Impacts of UV Irradiance and Medium-Energy Electron Precipitation on the North Atlantic Oscillation during the 11-Year Solar Cycle

Sigmund Guttu,Frode Stordal,Nazario Tartaglione,Pekka T Verronen,Mark A Clilverd,Nour-Eddine Omrani,Odd Helge Otterå,Craig J Rodger,Yvan Orsolini

doi:10.3390/atmos12081029

Abstract

Observational studies suggest that part of the North Atlantic Oscillation (NAO) variability may be attributed to the spectral ultra-violet (UV) irradiance variations associated to the 11-year solar cycle. The observed maximum surface pressure response in the North Atlantic occurs 2–4 years after solar maximum, and some model studies have identified that atmosphere–ocean feedbacks explain the multi-year lag. Alternatively, medium-to-high energy electron (MEE) precipitation, which peaks in the declining phase of the solar cycle, has been suggested as a potential cause of this lag. We use a coupled (ocean–atmosphere) climate prediction model and a state-of-the-art MEE forcing to explore the respective roles of irradiance and MEE precipitation on the NAO variability. Three decadal ensemble experiments were conducted over solar cycle 23 in an idealized setting. We found a weak ensemble-mean positive NAO response to the irradiance. The simulated signal-to-noise ratio remained very small, indicating the predominance of internal NAO variability. The lack of multi-annual lag in the NAO response was likely due to lagged solar signals imprinted in temperatures below the oceanic mixed-layer re-emerging equatorward of the oceanic frontal zones, which anchor ocean–atmosphere feedbacks. While there is a clear, yet weak, signature from UV irradiance in the atmosphere and upper ocean over the North Atlantic, enhanced MEE precipitation on the other hand does not lead to any systematic changes in the stratospheric circulation, despite its marked chemical signatures.

Highlights

On the basis of statistical analysis of surface pressure data extending back several centuries, the ultra-violet (UV) component of the spectral solar irradiance (SSI) forcing associated with the 11-year solar cycle has been suggested to have significant impacts on the extratropical regional climate during winter, in particular over the oceans in the North Atlantic and in the North Pacific as well as over Eurasian landmasses [4,5]
To further elucidate whether the combined effects of the UV irradiance variations and energetic particle precipitation (EPP) ( medium-to-high energy electron (MEE) precipitation) throughout the 11-year solar cycle had a notable impact on the North Atlantic winter climate and ocean-atmosphere interactions over the North Atlantic Ocean, we focused on solar cycle 23 in an idealized framework
We used a configuration of Norwegian Climate Prediction Model (NorCPM), wherein a high-top chemistry-climate model (WACCM) was coupled to an isopycnic coordinate ocean model (MICOM)

Summary

Introduction

On the basis of statistical analysis of surface pressure data extending back several centuries, the ultra-violet (UV) component of the spectral solar irradiance (SSI) forcing (hereafter called UV irradiance) associated with the 11-year solar cycle has been suggested to have significant impacts on the extratropical regional climate during winter, in particular over the oceans in the North Atlantic and in the North Pacific (see either of [1] for a review or [2,3]) as well as over Eurasian landmasses [4,5]. Previous studies [17] suggested that other external (i.e., volcanic) forcing can synchronize internal decadal fluctuations of the coupled atmosphere–ocean system Another complicating factor is that the winter NAO is potentially influenced by a variety of forcings, ranging from sea ice variability in the Atlantic sector of the Arctic [18] to Eurasian snow cover [18,19] and overall polar stratospheric variability [20], and that the winter circulation over the North Atlantic at large is influenced by tropical teleconnections originating in the ENSO phenomenon [21] and by teleconnections originating in the North Pacific [22,23]. We consider the boreal winter season (December–January–February or DJF)

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