Kelvin Waves and Tropical Cyclogenesis: Connections to Convection and Moisture

In recent years, research has illuminated a distinct relationship between Convectively Coupled Kelvin Waves (CCKWs) and tropical cyclone (TC) formation. In basins that support TCs, there is a pronounced increase in the number of TC genesis events 1-3 days following the passage of a CCKW’s convectively-active phase. It has been hypothesized that this lagged relationship could be the result of the modification of environmental and kinematic factors by the CCKW. However, little work has been done to try to connect these environmental changes to the processes involved in TC genesis. Observational and modeling studies alike have indicated that the development of TCs may be intimately tied to convective-radiative feedbacks and pre-moistening of the atmosphere. How might CCKWs be impacting these processes?

To investigate this, we leverage a 39-year database of African Easterly Waves (AEWs) and associated TC genesis events in the Atlantic Ocean basin from 1981 to 2019. Environmental composites of ERA5 reanalysis and satellite data show an increase in column specific humidity and convective coverage beginning two days prior to TC genesis. This supports previous hypotheses of AEW trough preconditioning. A moist static energy (MSE) variance budget surrounding AEWs is also calculated. This analysis indicates that the dominant source of MSE variance during TC genesis – a proxy for convective aggregation – are longwave-radiative feedbacks, further solidifying the role of convection-related feedbacks in TC development.

Environmental fields around developing AEWs are then composited relative to passing CCKWs. Convectively-active CCKWs temporarily promote an increase in convection, specific humidity, and relative vorticity around AEWs. AEW-CCKW passages are shown to be quite common, with 76% of all developing AEWs passing at least one CCKW in their lifetime. We also compare AEW-CCKW passages that result in TC genesis versus those that do not. The primary discriminator between these two outcomes appears to be convective coverage and diabatic heating at the time of CCKW passage. There is also a pronounced increase in the longwave-radiative feedback term following the CCKW passage for cases that result in TC genesis.

While it is hard to separate the simultaneous effects of a multi-day TC genesis process from that of passing CCKWs, this analysis provides at least circumstantial evidence that CCKW-related modifications to convection and humidity could play an indirect role in preconditioning the AEW and a direct role in strengthening radiative-convective feedbacks. These results also motivate investigation of AEW-CCKW interactions in numerical simulations, which may be more suited to investigate cross-scale interactions and better determine causality.