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

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.