Normalized Hough functions , , and corresponding to the Earth’s semidiurnal tide (m = 2 and ν = 1.0030). We note that ν = 2Ω /σ. Left: first gravity modes; 0 ≤ n ≤ 5. Right: first Rossby modes: − 6 ≤ n ≤ −1. In this case, |ν| is very close to 1 and the Rossby modes are trapped at the poles. The Earth’s semidiurnal tide is mainly described by gravity modes. For more details about the behaviour of Hough functions, see Fig. A.2.