Surprisingly low near-inertial energy flux leaving the ocean’s mixed layer

We present a renewed estimate of the fraction of the total Near-Inertial (NI) energy flux leaving the ocean's Mixed Layer (ML) based on simulations performed by a global high-resolution (1/10°) ocean general circulation model. The quantification of this fraction is important for understanding the global general circulation and the energetics of the ocean. At the same time NI waves that are excited at the ocean surface by winds are a major power source for deep ocean mixing. NI waves that escape the turbulent ML can propagate freely into the stratified deep ocean. When they break in the oceanic interior, they can contribute to deep mixing.

The previous studies on the subject indicate that a large portion of wind-power input to NI motions is dissipated in a surface layer of fixed depth and in specific regions in the ocean. In comparison, we study in detail the spatial and temporal variability of the ocean’s ML and gives an estimate on a global scale, which allows for the first time for a global quantification of the transfer of NI energy into the deep layers of the ocean. We address the following questions: What is the magnitude of the wind-power input to NI motions simulated with our high-resolution ocean model? How large is the fraction of the total wind-induced NI energy flux that leaves the ML? What are the main factors that control this fraction?

The wind-power input to surface NI motions amounts to 0.35 TW, of which a fraction of about 11% i.e., 0.04 TW leaves the ML and propagates into the stratified interior of the ocean. Locally, this fraction tends to decrease with increasing depth of the ML and with increasing strength of wind stress variability, indicating the strong control of the turbulent dissipation inside the ML on the fraction. The low power input by NI waves points towards the importance of other energy sources (like the dissipation of mesoscale eddy energy) for interior mixing to sustain the global general circulation in the ocean.