Intraseasonal Behavior of Clouds, Temperature, and Motion in the Tropics

The spectral character of tropical convection is investigated in an 11-yr record of outgoing longwave radiation from the Advanced Very High Resolution Radiometer to identify interaction with the tropical circulation. Along the equator in the eastern hemisphere, the space–time spectrum of convection possesses a broad peak at wave-numbers 1–3 and eastward periods of 35–95 days. Significantly broader than the dynamical signal of the Madden–Julian oscillation (MJO), this quasi-discrete convective signal is associated with a large-scale anomaly that propagates across and modulates time mean or “climatological convection” over the equatorial Indian Ocean and western Pacific. Outside that region the convective signal is small, even though, under amplified conditions, coherence can be found east of the date line and in the subtropics. Having a zonal scale of approximately wavenumber 2, anomalous convection propagates eastward at some 5 m s−1 and suppresses as well as reinforces climatological convection in the eastern hemisphere. The convective signal amplifies to a seasonal maximum near vernal equinox and, to a weaker degree, again near autumnal equinox, when climatological convection and warm SST cross the equator. Contemporaneous records of motion from ECMWF analyses and tropospheric-mean temperature from Microwave Sounding Unit reveal an anomalous component of the tropical circulation that coexists with the convective signal and embodies many of the established properties of the MJO. Unlike anomalous convection, that dynamical signal extends globally around the Tropics. The anomalous circulation differs fundamentally between the eastern and western hemispheres. In the eastern hemisphere, subtropical Rossby gyres and zonal Kelvin structure along the equator flank the convective anomaly as it tracks eastward, giving the anomalous circulation the form of a “forced response.” In the western hemisphere, the dynamical signal is composed chiefly of wavenumber−1 Kelvin structure, which has the form of a “propagating response” that is excited in and radiates away from anomalous convection at some 10 m s−1. Kelvin structure comprising the propagating response appears in 850-mb and 200-mb zonal winds even when the convective signal is absent, albeit with much smaller amplitude. In contrast, the signal in 1000-mb convergence appears only when accompanied by anomalous convection, which suggests that convergence in the boundary layer is instrumental in achieving strong interaction with the convective pattern.