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Sarah F. Kew
,
Michael Sprenger
, and
Huw C. Davies

Abstract

Inspection of the potential vorticity (PV) distribution on isentropic surfaces in the lowermost stratosphere reveals the ubiquitous presence of numerous subsynoptic positive PV anomalies. To examine the space–time characteristics of these anomalies, a combined “identification and tracking” tool is developed that can catalog each individual anomaly’s effective amplitude, location, overall spatial structure, and movement from genesis to lysis. A 10-yr winter climatology of such anomalies in the Northern Hemisphere is derived for the period 1991–2001 based upon the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The climatology indicates that the anomalies are frequently evident above high topography and in a quasi-annular band at about 70°N, are long lived (days to weeks), and that their effective amplitude is typically 2 PV units (PVU) higher than that of the ambient environment. In addition, the derived climatologies and associated composites pose questions regarding the origin of the anomalies, detail their life cycle, and shed light on their dynamics and role as long-lived precursors of surface cyclogenesis.

Full access
Sjoukje Philip
,
Sarah F. Kew
,
Geert Jan van Oldenborgh
,
Emma Aalbers
,
Robert Vautard
,
Friederike Otto
,
Karsten Haustein
,
Florence Habets
, and
Roop Singh

Abstract

The extreme precipitation that resulted in historic flooding in central-northern France began 26 May 2016 and was linked to a large cutoff low. The floods caused some casualties and over a billion euros in damage. To objectively answer the question of whether anthropogenic climate change played a role, a near-real-time “rapid” attribution analysis was performed, using well-established event attribution methods, best available observational data, and as many climate simulations as possible within that time frame. This study confirms the results of the rapid attribution study. We estimate how anthropogenic climate change has affected the likelihood of exceedance of the observed amount of 3-day precipitation in April–June for the Seine and Loire basins. We find that the observed precipitation in the Seine basin was very rare, with a return period of hundreds of years. It was less rare on the Loire—roughly 1 in 20 years. We evaluated five climate model ensembles for 3-day basin-averaged precipitation extremes in April–June. The four ensembles that simulated the statistics agree well. Combining the results reduces the uncertainty and indicates that the probability of such rainfall has increased over the last century by about a factor of 2.2 (>1.4) on the Seine and 1.9 (>1.5) on the Loire due to anthropogenic emissions. These numbers are virtually the same as those in the near-real-time attribution study by van Oldenborgh et al. Together with the evaluation of the attribution of Storm Desmond by Otto et al., this shows that, for these types of events, near-real-time attribution studies are now possible.

Open access
Sarah f. Kew
,
Sjoukje Y. Philip
,
Geert Jan van Oldenborgh
,
Gerard van der Schrier
,
Friederike E. L. Otto
, and
Robert Vautard
Full access
Sjoukje Philip
,
Sarah F. Kew
,
Geert Jan van Oldenborgh
,
Friederike Otto
,
Sarah O’Keefe
,
Karsten Haustein
,
Andrew King
,
Abiy Zegeye
,
Zewdu Eshetu
,
Kinfe Hailemariam
,
Roop Singh
,
Eddie Jjemba
,
Chris Funk
, and
Heidi Cullen

Abstract

In northern and central Ethiopia, 2015 was a very dry year. Rainfall was only from one-half to three-quarters of the usual amount, with both the “belg” (February–May) and “kiremt” rains (June–September) affected. The timing of the rains that did fall was also erratic. Many crops failed, causing food shortages for many millions of people. The role of climate change in the probability of a drought like this is investigated, focusing on the large-scale precipitation deficit in February–September 2015 in northern and central Ethiopia. Using a gridded analysis that combines station data with satellite observations, it is estimated that the return period of this drought was more than 60 years (lower bound 95% confidence interval), with a most likely value of several hundred years. No trend is detected in the observations, but the large natural variability and short time series means large trends could go undetected in the observations. Two out of three large climate model ensembles that simulated rainfall reasonably well show no trend while the third shows an increased probability of drought. Taking the model spread into account the drought still cannot be clearly attributed to anthropogenic climate change, with the 95% confidence interval ranging from a probability decrease between preindustrial and today of a factor of 0.3 and an increase of a factor of 5 for a drought like this one or worse. A soil moisture dataset also shows a nonsignificant drying trend. According to ENSO correlations in the observations, the strong 2015 El Niño did increase the severity of the drought.

Open access
Urs Neu
,
Mirseid G. Akperov
,
Nina Bellenbaum
,
Rasmus Benestad
,
Richard Blender
,
Rodrigo Caballero
,
Angela Cocozza
,
Helen F. Dacre
,
Yang Feng
,
Klaus Fraedrich
,
Jens Grieger
,
Sergey Gulev
,
John Hanley
,
Tim Hewson
,
Masaru Inatsu
,
Kevin Keay
,
Sarah F. Kew
,
Ina Kindem
,
Gregor C. Leckebusch
,
Margarida L. R. Liberato
,
Piero Lionello
,
Igor I. Mokhov
,
Joaquim G. Pinto
,
Christoph C. Raible
,
Marco Reale
,
Irina Rudeva
,
Mareike Schuster
,
Ian Simmonds
,
Mark Sinclair
,
Michael Sprenger
,
Natalia D. Tilinina
,
Isabel F. Trigo
,
Sven Ulbrich
,
Uwe Ulbrich
,
Xiaolan L. Wang
, and
Heini Wernli

The variability of results from different automated methods of detection and tracking of extratropical cyclones is assessed in order to identify uncertainties related to the choice of method. Fifteen international teams applied their own algorithms to the same dataset—the period 1989–2009 of interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERAInterim) data. This experiment is part of the community project Intercomparison of Mid Latitude Storm Diagnostics (IMILAST; see www.proclim.ch/imilast/index.html). The spread of results for cyclone frequency, intensity, life cycle, and track location is presented to illustrate the impact of using different methods. Globally, methods agree well for geographical distribution in large oceanic regions, interannual variability of cyclone numbers, geographical patterns of strong trends, and distribution shape for many life cycle characteristics. In contrast, the largest disparities exist for the total numbers of cyclones, the detection of weak cyclones, and distribution in some densely populated regions. Consistency between methods is better for strong cyclones than for shallow ones. Two case studies of relatively large, intense cyclones reveal that the identification of the most intense part of the life cycle of these events is robust between methods, but considerable differences exist during the development and the dissolution phases.

Full access