Transport of anomalous low-salinity waters from the Mississippi River flood of 1993 to the Straits of Florida

Abstract

Recent field studies in the southern Straits of Florida revealed the existence of Mississippi River outflow embedded in the Florida Current and adjacent coastal waters. Surface thermosalinograph measurements for the period of 10–13 September 1993 indicated a band of low-salinity water measuring approximately 40 km wide and 30 m in depth extending from south of Key West to Miami, a distance of 260 km. Surface salinity values as low as 31 psu were found. The estimated volume of the band is approximately 33.3 × 10 10 m 3 for the Key West to Miami region, thereby requiring roughly 1.2 × 10 10 m 3 of fresh water to mix with oceanic waters to produce this low-salinity band. The only nearby, dynamically viable, source for such a large volume of fresh water is the Mississippi River during its flood stage in 1993. The proposed transport mechanism for the transport of flood waters from the shelf in the northeastern Gulf of Mexico to the Straits of Florida is via the Loop Current through entrainment. Salinity records from offshore C-MAN towers indicate that the low-salinity band persisted off the lower Florida Keys for approximately 3 months. The variability in the flow field in the southern Straits occurs in a 30–70 day band due to the meandering of the Florida Current and the subsequent formation, and propagation, of cyclonic gyres off the Dry Tortugas. This variability in the flow field had a clear affect on the evolution of the low-salinity band, as observed by the salinity records from the C-MAN towers. Because the band traveled as a lens in the upper 30 m of the water column and because its evolution was highly dependent on the variability within the Gulf Stream System, it was a good indicator of the mixing and exchange of offshore waters with shallow waters of the Florida reef tract and Florida Bay.