MODELING THE GLOBAL FORMATION OF TROPICAL CYCLONES

Abstract

The first part of the thesis examines the large-scale factors that govern global tropical cyclone (TC) formation. Using idealized simulations for an aqua-planet tropical channel, we show that the tropical atmosphere has a maximum capacity for generating TCs, even under ideal environmental conditions. Regardless of how favorable the tropical environment is, the total number of TCs generated in the tropics has a 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. Once TCs form and move 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 re-moisten and rebuild the largescale instability associated with the Intertropical Convergence Zone (ITCZ) before a new TC formation episode can occur. These results offer new insights into the processes that control the upper bound on the annual global TC number. In the second part of the thesis, we examine the clustering of global TC formation from theoretical and numerical perspectives. Using an analytical model and idealized simulations, we find 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 ITCZ model capture indeed some planetaryvii scale stationary modes whose zonal and meridional structures 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. These numerical results are consistent with the ITCZ breakdown model and reveal some forcing structures that can support stationary "hot spots" for global TC formation beyond the traditional explanation based on zonal SST anomalies. The last part of this thesis examines possible future changes in TC lifetime for the northwestern Pacific basin. Using high-resolution dynamical downscaling, we show that TC lifetime appears to increase under both future emission scenarios RCP4.5 and RCP8.5. This is mostly seen at the tail of TC lifetime distribution with more long-lived TCs (lifetime of 8-11 days) and less short-lived TCs (lifetime of 3-5 days). Unlike previous studies, the correlation between TC lifetime and the Nino 3.4 index shows a mixed value in the future climate, with much less statistical significance. Analyses of the TC track distribution reveal instead more noticeable northeastward shifting of TC tracks by the end of 2050. This change in TC track climatology results in an overall longer duration of TCs in the open ocean, thus increasing TC lifetime. Such a shift in TC track pattern is ultimately linked to the strengthening of the Western North Pacific Subtropical High (WPSH), which retreats to the south during July and to the east during August-September in the future. These findings provide different insights into how large-scale circulations can affect TC lifetime in the northwestern Pacific basin under different climate projection scenarios.