The Role of Moisture in the Convective Coupling of Equatorial Waves

Despite their widely varying characteristics, all convectively coupled equatorial waves share a common feature; a coupling between large-scale circulations and convection. Understanding of the mechanisms responsible for this coupling remains limited. Here the exponential increase in precipitation with increasing column saturation fraction is used to investigate the role of moisture in convective coupling.

Precipitation anomalies associated with the MJO, equatorial Rossby waves, and east Pacific easterly waves can be accurately diagnosed from moisture anomalies, strongly suggesting their primary coupling mechanism is the modulation of environmental moisture. In contrast, precipitation anomalies associated with Kelvin waves, African easterly waves, and mixed Rossby gravity wave can not be accurately diagnosed from moisture anomalies, suggesting other coupling mechanisms are playing first order roles. These latter phenomena have strong adiabatically forced vertical motions which could reduce static stability and convective inhibition while simultaneously moistening, creating a more favorable convective environment. Cross-spectra of precipitation and column integrated dry static energy show enhanced coherence and an out-of-phase relationship in the Kelvin wave, mixed Rossby gravity wave, and eastward inertio-gravity wave bands, supporting this narrative. This cooperative modulation of precipitation shortens the convective adjustment timescale (i.e. timescale for precipitation to remove a moisture perturbation) of these phenomena. Speeding the adjustment of moisture anomalies relative to that of temperature anomalies allows the latter to assume a more important role in driving moist static energy fluctuations, promoting the gravity wave character of these phenomena.

In a companion study, Part I, it was shown that the convective adjustment timescale lengthens with increasing spatiotemporal scale, helping moisture anomalies of larger spatial scale to persist longer than those of smaller spatial scale. Here it is shown that the gravity wave adjustment timescale increases more rapidly than the the convective adjustment timescale with increasing length scale. This finding emphasizes the importance of a low effective gross moist stability in large scale phenomena of moisture mode character such as the MJO, whose moist static energy fluctuations are dominated by moisture.