Preprints
https://doi.org/10.5194/cp-2021-164
https://doi.org/10.5194/cp-2021-164
 
16 Dec 2021
16 Dec 2021
Status: a revised version of this preprint was accepted for the journal CP and is expected to appear here in due course.

El Niño-like conditions and seasonal aridity in the Indo-Pacific Warm Pool during the Younger Dryas

Petter Lars Hällberg1, Frederik Schenk1,2, Kweku Afrifa Yamoah3, Xueyuen Kuang4, and Rienk Hajo Smittenberg1 Petter Lars Hällberg et al.
  • 1Department of Geological Sciences and Bolin Centre for Climate Research, Stockholm University, 106 91, Sweden
  • 2Rossby Centre, Swedish Meteorological and Hydrological Institute, 601 76, Sweden
  • 3School of Geography, Earth and Environmental Sciences, University of Birmingham, B15 2TT, UK
  • 4School of Atmospheric Sciences, Nanjing University, 210023, China

Abstract. Island South-East Asia (ISEA) is a highly humid region and hosts the world’s largest tropical peat deposits. Most of this peat accumulated relatively recently during the Holocene, suggesting a generally drier and/or more seasonal climate during earlier times. Although there is evidence for savanna expansion and drier conditions during the Last Glacial Maximum (LGM, 21 ka BP), the mechanisms behind hydroclimatic changes during the ensuing deglacial period has received much less attention and are poorly understood. Here we use CESM1 climate model simulations to investigate the key drivers behind ISEA climate at the very end of the last deglacial period, at 12 ka BP. A transient simulation (TRACE) is used to track the climate seasonality and orbitally driven change over time during the deglaciation into the Holocene. In agreement with proxy-evidence, CESM1 simulates overall drier conditions at 12 ka BP. More importantly, ISEA experienced extreme seasonal aridity, in stark contrast to the ever-wet modern climate. We identify that the simulated drying and enhanced seasonality at 12 ka BP is mainly the result of a combination of three factors: 1) large orbital insolation difference between summer and winter in contrast to the LGM and the present day; 2) a stronger winter monsoon caused by a larger interhemispheric thermal gradient in boreal winters; and 3) a major reorganization of the Walker Circulation with an inverted land-sea circulation with a complete breakdown of deep convection over ISEA. The altered atmospheric circulation mean state during winters led to conditions resembling extreme El Niño events in the modern climate and a dissolution of the Inter-Tropical Convergence Zone (ITCZ) over the region. From these results we infer that terrestrial cooling of ISEA and at least a seasonal reversal of land-sea circulation likely played a major role in delaying tropical peat formation until at least the onset of the Holocene period.

Petter Lars Hällberg et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2021-164', Anonymous Referee #1, 01 Apr 2022
  • RC2: 'Comment on cp-2021-164', Anonymous Referee #2, 05 Apr 2022

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2021-164', Anonymous Referee #1, 01 Apr 2022
  • RC2: 'Comment on cp-2021-164', Anonymous Referee #2, 05 Apr 2022

Petter Lars Hällberg et al.

Petter Lars Hällberg et al.

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Short summary
Here we analyse the climate in tropical Asia under late glacial conditions (12 000 years before present) using climate model simulations. We find that the climate was much more seasonal, with highly arid winter conditions in the region that is today humid year-around. The seasonal aridity was caused by orbital forcing, stronger Asian winter monsoon and a breakdown of deep convection which caused a reorganization of the Walker Circulation and a mean state resembling El Niño conditions.