Secondary cyclogenesis: The initiation phase of a frontal wave observed over the eastern Atlantic

Abstract The initiation phase of a secondary frontal wave in the wake of a primary cyclogenesis event over the Atlantic is documented. North of the trailing surface cold front and south‐west of the primary cyclone, an intense upper‐level jet generates a direct and transverse ageostrophic circulation in its entrance region. Evidence is shown for two conflicting roles of this circulation: first, stabilization of the front through frontogenetic cross‐frontal low‐level convergence; and second, an increase of the potential for instability of the front through diabatic (e.g. latent‐heat release) potential‐vorticity (PV) redistribution in the upward branch of the circulation. Following saturation of the primary cyclogenesis and the associated along‐front north‐eastwards shift of the upper‐level jet, the low‐level branch of the ageostrophic circulation behaves like a zipper closing on the front, while in the upward branch an anomalous strip of PV builds along the cold front. Isentropic PV maps show that parcles of stratospheric origin embedded in the downward branch of the ageostrophic circulation (a tropopause fold) may have ended in the vicinity of the strip of anomalous low‐level PV, perhaps contributing to an increase in the potential for instability of the surface front. Using the domain‐independent attribution method of Bishop, it is shown that the frontal wave forms on a part of the cold front where: first, the along‐front stretching rate due to the environmental flow decays below the critical threshold value (0.6 × 10 −5 s −1 ) prescribed by theoretical studies; and second, the across‐front frontogenetic component due to the ageostrophic circulation decays. This is consistent with semi‐geostrophic theory, where the cross‐frontal convergence and the stretching deformation are linked through the Sawyer‐Eliassen equation. Results add to other recent observational findings: that the potential for instability, the evolving environmental flow and the induced ageostrophic convergence each have a crucial role in differentiating between frontal waves that will grow and those that will be suppressed.