Intrinsic oceanic variability, tipping points and early warning signals: examples from the dynamics of the Gulf Stream and the Mediterranean Sea

The intrinsic variability of the Gulf Stream (GS), the abrupt transition that can occur at a certain tipping point and the early warning signals that precede it, are investigated through a process modeling study. The nonlinear reduced-gravity shallow water equations are solved with schematic but quite realistic geometric configuration and time-independent wind forcing. A reference simulation shows a GS with correct mean Florida Current (FC) transport, realistic separation at Cape Hatteras (CH) due to inertial overshooting, realistic northern recirculation gyre and a strong chaotic intrinsic variability.

To simulate the effect of anthropogenic forcing, a sensitivity analysis is performed by decreasing the forcing amplitude. The results show a gradual shift of the GS toward the coasts north of CH and, therefore, a connection between the decline of the FC and one of the most significant fingerprints of a weakened Atlantic Meridional Overturning Circulation (AMOC). Furthermore, at a tipping point there is an abrupt transition to a GS flowing along the coasts north of CH; this is preceded by an early warning characterized by the fluctuation-dissipation relation, which is revealed by an increase in the autocorrelation and variance of the signals. It is suggested that such critical behavior could impact the AMOC tipping element. As regards the intrinsic variability of the Mediterranean Sea circulation, preliminary results are presented based on ensemble simulations using the FESOM-C finite element model. This research was partially supported by the INVMED project funded by the Italian PRIN-2022.