Decadal to multi-decadal variability in the Indian Monsoon Rainfall (IMR) and teleconnection with Atlantic Multidecadal Oscillation (AMO)

In order to understand the observed relation between the Atlantic multi-decadal Oscillation/Variability (AMO/AMV) and multi-decadal variability in the Indian summer monsoon (ISM) rainfall, we have analyzed observational data (Abish et al, 2013, 2015, Joseph et al, 2013) proxy reconstructions (Svendsen et al, 2014 and Sankar, submitted to Tellus, 2016) and several coupled model simulations (Cui et al, 2014, Gong et al, 2014, Svendsen et al, 2013 and Luo et al, in prep. to GRL). To assess the persistency of this relation a new multi-proxy reconstruction of the AMV was made (Svendsen et al. 2014). The short observational data do not significantly resolve multidecadal variability, and the existing AMV proxy reconstructions are all heavily based on land records, such as tree rings. Since AMV is an ocean signal, we have extended the AMV record past the instrumental record with five marine-based proxy records using principle component analysis to identify the leading mode of variability. We find that multidecadal variability in the Atlantic seems to persist prior to the instrumental record. This marine-based AMV reconstruction together with previously published AMV reconstructions, were then compared with several ISM proxy reconstructions (Sankar et al., submitted to Tellus, 2016). We find that multidecadal variability in the ISM is persistent as in Atlantic SSTs, but the link between ISM and AMV is not. The correlation between the two regions is weak and even negative in some periods. The observed correlation between AMV and the ISM has also weakened in the last decade, further suggesting that the AMV-ISM link is not steady. However there are not many reconstructions for the ISM that are available, and a similar analysis should be repeated when more reconstructions for the ISM have been published. The analysis of the proxy reconstructions in Sankar et al. (submitted to Tellus, 2016) showed that the AMV-ISM relation have been weaker prior to the instrumental era. This hypothesis is further investigated in Luo et al. (in prep.). Here we analysed 12 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), selected based on how well they simulate both the AMV and the ISM. We analysed both pre-industrial control simulations with constant forcing, and historical (1850-2000) simulation with prescribed external forcings including the greenhouse gasses. We found that none of the models simulated the observed AMV-ISM relation in the pre-industrial control simulations, but 3 models were able to simulate the correlation in the historical simulations. The significant correlation between AMV and ISM in these three models is due to external forcing phasing both the AMV and ISM independently, and not due to internal climate variability. In addition to the possible impact from the AMV on to the ISM on multidecadal timescales, the AMV could also impact the ISM though modulating interannual variability in the tropics. In Svendsen et al., (2013) we use a so-called freshwater hosing experiment with the Bergen Climate model, where freshwater is artificially added to the North Atlantic resulting in a weakening of the thermohaline circulation and a colder North Atlantic comparable to a negative AMV phase. We find that when the Atlantic cools there is a synchronization of interannual variability in the tropical Atlantic and Pacific, with the Atlantic leading the Pacific with half a year. The ENSO variability shifts also to higher frequencies during this period. From earlier studies on the ENSO-ISM relation we expect this to impact the ISM as well.

The climate in India is very complex, with profound variations on seasonal, interannual and longer time scales primarily associated with the monsoon variability. On seasonal time scales, the Indian Summer Monsoon (ISM) is dominating, leading to heavy rain fall from June to September. For example, a large part of the country gets about 80% of the annual rainfall during the four months. Therefore, any change to the Indian Summer Monsoon can have significant implication for the region. On interannual time sc ales, variations in the El Nino-Southern Oscillation (ENSO) pattern are important. It is also known that remote regions, like the Atlantic, may influence the ISM through so-called teleconnection patterns. The long-term changes to seasonal and inter-annual variations, as well as variations on decadal to multi-decdal time scales - underpin the importance of understanding decadal to multi-decadal scale variations in Indian climate and investigating mechanism of the climate teleconnection between the ISM and AMO. In India Clim, a novel approach will be taken, linking Indian and Norwegian climate students and scientists by combining historical and instrumental observations, and advanced IPCC/CMIP5 simulations including statistical analysis. It is the project ambition to significantly contribute to the basic understanding of decadal to multi-decadal variations in the Indian climate during the last 600 years and to explore the teleconnection between the AMO and the ISM, and by that adding to the knowledge basis needed for improving Indian Monsoon Rainfall prediction which has strong impact on hydrological cycle in India.