Abstract
Changes in tropical zonal atmospheric (Walker) circulation induce shifts in rainfall patterns along with devastating floods and severe droughts that dramatically impact the lives of millions of people. Historical records and observations of the Walker circulation over the 20th century disagree on the sign of change and therefore, longer climate records are necessary to better project tropical circulation changes in response to global warming. Here we examine proxies for thermocline depth and rainfall in the eastern tropical Indian Ocean during the globally colder Last Glacial Maximum (19–23 thousand years ago) and for the past 3000 years. We show that increased thermocline depth and rainfall indicate a stronger-than-today Walker circulation during the Last Glacial Maximum, which is supported by an ensemble of climate simulations. Our findings underscore the sensitivity of tropical circulation to temperature change and provide evidence for a further weakening of the Walker circulation in response to greenhouse warming.
Highlights
Changes in tropical zonal atmospheric (Walker) circulation induce shifts in rainfall patterns along with devastating floods and severe droughts that dramatically impact the lives of millions of people
We show that the thermocline was deeper and the amount of rainfall was higher in the eastern tropical Indian Ocean during the Last Glacial Maximum (LGM) compared to the late Holocene
We present three sea surface temperature (SST) and five thermocline temperature records calculated from shell Mg/Ca of planktic foraminifera at several sites in the eastern tropical Indian Ocean since the LGM (Fig. 1a, Methods and Supplementary Table 1)
Summary
Changes in tropical zonal atmospheric (Walker) circulation induce shifts in rainfall patterns along with devastating floods and severe droughts that dramatically impact the lives of millions of people. Studies on the recent development and future projection of the Walker circulations remain contradictory and suggest either a reduced[2,3,4,5] or an enhanced[6,7,8,9] Walker circulation in response to global warming This controversy mainly arises from the shortness of the instrumental data covering only a few decades, and necessitates records of Walker circulation changes from the geological past to better understand the sensitivity of the Walker circulation and the hydrological cycle to temperature change[2,3,4]. A relatively dry IPWP during the LGM has been attributed to a slowdown of the Walker circulation[11,12,13] These studies further indicate that similar to the present-day situation[14, 15], records of rainfall and thermocline depth in the eastern tropical Indian Ocean are the most sensitive diagnostic tools to reconstruct past Indian Walker circulation changes. We infer a further weakening of the Walker circulation with increasing global temperatures during the 21st century
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