Impact of Indian Ocean surface temperature gradient reversals on the Indian Summer Monsoon

Syee Weldeab,Carsten Rühlemann,Qinghua Ding,Vyacheslav Khon,Birgit Schneider,William R Gray

doi:10.1016/j.epsl.2021.117327

Abstract

Indian Summer Monsoon (ISM) precipitation is the main determinant of livelihood in a densely populated world region. The interannual variability of the ISM is influenced by several modes of climate variability, including anomalous seasonal sea surface temperature (SST) gradient reversals between the eastern, western, and northeastern Indian Ocean. With global warming, the frequency of zonal and meridional Indian Ocean's SST gradient changes is projected to increase but its impact on the ISM is debated. Here we present a 25,000-year proxy record of SST and inferred Ganges-Brahmaputra-Meghna (GBM) River runoff that provides a spatially integrated measure of ISM precipitation changes. This record indicates a monotonic deglacial strengthening of the ISM system when the SST gradient between the Bay of Bengal surface water and the eastern equatorial Indian Ocean was reversed. We posit that the reversal in the meridional SST gradient reduced the impact of Heinrich Event 1 and Younger Dryas on the low elevation part of the ISM domain. Furthermore, the proxy record shows that the strongest Holocene ISM strengthening occurred between 7900±470 and 5700±360 years before present, coinciding with and causally linked to the reversal of the Indian Ocean zonal SST gradient and ensuing changes in the wind fields, a sequence of events that is inferred from and supported by the results of our climate simulation. Our study demonstrates that changes in the Indian Ocean's zonal and meridional thermal gradient strongly shaped the timing of Holocene monsoon strengthening and the response of ISM to the last deglacial freshwater forcing.

Highlights

Indian Summer Monsoon (ISM) precipitationThe onset and northward progression of the Indian Summer Monsoon (ISM) precipitation begins, on average, in middle May/early June and is triggered by reversal in the thermal gradient in response to a warmer Indian subcontinent relative to the adjacent ocean surface water (Noska and Misra, 2016)
Some studies have suggested that the increased occurrence of middle Holocene positive Indian Ocean Dipole (pIOD) was caused by the strengthening of the Asian Monsoon (Abram et al, 2007)
Asian Monsoon strengthening occurred in the early Holocene (8–10 kyr BP) (Cheng et al, 2012) and there is no evidence for a strong and sustained development of pIOD-like conditions during the early Holocence

Summary

Introduction

The onset and northward progression of the Indian Summer Monsoon (ISM) precipitation begins, on average, in middle May/early June and is triggered by reversal in the thermal gradient in response to a warmer Indian subcontinent relative to the adjacent ocean surface water (Noska and Misra, 2016). Interannual variability of ISM precipitation is strongly influenced by the El Nino/Southern Oscillation (ENSO) phenomenon, anomalies in the eastern tropical Indian Ocean’s north-south SST gradient, and the development of a positive Indian Ocean Dipole (pIOD) (Cai et al, 2014; Noska and Misra, 2016; Saji et al, 1999; Weller and Cai, 2014; Yadav et al, 2018).

Methods

Results

Conclusion