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

Abstract

A broad-scale circulation index representing the interannual variability of the Indian summer monsoon is proposed and is shown to be well correlated with the interannual variability of precipitation in the Indian monsoon region. Using monthly precipitation analysis based on merging rain-gauge data with satellite estimates of precipitation for the period 1979-96, it is shown that the variability of precipitation on seasonal to interannual time-scales is coherent over a large region covering the Indian continent as well as the north Bay of Bengal and parts of south China. A new index, termed Extended Indian Monsoon Rainfall (EIMR), is defined as the precipitation averaged over the region 70°E-110°E, 10°N-30°N. the EIMR index is expected to represent the convective heating fluctuations associated with the Indian monsoon better than the traditional all India Monsoon Rainfall (IMR) based only on the precipitation over the Indian continent. It is shown that large precipitation over the Bay of Bengal with significant interannual variability cannot be ignored in the definition of Indian summer monsoon and its variability. the June-to-September climatological mean EIMR is found to be larger than that of the IMR even though the former is averaged over a larger area. the dominant mode of interannual variability of the Indian summer monsoon is associated with a dipole between the EIMR region and the north-western Pacific region (110°E-160°E, 10°N-30°N) and a meridional dipole between the EIMR region and the equatorial Indian Ocean (70°E-110°E, 10°S-5°N). It is argued that the interannual variability of the monsoon circulation is primarily driven by gradients of diabatic heating associated with variations of the EIMR, and that the regional monsoon Hadley circulation is a manifestation of this heating. an index of the monsoon Hadley (MH) circulation is 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 two independent reanalysis products, namely the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis and the European Centre for Medium-Range Weather Forecasts reanalysis, it is shown that the MH index is significantly correlated with the EIMR. Also it is shown that both the EIMR and MH indices have a dominant quasi-biennial variability, consistent with previous studies of IMR. Teleconnections of IMR, EIMR 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 weak during the period under consideration primarily due to almost a reverse relationship between monsoon and El Nino and Southern Oscillation during the latest eight years.