Intra-Seasonal Variability of Cross Equatorial Flow, Mascarene High and Circumpolar Lows: Monsoon Season 2015

Abstract

Indian summer monsoon rainfall (ISMR) of the year 2015 has been deficient as emphasized by many seasonal forecasting communities. And, the prime reason for deficiency was the southern oscillations in the Pacific, where the eastern pacific is much warmer than the normal, leading to El-Nino phase. But there are other factors from the southern hemisphere contributed to the deficient rainfall over India. It was observed that the Mascerene High (MH), which acts as a prime engine that drives the whole circulation of the Indian summer monsoon, was very weak. The intensity and position of the MH decides the speed of the cross equatorial flow over Somalia region. Since cross equatorial flow (CEF) over Somalia region has good correlation with the ISMR, here weekly wind composites of Somalia region and the area weighted average rainfall of the Indian region for the monsoon 2015 were computed to investigate the Somali Jet Speeds with corresponding ISMR. Strong wind speed patterns are observed for the June and September whereas they were weak during August, July months. The CEF was related to MH, which is in turn regulated by the circumpolar lows (CPLs). Hence, the strength and locations of the CPLs have been studied to identify the corresponding temporal variations of the MH which in turn is manifested in the deficient sub periods during the current deficient monsoon. CPL has very good correlation with MH during the month of June, for which correlation coefficient is- 0.51 whereas correlation is almost negligible during Months of July and August, for which correlation coefficients are --0.04 and -0.1 respectively. And reverse correlation is observed for the month of September, it has correlation coefficient of 0.04 on the positive side.