Interdecadal changes in 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 fields in the Asian monsoon system are investigated using 850 hPa zonal wind data obtained from the National Centers for Environmental Prediction - National Center for Atmospheric Research (NCEP/NCAR) Reanalysis for a 51-year period. Decadal- interdecadal variations have considerable contribution to the total variability of the summer mean 850 hPa zonal wind (30-45%) and the ISO intensity (20-35%). These variations in the summer mean 850 hPa zonal wind and in intensity of 30-60 day ISO have a distinct zonal structure and associated with strength of low level westerlies and with meridional dynamics of the Tropical Convergence Zone (TCZ). Interdecadal changes in intensity of 10-20 day ISO are the most pronounced in the domain of its activity over the eastern Indian ocean and tropical western Pacific. Singular value decomposition (SVD) analysis has revealed a strong correlation on interdecadal time scale between the sea surface temperatures (SST) in the Indian ocean and the summer mean 850 hPa zonal wind and the ISO intensities in the Asian summer monsoon, whereas such links on decadal time scale are weak. Temporal behaviour of all expansion coefficients of the first SVD mode is characterized by a climate regime shift in the mid-late 1970s. During last few decades SST in the Indian ocean increased, resulting in decrease of land-sea heat contrast and associated decrease in the strength of low level westerlies over northern Indian ocean, Indian subcontinent, and Indochina. In response to sea surface warming, the 30-60 day ISO became more intensive over Indian ocean and less intensive over Indian subcontinent. A 10-20 day ISO has intensified over the eastern Indian ocean, South China Sea, and western tropical Pacific.