Preprints
https://doi.org/10.5194/esd-2022-30
https://doi.org/10.5194/esd-2022-30
 
25 Jul 2022
25 Jul 2022
Status: this preprint is currently under review for the journal ESD.

Origins and suppression of bifurcation phenomena in lower-order monsoon models

S. Krishna Kumar1 and Ashwin K. Seshadri1,2 S. Krishna Kumar and Ashwin K. Seshadri
  • 1Divecha Centre for Climate Change, Indian Institute of Science, Bengaluru, India
  • 2Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bengaluru, India

Abstract. South Asian monsoon rainfall varies rapidly in the paleoclimate record, and this has been interpreted using simple models as arising from tipping points. This study explores a class of simple monsoon models, based on convective quasi-equilibrium, and the bifurcations permitted by their mathematical forms. Specifically, low-order models are derived starting from the Quasi-equilibrium tropical circulation model (QTCM) to examine the bifurcations present. Previous studies that have pointed to an abrupt transition in low-order monsoon models typically identify a saddle node bifurcation occurring as a result of changes in the radiation budget. The present study shows how such saddle node structures arise across a wide range of modeling assumptions and parameter values, and yet permit a continuous transition into and out of precipitating regimes without any bifurcation being physically manifest. This is because the bifurcation points lie in a regime that is not physically relevant when the dry thermal stratification is sufficiently large. As a result, these low-order models can be interpreted as possessing abrupt transitions that are latent in the equations but do not express themselves physically. However, when the dry thermal stratification is reduced, bifurcations can occur. This paper also shows that these latent saddle-node structures are themselves part of the unfolding of a pitchfork bifurcation. These findings help understand the role of stabilizing phenomena on the general absence of abrupt monsoon transitions despite the presence of nonlinear terms in these models.

S. Krishna Kumar and Ashwin K. Seshadri

Status: open (until 09 Sep 2022)

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S. Krishna Kumar and Ashwin K. Seshadri

S. Krishna Kumar and Ashwin K. Seshadri

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Short summary
Abrupt seasonal onset of monsoon as well as abrupt changes in paleoclimatic records have sometimes been ascribed to bifurcations and characterized as tipping points. We describe a mechanism by which nonlinear lower-order monsoon models may not manifest bifurcations latent in their mathematical form. This contributes to resolving a long-standing contradiction between data/higher order models and lower-order monsoon models, regarding bifurcations explaining abrupt changes seen in monsoon.
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