Observed Tracer Fields Structuration by Middepth Zonal Jets in the Tropical Pacific

Abstract

AbstractThe middepth ocean circulation in the tropical Pacific is dominated by sets of alternating eastward and westward jets. The origin and transport properties of these flow features remain in many ways an open question, all the more crucial since their usual underestimation in ocean global circulation models has been identified as a potential bias for the misrepresentation of the oxygen minimum zones. In this study, we analyze the water mass properties associated with these systems of jets using velocity and hydrographic sections. Data acquired during a dedicated cruise carried out in the western part of the basin and supplemented by cross-equatorial sections from historical cruises in the central and eastern parts are analyzed. While it is confirmed that the near-equatorial jets carry oxygen anomalies, contributing to the ventilation of the eastern tropical Pacific, the data also revealed unexpected features. Tracer distributions (oxygen, salinity, and potential vorticity) show the presence of fronts extending from 500 to 3000 m and flanked by homogeneous regions. These structures define meridional staircase profiles that coincide with the alternating velocity profiles. Historical data confirm their presence in the off-equatorial deep tropical ocean with a zonal and temporal coherence throughout the basin. These observations support existing theoretical studies involving homogenization by isopycnic turbulent mixing in the formation of staircase profiles and maintenance of zonal jets. The effect of other processes on the equilibration of tracer structures is also discussed.