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

Decadal–interdecadal changes in the intensity of intraseasonal oscillations (ISO) and in the summer mean wind fields in the Asian monsoon system are investigated using 51 yr of 850 hPa zonal wind data obtained from the National Centers for Environmental Prediction — National Center for Atmospheric Research (NCEP/NCAR) Reanalysis. Decadal–interdecadal variations contribute 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 hPa zonal wind and in the intensity of the 30—60 d ISO have a distinct zonal structure and are associated with the strength of low level westerlies and with meridional dynamics of the Tropical Convergence Zone (TCZ). Interdecadal changes in the intensity of the 10—20 d ISO are most pronounced 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 interdecadal timescale 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, whereas such correlations on a decadal timescale are weak. The temporal expansion coefficients of the first 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 weaker low-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 became stronger over the equatorial central and western Indian Ocean and the South China Sea, and weaker 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, and the western tropical Pacific.