Spatial variability of daily rainfall over northeast India during summer monsoon season

Abstract

The summer monsoon rainfall over northeast India shows characteristic spatial and temporal variability due to the interaction of basic monsoon flow with orography and the synoptic scale systems developing over Indian region. The aim of this study is to find out the main features of spatial variability of daily monsoon rainfall over northeast India and associated synoptic systems. The principal component analysis (PCA) is a good tool to filter out the main components from the noise and this is applied to daily monsoon rainfall (June-September) data of 50 almost uniformly distributed stations of northeast India over a period of 10 years (1991-2000). The association of synoptic systems with significant principal components (PC) has been confirmed by analyzing daily synoptic systems over northeast India and neighbourhood during the same period.

Eight PCs explaining about 46% of the total variance of daily monsoon rainfall over northeast India are significant and can be attributed to different physical processes. The first PC associated with good monsoon condition over northeast India may be attributed to low over Sub Himalayan West Bengal (SHWB). Similarly, the second PC may be attributed to good monsoon condition due to low level cyclonic circulation (cycir) over East Madhya Pradesh and Chattisgarh (EMPC), third PC to good monsoon condition due to depression over Bihar, fourth PC to good monsoon condition due to low level cycir over Arunachal Pradesh, fifth and sixth PCs to weak monsoon condition due to low over west central Bay and cycir extending upto mid tropospheric level (MTL) over North Coastal Andhra Pradesh respectively, seventh and eighth PCs to rainfall activity due to cycir extending upto MTL over Bangladesh and northwest Bay respectively.

According to rotation of these significant PCs in S-mode, northeast India consists of eight homogeneous regions with the rainfall over each region being dominantly associated with one significant PC.