Forcing of Stratospheric Kelvin Waves by Convective Activity

Kelvin waves are important atmospheric disturbances in the equatorial troposphere, where they are coupled to convection and account for a substantial fraction of tropical rainfall activity. Dry Kelvin waves are the most dominant disturbance in the equatorial stratosphere, where they are critical components in the forcing the stratospheric Quasi-Biennial Oscillation (QBO), which is in turn an important modulating factor in modulating tropospheric variability such as the North Atlantic Oscillation and the Madden-Julian Oscillation (MJO) on subseasonal timescales. In the lower stratosphere, Kelvin wave activity can be characterized as either “forced” or “free”. The forced Kelvin waves are the result of the well-documented responses to propagating convective heating associated with convectively coupled waves in the troposphere, and their scales are well predicted by the phase speed of the convective forcing according to linear theory. This is contrasted with vertically propagating free Kelvin wave activity that may be launched by tropospheric convective events. Along with gravity waves, these upward propagating free modes are the primary source of the eastward momentum forcing of the QBO. The free Kelvin waves are generally thought to be primarily a response to stochastic forcing from a broad spectrum of convective events in the troposphere, as outlined in the classic 1987 study by Salby and Garcia. We investigate the variability of free Kelvin wave activity from the lower to mid-stratosphere. As is well-documented, vertically propagating free Kelvin waves are strongly modulated by the QBO in the middle stratosphere, with greatly reduced activity at levels above QBO westerlies due to critical layer absorption. However, this relationship is much reduced to non-existent below around 50 hPa. Based on space-time spectral analysis of Outgoing Longwave Radiation (OLR) and IMERG precipitation conditioned on wave activity, we show that enhanced Kelvin activity in the lower stratosphere is indeed associated with an enhancement of convective activity in the troposphere. Interestingly, there appears to be no preferred scale for this enhancement, but instead the signal spans all of the wavenumbers and frequencies that can be resolved by the OLR and IMERG datasets.