Positive Storm Surges in the Río de la Plata Estuary: forcings, long-term variability, trends and linkage with Southwestern Atlantic Continental Shelf dynamics

The Rio de la Plata Estuary (RdP), one of the most populated and developed areas of Southern South America, often experiences positive storm surges (PSS). These episodic rises of sea level due to meteorological forcing drive floods that endanger human lives and cause property damage. In this work, PSS are studied and contextualized in both the dynamics of the adjacent Southwestern Atlantic Continental Shelf (SWACS) and the synoptic dynamics of the atmosphere. The study is based on statistical analyses of tide gauge observations gathered at the upper RdP (period 1934-2020), numerical simulations of sea level in the SWACS and atmospheric reanalysis products. Results reveal hitherto unknown aspects of ocean dynamics in the SWACS forced by the atmosphere in which PSS events impacting the RdP are embedded, showing that: (1) Strong PSS in the RdP can be locally forced by cyclogenesis; nevertheless, most of PSS events affecting the estuary are remotely forced at the southern SWACS and reach the estuary as free propagating coastal waves, producing a surge that can be as strong as the locally forced ones; (2) the spatial pattern of the sea level anomalies in the SWACS during the PSS in the RdP can be described in terms of three distinctive modes (or spatial structures), each one of them related to characteristic regional atmospheric synoptic processes (in particular traveling Rossby waves and cyclogenesis); this provides clues to both better understand the surges and to extend their predictability; and (3) the PSS extension in the SWACS and the RdP depends on the speed at which atmospheric systems pass through the region. These findings provide valuable insights that can aid in anticipating extreme situations several days in advance to the numerical ocean forecast systems. The analysis of the gauge observations at Buenos Aires shows that (1) the number of cases of PSS per year presents large multidecadal and interannual (pseudo-cycle at 5 years) variability; (2) in this frame, it is not reasonable to estimate long-term lineal trends and/or extrapolate tendencies; therefore, our results refute the suggestion that the number of PSS cases in the RdP has been increasing over time, as reported in previous works.