PNNL

Abstract

CMIP5 models exhibit a mean dry bias and a large inter-model spread in simulating Indian monsoon precipitation but the origins of the bias and spread have not been well understood. Using moisture and energy budget analysis that exploits the weak temperature gradients in the tropics, we show that they are linked to the convection simulated over the equatorial Indian Ocean. About half of the 21 models analyzed operate at the steep gradient of the non-linear relationship between the normalized precipitable water and normalized precipitation, where small differences in the former are amplified as large differences in precipitation. These models preferentially produce higher precipitation over the equatorial Indian Ocean, and contribute disproportionately to the inter-model spread and multi-model mean dry bias in monsoon precipitation. Conversely, models on the flat side of the relationship are in better agreement with each other and with observations. Under the RCP8.5 forcing scenario, models on the steep part of the relationship project stronger response to warming and dominate the inter-model spread in the projection. This study identified the normalized precipitable water as a key metric for understanding the behavior of model simulated equatorial and Indian monsoon precipitation.