Continental Runoff and Effects on the North Atlantic Ocean Subtropical Mode Water

Interannual salinity variations in North Atlantic Subtropical Mode Water (STMW) are well known although the cause is less well understood. Attempts to model local salinity variation with local evaporation and precipitation have not been successful and some authors invoke advection of low salinity water as the cause. Examination of the STMW and North American river runoff data suggests that runoff may partly explain the salinity variations. It is known that low salinity water resulting from Mississippi River outflow is transported well past Cape Hatteras. Spearman Rank Correlation analysis and spectra and crossspectra Fourier correlation analysis both show that river flow is significantly inversely correlated with STMW salinity. This result suggests that North American river flow may have an influence on the salinity of STMW. [Talley and Rayruer, 1982; Talley, 1996; Joyce and Robbins, 1996]. Talley [Talley, 1996] noted that" the source of fresher water would be the Gulf Stream and slope water region to its north." while Joyce and Robbins [Joyce and Robbins, 1996] noted that" the T/S variability in the surface layer is uncorrelated, which suggests some other processes uch as precipitation, runoff, or possibly inter gyre exchange with the subpolar North Atlantic [Talley, 1996] might be key factors as well." In this paper we find a strong relationship between periods of high Mississippi River runoff and low STMW salinity and vice versa. These results strongly suggest that variations in continental runoff may be an important cause of STMW salinity variability.