Tunnels in the ocean: formation and propagation of interannual water mass anomalies in global subtropical cells

Abstract

Subtropical cells connect subducting subtropical waters to upwelling sites along the equator. Water mass properties (e.g. salinity, temperature) are set at the surface and subducted along ventilated pathways that flow toward upwelling sites in the tropics. 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. In Chapter 2, 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. We find long-lived interannual water mass anomalies that translate along mean advective pathways in all ventilated subtropical gyres. These anomalies, in spice (densitycompensated temperature and salinity) and potential vorticity (ratio of Coriolis parameter to density layer thickness), are detectable over multiple years and several thousand kilometers. Anomalies sometimes reach western boundary and equatorial current systems before being entirely eroded, and thus could form ocean “tunnels” to impact remote climate variability. ECCO successfully captures these phenomena, motivating further study of subtropical thermocline variability using modified-forcing experiments in the ECCO-configured Massachusetts Institute of Technology General Circulation Model (MITgcm) in subsequent chapters. In Chapter 3, we remove the interannual frequency of the input surface forcing over different ocean regions. This allows us to describe the pathways and quantify the impact strength of subtropically-forced water mass anomalies on equatorial variability. We find that interannual forcing over each of the five global subtropical basins creates a detectable impact on equatorial water properties, although the strength varies greatly between basins. We also investigate impacts in the reverse direction, from variability forced over the equator toward the subtropics, and demonstrate potential ocean-based mechanisms through which such variability can propagate. Finally, in Chapter 4, we perform additional ECCO experiments in which we remove the interannual band of wind or buoyancy forcing in order to understand surface forcing drivers of the observed spice variability. We see that while the interannual wind forcing is responsible for fast-moving, high-frequency wave perturbation features in the spice record along subducting pathways, it also creates large, low-frequency spice variability which is anticorrelated with the buoyancy-forced variability. These experiments emphasize the role of the outcrop migration mechanism of spice formation, in which sea surface temperature anomalies cause an isopycnal outcrop to move across the background surface salinity field, particularly in the South Pacific.