Effects of a frontal passage on surface salinity distribution along the Louisiana-Texas coast, USA, from ocean color and model outputs

Abstract

Short-term and seasonal estimates of colored dissolved organic matter (CDOM) absorption at 412 nm and surface salinity were derived from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite data for the Louisiana-Texas coast during 2005 using an empirical CDOM algorithm and a conservative CDOMsalinity relationship. Field measurements obtained during various seasons in 2005 indicated high correlations between field and satellite estimates of CDOM suggesting satellite estimates to be good representation of the surface CDOM and salinity fields. Discharge from the Mississippi and Atchafalaya rivers strongly influenced the seasonal surface CDOM distribution as well as during a frontal passage. Clear satellite imagery obtained before and after the passage of a cold front in March 2005 indicated a general decrease in surface CDOM and an offshore increase of elevated CDOM suggesting that frequent frontal passages contribute to mixing of riverine CDOM and its offshore transport. A comparison of SeaWiFS-derived salinity with the salinity outputs of a three-dimensional Navy Coastal Ocean Model (NCOM) indicated similar salinity trends offshore and a region of freshwater influence along the inner shelf. Following the frontal passage, changes in the surface salinity were observed mainly closer to the coast with a general increase in mid-shelf waters, likely due to mixing of lower salinity surface waters with higher salinity sub-surface waters. Short-term salinity model simulation could be improved with daily assimilation of river discharge data. SeaWiFS derived seasonal salinity estimates provided a synoptic view of the effects of coastal circulation and riverine discharge on shelf variability.