Astronomic influences on seasonal to interannual variability in saltwater intrusion in a subtropical estuary

A 10-year time series of surface salinity values along an estuary-river transition documents their spatial structure and their temporal variability. Decomposition of detided time series into empirical modes indicates that the first mode, explaining almost 95% of the overall variance, has the typical sigmoid spatial structure found in estuaries. The temporal variability of mode 1, referred to as salinity index, displays annual and semiannual signals that are modulated from year to year. A fit of the salinity index to harmonics associated with luni-solar orbital motions and solar activity explain more than 70% of the index variance. This suggests that a) gravitational forcing from moon and sun, and b) the thermodynamic influence of solar activity, both impact salinity intrusions into rivers. The mechanistic linkage is unknown, but it is likely that it develops through atmospheric pressure, winds, air temperature, and water level. These variables are influenced by gravitational forcing and solar activity as observed elsewhere. The main finding is that the salinity variability at an estuary-river transition is influenced by gravitational forces and solar activity, i.e., saltwater intrusion could be determined by the modulation of astronomic influences. Similar variability may be found for mixing processes in coastal regions.