Tropical Multi-Scale Oscillations and TC activities in Western Pacific

This study investigates the influence of large-scale background state on tropical cyclone (TC) formation in western Pacific, using OLR and the ERA-Interim reanalysis for the warm season (MJJAS) of ten years (2000-2009). First, we identify synoptic-scale cyclonic disturbances by 3-8 day filtered 850-hPa vorticity centers that attain certain strength and area during its life longer than three days. We identify a total of 166 cyclonic disturbances that eventually developed into named tropical cyclones (TC_d in the JTWC best track data) and 315 cyclonic disturbances that did not (TC_n) in the ten years. This gives a TC formation probability [P_TcF =TC_d/(TC_d+TC_n)] at 34%. We further analyze P_TcF in the large-scale background state including low-frequency flow associated with ENSO and MJO, and n=1 equatorial Rossby (ER) waves. On the low-frequency scale, corresponding to the broad-scale changes from unfavorable condition to favorable condition (cold to warm years, suppressed to convective phase of MJO), the number of TC_d increases and TC_n decreases but the total number of disturbances remains near unchanged (~30 per 100 days), so P_TcF increases from 30% to 40%. For ER waves, both TC_d and TC_n increases as the wave evolves from anticyclonic to cyclonic phase but TC_d increases more so P_TcF increases from 23% to 40%. The influence of ER waves is more significant in MJO suppressed phase: P_TcF increases from 17% to 53% corresponding to ER- and ER+ phases during three warm years, 8% to 33% during three cold years.

Apparently both MJO and ER waves are effective in fostering tropical disturbances into developing TCs. We analyze the wave structure corresponding the TC_d and TC_n in ER- and ER+, respectively. Major Features of composite ER wave are summarized as the following: most TCs (TC_d at warning time or TC_n at peak time) occur within the NW Pacific monsoon region (5^o~20^oN, 120^o ~155^oE) where ER waves tend to grow and attain maximum amplitude; convection centers of TCs are co-located with centers of low-level convergence and vorticity of ER waves that exhibit unstable features (zonal wave # 6, SW to NE tilt, convection coupling, vertical and horizontal shear). The unstable ER waves are attributed to the monsoon trough with cyclonic shear (-∂u /∂y> 0) and zonal convergence (∂u /∂x < 0) that favor the growth of synoptic-scale cyclonic disturbances through wave accumulation (waves increase in amplitude and decrease in scale). A growing disturbance can be further intensified by convection through abundant surface moisture and heat fluxes over the NW Pacific warm pool. The vertical and horizontal shear in the monsoon flow also causes wave energy trapping in the lower troposphere (thus an unstable wave growth), poleward wave propagation, and meridional asymmetric wave structure.