Vertical Structure of Moisture and Temperature Advection of Kelvin Waves Propagating Through Different Phases of the Madden-Julian Oscillation

The Madden Julian Oscillation (MJO) modulates extratropical Rossby waves which can propagate equatorward to initiate vertical motions that generate upper tropospheric moisture signals. Such moisture flow can modify into or merge with eastward-moving convectively coupled tropical Kelvin waves, which together can develop upward motion that cools the deep troposphere. The advection of moisture and temperature by propagating waves interacts with and initiates further convection that can modify the dynamics of the MJO.

The MJO alternates between patterns of easterly or westerly wind shear that can alter the vertical structure and tilt patterns of Kelvin waves. Such changes in vertical structure can lead to nonlinear advection of temperature and specific humidity that modify the moisture budget on the timescale of the MJO itself. Studying interactions between the MJO and Kelvin waves can lead to better understanding of extratropical wave formation and feedback between moisture and temperature advection and convection, and a more accurate representation of Kelvin waves and the MJO as propagating disturbances of moisture and temperature in global and regional models.

This study investigates the spatial and temporal evolution of temperature and specific humidity associated with Kelvin waves propagating along the tropics at different Real-time Multivariate MJO (RMM) phases. This study uses a real-valued Morlet wavelet decomposition of Outgoing Longwave Radiation (OLR) to distinguish Kelvin waves from other convectively coupled tropical waves and the MJO. OLR approximates convection associated with propagating waves. Compared to the classic Fourier transform filtering, which diagnoses a range of phase speeds and wavenumbers, Morlet wavelet decomposition can prescribe specific phase speeds and wavenumbers to investigate how Kelvin waves behave. This important feature determines how Kelvin waves at specific phase speeds and wavenumbers project onto the MJO subseasonal time scale during different RMM phases.

To illustrate the evolution of temperature and specific humidity, this study uses composites and time lagged regressions between the filtered Kelvin wavelet indices and temperature and specific humidity advection. This study determines how much the temperature and specific humidity variability contributed by Kelvin waves project on the MJO timescale, and how different moisture patterns can generate an environment favorable for further convection. This study also relates temperature and specific humidity to results from previous work on the wind flow and vertical advection associated with Kelvin waves, which could explain how the Kelvin wave projects its moisture potential onto the MJO timescale.