The Response of Atlantic Tropical Cyclones to Suppression of African Easterly Waves

Abstract

Atlantic tropical cyclone (TC) genesis is strongly linked with African easterly waves (AEWs) on the synoptic time scale. However, the TC‐AEW relationship is unclear on interannual to climate time scales, and it is unknown whether AEWs are necessary to maintain climatological TC frequency, that is, whether TCs are limited by AEWs. We investigated the impact of AEW suppression on seasonal Atlantic TC activity using a 10‐member ensemble of regional climate model simulations in which AEWs were either prescribed or removed through the lateral boundary condition. The climate model experiments produced no significant change in seasonal Atlantic TC number, indicating that AEWs are not necessary to maintain climatological basin‐wide TC frequency even though TCs readily originate from these types of disturbances. This suggests that the specific type of “seedling” disturbance is unimportant for determining basin‐wide seasonal Atlantic TC number and that in the absence of AEWs, TCs will generate by other mechanisms. The results imply that changes in the presence of AEWs may not be reliable predictors of seasonal variability and future change in Atlantic TC frequency.

Large-scale dynamics of tropical cyclone formation associated with ITCZ breakdown

“…This number is quite consistent with a simple scale analysis based on the typical scale of TCs with a diameter ∼ 3000 km, which shows that there should have been less than 14 TCs on the Earth's atmosphere at any given time, assuming that the radius of the Earth is ∼ 6400 km. Using idealized simulations for a tropical channel, Kieu et al (2018) showed in fact that genesis occurs in episodes of 7-10 storms each time with a frequency between the episodes of 12-16 d. This episodic development at the global scale as well as the upper bound of ∼ 10 storms for each episode as obtained from these idealized experiments suggests that there must have some large-scale environmental conditions or intrinsic properties of the tropical dynamics, which control the genesis processes beyond the basin-specific mechanisms as discussed in Patricola et al (2018) .…”

“…Specifically, a theoretical estimation of the largest zonal wavenumber k that can allow an unstable structure as a re- sult of the ITCZ breakdown is k = 12, assuming the typical scales of the Earth's tropical channel in which the zonal scale of the tropical channel is about an order of magnitude larger than the width of the channel. Such a dynamical constraint on the maximum number of TC disturbances is remarkable, as it suggest an intrinsic large-scale mechanism that controls the climatology of the TC numbers beyond the basin-specific features as recently noted in Patricola et al (2018). Of interest is that this constraint on the largest wavenumber of the unstable eigenmodes imposes not only an upper limit on the number of TC disturbances in the tropical region, but also a lower bound on the size of TC disturbances.…”

“…Recent efforts in the TC genesis research have been shifted from examining local mechanisms to a broader perspective of how environmental conditions can produce and maintain TC disturbances during TC early development (Wang and Magnusdottir, 2006;Dunkerton et al, 2009;Montgomery et al, 2010;Wang et al, 2012;Lussier et al, 2014;Zhu et al, 2015;Wu and Shen, 2016;Patricola et al, 2018). The most current attempt in quantifying the large-scale factors governing the genesis in the North Atlantic basin focuses on the so-called "pouch" conceptual model, which treats an early TC embryo as a protected region within large-scale easterly waves (Wang et al, 2010(Wang et al, , 2012Dunkerton et al, 2009;Montgomery et al, 2010).…”

Impact of Model Resolution on Tropical Cyclone Simulation Using the HighResMIP–PRIMAVERA Multimodel Ensemble

“…13a) are consistent with the shear, with several models (e.g., MPI-ESM1.2) having the mean jet somewhat farther north than indicated by the reanalyses, while HadGEM3-GC31-LM is too far to the south. Some previous work (Patricola et al 2018) has suggested that African easterly waves (AEWs) play little role in setting North Atlantic tropical cyclone numbers, while Thorncroft and Hodges (2001) and Roberts et al (2015) showed some relationship with TC variability at higher resolutions for storms with genesis in the eastern Atlantic. The mean number of AEWs is shown in Fig.…”

Atlantic Hurricane response to Sahara greening and reduced dust emissions during the mid-Holocene

“…Their results suggest a large increase in TC activity worldwide and in particular in the Atlantic Ocean in the MH climate. However, this kind of downscaling approach does not consider how the TC genesis may have been affected by changes in atmospheric dynamics, such as those associated to the African Easterly Waves (AEWs; Gaetani et al, 2017) that are known to seed TC genesis (Caron & Jones, 2012;Frank & Roundy, 2006;Landsea, 1993;Thorncroft & Hodges, 2001;Patricola et al, 2018). Here, we use the same modeling simulations as in Pausata et al (2017) to drive a high-resolution regional climate model to investigate the impact of the atmospheric dynamics changes induced by Saharan vegetation and dust reduction on TC activity during the MH compared to the PI climate.…”