The Indian summer monsoon climate during the Last Millennium, as simulated by the PMIP3

Abstract. In this study, using the available model simulations from the PMIP3, we study the mean summer (June–September; JJAS) climate and its variability in India during the Last Millennium (CE 850–1849; LM) for which conventional observations are unavailable, with emphasis on the Medieval Warm Period (MWP; CE 1000–1199 as against the CE 950–AD1350 from the proxy-observations) and Little Ice Age (LIA; CE 1550–1749 as against the CE 1500–1850 proxy observations. Out of the eight available models, by validating the corresponding simulated global and Indian mean summer temperatures and mean Indian summer monsoon rainfall (ISMR), and their respective trends, from historical simulations (CMIP5) against the various observed/reanalysed datasets for the 1901–2005 period. From this exercise, we identify seven realistic models.

The models simulate higher (lower) mean summer temperatures in India as well as globally during the MWP (LIA) as compared to the corresponding LM statistics, in conformation with several proxy studies. Our Analysis shows a strong negative correlation between the NINO3.4 index and the ISMR and a positive correlation between NINO3.4 and summer temperature over India during the LM, as is observed in the last one-and-half centuries. The magnitude of the simulated ISMR-NINO3.4 index correlations, as seen from the multi-model mean, is found to be higher for the MWP (−0.19; significant at 95 % confidence level) as compared to that for the LIA (−0.09; insignificant). Our analysis also shows that the above (below) LM-mean summer temperatures during the MWP (LIA) are associated with relatively more (less) number of concurrent El Niños as compared to the La Niñas. Distribution of boreal summer velocity potential at 850 hPa in the central tropical pacific and a zone of anomalous convergence in the central tropical pacific, flanked by two zones of divergence in the equatorial pacific, suggesting a westward shift in Walker circulation as compared to the current day signal. The anomalous divergence centre in the west also extends into the equatorial eastern Indian Ocean, which results in an anomalous convergence zone over India and therefore excess rainfall during the MWP as compared to the LM. The results are qualitative, given the inter-model spread.