Basinwise Statistical Analysis of Factors Limiting Tropical Storm Formation From an Initial Tropical Circulation

Abstract

AbstractTropical cyclones (TCs) form within a closed circulation region of large‐scale disturbances under certain dynamical and thermodynamical conditions. This study investigates differences in the large‐scale environmental conditions of a TC 48 hr before the declaration of tropical depression, comparing those that developed into tropical storms (TSs) with those depressions that did not develop. Here, we apply the Okubo‐Weiss Zeta Parameter detection and tracking scheme to ERA‐interim reanalysis from 1989–2018, across different ocean basins. The method detects storm‐system‐scale environmental conditions that favor TC formation. We construct spatial composites of thermodynamical and dynamical quantities for both developing and nondeveloping depressions, as well as storm‐relative streamlines. A statistical index (the Box Difference Index) is used to quantitatively estimate the dominant limiting factors of TS formation from the area‐averaged quantities of large‐scale variables. The relative contribution of large‐scale environmental variables impeding the development of an initial tropical depression to TS differs between ocean basins perhaps due to regional variations in the characteristics of large‐scale disturbances and the surrounding environmental conditions. A streamline analysis shows that the developing storms have a more pronounced cyclonic core and are protected from external influences by a stronger shear sheath layer surrounding the core extending from the lower to middle troposphere. This study, therefore, identifies the potential limiting variables and structural differences in an initial circulation that impede TS formation across different ocean basins.