Abstract
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 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” equivalent to the well-studied atmospheric bridge to impact the remote climate variability. The 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.
Significance Statement
Surface waters in the subtropical oceans flow below the surface layer and travel for long distances westward and equatorward until they reemerge at the surface. It is known that average water properties can persist for thousands of kilometers along this “ocean tunnel,” but how much the year-to-year variability can survive without being mixed away is less understood. We use observational data and global model output to estimate the frequency and survival of interannual anomalies of water properties along ocean tunnels. We find that such anomalies are common in all subtropical basins, that they have the potential to persist until they reach the tropics, and that the global model captures the variability seen in observations.
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