The response of the tropical climate in the Indian Ocean realm to abrupt climate change events in the North Atlantic Ocean is contentious. Repositioning of the intertropical convergence zone is thought to have been responsible for changes in tropical hydroclimate during North Atlantic cold spells 1-5 , but the dearth of high-resolution records outside the monsoon realm in the Indian Ocean precludes a full understanding of this remote relationship and its underlying mechanisms. Here we show that slowdowns of the Atlantic meridional overturning circulation during Heinrich stadials and the Younger Dryas stadial affected the tropical Indian Ocean hydroclimate through changes to the Hadley circulation including a southward shift in the rising branch (the intertropical convergence zone) and an overall weakening over the southern Indian Ocean. Our results are based on new, high-resolution sea surface temperature and seawater oxygen isotope records of well dated sedimentary archives from the tropical eastern Indian Ocean for the past 45,000 years, combined with climate model simulations of Atlantic circulation slowdown under Marine Isotope Stages 2 and 3 boundary conditions. Similar conditions in the east and west of the basin rule out a zonal dipole structure as the dominant forcing of the tropical Indian Ocean hydroclimate of millennial-scale events. Results from our simulations and proxy data suggest dry conditions in the northern Indian Ocean realm and wet and warm conditions in the southern realm during North Atlantic cold spells.In the North Atlantic, the most recent glacial and deglacial periods are characterized by a series of abrupt and severe cold snaps of millennial duration associated with either iceberg instabilities and surges (Heinrich events) or freshwater input from the Arctic Ocean 6 (the Younger Dryas). These abrupt events are of particular interest because they were rapidly communicated through the ocean by a slowdown, or potentially a shutdown, of the Atlantic meridional overturning circulation 7 (AMOC) and through the atmospheric circulation 8 causing climate anomalies worldwide. Climate archives document a significant tropical hydrologic response to these events. Dry Younger Dryas and Heinrich stadials have been reported from various marine and terrestrial archives across the tropical Indian Ocean 4,9-14 .However, a few records suggest wet Younger Dryas or Heinrich stadials over northeast Australia 15 , southern Indonesia 5,16 and southeast Africa 12,17 . Although there seems to be strong evidence that the intertropical convergence zone (ITCZ) moved southwards in the tropical Atlantic 2 , a wide range of mechanisms have been offered to explain the connection between the cooling of the North Atlantic and tropical Indian Ocean hydroclimates: a weakening of the rainfall system in response to regional sea surface cooling 13,14 ; and changes in the monsoon intensity 4,10,16 associated with a southward shift in the mean 1 or winter 4,5,15 position of the ITCZ or in the position of oceanic fronts 18 .How...

The Australian-Indonesian monsoon is an important component of the climate system in the tropical Indo-Pacific region 1 . However, its past variability, relation with northern and southern high-latitude climate and connection to the other Asian monsoon systems are poorly understood. Here we present high-resolution records of monsoon-controlled austral winter upwelling during the past 22,000 years, based on planktic foraminiferal oxygen isotopes and faunal composition in a sedimentary archive collected offshore southern Java. We show that glacial-interglacial variations in the Australian-Indonesian winter monsoon were in phase with the Indian summer monsoon system, consistent with their modern linkage through cross-equatorial surface winds. Likewise, millennial-scale variability of upwelling shares similar sign and timing with upwelling variability in the Arabian Sea. On the basis of element composition and grain-size distribution as precipitation-sensitive proxies in the same archive, we infer that (austral) summer monsoon rainfall was highest during the Bølling-Allerød period and the past 2,500 years. Our results indicate drier conditions during Heinrich Stadial 1 due to a southward shift of summer rainfall and a relatively weak Hadley cell south of the Equator. We suggest that the Australian-Indonesian summer and winter monsoon variability were closely linked to summer insolation and abrupt climate changes in the northern hemisphere.The seasonality of the global monsoon is of central importance to the global hydrologic cycle and its environmental influence on human societies. Palaeoclimate reconstructions generally agree that the strengths of the East Asian monsoon and the Indian monsoon are tightly coupled to the Northern Hemisphere climate through zonal migrations of the monsoon convection centres, also interpreted as Intertropical Convergence Zone (ITCZ) migration 2-6 . The mechanism of the Australian-Indonesian monsoon (AIM) in both time and space is, however, poorly understood [7][8][9][10][11][12] . Australian records 9-11 indicate wetter-than-today conditions during the early and middle Holocene. These changes have been explained through a Northern Hemisphere insolation control on the AIM (refs 8, 10), regional sea-surface-temperature (SST) feedback 7 or human impact on the late Holocene vegetation cover 8 , countering or even cancelling the response of the AIM to local insolation. Deglacial sea-level rise and its effect on exposed land surface may also be an important influence on southeast Indonesian rainfall 12,13 . Moreover, many records suggest that the Asian monsoons, including the AIM, changed in association with North Atlantic deglacial climate

Monsoons are the dominant seasonal mode of climate variability in the tropics and are critically important conveyors of atmospheric moisture and energy at a global scale. Predicting monsoons, which have profound impacts on regions that are collectively home to more than 70 per cent of Earth's population, is a challenge that is difficult to overcome by relying on instrumental data from only the past few decades. Palaeoclimatic evidence of monsoon rainfall dynamics across different regions and timescales could help us to understand and predict the sensitivity and response of monsoons to various forcing mechanisms. This evidence suggests that monsoon systems exhibit substantial regional character.

[1] Results from sediment trap experiments conducted in the seasonal upwelling area off south Java from November 2000 until July 2003 revealed significant monsoon-, El Niño-Southern Oscillation-, and Indian Ocean Dipole-induced seasonal and interannual variations in flux and shell geochemistry of planktonic foraminifera. Surface net primary production rates together with total and species-specific planktonic foraminiferal flux rates were highest during the SE monsoon-induced coastal upwelling period from July to October, with three species Globigerina bulloides, Neogloboquadrina pachyderma dex., and Globigerinita glutinata contributing to 40% of the total foraminiferal flux. Shell stable oxygen isotopes (d 18 O) and Mg/Ca data of Globigerinoides ruber sensu stricto (s.s.), G. ruber sensu lato (s.l.), Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globorotalia menardii in the sediment trap time series recorded surface and subsurface conditions. We infer habitats of 0-30 m for G. ruber at the mixed layer depth, 60-80 m (60-90 m) for P. obliquiloculata (N. dutertrei) at the upper thermocline depth, and 90-110 m (100-150 m) for G. menardii in the 355-500 mm (>500 mm) size fraction corresponding to the (lower) thermocline depth in the study area. Shell Mg/Ca ratio of G. ruber (s.l. and s.s.) reveals an exponential relationship with temperature that agrees with published relationships particularly with the Anand et al. (2003) equations. Flux-weighted foraminiferal data in sediment trap are consistent with average values in surface sediment samples off SW Indonesia. This consistency confirms the excellent potential of these proxies for reconstructing past environmental conditions in this part of the ocean realm.

The tropical ocean plays a major role in global climate. It is therefore crucial to establish the precise phase between tropical and high-latitude climate variability during past abrupt climate events in order to gain insight into the mechanisms of global climate change. Here we present alkenone sea surface temperature (SST) records from the tropical South China Sea that show an abrupt temperature increase of at least 1 degrees C at the end of the last glacial period. Within the recognized dating uncertainties, this SST increase is synchronous with the Bølling warming observed at 14.6 thousand years ago in the Greenland Ice Sheet Project 2 ice core.