1)Influence of eddy-turbulence on the basin modes
Sea surface temperature is the main link of the ocean with the overlying atmosphere and therefore the questions that are raised are at the heart of ocean-atmosphere climate evolutions.
The mode's existence was first shown in a rectangular flat-bottomed single-hemispheric basin, with prescribed surface heat fluxes. In this configuration, a large scale baroclinic instability continuously feeds large scale planetary waves. The planetary waves give rise to SST and Meridional Overturning Circulation variability. Figure 1 shows the variability that spontaneously emerges in this configuration.
Our approach is to simulate the large scale mode using this relatively simple configuration and to add a mesoscale eddy turbulence field by employing the MITgcm (an Ocean General Circulation Model) at eddy-resolving resolution.
The main outcome of this workpackage is that we find a transfer of temperature variance from low to high frequencies associated with meso-scale eddy turbulence which acts as a sink of temperature variance for the low-frequency large-scale mode.
2)Forcing of the basin modes
Identifying sources, sinks and pathway of energy (Kinetic (KE) and Available Potential Energy (APE)) in frequency space is essential to the understanding of low frequency variability mechanisms. We have shown that the non-linear APE transfer is much larger than the non-linear KE transfer and that it is in the opposite direction. The main source of mechanical energy (APE+KE) for the low frequency mode is the large scale baroclinic instability of the mean flow transferring energy between the mean APE and low frequency APE reservoirs. The non-linear transfer of kinetic energy contribution is negligible.
The main difficulty in establishing this energy budget lies in the Background Potential Energy (BPE one of the two components of APE with potential energy) decomposition. Previous methods described in the literature led to spurious transfer and fictitious conversion between the different energy reservoirs. A new decomposition method has been developed to fix this issue.
The stratification is controlled by the vertical diffusion parameter. At low resolution (i.e. without resolved meso-scale variability), increasing the stratification increases the mean circulation instability and thus increases the large scale mode amplitude. We showed that when mesoscale variability is permitted, the low frequency mode amplitude decreases with increasing stratification, at odds with previous results from low resolution studies.
3) Understanding the QG APE/KE transfers in frequency space
The primitive equation of the MITgcm simulations are too computationally demanding to investigate APE/KE transfers sensibility to external parameters (forcing and friction in that case). Using a simpler doubly-periodic, two layers QG setup we investigate the role of friction and mean flow forcing intensity. Preliminary results show that if the kinetic energy is indeed transferred to lower frequencies, the transfer of mechanical energy is toward higher frequencies because the NL APE transfer is larger than the KE transfer and in the opposite direction.
Although the work described above has shown that the role of mesoscale eddies is to dissipate the low frequency mode, we show in a simple 1 layer QG model, that mesoscale eddies are able to force large scale basin modes. We also focus on the inverse cascade of kinetic energy and direct cascade of enstrophy and describe for the first time the differences that appears between the classical doubly periodic QG model and our more realistic boundary enclosed QG model.
