Wave breaking and critical levels for propagating inertio-gravity waves in the lower stratosphere

This paper analyses and interprets the formation of mixed layers due to propagating inertio-gravity waves observed during the Fronts and Atlantic Storm-Track EXperiment. The data used are high-vertical-resolution soundings launched from different sites located in the North Atlantic sector. In agreement with other studies, attributing the origin of the inertio-gravity waves to the adjustment of the jet stream near fronts, the largest waves observed are often located in the vicinity of and above frontal bands. The amplitude of these waves is large enough that they give rise to narrow mixed layers. By a detailed analysis of the waves' characteristics, we can distinguish whether such a mixing is favoured by the presence of an inertia-critical level, where Ω = f, or results from wave-induced convective instability occurring far from critical levels. Here, Ω is the wave intrinsic frequency and f is the Coriolis parameter. These observations highlight the importance of inertio-gravity waves in the vertical exchanges that occur in the stratosphere.