The seasonal and interannual variability of an index measuring the potential for deep (surface-to-tropopause) convection over the extratropical oceans is studied using reanalysis data. It is found that most of the conditional instability is concentrated over the world's western boundary currents during winter, but shifts equatorward of the currents in summer. Conditional instability is only detected over the Gulf Stream and the East Australian Current in their respective summer season.
The coupled ocean–atmosphere mechanisms controlling the variability of the convective index are then studied. It is found that the convective index displays a large interannual variability, which is primarily controlled by the erratic displacements of the storm tracks. Only weak negative feedback from the oceans is singled out on short (intraseasonal) time-scales, reflecting the stabilization of the troposphere through the development of cold sea-surface temperature anomalies. A larger role for warm oceanic advection in destabilizing the troposphere is, however, suggested on longer (interannual and decadal) time-scales.