Fig. 4. Co-existing and pre-existing equatorial waves related to pre-tropical cyclogenesis (pre-TCG) events in the Northern Hemisphere (NH).

a Composite means of combined wave horizontal winds (vectors) and relative vorticity (RV, in the units of 10⁻⁶ s⁻¹; shading) in the lower troposphere onto pre-TCG events that are simultaneously matched in-phase to Westward-moving Mixed Rossby-Gravity wave (WMRG), mode number 1 Rossby wave (R1) and mode number 2 Rossby wave (R2), with respect to the vortex longitude and event time of pre-TCG, in the NH. Only composite of relative vorticity of wave winds significant at the 95% confidence level is shown. Winds are combined from WMRG, R1 and R2 wave winds. Red dot shows the averaged latitude of pre-TCG vortex. b–d as a, but for composite means 1–3 days before pre-TCG events. e–h as a–d, but with shading showing composite means of relative humidity anomaly at 700 hPa (RH, in the units of %). The lower troposphere is equally averaged over 900, 850, 800, 750, and 700 hPa.