Why the droughts of the Indian summer monsoon are more severe than the floods.

Vertically integrated eastward moisture flux at 70° E over the Arabian Sea (Q) is the dominant factor that drives seasonal Indian summer monsoon rainfall (ISMR). Yet, the nature of this relationship depends on the phase of El-Nino Southern Oscillation (ENSO). The conversion efficiency of Q to rainfall is twice as high during the El-Nino years than La-Nina years. This increased sensitivity of ISMR to Q during the El-Nino years causes positive feedback to reduce Q further. As a result, droughts are more frequent and more vigorous than floods, manifesting as a negatively skewed non-Gaussian probability distribution function of ISMR. A non-linear multi regression model using Q and ENSO explained the observed skewness of ISMR with the severity of droughts. Besides, droughts have frequent long break-spells of rainfall, characterized by zonal circulation with an ascending branch over the western North Pacific Ocean and descending branch over the Indian region, which may cause moisture divergence and hence the moist stability over India. The increased stability in droughts might determine the conversion efficiency of Q to rainfall. This zonal dipole pattern of circulation anomaly over India and the western North Pacific Ocean in long break spells is manifest about 10-days before the advent of break conditions. [ABSTRACT FROM AUTHOR]