Interplay between Stratospheric Kelvin Waves, Tropical Convection, and the Quasi-Biennial Oscillation
NOAA Physical Sciences Laboratory
Tuesday, Feb 27, 2024, 2:00 pm MT
DSRC Room 2A305
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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. Kelvin waves are the most dominant large-scale disturbance in the equatorial stratosphere, where they are critical components in the forcing the stratospheric Quasi-Biennial Oscillation (QBO), which is in turn important for modulating tropospheric variability on subseasonal timescales.
In the lower stratosphere, Kelvin waves can be characterized as either "forced" or "free". The forced Kelvin modes are the result of the well-documented response to eastward propagating convective heating associated with convectively coupled Kelvin waves, and their scales are well predicted by the phase speed of the convective forcing according to linear theory. This is in contrast to the faster eastward propagating free Kelvin waves that populate the middle and upper stratosphere.
Along with gravity waves, these Kelvin modes are a primary source of the eastward momentum forcing of the QBO. The free Kelvin waves are generally thought to be a response to stochastic forcing from the broad spectrum of convective events in the troposphere. We investigate the structure and variability of free and forced Kelvin wave activity from the lower to mid-stratosphere and its relationship to the QBO.
Changes in the structure of Kelvin waves due to the QBO are well-predicted by linear theory. While vertically propagating free Kelvin wave activity is strongly modulated by the QBO in the middle stratosphere, this relationship is much reduced to non-existent below around 50 Pa.
Based on space-time spectral analysis of satellite brightness temperature and IMERG precipitation, we show that increased stratospheric Kelvin activity is indeed associated with an overall enhancement of convective activity in the troposphere. Interestingly, while convectively coupled Kelvin waves are shown to be an important source, the enhancement also spans all of the resolved wavenumbers and frequencies in the satellite data.
These results provide targets for model development, since current operational forecast and climate models struggle to reproduce realistic stratospheric equatorial waves and, consequently, the QBO.
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