Sub-Monthly Circulation Features Associated with Tropical Cyclone Tracks over the East Asian Monsoon Area during July-August Season

Abstract

This study explores sub-monthly variation for the circulation and convection in the western North Pacific during July and August. The sub-monthly variability is dominated by a wave-like pattern that propagates north-northwestward from the northeast of Papua New Guinea, to the East China Sea. The wavelength and phase speed are about 4000 km and 5 m s À1 , respectively. This feature fluctuates in a 7-30 day timescale, with the spectral peak at 12.5 days. The vertical structure exhibits a barotropic structure below 500 hPa, which gradually tilts northwestward in upper levels near Taiwan and southern Japan. For more than 70% of cases which reveal the wave-like fluctuation, recurving tropical cyclones are found to occur concurrently with the wave-like pattern. These recurving tropical cyclones are likely part of the wave-like pattern, and may not be regarded as isolated vortices. The wave-like pattern occurs in a large-scale flow, which is characterized by an enhanced monsoon trough extending eastward into the Philippine Sea, and a strong ridge protruding westward from the subtropical high. It is suggested that the cyclonic phase of the wave-like pattern provides a favorable environment for the genesis, growth, and movement of the recurving tropical cyclones. The monsoon trough/subtropical ridge fluctuation, the wave-like pattern, and tropical cyclones, which represent phenomena in three different time and spatial scales, form a multi-scale system that affects the sub-monthly variability of the circulation, and convection, in the western North Pacific during the boreal summer.

Characteristics of Summertime Circulation Patterns for Southern Taiwan’s Monsoon Rainfall from July to September

“…According to Holland (1995), this kind of TCs is less understood and can possibly bring more damage to this area. Ko and Hsu (2006) also find that the recurving TCs are usually associated with periodic oscillations that propagate to the Taiwan-Japan area approximately every two weeks.…”

Climatological analysis of passage-type tropical cyclones from the Western North Pacific into the South China Sea

“…The appearance of an anomalous anticyclonic circulation over the WNP causes vertical wind shear enhancement and thus suppresses TC genesis (e.g., Chen et al 2005). On the other hand, monsoon trough enhancement provides a favorable environment for the formation and growth of TCs (e.g., Ko and Hsu 2006). During the warm years, the eastward displacement of warm waters in the tropical Pacific is accompanied with eastward extensions of the monsoon trough and equatorial westerlies (e.g., Lander 1994a).…”

Barotropic Interactions Between Summertime Tropical Cyclones/Sub-Monthly Wave Patterns and Intraseasonal Oscillations over the Western North Pacific

“…The western North Pacific (WNP) exhibits a wide range of multi-scale phenomena (hereafter KH06: Lau and Lau 1990;Liebmann et al 1994; Ko and Hsu 2006;Hsu 2012). A crucial issue over the last decade has been the interaction between phenomena of different scales (e.g., hereafter KH09: Maloney and Dickinson 2003;Ko and Hsu 2009;hereafter KHC12: Maloney and Hartmann 2001;Chen and Sui 2010;Zhou and Li 2010;Ko et al 2012).…”