean general circulation model (OGCM) simulations and specific sensitivity studies, and (3) in land precipitation data to infer estimates of the river discharge in absence of gauged station data. The river effects on 1997–2001 sea surface temperature (SST) and surface currents are also examined. Both in situ SSS‐SST measurements and OGCM simulations clearly indicate that the integration of the 1997 drought signal and 1998–1999 flooding signal over the catchment area of the Yangtze River are manifest in the ocean where local precipitation played a rather modest role. Sensitivity studies with the OGCM with and without the river discharge further reveal that these signals can extend over several hundred kilometers off the river mouth, consistent with the along track in situ measurements. In particular, SSS along the ship track can drop from about 34 psu in August 1997 to less than 26 psu in July 1998 (almost the same for 1999) at the latitude of the river mouth although located 500 km to the west of the ship track. Model results indicate that the dynamic and thermodynamic effects of the freshwater input from the river may be phase lagged by up to a season with maximum surface warming occurring in the summer months when the river discharge is maximum whereas the largest differences in mixed layers with and without the river input occur during later winter and late autumn. The possible positive feedback between the increase of river runoff yielding warm SST anomaly off the river mouth and so enhanced atmospheric convection and precipitation is discussed.