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 thermocline, 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 0 degrees, 140 degrees W. 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 0 degrees, 140 degrees W. 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 (similar to 100 W m(-2)) within 2 degrees of the equator, slightly (similar to 10 W m(-2)) stronger in the northern than Southern Hemisphere throughout the cold tongue, and correlated with surface heat fluxes (r(2) = 0.7). The seasonal cycle of the subsurface heat flux, which does not covary with the surface heat flux, ranges from 150 W m(-2) near the equator to 30 and 10 W m(-2) at 4 degrees N and 4 degrees S, respectively. Aseasonal variability of the subsurface heat flux is logarithmically distributed, covaries spatially with the time-mean flux, and is highlighted in 34-day LES of boreal autumn at 0 degrees and 3 degrees N, 140 degrees W. Intense DCT occurs frequently above the undercurrent at 0 degrees and intermittently at 3 degrees N. Daily mean heat fluxes scale with the bulk vertical shear and the wind stress, which together explain similar to 90% of the daily variance across both LES. Observational validation of the scaling at 0 degrees, 140 degrees W is encouraging, but observations beyond 0 degrees, 140 degrees W are needed to facilitate refinement of mixing parameterization in ocean models.

Automated summary

The study analyzes mixing and deep-cycle turbulence in the Pacific cold tongue region, finding that subsurface turbulent fluxes vary with geographical location and season, and are correlated with surface heat fluxes. The results show that intense deep-cycle turbulence occurs frequently in the region of 0 degrees, 140 degrees W and 3 degrees N.