A reconciling vision of the Adriatic-Ionian Bimodal Oscillating System

Abstract

The bimodal oscillating system (BiOS) consists in an oscillation of the Ionian Sea surface structure with period of 12–13 years, which reflects in a near-surface circulation inversion. BiOS regimes are deeply interconnected with the circulation patterns of the Eastern Mediterranean, and it is a dominant process governing water masses formation, air-sea fluxes and bio-geochemical properties, which impacts living organisms. The BiOS has been partially explained as a self-sustained oscillation maintained by the interplay between Adriatic dense water formation and changes in the relative volume of waters of Levantine and Atlantic origin entering the Adriatic; however, attempts have also been made to explain the BiOS in terms of atmospheric-related processes. Despite the intensive research aiming at reproducing this oscillating system, the fundamental question “which is the source of energy necessary to initiate the BiOS?” has, until now, remained unanswered. The scope of this paper is two-fold. First, we document that, since 1993, two periods in the BiOS can be observed: a first one, between 1993 and 2017, during which the BiOS damped up to nearly disappear, with e-folding time of 11 years; and a second one, starting in 2017, during which the BiOS revitalized. Then, we propose here an analytical model that, under a two-layer ocean assumption, shows how it is possible for winds rotating in the same direction to initiate oscillations of the free surface, as result of the competing effects of wind and internal fluid pressure fields. The proposed model forced with wind data could successfully reproduce the characteristic time scales of the BiOS cycle over the period 1993–2019, and is therefore offered as a novel vision explaining the originating mechanism as the basis of its initiation, as well as a fundamental tool to address possible BiOS regimes in future climate scenarios.

The potential recording of North Ionian Gyre’s reversals as a decadal signal in sea level during the instrumental period

“…During this time, in fact, a different behavior in the signal is noticeable, mostly characterized by uncertain and frequent shifts, and sometimes opposite phases between the sub-basins. The periodicity of the second IMF signals from TGs in the Mediterranean Sea has been averaging around 12.0 ± 4.8 years since 1900, in agreement with 18 . The spatial distribution of the second IMFs extracted from SA, illustrated in Fig.…”

“…

Figure 2 The ensemble mean of the second IMF at the sub-basin scale, computed from TGs, is represented by the solid black line, accompanied by the ± 1 range (shaded in gray). The red hatched lines depict the basin average mean of the second IMF derived from the altimetry data, whereas the vertical black hatched lines indicate occurrences of NIG reversal episodes documented in literature 6 , 18 , 19 . The cyan and orange sectors respectively symbolize the likely periods of the cyclonic and anticyclonic patterns of Ionian circulation.

…”

“…The effect of reversal episodes on sea level has also been observed in the altimetric signal over the past three decades, as hypothesized by Refs. 18 , 20 , 25 ; this led to the so-called ‘breathing oscillation’ 26 in sea level throughout the domain, i.e., a rising (falling) sea-level trend in the Ionian Sea and a simultaneous falling (rising) trend in the other Mediterranean sub-basins during cyclonic (anticyclonic) phases. This phenomenon has been attributed to the effects of the NIG reversals on changes in the sterodynamic component, controlling the short-term trends in sea level within the Mediterranean Sea 21 .…”

“…Occurrences of reversals have been documented since the late 1980s, based on modeling, experimental studies and direct observations. These reversals have transitioned from a cyclonic to an anticyclonic state around the years 1987, 1998, 2010, and 2019, and vice versa around 1991, 2006, and 2016 (see 6,[16][17][18][19] and references therein). A real periodicity of this phenomenon, however, remains uncertain 3 .…”

Critical analysis for nonlinear oscillations by least square HPM

“…Dynamic systems that manifest oscillations or vibrations departing from linearity are referred to as nonlinear oscillating systems.These systems are commonly characterized by nonlinear differential equations, capturing intricate and at times unpredictable dynamics. These kinds of nonlinear oscillatory systems are found in many different scientific and engineering fields, including mechanics 1 , electromagnetic phenomena 2 , fluid dynamics 3 , biology 4 – 7 , chemistry 8 , and other fields 9 , including particle physics and cosmology 10 . These systems describe several applications of nonlinear oscillating dynamics.…”