Intraseasonal fluctuations in low-level meridional winds over the south China Sea and the western Pacific and monsoonal convection over Indonesia and northern Australia

Abstract

Outgoing long‐wave radiation (OLR), objectively analyzed daily winds (u, v) and geopotential height data for 5 northern winters are used to study the relationship between low‐level equatorward (meridional) winds and tropical convection on the 30–60 day time scale. The general tendency of equatorial 30–60 day OLR perturbations is to propagate eastward. Occasionally, these perturbations exhibit irregular movement. This study is limited to the periods when the eastward propagation is clearly defined. Correlation maps indicate that the low‐frequency modes are of global‐scale character with zonal wavenumber one. The low‐level 30–60 day northerly wind along the eastern edge of the Siberian high (anomaly) does not contribute to convection and monsoon activity in the Indonesian‐northern Australian region. However, 30–60 day northeasterly flows of subtropical Pacific origin play a significant rôle in monsoon activity. An active monsoon condition with strong upper‐level southerly outflow from the monsoon region and an intense westerly jet near Japan, is observed approximately 10 days prior to the maximum low‐level northerly currents from Siberia. Fluctuations with periods less than 30 days are considered as transient disturbances. The square of transient meridional winds shows pronounced spectral peaks in the 30–60 day period range. This is associated with an amplitude modulation of synoptic‐scale transient disturbances, which is most pronounced in both the NH and SH extratropics. Perhaps, transient disturbances, as a group, can make a contribution toward the enhancement and/or maintenance of extratropical low‐frequency perturbations.

Nonlinear Dynamics and Predictability in the Atmospheric Sciences

“…[1989] and in the intermediate model of Mak [1989]. Equatorward propagation of intraseasonal oscillations is apparent in the observations of Sumathipala and Murakami [1988] and of Magaña and Yanai [1990].…”

Monsoon intra-seasonal variability in a high-resolution version of Met Office Global Coupled model

“…The East Asian (EA) and the western North Pacific (WNP) summer monsoon is one of the most important land-atmosphereocean coupled climate system over the subtropics and tropics and exhibits substantial variability at seasonal and intra-seasonal time scales (Ding, 2007). During boreal summer (May-September), the rainfall fluctuations over the EAWNP region are manifested in the form of active-break cycles intimately associated with the monsoon intra-seasonal oscillations (ISOs) (Chen and Murakami, 1988;Wu, 1993, Sumathipala andMurakami, 2010). The monsoon ISO with periodicity between 10 and 90 days has a more complex structure compared to the wintertime MJO (Madden and Julian, 1972), in which eastward propagation dominates, by also exhibiting northward or northwest propagation of convective anomalies (Kayano and Kousky, 1999;Hsu and Weng, 2001).…”

How much does heat content of the western tropical Pacific Ocean modulate the South China Sea summer monsoon onset in the last four decades?

“…There are a number of factors affecting the SCSSM onset: (1) land‐sea thermal contrast and Tibetan plateau [e.g., Ye , ; Murakami and Ding , ; He et al ., ; Yanai et al ., ; Sun and Ding , ; Zhang et al ., ], (2) tropical oceans [e.g., Wang et al ., ; Chao and Chen , ; Wang and Zhang , ; Chen and Hu , ; Annamalai et al ., ; Hu and Yu , ; Yu and Hu ,], (3) the midlatitude trough and front system [e.g., Wu et al ., ], (4) high latitude influence, such as Arctic Oscillation [e.g., Gong and Ho , ] and Antarctic Oscillation [e.g., Nan and Li , ], (5) intraseasonal oscillations with 10–20 day period and Madden‐Julian Oscillation [e.g., Chen and Chen , ; Lau et al ., ; Lin , ; Sumathipala and Murakami , ; Chen et al ., ; Li et al ., ; Mao and Chan , ].…”