Influence of global sea-surface temperature on ultra-low-frequency variability in Indian summer monsoon rainfall

Karmakar, N and Chakraborty, A and Nanjundiah, RS (2019) Influence of global sea-surface temperature on ultra-low-frequency variability in Indian summer monsoon rainfall. In: Quarterly Journal of the Royal Meteorological Society .

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Abstract

A multidecadal or ultra-low-frequency mode of variability in Indian summer monsoon rainfall (ISMR) with a spectral peak at nearly 67 years is identified using a nonparametric approach, namely, singular spectrum analysis. Not only does this mode modulate the seasonal mean rainfall over India, but also frequent occurrences of excess or below normal rainfall over India are favored by the phases of this mode. We show that 80 of all droughts occur during the negative phase of this mode, compared with 60 of all floods during its positive phase. Although the existence of this mode was reported in many previous studies, proper understanding of its exact nature and driving mechanism, and how it relates to global sea-surface temperature (SST), is still not clear. Here, we show that the 67-year variability in ISMR is associated with a SST mode of similar periodicity, which primarily exhibits a hemispheric SST difference, with the strongest signals over the northern Pacific and Atlantic. The positive phase of this SST mode induces low surface pressure over the northern Arabian Sea. Changes in surface pressure over the northern Arabian Sea govern changes in ISMR. The bridge between the SST mode and central Asian circulation is possibly established through the North Pacific Oscillation. The positive phase of this mode enhances the meridional temperature gradient over India, which acts to strengthen the tropical easterly jet, and subsequently the low-level westerly jet over India. These lead to a negative surface pressure anomaly over the Indian subcontinent and an associated anomalous cyclonic circulation. The environment thus created is conducive to increased seasonal mean monsoon rainfall over India. The phase of the SST mode is remarkably similar to, and leading by 2�3 years, that of the multidecadal mode in ISMR up to the 1990s. However, the relationship weakens after the early 1990s.

Item Type:	Journal Article
Publication:	Quarterly Journal of the Royal Meteorological Society
Publisher:	John Wiley and Sons Ltd
Additional Information:	Copyright of this article belongs to John Wiley and Sons Ltd
Keywords:	Atmospheric temperature; Atmospheric thermodynamics; Rain; Spectrum analysis; Submarine geophysics; Surface properties; Surface waters, Indian monsoon rainfall; Indian summer monsoon rainfall; Meridional temperature gradient; Multidecadal variability; Nonparametric approaches; North Pacific; Sea surface temperature (SST); Singular spectrum analysis, Oceanography
Department/Centre:	Division of Mechanical Sciences > Divecha Centre for Climate Change Division of Mechanical Sciences > Centre for Atmospheric & Oceanic Sciences
Date Deposited:	19 Feb 2020 05:39
Last Modified:	19 Feb 2020 05:39
URI:	http://eprints.iisc.ac.in/id/eprint/64564

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