Water Exchange between the Patos Lagoon and the Atlantic Ocean Through a Narrow Strait

The Patos Lagoon is located in southern Brazil and is the largest freshwater lagoon in the world; it is connected to the Atlantic Ocean by a narrow strait. Through this strait, saline water flows from the sea into the lagoon and fresh water from the lagoon into the sea. Based on long-term in situ data from sea mooring and river gauge stations, as well as wind reanalysis data, the influence of local meteorological and hydrological conditions on water exchange through this strait has been studied. It was revealed that the distinct seasonal variability of water exchange is governed mostly by the variability of seasonal river discharge, but not at all by the variability of local atmospheric circulation. Similarly, the synoptic variability of water exchange between the Patos Lagoon and Atlantic Ocean on an annual scale depends mostly on the variability of river runoff and shows little relation to the variability of synoptic wind forcing. The water exchange responds to the variability of synoptic wind forcing in 24–48 h depending on the intensity of water exchange preceding the change in atmospheric circulation. Outflow of lagoon water to the sea is typical during the high river discharge period, while inflow of seawater to the lagoon is rare, occurring under specific wind conditions. Under strong wind, blowing along the Patos Lagoon from the strait to the continent, a negative surge of lagoon water near the strait and a positive surge on the opposite side of the lagoon are formed. As a result, seawater flows into the lagoon. During low river discharge periods, seawater inflow into the lagoon is typical, while short-term outflow is induced by the increase in river discharge.