A study of oceanic mixing processes: moderate wind regime

Oceanic mixing processes in moderate wind regime are studied in this work through 1D- model response experiments to surface forcing. We estimate surface momentum and heat fluxes during summer of 2000 (June-September) from the TAO moorings and ancillary satellite data in the tropical western Pacific region. The surface fluxes are used to force two 1D models: a mixed layer model (the Hybrid version developed by D. Chen) and a turbulence closure model (a Mellor-Yamada level 2.5 type, MY2.5). A careful evaluation of the observed surface variables and estimated fluxes reveals the following: i) TRMM SST fluctuates more frequently with larger amplitude than TAO SST. But the two SST data sets have similar low-frequency variability with a seasonal warming trend; ii) The heat fluxes based on TRMM SST and assimilated surface parameters (ECMWF) are comparable to TAO fluxes, but the former has larger variability due to TRMM SST; iii) The wind stresses based on TAO and EC data are close to each other, featuring summer monsoon with dominant easterly winds and frequent southerly winds. The simulated evolution of mixed layer by the two 1D models in response to the observed surface fluxes are compared against observed mixed layer. Simulated and observed mixed layer (SST and h) generally have similar variability and magnitude, yet a deficit between observed and simulated temperature appears near the end period of the simulation. The temperature deficit is negatively correlated with temperature advection. Relative to observed variability of mixed layer, MY2.5 tends to produce a shallower mixed layer than the Hybrid model does in the shoaling stages when thermal buoyancy forcing is dominant. The surface temperature changes in this stage are critically dependent on 1) the parameterization of solar irradiance in the upper ocean, 2) wave breaking effect. In the deepening stage, the Hybrid model's response is stronger than the MY2.5's response, obviously due to the different treatment of wind mixing in the two models.