Regression Analysis of Zonally Narrow Components of the MJO

Abstract

Abstract Recent works have demonstrated that eastward-propagating features smaller than zonal wavenumber 3 but with spatial structures similar to those of the Madden–Julian oscillation (MJO) frequently develop over the Indo-Pacific warm pool. These signals are characterized by periods shorter than 4 weeks, but since they occur as part of a spectral peak of the MJO, they might be characterized by similar physics. These zonally narrow features occur at any phase of traditionally defined 30–60-day MJO events, but they occur most frequently in its active convective phase. This work presents a linear regression analysis based on filtering in the wavenumber–frequency domain to compare such signals with traditionally defined MJOs and 15–30 m s−1, convectively coupled Kelvin waves. Results show that the trough collocated with the easterly wind anomaly extends westward into the region of lower-tropospheric westerly wind and deep convection in the zonally narrow slow signals and MJOs. The fast Kelvin waves have a ridge anomaly collocated with the westerly wind anomaly. The zonally narrow slow signals and MJOs include a warm anomaly in the boundary layer west of the deep convection that is absent in fast Kelvin waves. Results suggest that MJO dynamics are not confined to the 30–60-day band and that time scales as short as 2 weeks could be considered in wavenumber–frequency diagnostics for the MJO.