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Abstract:
Influences of the 11-y solar cycle (SC) on climate have been speculated, but here we provide robust evidence that the SC affects decadal variability in the tropical Pacific. By analyzing independent observations, we demonstrate a slowdown of the Pacific Walker Circulation (PWC) at SC maximum. We find a muted hydrological cycle at solar maximum that weakens the PWC and this is amplified by a Bjerknes feedback. Given that a similar muted hydrological cycle has been simulated under increased greenhouse gas forcing, our results strengthen confidence in model predictions of a weakened PWC in a warmer climate. The results also suggest that SC forcing is a source of skill for decadal predictions in the Indo-Pacific region.The Pacific Walker Circulation (PWC) fluctuates on interannual and multidecadal timescales under the influence of internal variability and external forcings. Here, we provide observational evidence that the 11-y solar cycle (SC) affects the PWC on decadal timescales. We observe a robust reduction of east–}west sea-level pressure gradients over the Indo-Pacific Ocean during solar maxima and the following 1{–}2 y. This reduction is associated with westerly wind anomalies at the surface and throughout the equatorial troposphere in the western/central Pacific paired with an eastward shift of convective precipitation that brings more rainfall to the central Pacific. We show that this is initiated by a thermodynamical response of the global hydrological cycle to surface warming, further amplified by atmosphere{–}ocean coupling, leading to larger positive ocean temperature anomalies in the equatorial Pacific than expected from simple radiative forcing considerations. The observed solar modulation of the PWC is supported by a set of coupled ocean{–atmosphere climate model simulations forced only by SC irradiance variations. We highlight the importance of a muted hydrology mechanism that acts to weaken the PWC. Demonstration of this mechanism acting on the 11-y SC timescale adds confidence in model predictions that the same mechanism also weakens the PWC under increasing greenhouse gas forcing.
