The Zapiola Anticyclone: A Lagrangian Study of its Kinematics in an Eddy-Permitting Ocean Model
The Zapiola Anticyclone (ZA) is a strong, O (100 Sv), barotropic vortex in the center of the Argentine Basin that is tied to a bathymetric feature called the Zapiola Rise. It is regionally significant for two reasons: first, the strong vortex is a dynamical barrier that inhibits the lateral exchange of water, and hence has the ability to trap water for a long period of time. Second, its dynamics is governed by a balance between eddy-driven mass convergence and divergent Ekman transport, which gives rise to strong downwelling and Ekman pumping into the bottom boundary layer. This study investigates the kinematics of the ZA by studying the fate of the water parcels that are trapped by the ZA. We use output from a five-year simulation with an eddy-permitting ocean model, and we use a Lagrangian approach to track water parcels originating from within the ZA. We determine basic statistics of the parcel trajectories, including retention time, number of revolutions, vertical displacement, and temperature and salinity changes. The picture that emerges is one of water parcels spiraling downward through the water column, undergoing downwelling while they revolve anticyclonically around the center of the ZA. In our experiment, water parcels spend on average 451 days within the ZA, and make 2.6 revolutions around its center, with each revolution taking somewhere between 100 and 200 days. On average, parcels undergo a 94 m descent, 0.03°C cooling and 0.0042 psu freshening. But individual parcels can undergo more than 800 m of downwelling, 0.2°C of cooling, and ± 0.02 psu of salinity change. We believe that vertical motions of this order of magnitude, and the associated water mass transformations, are unique in the abyssal mid-latitude oceans.