Indian summer monsoon during the last two millennia

The monsoon is a large-scale feature of the tropical atmospheric circulation, affectingpeople and economies in the world’s most densely populated regions. Future trends due to naturalvariabilityandhuman-inducedclimatechangesareuncertain.Palaeoclimaterecordscanimprove ourunderstanding of monsoon dynamics and thereby reduce this uncertainty. Palaeoclimate records haverevealed a dramatic decrease in the Asian summer monsoon since the early Holocene maximum9kaBP.Herewefocusonthelast2ka,wheresomerecordsindicateanincreasingtrendinthesummermonsoon. Analysing Globigerina bulloides upwelling records from the Arabian Sea, we find theweakest monsoon occurred 1500 a BP, with an increasing trend towards the present. Copyright # 2010 John Wiley & Sons, Ltd. KEYWORDS: monsoon; Holocene; Tropics; India; Arabian Sea. Introduction Earth’s climate is changing as a result of both natural andanthropogenic causes. One region of the world that isparticularly susceptible to shifts in climate and extremeweather events is the Indian subcontinent. In this denselypopulated region both droughts and floods, as well as trends inthe seasonal receipt of rainfall, have the potential to bedevastating from economic, agricultural and public healthperspectives. The Southwest Indian Monsoon is the maincontributor to India’s yearly rainfall budget. Therefore,unravelling past trends of the monsoon rains is critical tounderstanding future monsoon trends.India ishome tooneofthelongesttimeseriesofinstrumentalclimate observations, the all-India monsoon rainfall index,spanning 1844 to the present (Sontaake et al., 1993;Parthasarathy et al., 1994). This record is remarkable bothfor showing an absence of a long-term trend and for showing alarge variation in rainfall from one year to the next. Records ofthe atmospheric circulation such as the Indian Monsoon Index(IMI), based on the Arabian Sea winds, are much shorter andspan only the past 50 a (Wang et al., 2001). These records alsoindicate large variations from one year to the next and do notreveal a trend. Blending appropriate instrumental records withpalaeo observations, from ocean sediments, cave deposits orotherproxies,canprovidealongerperspectiveonthemonsoonand its natural variability than otherwise would be available.The monsoon is the result of both the seasonally changingthermal contrast between the land and ocean, and the seasonalmigration of the Intertropical Convergence zone (ITCZ), whichreaches its northernmost location in summer, exacerbating thethermally driven pressure gradient (Gadgil, 2003). Duringwinter, the air over the land is cooler and denser than the airover the water. This creates a surface pressure contrast that isenhanced by the position of the ITCZ over the ocean south ofthe continent of India, resulting in a dry northeasterly wind thatblows across the continent. In spring the continent warmsquickly compared to the surrounding water. The winds thenreverse, and a southwesterly wind blows across the ArabianSea, bringing moisture-laden air towards India. The south-westerly winds that bring the monsoon rains also causeupwelling and sea surface cooling in the Arabian Sea,producing a distinctive fossil record that can be tied to themonsoon winds via well-known physical mechanisms.A variety of Indian and Asian summer monsoon indices havebeen developed from instrumental records. In general,throughout India and Asia in summer, stronger winds overthe ocean and increased rainfall over land are correlated withthe large-scale atmospheric circulation (Fig. 1). The Webster–Yang Index (WYI) is a broad-scale index of the monsooncirculation, covers a large continuous area (40–1108 E, 0–208N), and measures the difference in strength between low-level(850hPa) and upper-level (200hPa) winds (Webster and Yang,1992). In contrast, the IMI measures the Arabian Sea windsby subtracting the low-level (850hPa) zonal wind over land(20–308 N, 60–908 E) from the low-level (850hPa) winds in