Simulation and Scaling of the Turbulent Vertical Heat Transport and Deep-Cycle Turbulence across the Equatorial Pacific Cold Tongue

Microstructure observations in the Pacific cold tongue reveal that turbulence often penetrates into the ther- mocline, producing hundreds of watts per square meter of downward heat transport during nighttime and early morning. However, virtually all observations of this deep-cycle turbulence (DCT) are from 08, 1408W. Here, a hierarchy of ocean process simulations, including submesoscale-permitting regional models and turbulence-permitting large-eddy simulations (LES) embedded in a regional model, provide insight into mixing and DCT at and beyond 08, 1408W. A regional hindcast quantifies the spatiotemporal variability of subsurface turbulent heat fluxes throughout the cold tongue from 1999 to 2016. Mean subsurface turbulent fluxes are strongest (100 W m22) within 28 of the equator, slightly (10 W m22) stronger in the northern than Southern Hemisphere throughout the cold tongue, and correlated with surface heat fluxes (r2 5 0.7). The seasonal cycle of the subsurface heat flux, which does not covary with the surface heat flux, ranges from 150 W m22 near the equator to 30 and 10 W m22 at 48N and 48S, respectively. Aseasonal variability of the subsurface heat flux is loga- rithmically distributed, covaries spatially with the time-mean flux, and is highlighted in 34-day LES of boreal autumn at 08 and 38N, 1408W. Intense DCT occurs frequently above the undercurrent at 08 and intermittently at 38N. Daily mean heat fluxes scale with the bulk vertical shear and the wind stress, which together explain 90% of the daily variance across both LES. Observational validation of the scaling at 08, 1408W is encouraging, but observations beyond 08, 1408W are needed to facilitate refinement of mixing parameterization in ocean models.