Extratropical cyclones over the North Atlantic and western Europe during the Last Glacial Maximum and implications for proxy interpretation

Joaquim G Pinto,Patrick Ludwig

doi:10.5194/cp-16-611-2020

Abstract

Abstract. Extratropical cyclones are a dominant feature of the midlatitudes, as their passage is associated with strong winds, precipitation and temperature changes. The statistics and characteristics of extratropical cyclones over the North Atlantic region exhibit some fundamental differences between pre-industrial (PI) and Last Glacial Maximum (LGM) climate conditions. Here, the statistics are analysed based on results of a tracking algorithm applied to global PI and LGM climate simulations. During the LGM, both the number and the intensity of detected cyclones were higher compared to PI. In particular, increased cyclone track activity is detected close to the Laurentide ice sheet and over central Europe. To determine changes in cyclone characteristics, the top 30 extreme storm events for PI and LGM have been simulated with a regional climate model and high resolution (12.5 km grid spacing) over the eastern North Atlantic and western Europe. Results show that LGM extreme cyclones were characterised by weaker precipitation, enhanced frontal temperature gradients and stronger wind speeds than PI analogues. These results are in line with the view of a colder and drier Europe, characterised by little vegetation and affected by frequent dust storms, leading to reallocation and build-up of thick loess deposits in Europe.

Highlights

The day-to-day weather conditions in the midlatitudes are strongly affected by the passage of extratropical cyclones, which are typically associated with precipitation, strong winds and changes in temperature and cloudiness
To analyse the characteristics of cyclones for the Last Glacial Maximum (LGM), a subset of 30 extreme cyclones passing over the eastern North Atlantic is selected for both the PI and LGM periods based on the highest values for vorticity (Laplacian of mean sea level pressure (MSLP)) within the selected box
The statistics and characteristics of extratropical cyclones over the North Atlantic and western Europe were analysed for time-slice experiments for PI and LGM conditions

Summary

Introduction

The day-to-day weather conditions in the midlatitudes are strongly affected by the passage of extratropical cyclones, which are typically associated with precipitation, strong winds and changes in temperature and cloudiness. Under the influence of the continental ice sheets and extended sea ice, the PMIP3 general circulation models (GCMs) show stronger meridional temperature gradients, leading to a southward-displaced, more intense and less variable North Atlantic jet than under current climate conditions (Löfverström et al, 2014, 2016; Merz et al, 2015; Wang et al, 2018) These differences have been related, e.g. to more dominant cyclonic Rossby wave breaking near Greenland (Riviére et al, 2010), stationary wave packets trapped in the midlatitude wave guide (Löfverström, 2020) and enhanced meridional eddy momentum flux convergence over the North Atlantic (Wang et al, 2018). The present work aims to advance our understanding of the LGM climate over the North Atlantic and Europe through a more detailed analysis of the cyclonic activity and its associated impacts, notably in terms of precipitation, temperature and wind speed.

Data and methods

Northern Hemisphere cyclone statistics for PI and LGM conditions

Characteristics of extreme cyclones over the eastern North Atlantic

Discussion with available proxy-based climate reconstructions

Summary and conclusions