An Initiation Process of Tropical Depression–Type Disturbances under the Influence of Upper-Level Troughs

Abstract

Abstract In this study, the statistical relationship between tropical upper-tropospheric troughs (TUTTs) and the initiation of summertime tropical depression–type disturbances (TDDs) over the western and central North Pacific is investigated. By applying a spatiotemporal filter to the 34-yr record of brightness temperature and using JRA-55 products, TDD-event initiations are detected and classified as trough-related (TR) or non-trough-related (non-TR). The conventional understanding is that TDDs originate primarily in the lower troposphere; our results refine this view by revealing that approximately 30% of TDDs in the 10°–20°N latitude ranges are generated under the influence of TUTTs. Lead–lag composite analysis of both TR- and non-TR-TDDs clarifies that TR-TDDs occur under relatively dry and less convergent large-scale conditions in the lower troposphere. This result suggests that TR-TDDs can form in a relatively unfavorable low-level environment. The three-dimensional structure of the wave activity flux reveals southward and downward propagation of wave energy in the upper troposphere that converges at the midtroposphere around the region where TR-TDDs occur, suggesting the existence of extratropical forcing. Further, the role of dynamic forcing associated with the TUTT on the TR-TDD initiation is analyzed using the quasigeostrophic omega equation. The result reveals that moistening in the mid- to upper troposphere takes place in association with the sustained dynamical ascent at the southeast side of the TUTT, which precedes the occurrence of deep convective heating. Along with a higher convective available potential energy due to the destabilizing effect of TUTTs, the moistening in the mid- to upper troposphere also helps to prepare the environment favorable to TDDs initiation.