TY  - JOUR
A1  - Coumou, Dim
A1  - Lehmann, Jascha
A1  - Beckmann, Johanna
T1  - The weakening summer circulation in the Northern Hemisphere mid-latitudes
JF  - Science
N2  - Rapid warming in the Arctic could influence mid-latitude circulation by reducing the poleward temperature gradient. The largest changes are generally expected in autumn or winter, but whether significant changes have occurred is debated. Here we report significant weakening of summer circulation detected in three key dynamical quantities: (i) the zonal-mean zonal wind, (ii) the eddy kinetic energy (EKE), and (iii) the amplitude of fast-moving Rossby waves. Weakening of the zonal wind is explained by a reduction in the poleward temperature gradient. Changes in Rossby waves and EKE are consistent with regression analyses of climate model projections and changes over the seasonal cycle. Monthly heat extremes are associated with low EKE, and thus the observed weakening might have contributed to more persistent heat waves in recent summers.
Y1  - 2015
U6  - https://doi.org/10.1126/science.1261768
SN  - 0036-8075
SN  - 1095-9203
VL  - 348
IS  - 6232
SP  - 324
EP  - 327
PB  - American Assoc. for the Advancement of Science
CY  - Washington
ER  - 
TY  - GEN
A1  - Lehmann, Jascha
A1  - Coumou, Dim
A1  - Frieler, Katja
T1  - Increased record-breaking precipitation events under global warming (vol 132, pg 501, 2015)
T2  - Climatic change : an interdisciplinary, intern. journal devoted to the description, causes and implications of climatic change
Y1  - 2015
U6  - https://doi.org/10.1007/s10584-015-1466-3
SN  - 0165-0009
SN  - 1573-1480
VL  - 132
IS  - 4
SP  - 517
EP  - 518
PB  - Springer
CY  - Dordrecht
ER  - 
TY  - JOUR
A1  - Lehmann, Jascha
A1  - Coumou, Dim
A1  - Frieler, Katja
T1  - Increased record-breaking precipitation events under global warming
JF  - Climatic change : an interdisciplinary, intern. journal devoted to the description, causes and implications of climatic change
N2  - In the last decade record-breaking rainfall events have occurred in many places around the world causing severe impacts to human society and the environment including agricultural losses and floodings. There is now medium confidence that human-induced greenhouse gases have contributed to changes in heavy precipitation events at the global scale. Here, we present the first analysis of record-breaking daily rainfall events using observational data. We show that over the last three decades the number of record-breaking events has significantly increased in the global mean. Globally, this increase has led to 12 % more record-breaking rainfall events over 1981-2010 compared to those expected in stationary time series. The number of record-breaking rainfall events peaked in 2010 with an estimated 26 % chance that a new rainfall record is due to long-term climate change. This increase in record-breaking rainfall is explained by a statistical model which accounts for the warming of air and associated increasing water holding capacity only. Our results suggest that whilst the number of rainfall record-breaking events can be related to natural multi-decadal variability over the period from 1901 to 1980, observed record-breaking rainfall events significantly increased afterwards consistent with rising temperatures.
Y1  - 2015
U6  - https://doi.org/10.1007/s10584-015-1434-y
SN  - 0165-0009
SN  - 1573-1480
VL  - 132
IS  - 4
SP  - 501
EP  - 515
PB  - Springer
CY  - Dordrecht
ER  - 
TY  - JOUR
A1  - Lehmann, Jascha
A1  - Coumou, Dim
T1  - The influence of mid-latitude storm tracks on hot, cold, dry and wet extremes
JF  - Scientific reports
N2  - 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.
Y1  - 2015
U6  - https://doi.org/10.1038/srep17491
SN  - 2045-2322
VL  - 5
PB  - Nature Publ. Group
CY  - London
ER  -