Dynamics of the Southern Tsuchiya Jet*

Abstract

The Tsuchiya jets (TJs) are narrow eastward currents, located a few degrees on either side of the equator at depths from 200 to 500 m in the Pacific Ocean. In this study, non-eddy-resolving, oceanic general circulation models (OGCMs) are used to investigate the dynamics of the southern TJ. Most solutions are found in a rectangular basin extending 100°zonally and from 40°S to 10°N. They are forced by idealized zonal and meridional winds representing the trades and the southerly winds near the South American coast, by a prescribed interocean circulation (IOC) that enters the basin through the southern boundary and exits through the western boundary from 2°to 6°N (the model's Indonesian passages), and by surface heating that warms the ocean in the Tropics. A suite of solutions is presented to isolate effects of each forcing and mixing process. A few solutions are also found to a global OGCM driven by realistic forcings. Solutions forced by all of the aforementioned processes and with minimal diffusion resemble the observed flow field in the tropical South Pacific. A narrow eastward current, the model southern TJ, flows across the basin along the southern edge of a thick equatorial thermostad, and upwells at the eastern boundary. Its deeper part is supplied by water that leaves the western boundary current somewhat south of the equator. Its shallower part originates from water that diverges from the deep portion of the Equatorial Undercurrent (EUC); as a result, the TJ transport increases to the east and the TJ warms as it flows across the basin. A major part of the water that upwells at the eastern boundary is supplied by the TJ with a minor contribution from the southern boundary region. In idealized-basin solutions without forcing either by the IOC or meridional wind, the TJ is weak or absent. These, and other, properties suggest that the dynamics of the model's TJ are those of an arrested front, which in a 2 1 ⁄2-layer model are generated when characteristics of the flow merge or intersect. When diffusivity is increased to commonly used values, the thermostad is less well defined or even absent and the TJ is weak, suggesting that excessive diffusion is the reason why TJs are not present in many previous OGCMs. In the solution to a global OGCM, the southern TJ still exists without the IOC, although it is warmed by 1°C, indicating that much of its water is supplied by an overturning cell confined within the Pacific basin.

The OMZ and nutrient features as a signature of interannual and low-frequency variability in the Peruvian upwelling system

“…It is wide in vertical extension (∼ 500 m) and intense (< 22.5 µmol kg −1 ), and at some latitudes the upper boundary could be very shallow (25-50 m), intersecting the euphotic zone and impinging the continental shelf (Morales et al, 1999;Schneider et al, 2006;Fuenzalida et al, 2009;Paulmier and Ruiz-Pino, 2009;Ulloa and Pantoja, 2009). The OMZ off Peru is associated with the presence of nutrient-rich equatorial subsurface water (ESSW) transported poleward by the Peru-Chile Undercurrent (PCU; Strub et al, 1998;Fuenzalida et al, 2009;Silva et al, 2009) and the transport of low-oxygenated waters by the narrow primary and secondary Southern Subsurface Countercurrents near 4 and 7 • S, respectively, also known as Tsuchiya jets (Furue et al, 2007;Montes et al, 2010). Recent modeling studies also highlight the important role of submesoscale to mesoscale dynamics in constraining the upper OMZ meridional boundaries (Bettencourt et al, 2015;Vergara et al, 2016).…”

Origin and Pathway of Equatorial 13°C Water in the Pacific Identified by a Simulated Passive Tracer and Its Adjoint*

“…Variations in this lower thermocline cell are likely to be related to variations in the eastern equatorial Pacific surface temperature (cf. Schott et al 2004), where mixed layers are typically very shallow (e.g., Johnson and McPhaden 1999;Furue et al 2007), implying a potential role in the temporal evolution of the ''cold tongue.' ' The 138C Water also affects the eastern boundary SST in the tropical South Pacific, a region well known to be climatologically and ecologically important.…”

Impacts of regional mixing on the temperature structure of the equatorial Pacific Ocean. Part 1: Vertically uniform vertical diffusion

“…The eastward currents at 200-450 m depths and 3-5°from the equator are the northern and southern Tsuchiya jets (TJs; Tsuchiya, 1972Tsuchiya, , 1975Tsuchiya, , 1981McCreary et al, 2002;Furue et al, 2007Furue et al, , 2009. The structures of the model TJs agree fairly well with observations, except that the model has westward currents on the equatorward sides of the TJs.…”

“…6a (McCreary, 1981;Kawase, 1987). An alternative explanation for the wedge shape is that it represents the pathways of higherorder vertical-mode (n ≥ 2) Rossby waves emanating from the eastern boundary, which are bent equatorward by the background Sverdrup circulation (Luyten and Stommel, 1986;McCreary et al, 2002;Furue et al, 2007Furue et al, , 2009. Consistent with this idea, the poleward edge of the positive δ T SE signal farther west is tilted somewhat equatorward (top-right panel of Fig.…”

“…In the western ocean, the deep, positive δ T ESE anomaly (300-400 m, 150 • E-160 • W) is the westward extension of the locallygenerated deep anomaly (Fig. B.3a); it appears only in the west because the southern Tsuchiya Jet shifts equatorward toward the west due to the equatorward bending of Rossby-wave characteristics (McCreary et al, 2002;Furue et al, 2007Furue et al, , 2009.…”