Modulation of diurnal convection by convectively coupled equatorial waves

The geographical distribution of the amplitude and phase of the diurnal cycle of deep tropical convection is not simply governed by the diurnal cycle of radiation. Daytime convection over continents due to strong solar heating gives way to complex patterns over the oceans controlled by nocturnal radiational cooling, propagating gravity waves forced by land-sea contrasts, and apparent phase locking between the diurnal cycle and longer period waves, such as westward-inertio gravity modes. An example of the latter involves the so-called "two day waves" observed over the western Pacific warm pool during TOGA COARE. The Madden-Julian Oscillation is also known to strongly modulate the diurnal cycle, particularly over the Indonesian region. We present statistical evidence that higher frequency equatorial waves, such as convectively coupled Kelvin waves, strongly modulate the diurnal cycle within their convective envelopes. As an example, Kelvin waves at times appear to suppress diurnal convection over South America, while they enhance the diurnal cycle once they propagate offshore into the Atlantic ITCZ and Africa. These diurnal pulses of convection are frequently observed to launch convectively coupled inertio-gravity waves that propagate westward at speeds of up to 30 m/s or greater. Over some regions such as Africa these disturbances can be identified as squall lines that propagate westward much faster than can be explained by advection. These relationships illustrate potential mechanisms of scale interactions which can take place within the tropics through complex interplays between wave dynamics and diurnal forcing.