The Origin and Dispersion Characteristics of the Observed Tropical Summertime Synoptic-Scale Waves over the Western Pacific*

Abstract

The origin, initiation, and dispersion behavior of the observed summertime synoptic-scale disturbances in the tropical western Pacific are studied. These westward-propagating disturbances have the strongest growth rate over the region of ϳ130°-160°E off the equator. The three-dimensional wave activity flux associated with a wave packet in the vicinity of this region is computed. In general, wave activity is directed westward. There is accumulation of activity flux, which gives rise to the amplification of waves. In the low levels, such accumulation can be attributed to the convergence of both the mean flow and the intrinsic group velocity. Diabatic forcing also contributes to the growth of disturbances and is most important in the 500-600-hPa layer. Along the east-west-oriented "storm tracks" of the synoptic-scale disturbances, there are two different dynamical regimes. West of ϳ150°E, enhanced convection is associated with increased specific humidity at the top of the planetary boundary layer and is in phase with positive low-level vorticity anomalies. To the east of 150°E the vorticity leads the convection by about one-quarter of a wavelength. This phase relationship can be explained by adiabatic dynamics and is related to the positive vertical shear of the mean zonal flow in the latter region.Near and to the east of the date line where disturbances are initiated in the low levels, the heat flux associated with the synoptic-scale eddies is negative (i.e., ЈT Ј Ͻ 0) from about 300 to 700 hPa. This implies downward-directed wave activity. In the upper troposphere at the same geographical location, there is southward wave activity from the extratropics penetrating into the Tropics. These findings suggest that summertime synoptic-scale disturbances may originate from extratropical forcing. This hypothesis is supported by a case study. Intrusion of high potential vorticity into the Tropics was seen to be followed by downward development, resulting in low-level disturbances that subsequently moved westward in the western Pacific and grew.

An Observational Study of the Extremely Heavy Rain Event in Northern Vietnam during 30 October-1 November 2008

“…Meanwhile, on 30 and 31 October a warm core structure in the upper levels of the troposphere was clearly evident, as the equivalent potential temperatures increased from 500 to 200 hPa in the area just above northern Vietnam. This structure is also seen in other convectively coupled tropical waves in the northwestern Pacific region (Lau and Lau 1990;Tam and Li 2006;Serra et al 2008). The upper warm core is attributable to warming by the latent heat of condensation in cloud formation and increases of moisture in the atmosphere associated with the disturbances.…”

“…So, this was a rather deep disturbance, since most tropical waves across the northwestern Pacific are confined to the lower troposphere, generally extend vertically not higher than 500 hPa. (e.g., Chang 1970;Takayabu and Nitta 1993;Tam and Li 2006;Serra et al 2008).…”

“…The wavelength of a TD-type disturbance is 2000-3000 km, with a westward phase speed of approximately 7-8 m s À1 at around 150 E over the Pacific, and slower at more westerly longitudes near the Philippines (e.g. Nitta and Takayabu 1985;Tam and Li 2006). Convective variations in the tropical northwestern Pacific are closely associated with TD-type disturbances (e.g.…”

Precursor synoptic‐scale disturbances associated with tropical cyclogenesis in the South China Sea during 2000–2011

“…Kuo et al, 2001), or attributed to wave activity flux convergence associated with the decrease of zonal wavelength (e.g. Tam and Li, 2006).…”

“…Tam and Li (2006) showed that the wave energy comes from Pacific subtropical Jet, through southward and downward group velocity. Dunkerton et al (2009) proposed a 'pouch' theory in which EW acts to protect vortex development from hostile air invasion.…”