Abstract
Within the context of anthropogenic warming, rainfall oscillations may induce especially important societal impacts worldwide. In this article, we propose to study potential underlying mechanisms related to precipitation changes on a planetary scale by taking advantage of the recent theory of Rossby waves of long periods winding around subtropical gyres, the Gyral Rossby Waves (GRWs). The stable oxygen isotopic compositions of speleothems are used to regionalize and reconstruct the evolution of long-term rainfall oscillation during the Holocene. The method applied here consists in estimating the wavelet power of dated series of stable oxygen isotopic composition (δ18O) in speleothems within period bands representative of subharmonic modes. Our findings highlight: (1) hydrological processes resulting from friction between the North Equatorial Current (NEC) and the North Equatorial Counter Current (NECC) to explain the weakening of ENSO activity in mid-Holocene, and (2) the quasi-resonance of the equatorward migration of the summer Inter Tropical Convergence Zone (ITCZ) during the Holocene, because of the progressive decrease of the thermal gradient between the low and high latitudes of the gyres. The results of this study suggest that the spatial and temporal variations in the amplitude of the rainfall oscillations are related both on the acceleration/deceleration phases of the western boundary currents and on the shrinkage of the Hadley cell. The latitudinal shift of the summer ITCZ in response to changes in the thermal gradient is of the utmost importance in predicting the expansion of deserts resulting from anthropogenic warming.
Highlights
IntroductionOutstanding progress has been made over the past half century in understanding how climate responds to external and internal forcing, many questions remain unanswered
Following on from the various works on the interannual rainfall oscillation, whether it is related to El Niño Southern Oscillation (ENSO) [1,2,3] or the Interdecadal Pacific Oscillation [4], this study focuses on the evolution of long-term ENSO activity and rainfall oscillation on a planetary scale throughout the Holocene
Evolution of long-period rainfall oscillation during the Holocene is highlighted from the wavelet power of dated δ18O series in speleothems within period bands representative of subharmonic modes
Summary
Outstanding progress has been made over the past half century in understanding how climate responds to external and internal forcing, many questions remain unanswered This is the case, for example, of the role played by subtropical gyres on a planetary scale or of the drivers of long-term evolution of ENSO activity and rainfall oscillation, as well as migration of the summer inter-tropical convergence zone (ITCZ) at the centennial timescale. Studies focused on these open issues demand novel approaches to clarify the contribution of natural cycles in the current context of anthropogenic warming, and for a better understanding of the impact of human activities on the natural dynamics of the climate system. The variability observed in both South America and Asia in contrast to a remarkably stable trend in other regions like Europe may embed the existence of other underlying mechanisms driving such climate changes
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