Experimental evidence of long-term oceanic circulation reversals without wind influence in the North Ionian Sea

Angelo Rubino,Joël Sommeria,Boris Petelin,Vlado Malačič,Mirko Orlić,Vedrana Kovačević,Milena Menna,Miroslav Gačić,Giuseppe Siena,Manuel Bensi,Giuseppe Civitarese,Ricardo V Barreto,Vanessa Cardin,Laura Ursella,Maria Eletta Negretti,Davide Zanchettin

doi:10.1038/s41598-020-57862-6

Abstract

Under the emerging features of interannual-to-decadal ocean variability, the periodical reversals of the North Ionian Gyre (NIG), driven mostly by the mechanism named Adriatic-Ionian Bimodal Oscillating System (BiOS), are known as impacting on marine physics and biogeochemistry and potentially influencing short-term regional climate predictability in the Eastern Mediterranean. Whilst it has been suggested that local wind forcing cannot explain such variability, aspects of the alternative hypothesis indicating that NIG reversals mainly arises from an internal ocean feedback mechanism alone remain largely debated. Here we demonstrate, using the results of physical experiments, performed in the world’s largest rotating tank and numerical simulations, that the main observed feature of BiOS, i.e., the switch of polarity of the near-surface circulation in the NIG, can be induced by a mere injection of dense water on a sloping bottom. Hence, BiOS is a truly oceanic mode of variability and abrupt polarity changes in circulation can arise solely from extreme dense water formation events.

Highlights

Under the emerging features of interannual-to-decadal ocean variability, the periodical reversals of the North Ionian Gyre (NIG), driven mostly by the mechanism named Adriatic-Ionian Bimodal Oscillating System (BiOS), are known as impacting on marine physics and biogeochemistry and potentially influencing short-term regional climate predictability in the Eastern Mediterranean
During the following spring season, these newly formed waters spread into the Ionian Sea, simultaneously with the occurrence of a circulation reversal of the NIG14
Controlled experiments in a simplified framework remain to be performed in order to demonstrate, beyond any doubts, that an exclusive causal mechanism exists between spreading of dense Adriatic bottom waters into the Ionian and NIG reversals

Summary

Introduction

Under the emerging features of interannual-to-decadal ocean variability, the periodical reversals of the North Ionian Gyre (NIG), driven mostly by the mechanism named Adriatic-Ionian Bimodal Oscillating System (BiOS), are known as impacting on marine physics and biogeochemistry and potentially influencing short-term regional climate predictability in the Eastern Mediterranean. The circulation of the upper layer of the North Ionian Gyre (NIG), in the Eastern Mediterranean, shows periodic reversals[7,8,9,10] that have been demonstrated to be associated with major variations in physical and biogeochemical properties of the local water masses, with potential impacts on the broad marine (eco)system functioning[11,12,13] Observations indicate that this phenomenon, baptized Adriatic-Ionian Bimodal Oscillating System (BiOS), generally occurs about every five years, a remarkable irregularity in the timing of phase reversals has been observed as well[14]

Methods

Results

Conclusion