A broad-scale circulation index for the interannual variability of the Indian summer monsoon

A broad-scale circulation index representing the interannual variability of the Indian summer monsoon is proposed and is shown to be well correlated w ith the interannual variability of precipitation in the Indian monsoon regi on. Using monthly precipitation analysis based on merging rain-gauge data w ith satellite estimates of precipitation for the period 1979-96, it is show n that the variability of precipitation on seasonal to interannual time-sca les is coherent over a large region covering the Indian continent as well a s the north Bay of Bengal and parts of south China. A new index, termed Ext ended Indian Monsoon Rainfall (EIMR), is defined as the precipitation avera ged over the region 70 degrees E-110 degrees E, 10 degrees N-30 degrees N. The EIMR index is expected to represent the convective heating fluctuations associated with the Indian monsoon better than the traditional all India M onsoon Rainfall (IMR) based only on the precipitation over the Indian conti nent. It is shown that large precipitation over the Bay of Bengal with sign ificant interannual variability cannot be ignored in the definition of Indi an summer monsoon and its variability. The June-to-September climatological mean EIMR is found to be larger than that of the LMR even though the forme r is averaged over a larger area. The dominant mode of interannual variabil ity of the Indian summer monsoon is associated with a dipole between the EI MR region and the north-western Pacific region (110 degrees E-160 degrees E , 10 degrees N-30 degrees N) and a meridional dipole between the EIMR regio n and the equatorial Indian Ocean (70 degrees E-110 degrees E, 10 degrees S -5 degrees N). It is argued that the interannual variability of the monsoon circulation is primarily driven by gradients of diabatic heating associated with variatio ns of the EIMR, and that the regional monsoon Hadley circulation is a manif estation of this heating. An index of the monsoon Hadley (MH) circulation i s defined as the meridional wind-shear anomaly (between 850 hPa and 200 hPa ) averaged over the same domain as the EIMR. Using circulation data from tw o independent reanalysis products, namely the National Centers for Environm ental Prediction/National Center for Atmospheric Research reanalysis and th e European Centre for Medium-Range Weather Forecasts reanalysis, it is show n that the MH index is significantly correlated with the EIMR. Also it is s hown that both the EIMR and MH indices have a dominant quasi-biennial varia bility, consistent with previous studies of IMR. Teleconnections of IMR, EI MR and MH indices with summer sea surface temperature (SST) have also been investigated. There are indications that the south equatorial Indian Ocean SST has a strong positive correlation with the EIMR. Also it is noted that the correlation of the monsoon indices with the eastern Pacific SST was wea k during the period under consideration primarily due to almost a reverse r elationship between monsoon and El Nino and Southern Oscillation during the latest eight years.