Abstract. This study examines the formation of tropical cyclones (TCs) from the large-scale perspective. Using the nonlinear dynamical transition framework recently developed by Ma and Wang, it is shown that the large-scale formation of TCs can be understood as a result of the principle of exchange of stabilities in the barotropic model for the intertropical convergence zone (ITCZ). Analyses of the transition dynamics at the critical point reveal that the maximum number of TC disturbances that the Earth's tropical atmosphere can support at any instant of time has an upper bound of ∼12 for current atmospheric conditions. Additional numerical estimation of the transition structure on the central manifold at the critical point of the ITCZ model confirms this important finding, which offers an explanation for a fundamental question of why the Earth's atmosphere can support a limited number of TCs globally each year.
The tropical atmosphere has a maximum capacity in producing global tropical cy-9 clones, with an order of ∼ O(10 2) in an aquaplanet setting. • Global tropical cyclone formation occurs intermittently in episodes with a frequency of approximately 2 weeks, instead of continuously throughout the year. • Mid-level moisture content and 850 hPa absolute vorticity associated with the ITCZ instability are key factors determining the episodic nature of global tropical cyclone formation.
This study examines the climatic shift of the tropical cyclone (TC) frequency affecting Vietnam’s coastal region during 1975-2014. By separating TC databases into two different 20-year epochs, it is found that there is a consistent increase in both the number of strong TCs as well as the number of TC occurrences during the recent epoch (1995-2014) as compared to the reference epoch (1975-1994) across different TC databases. This finding suggests that not only the number of strong TCs but also the lifetime of strong TCs affecting Vietnam’s coastal region has been recently increasing as compared to the reference epoch from 1975-1994. To understand the physical connection of these shifts in the TC frequency and duration, large-scale conditions obtained from reanalysis data are analyzed. Results show that meridional surface temperature gradient (STG) during the recent epoch is substantially larger than that during 1975-1994. Such an increase in the meridional STG is important, because it is potentially linked to the increase in large-scale vertical wind shear as well as the reduced intensity of summer monsoon in the South China Sea between the two epochs.
This study examines the large-scale factors that govern global tropical cyclone (TC) formation and an upper bound on the annual number of TCs. Using idealized simulations for an aquaplanet tropical channel, it is shown that the tropical atmosphere has a maximum capacity in generating TCs, even under ideal environmental conditions. Regardless of how favorable the tropical environment is, the total number of TCs generated in the tropical channel possesses a consistent cap across experiments. Analyses of daily TC genesis events reveal further that global TC formation is intermittent throughout the year in a series of episodes at a roughly 2-week frequency, with a cap of 8-10 genesis events per day. Examination of different large-scale environmental factors shows that 600-hPa moisture content, 850-hPa absolute vorticity, and vertical wind shear are the most critical factors for this global episodic TC formation. Specifically, both the 850-hPa absolute vorticity and the 600-hPa moisture are relatively higher at the onset of TC formation episodes. Once TCs form and move to poleward, the total moisture content and the absolute vorticity in the main genesis region subside, thus reducing large-scale instability and producing an unfavorable environment for TCs to form. It takes ∼2 weeks for the tropical atmosphere to remoisten and rebuild the large-scale instability associated with the Intertropical Convergence Zone before a new TC formation episode can occur. These results offer new insight into the processes that control the upper bound on the global number of TCs in the range of 80-100 annually. Plain Language Summary Understanding the mechanisms behind the upper bound on the global tropical cyclone (TC) number is important yet still elusive. Using TC simulations on an aquaplanet tropical channel, this study shows that the tropical atmosphere possesses a maximum capacity in producing TCs globally, regardless of how ideal the environmental conditions are. In particular, global TC formation does not occur throughout the year but in a series of episodes with a frequency of about 2 weeks, with ∼8-10 TCs per episode. Examination of the TC genesis index shows that midlevel moisture content and 850-hPa absolute vorticity associated with the ITCZ instability are key factors governing the episodic global TC formation.
Abstract. This study examines the role of tropical dynamics in the formation of global tropical cyclone (TC) clusters. Using theoretical analyses and idealized simulations, it is found that global TC clusters can be produced by the internal dynamics of the tropical atmosphere, even in the absence of landmass surface and zonal sea surface temperature (SST) anomalies. Our analyses of a two-dimensional InterTropical Convergence Zone (ITCZ) model capture indeed some planetary-scale stationary modes whose zonal and meridional structures can support the formation of TC clusters at the global scale. Additional idealized simulations using the Weather Research and Forecasting (WRF) model confirm these results in a range of aqua-planet experiments. Specifically, the examination of two common tropical waves including the equatorial Rossby (ER) wave and the equatorial Kelvin (EK) wave shows that ER waves could develop and maintain a planetary-scale stationary structure for a range of zonal wavenumbers [5–11], while EK waves do not. This numerical result is consistent with the ITCZ breakdown model and reveals some forcing structures that can support stationary "hot spots" for global TC formation. The findings in this study offer different insights into the importance of tropical waves in producing global TC clusters beyond the traditional explanation based on zonal SST anomalies.
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