Extended North Atlantic Oscillation and Greenland Blocking Indices 1800–2020 from New Meteorological Reanalysis

Edward Hanna,Mariano Barriendos,Thomas E Cropper,Richard C Cornes,Richard J Hall

doi:10.3390/atmos13030436

Abstract

Based on newly-available meteorological reanalysis, we compile and present extended seasonal series of the North Atlantic Oscillation (NAO) and Greenland Blocking indices spanning 1800–2020, which we analyse for evidence of significant trends. This represents a major backward extension of the previously available instrumental-/reanalysis-based Azores–Iceland and principal component-based NAO indices, and allows us to evaluate the potential effect of natural climate perturbations, especially the 1809 and 1815 major volcanic eruptions and ~1790s–1830 Dalton solar minimum, on North Atlantic atmospheric circulation. We find that winters 1809/10 and 1816/17 mark positive NAO peaks, relative to several years before and afterwards, which is in accordance with the theory of volcanic forcing of climate. However, there is little evidence of a summer NAO volcanic signature. Overall, based on the significantly longer new reanalysis time series, the new series presented here corroborate and extend our previous results of: (1) a significantly more variable year-to-year NAO with a recent exceptional clustering of extreme events since 2000 for winter; (2) a significant increasing trend in blocking over Greenland in summer. These trends have major repercussions for the probability of the occurrence of extreme weather events over northwest Europe and for the sensitivity and response of the Greenland Ice Sheet to global warming, especially if they continue as an integral part of anthropogenic climate change.

Highlights

The North Atlantic Oscillation (NAO) is a widely-used measure of North Atlantic polar jet stream variation, which is an important control of climatic conditions and extreme weather fluctuations across the UK, Northwest
We have presented newly developed Greenland Blocking and NAO indices based on recently released climate reanalyses, that exceed the span of previous similar records by
NAOPC, which was previously highlighted for 1899–2014 for the month of December by [7], and here culminates in a significant clustering of extreme low/high values during

Summary

Introduction

The North Atlantic Oscillation (NAO) is a widely-used measure of North Atlantic polar jet stream variation (mainly the north–south position of the jet), which is an important control of climatic conditions and extreme weather fluctuations across the UK, NorthwestEurope and eastern North America [1–4]. Ice extent and snow cover; oceanic effects from North Atlantic sea-surface temperatures, which may for example, influence the NAO teleconnection with Tibetan Plateau surface air temperatures [12]; tropical influences such as the El-Niño Southern Oscillation; stratospheric effects due to stratospheric circulation variability, solar variability, volcanic eruptions and the Quasi-Biennial Oscillation (downward-propagating zonal wind variation in the equatorial stratosphere, which typically reverses phase every ~28 months between east and west flowing). These drivers can oppose or reinforce one another, and there are indications of interactions between them [13]. Recent work has shown an ability to provide skilful NAO winter seasonal forecasts from both dynamical [14,15] and statistical/complex systems [13,16] modelling, and provides a basis for significantly improving North Atlantic seasonal weather prediction

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