Distinguishing Features of Ensemble Spread between Drought and Flood Years of Indian Summer Monsoon in the Past 58 Years (1958–2015) Reforecasts

Abstract:

This study examines the different characteristics of ensemble spread between drought and flood years of Indian summer monsoon (ISM) using the National Centers for Environmental Prediction (NCEP) Climate Forecast System version-2 (CFSv2). We have analyzed a set of 20-member ensemble seasonal reforecasts for 58-years (1958-2015) using CFSv2 initialized in April, which COLA/GMU have conducted. An ensemble reforecast of 20 members is generated by matching each of the five ocean initial states at 00Z 1st of April with the atmospheric and land initial states at 00Z of the first four days of April while the sea ice initial state is fixed at 00Z 1st for all ensemble-members (Huang et al. 2017).

ISM rainfall (ISMR) is defined by June to September (JJAS) mean rainfall over Indian landmass. It is confirmed that in the reforecasts, India tends to experience depressed (enhanced) rainfall in summer when the tropical eastern Pacific SST is above (below) normal, showing overly strong negative correlation between the ensemble mean ISMR anomaly and JJAS mean NINO3.4 index, compared with the observation. On the other hand, ensemble spread of ISMR is positively correlated (correlation coefficient = 0.63) with the NINO3.4 index, indicating that ensemble spread in drought ISMR years seems to be larger than that in flood years. To quantify the ensemble spread of ISMR during flood and drought years, we have selected 13 strong ISMR years (e.g., 1973 and 2008 years) in which the ensemble mean rainfall anomaly is greater than +1 standard deviation and 11 deficient ISMR years (e.g., 1965 and 2014 years) in which the ensemble mean rainfall anomaly is less than -1 standard deviation over Indian landmass in the reforecasts. The mean value of ensemble spread for the drought ISMR years is 0.68 but 0.57 for the flood years, demonstrating lager ensemble spread of ISMR in drought years than flood years. The spatial structure of ensemble mean ISMR composite anomalies during drought years shows better agreement with observed rainfall anomalies in comparison to flood years. As a result, smaller ensemble spread in the flood years may suggest that the ensemble prediction of ISMR in flood years tends to be overconfident and less reliable due to the underestimate of the forecast uncertainty, compared to drought years.

References:

Huang, B., Shin CS, Shukla J, Marx L, Balmaseda M, Halder S, Dirmeyer PA, Kinter JL III (2017). Reforecasting the ENSO events in the past fifty-seven years (1958–2014). J Clim. doi: 10.1175/JCLI-D-16-0642.1

Shukla, RP and Shin CS (2019) Distinguishing features of ensemble spread between drought and flood years of Indian summer monsoon in the past 58 years (1958–2015) reforecasts. (preparing manuscript)