発表要旨
1. プロジェクトの目的
The big challenge of the next decade for the oceanic sciences is to adopt a multi-scale approach because of the strong nonlinearity of the oceanic fluid, which requires numerical simulations with ultra-high resolution. Purpose of this project is to fully explore two energetic nonlinear oceanic regimes that have a major impact on the general oceanic circulation: the mesoscale eddy regime at mid-latitudes and the equatorial regime. The expected results will help for the configuration of realistic numerical simulations to be performed in 2007 by the OFES group on the Earth Simulator and they should benefit to future climate models.
2. 今年度当初の計画
Planning for the mid-latitude project was to perform several high resolution (1/100° and 200 levels) simulations of mesoscale eddy turbulence. A tracer equation is activated to characterize the effects of the dynamics on the vertical and horizontal exchanges of any properties some and simulations are forced by realistic atmospheric forcings.
Planning for the equatorial project was to perform simulations in a bihemispheric basin of idealized geometry, centered about the equator and of comparable size either to the Atlantic and Pacific basin’s with a resolution of 1/12° in the horizontal and 400 levels in the vertical.
3. 今年度得られた成果
a. Impact of the vertical pump due to oceanic eddies
All simulations performed have led to important results. The dynamics of the upper layers is much more energetics than expected, because of the small-scale frontogenesis mechanisms explicitly resolved. Consequences are much stronger vertical fluxes and also a warming of oceanic surface layers of nearly one degree Celsius compensated for by a cooling in the interior. At last wind energy is found to penetrate much deeper in the ocean interior (down to 3000m), which represents a potential route to mixing in the deep ocean.
b. Dynamics of deep equatorial transport and mixing
Numerical solutions have enabled us to identify the main parameters which govern the formation mechanisms of alternate equatorial jets and we have been able to reproduce the very different characteristics of the jets which are observed in the equatorial Atlantic and Pacific oceans. We next started to study the quantification of their associated mixing and the influence of the longitudinal extent of the basin (Atlantic vs. Pacific) on the jets characteristics. Enhanced vertical mixing of the tracer field has been observed, with time-dispersion laws much faster than normal diffusion laws. The basin extent impacts the low-frequency behaviour of the jets.