Mississippi Waters Reaching South Florida Reefs Under No Flood Conditions: Synthesis of Observing and Modeling System Findings

In August 2014, in situ measurements revealed an intense salinity drop impacting South Florida coral reefs. Satellite observations showed that this drop in salinity was due to a southeastward export of Mississippi waters from the Northern Gulf of Mexico. Unlike previous events of long-distance Mississippi water export, this episode is not marked by Mississippi flooding conditions, which makes it a unique study case.

We have developed a high-resolution (~2 km) numerical model of the Gulf of Mexico to study the conditions that controlled the 2014 Mississippi water export episode. It is based on the HYbrid Coordinate Ocean Model (HYCOM), which has a detailed representation of coastal physics (especially river plume dynamics) and employs high frequency river discharge and atmospheric forcing. In addition, it assimilates remotely sensed altimetry and sea surface temperature observations. The simulation reveals a unique pathway that brought Mississippi waters along the Northern Gulf continental shelf, before reaching the deep Gulf. In the Florida Straits, Mississippi waters were advected from the deep ocean to the continental shelf under the influence of both deep sea (frontal dynamics of the local western boundary current) and shelf flows (wind-induced Ekman transport). The combined use of a regional, data-assimilative nested simulation and available observations followed best practices recommended under the Coastal Ocean and Shelf Seas Task Team of the GODAE (Global Data Assimilation Experiment) OceanView initiative. It allowed identifying key processes and features that characterize the unique episode of Mississippi River waters export of 2014, and helped analyze the wide range of processes affecting the connectivity at both the local and basin scale in the Gulf of Mexico.