Sea-level variability as a proxy for ocean dynamics in the Mediterranean Sea

The Mediterranean Sea accounts for less than 0.5% of the global ocean's total volume and is characterized by unique significance in terms of oceanographic complexity. Indeed, an internal conveyor belt, together with complex circulation patterns and gyres, defines the entire domain. The understanding of Mediterranean oceanography has evolved significantly in recent times, transitioning from a static perspective to a dynamic one, as circulation patterns and thermohaline properties in the basin are now acknowledged to vary over time. Within this dynamic framework, the North Ionian Gyre (NIG) emerges as one of the most intriguing oceanographic features. Situated in the Ionian Sea, the NIG is known to reverse its circulation between cyclonic and anticyclonic modes on a quasi-decadal scale. This fluctuation results in significant variations in the redistribution of water masses and thermohaline properties throughout the Mediterranean Sea. Although various hypotheses have been proposed to explain the causes of these reversal episodes, a widely accepted consensus has yet to be reached. Moreover, reversal episodes have been documented only since the late 1980s through direct observations, modeling, and experimental studies, while the historical variability of this phenomenon remains poorly understood.

In this study, to enhance the understanding of the NIG evolution over time, information about sea-level changes has been accounted for. Indeed, variations in thermohaline properties and water mass redistribution, induced by NIG state shifts, might have been recorded in sea-level changes as a response to these modifications. A total of 46 tide gauges, distributed across the entire domain, have been considered, providing signals that often date back decades or even cover the entire 20th century. Furthermore, information from satellite altimetry has been included to provide a detailed spatial view of sea-level changes in recent decades across the Mediterranean Sea. After the removal of effects such as atmospheric pressure, glacial isostatic adjustment, and the sea-level response induced by the water mass exchange from continents, all signals were decomposed into a finite number of mode functions, each theoretically related to a specific phenomenon. At this stage, the influence of vertical land movements recorded in tide gauges has been isolated and attributed to residual signals, while the oscillatory modes primarily represent sea-level changes associated with thermohaline variations and the dynamic redistribution of seawater. 

An interesting oscillatory, quasi-decadal signal emerged as the second mode of variability within all datasets considered. Inflections within this signal provide a notable match, both in time and space, with all known NIG reversal episodes, particularly in the eastern Mediterranean sub-basins. These inflections manifest as an acceleration (or deceleration) in sea-level rise during anticyclonic (cyclonic) NIG phases. Despite their low magnitude in terms of amplitude (approximately 4 cm), they appear to be associated with the main driver of short-term variability in sea-level trends across the domain. Since signals from tide gauges provide long-term time series, this correlation enables the reconstruction of the NIG reversal history over the past 120 years based on direct observations.