Interdecadal changes in the zonal wind and the intensity of intraseasonal oscillations during boreal summer Asian monsoon

Abstract

Decadal’interdecadal changes in the intensity of intraseasonal oscillations (ISO) and in thesummer mean wind fields in the Asian monsoon system are investigated using 51 yr of 850 hPazonal wind data obtained from the National Centers for Environmental Prediction — NationalCenter for Atmospheric Research (NCEP/NCAR) Reanalysis. Decadal’interdecadal variationscontribute significantly to the total variability of the summer mean 850 hPa zonal wind(30—45%) and the ISOs intensity (20—35%). These variations in the summer mean 850 hPazonal wind and in the intensity of the 30—60 d ISO have a distinct zonal structure and areassociated with the strength of low level westerlies and with meridional dynamics of the TropicalConvergence Zone (TCZ). Interdecadal changes in the intensity of the 10—20 d ISO are mostpronounced over the eastern Indian Ocean, the South China Sea and the western tropical Pacific.Singular value decomposition (SVD) analysis revealed a strong correlation on an interdecadaltimescale between the sea surface temperatures (SST) in the Indian Ocean and the summermean 850 hPa zonal wind and the intensity of the ISOs in the Asian summer monsoon, whereassuch correlations on a decadal timescale are weak. The temporal expansion coefficients of thefirst SVD mode show a climate regime shift in the mid-late 1970s. During the last few decades, SST in the Indian Ocean increased, resulting in a decreased land’sea heat contrast and weakerlow-level westerlies over northern Indian Ocean, the Indian subcontinent, and Indochina. Inresponse to sea surface warming and associated enhanced convection, the 30—60 d ISO becamestronger over the equatorial central and western Indian Ocean and the South China Sea, andweaker over the Indian subcontinent, the northern Arabian Sea and the Bay of Bengal.Meanwhile a 10—20 d ISO intensified over the eastern Indian Ocean, the South China Sea, andthe western tropical Pacific.