Energy Transfer into the Interior Ocean Through Near-Inertial Waves After an Extreme Wind Event

Winds play a substantial role in the energetic balance of the ocean-atmosphere coupled system. They are known to largely influence ocean dynamics, namely by cooling the water surface, or inducing upper ocean turbulence. Another consequence of the passing of a wind event is the excitation of internal waves that oscillate at a frequency close to the inertial frequency (near inertial waves, NIWs). These waves carry energy into the different ocean layers and participate in their vertical mixing by generating shear instability.

Both observations and models show that wind energy input in the mixed layer is well dominated by strong wind events, such as midlatitude storms, tropical cyclones or hurricanes. While most of the energy is dissipated in the mixed layer, a portion is assumed to reach the interior ocean. For strong events, this energy input is locally comparable to -or even in some cases greater than – the contribution of internal tides, and therefore assumed to play a key role in maintaining abyssal stratification. However, observations and in-depth studies regarding the understanding and weight of this energy transfer are currently lacking. Such considerations lead to the following question: What are the necessary conditions for near inertial energy to become significant in the interior ocean?

Therefore, in this study, we focus on quantifying the propagation of near inertial wave energy below the mixed layer, and the associated mixing, using a time series derived from a mooring southwest of a seamount chain south of the Azores Islands at 30.49°N, 30.20°W. The dataset consists of vertically high-resolution measurements from May 18, 2018 to March 29, 2019. Hurricane Leslie, a category one hurricane, passed north of the mooring during the second half of October, 2018.

A comprehensive analysis of the observed kinetic energy below the mixed layer is conducted using two complementary methods. First, through rotary spectral analysis, kinetic energy is separated into downward and upward going components. Then, a normal mode analysis is employed to examine the contribution of different modes to the energy flux. We thus aim to determine the potential parameters influencing this complex energy distribution to later infer a parametric source model that will provide a more refined representation of the influence of extreme wind events in near inertial energy transfer into the interior ocean.