The School of Earth and Atmospheric Sciences Presents, Dr. Paola Cessi, UCSD

The Global Routes of the Mid-depth Meridional Overturning Circulation

Using velocities from the ECCO state estimate, Lagrangian analysis maps the global routes of the lower branch of the Atlantic Meridional Overturning Circulation (AMOC) as it exits and reenters the South Atlantic to close the southern branch of the AMOC. Virtual parcel trajectories followed for 8100 years identify an upper route (32%) and a lower route (68%). The latter samples sigma_2>37.07 and is further divided into subpolar (20%) and abyssal cells (48%).  Parcels in the abyssal cell detour into the abyssal Indo-Pacific as Circumpolar Water, directly from the northern Antarctic Circumpolar Current.

The region were Antarctic bottom water is formed is rarely sampled, and typically only after entering and exiting the abyssal Indo-Pacific. Parcels in the subpolar cell sink to abyssal density exclusively in the Southern Ocean partly as Circumpolar Water and partly as Antarctic Bottom Water. Parcels in the upper route never reach densities in the abyssal range. Typical transit times are: 300, 700 and 3600 years for the upper route, subpolar and abyssal cells respectively. In all three groups, after parcels upwell in the Southern Ocean, they undergo additional diabatic transformations, with higher diabatic change north of 34oS than elsewhere.

Total diapycnal transformations are largest in the lower route, but of comparable in magnitudes to the upper route, challenging its previous characterization as  ``quasi-adiabatic’'. The additional diabatic changes are predominantly  in the mixed layer of the tropical and subpolar gyres of the Indo-Pacific, enabled by Ekman suction. A kinematic model of the wind-driven gyres and associated Ekman transport illustrates that parcels always reach the surface in the tropical and subpolar gyres, regardless of their initial condition, because of the coupling between the gyral velocity, the Ekman transport and its return.