The 11 year solar cycle UV irradiance effect and its dependency on the Pacific Decadal Oscillation

Sigmund Guttu,Yvan Orsolini,Odd Helge Otterå,Frode Stordal,Nour-Eddine Omrani

doi:10.1088/1748-9326/abfe8b

Abstract

The stratospheric, tropospheric and surface impacts from the 11 year ultraviolet solar spectral irradiance (SSI) variability have been extensively studied using climate models and observations. Here, we demonstrate using idealized model simulations that the Pacific Decadal Oscillation (PDO), which has been shown to impact the tropospheric and stratospheric circulation from sub-decadal to multi-decadal timescales, strongly modulates the solar-induced atmospheric response. To this end, we use a high-top version of the coupled ocean–atmosphere Norwegian Climate Prediction Model forced by the SSI dataset recommended for Coupled Model Intercomparison Project 6. We perform a 24-member ensemble experiment over the solar cycle 23 in an idealized framework. To assess the PDO modulation of the solar signal, we divide the model data into the two PDO phases, PDO+ and PDO−, for each solar (maximum or minimum) phase. By compositing and combining the four categories, we hence determine the component of the solar signal that is independent of the PDO and the modulation of the solar signal by the PDO, along with the solar signal in each PDO phase. Reciprocally, we determine the PDO effect in each solar phase. Our results show that the intensification of the polar vortex under solar maximum is much stronger in the PDO− phase. This signal is transferred into the troposphere, where we find a correspondingly stronger polar jet and weaker Aleutian Low. We further show that the amplification of the solar signal by the PDO− phase is driven by anomalous meridional advection of solar-induced temperature anomalies over northern North America and the North Pacific, which contributes to a decreased meridional eddy heat flux and hence to a decreased vertical planetary wave flux into the stratosphere.

Highlights

The stratospheric, tropospheric and surface impacts from the 11 year ultraviolet (UV) solar spectral irradiance (SSI) variability have been extensively studied during recent years using climate models and observations (see Gray et al (2010) or Matthes et al (2017) for reviews; Mitchell et al 2015, Misios et al 2015)
Our results show that the intensification of the polar vortex under solar maximum is much stronger in the Pacific Decadal Oscillation (PDO)− phase
We further show that the amplification of the solar signal by the PDO− phase is driven by anomalous meridional advection of solar-induced temperature anomalies over northern North America and the North Pacific, which contributes to a decreased meridional eddy heat flux and to a decreased vertical planetary wave flux into the stratosphere

Summary

Introduction

The stratospheric, tropospheric and surface impacts from the 11 year ultraviolet (UV) solar spectral irradiance (SSI) variability have been extensively studied during recent years using climate models and observations (see Gray et al (2010) or Matthes et al (2017) for reviews; Mitchell et al 2015, Misios et al 2015). The SSI-induced top-down stratospheric influence manifests itself prominently in the North Atlantic region, where both observational and model studies (Gray et al 2013, Scaife et al 2013, Andrews et al 2015, Ma et al 2018) have suggested a tendency for a more positive phase of the North Atlantic Oscillation, albeit with a lag of a few years with respect to the solar maximum, explained in terms of ocean–atmosphere coupling. Over the tropical Pacific, the impact of the TSI variability drives a so-called bottom-up mechanism, by which direct solar heating of the sea surface in cloud free areas and subsequent ocean–atmosphere interactions involving water vapor, convection and clouds, modify the Hadley and Walker circulations and amplify the solar signal (Meehl et al 2009, Misios et al 2015). The changes in precipitation over the tropical and subtropical Pacific induce propagation of quasi-stationary wavetrains northeastwards into the extratropics (Meehl et al 2008)

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