The present study examined the ability of Coupled Model Intercomparison Project phase 5 (CMIP5) models in representing the differences in decaying phases of El Niño and their impact on south Asia and east Asia monsoon during June and July (JJ), the early summer monsoon months. El Niño decay is classified into three categories, based on the timing of decay with respect to the summer season after the peak phase of El Niño. Analysis suggests that many CMIP5 models are able to capture the differences in the decaying phase of El Niño. Observed rainfall anomalies are positive over most parts of south and east Asia regions during the early decay (ED) years mainly due to anomalous Tropical Indian Ocean (TIO) Sea Surface Temperature (SST) warming and La Niña like forcing from the eastern Pacific. Most models show significant skill in capturing the positive rainfall anomalies over south Asia and TIO SST warming in ED years. However, many models have failed to represent east Asian rainfall anomalies due to weak Western North Pacific (WNP) anticyclone and its association with El Niño‐Southern Oscillation. In case of mid‐decay (MD) years, observed rainfall anomalies over south Asia is negative especially in the monsoon trough region and positive over east Asian region. Low‐level divergence induced by anomalous WNP anticyclone extending to head Bay of Bengal and Gangetic Plain region in MD years caused low rainfall over south Asia. These features are however not well organized in many CMIP5 models. In no decay (ND) years, the rainfall anomalies over south and east Asia regions are negative in almost all the CMIP5 models, which is consistent with the observations. This study highlights the importance of proper representation of differences in the decaying phase of El Niño and associated teleconnections in CMIP5 models.
The Indian summer monsoon (ISM) rainfall is often influenced by El Niño‐Southern Oscillation (ENSO) on the interannual time scale. Physical mechanisms that link the canonical El Niño and ISM rainfall have been well documented. Very few studies have discussed the pathways that link El Niño Modoki and ISM rainfall variations. In this study using long period (1901–2014) reanalysis and observed rainfall data, it is found that rainfall anomalies are negative over the southern peninsular India and positive over the central parts of India during El Niño Modoki years. Detailed analysis suggests that the El Niño Modoki modulates ISM rainfall by inducing changes in the western north Pacific (WNP) low‐level circulation. The weak positive rainfall over the monsoon trough region is due to weak moisture convergence associated with the westward extension of WNP cyclonic circulation corroborated by lows and depressions. Anomalous WNP cyclonic circulation is induced by the central Pacific Sea Surface Temperature (SST) warming associated with the El Niño Modoki. Further, strong moisture divergence over south peninsular India caused negative rainfall anomalies during the El Niño Modoki years. Thus, WNP circulation plays a critical role in determining the ISM rainfall patterns during the El Niño Modoki years as evidenced by significant correlation between WNP circulation pattern and EMI. These regional rainfall patterns over the Indian subcontinent corresponding to El Niño Modoki are not well explained in previous studies. It is found that many Coupled Model Intercomparison Project Phase 5 (CMIP5) models displayed poor skill in representing the relation between ISM rainfall and El Niño Modoki events due to unrealistic simulation of SST and circulation patterns over the Indo‐Pacific region. This suggests that CMIP5 models tend to have a large diversity in representing the El Niño Modoki teleconnections to ISM rainfall.
Abstract. We describe the Monsoon Mission Coupled Forecast System version 2 (MMCFSv2) model, which substantially upgrades the present operational MMCFSv1 (version 1) at the India Meteorology Department. We evaluate MMCFSv2 based on the latest 25 years (1998–2022) of retrospective coupled hindcast simulations of the Indian Summer Monsoon with April initial conditions from Coupled Forecast System Reanalysis. MMCFSv2 simulates the tropical wind, rainfall, and temperature structure reasonably well. MMCFSv2 captures surface winds well and reduces precipitation biases over land, except in India and North America. The dry bias over these regions remained similar to MMCFSv1. MMCFSv2 captures significant features of the Indian monsoon, including the intensity and location of the maximum precipitation centres and the large-scale monsoon circulation. MMCFSv2 improves the phase skill (anomaly correlation coefficient) of the interannual variation of ISMR by 17 % and enhances the amplitude skill (Normalized Root Mean Square Error) by 20 %. MMCFSv2 shows improved teleconnections of ISMR with the equatorial Indian and Pacific oceans. This 25-year hindcast dataset will serve as the baseline for future sensitivity studies of MMCFSv2.
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