The influence of mid-latitude storm tracks on hot, cold, dry and wet extremes.

Jascha Lehmann,Dim Coumou

doi:10.1038/srep17491

Jascha Lehmann, Dim Coumou

Open Access

https://doi.org/10.1038/srep17491

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Abstract

Changes in mid-latitude circulation can strongly affect the number and intensity of extreme weather events. In particular, high-amplitude quasi-stationary planetary waves have been linked to prolonged weather extremes at the surface. In contrast, analyses of fast-traveling synoptic-scale waves and their direct influence on heat and cold extremes are scarce though changes in such waves have been detected and are projected for the 21st century. Here we apply regression analyses of synoptic activity with surface temperature and precipitation in monthly gridded observational data. We show that over large parts of mid-latitude continental regions, summer heat extremes are associated with low storm track activity. In winter, the occurrence of cold spells is related to low storm track activity over parts of eastern North America, Europe, and central- to eastern Asia. Storm tracks thus have a moderating effect on continental temperatures. Pronounced storm track activity favors monthly rainfall extremes throughout the year, whereas dry spells are associated with a lack thereof. Trend analyses reveal significant regional changes in recent decades favoring the occurrence of cold spells in the eastern US, droughts in California and heat extremes over Eurasia.

Highlights

In the mid-latitudes, synoptic storms bring moist air from the ocean to the land and in regions like Europe and North America they account for over 70% of total precipitation[27]
The North Atlantic Oscillation (NAO) index is often defined as the mean winter (December to March) standardized air pressure difference between the Azores High and the Icelandic Low with positive index values indicating that air pressure is anomalously high over the Azores and anomalously low over Iceland and vice versa for negative index values
Storm track activity is represented by monthly eddy kinetic energy (EKE), which is computed by bandpass filtering daily wind field data thereby extracting wind speed variability on 2.5–6 days associated with synoptic-scale activity[21,40]

Summary

Introduction

In the mid-latitudes, synoptic storms bring moist air from the ocean to the land and in regions like Europe and North America they account for over 70% of total precipitation[27]. During negative NAO phases, the westerly winds weaken associated with more frequent blocking conditions over Greenland[32,34] In these situations cold air from the Arctic can be dragged to northern Europe leading to anomalously cold winter temperatures over the UK and Scandinavia[35]. Climate models project a substantial weakening of summer storm track activity over essentially all of the mid-latitudes[21] and a poleward shift and downstream extension into Europe during winter[20,21,37,38] which will likely alter the intensity and frequency of surface weather extremes in these regions. Sensitivity analyses performed at the 500 mb pressure level lead to similar results (see Methods)

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Conclusion