The impact of internal lee wave closure in an idealised global model

Internal lee waves, with scales typically ranging from 1 to 10 km, play an important role in modulating ocean energy budget and deep ocean stratification. However, these small-scale motions have not yet been resolved in global ocean circulation models. Here we conduct a suite of numerical experiments using an idealised global model that incorporates an energetically and dynamically consistent lee wave closure. The effects of mean-wave interaction, wave drag and wave-driven mixing are considered separately through different experiments. The results show that wave drag plays a dominant role in modulating ocean energy and overturning circulation, while the effects of mean-wave interaction and wave-driven mixing are secondary. In the presence of wave drag, the (eddy) kinetic energy is significantly reduced by ~30%. In the Southern Ocean, enhanced eddy dissipation due to wave drag weakens eddy-induced meridional overturning circulation and reduces meridional eddy heat transport, leading to greater ocean heat uptake. To compensate for the enhanced eddy dissipation, isopycnals in the Southern Ocean steepen, which results in a deepening of the Atlantic pycnocline and a strengthening and deepening of the upper cell of the Atlantic meridional overturning circulation. Our study highlights the role of lee waves in modulating overturning circulation which could have important implications for global heat storage and climate change.