The long lives of subducted spice and vorticity anomalies in the subtropical oceans

Subtropical cells, which exist in nearly all ocean basins, connect subducting subtropical waters to upwelling sites along the equator. This tight link between the subtropics and the tropics, on a scale of 5-15 years, is well-established in a time-averaged sense by modeling and observations. Recently, evidence has emerged of spice (density-compensated temperature and salinity variations) and potential vorticity anomaly persistence along mean flow pathways on isopycnals. We provide the first global view of subtropical water mass anomaly propagation, using both an observational dataset and the Estimating the Circulation and Climate of the Ocean (ECCO) state estimate Version 4 Release 4. In this global synthesis that complements the existing body of largely regional studies, we find long-lived interannual water mass anomalies that translate along mean advective pathways in all ventilated subtropical gyres. They are detectable over multiple years and several thousand kilometers. Some anomalies are persistent enough to reach both the western boundary and equatorial current systems before being entirely eroded, and thus could form ocean “tunnels” along which heat anomalies could travel to impact remote climate variability. Analysis of ocean tunnel propagation of a passive tracer (spice) and an active tracer (potential vorticity) confirms earlier model results that the active tracer decays more quickly than the passive tracer. Similarities and differences between timing and frequency of the two tracers could provide clues to anomaly formation mechanisms in various subduction regions. The success of ECCO in capturing these phenomena is encouragement to further explore their upstream sources and downstream impacts within this framework.