Biases in Indian Summer Monsoon Precipitation Forecasts in the Unified Model and Their Relationship With BSISO Index

This study shows that the Boreal Summer Intraseasonal Oscillation (BSISO) dominates the Indian summer monsoon low-precipitation bias in the Met Office Unified model. Analyzing a recent 9-year period (June, July, August only), it is found that the precipitation bias is dominated by break and break-to-active transition BSISO phases, while some of the other phases have no bias at all over a 7-day forecast. Evidence of a link to upstream effects is found, in that there is a delayed reduction in the moisture flux entering India from the west. It is also shown that an increase in the net flow of moisture out of India to the east is strongly linked to the low-precipitation bias, and there is some evidence that this is related to a lack of low-pressure systems over India. Most atmospheric models have substantial rainfall biases over India, and these results may indicate the circulation patterns responsible.

Automated summary

The study reveals that BSISO plays a dominant role in the Indian summer monsoon low-precipitation bias in the Met Office Unified model. It shows that the bias is mainly associated with break and break-to-active transition BSISO phases, potentially linked to a lack of low-pressure systems over India.