Our Common Future Under Climate Change

International Scientific Conference 7-10 JULY 2015 Paris, France

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Wednesday 8 July - 17:30-19:00 UPMC Jussieu - Amphi Durand

1113 - Climate Extremes: Patterns, Mechanisms and Impacts

Parallel Session

Lead Convener(s): S.S.Y. Wang (Utah State University, Logan, Utah, United States of America)

Convener(s): R. Nelson Guillermo (Universidad del Atlantico, Barranquilla, Colombia), J.H. Yoon (Pacific Northwest National Laboratory , Richland, WA, United States of America), A. Giorgio (Cadiz University, Cadiz, Spain)

The timing of anthropogenic emergence in climate extremes

A. King (University of Melbourne, Melbourne, Victoria, Australia), M. Donat, (University of New South Wales, Sydney, Australia), E. Fischer, (ETH Zurich, Zurich, Switzerland), E. Hawkins (University of Reading, Reading, United Kingdom), L. Alexander (University of New South Wales, Sydney, Australia), D. Karoly (University of Melbourne, University of Melbourne, VIC, Australia), A. Dittus, (University of Melbourne, Melbourne, Victoria, Australia), S. Lewis, (Australian National University, Canberra, Australia), S. Perkins, (University of New South Wales, NSW, Sydney, Australia)

Abstract details
The timing of anthropogenic emergence in climate extremes

A. King (1) ; M. Donat, (2) ; E. Fischer, (3) ; E. Hawkins (4) ; L. Alexander (2) ; D. Karoly (5) ; A. Dittus, (1) ; S. Lewis, (6) ; S. Perkins, (7)
(1) University of Melbourne, School of Earth Sciences, Melbourne, Victoria, Australia; (2) University of New South Wales, Climate change research centre, Sydney, Australia; (3) ETH Zurich, Zurich, Switzerland; (4) University of Reading, Dept. of meteorology, Reading, United Kingdom; (5) University of Melbourne, School of Earth Sciences, University of Melbourne, VIC, Australia; (6) Australian National University, School of earth sciences, Canberra, Australia; (7) University of New South Wales, Climate change research centre, NSW, Sydney, Australia

Abstract content

Many extreme events can be attributed to anthropogenic climate change whilst others can not. This has motivated us to study the time of an anthropogenic emergence (TAE) of six indices representing temperature and precipitation extremes. We used multiple historical runs and RCP8.5 projections from six CMIP5 models. We define a quasi-natural variability for each of these indices at gridbox level and for sub-continental regions and the globe as a whole. We determine when an anthropogenic emergence occurs by comparing index distributions across moving windows with the quasi-natural variability. We also investigated how TAE compared for mean temperature and precipitation with extremes. We found earlier emergence of extreme temperatures in equatorial regions compared to other parts of the world and some seasonal variability in TAE. Spatial aggregation reduces variability in mean and extreme temperature and precipitation leading to earlier TAE values. Using limited observational datasets, the same TAE methodology was applied and signs of emergence found. Finally, using the CMIP5 models, we show the regions of the world where anthropogenic signals can already be detected in our temperature and precipitation indices to aid in the study of attribution of extreme events to climate change.

Extreme heat waves with the Heat Wave Magnitude Index and their occurrence in the future

A. Dosio (European Commission Joint Research Centre, Ispra, Italy), S. Russo, (European Commission Joint Research Centre, Ispra, Italy), J. Sillmann (CICERO (Center for International Climate and Environmental Research - Oslo), Oslo, Norway)

Abstract details
Extreme heat waves with the Heat Wave Magnitude Index and their occurrence in the future

A. Dosio (1) ; S. Russo, (1) ; J. Sillmann ()
(1) European Commission Joint Research Centre, Institute for Environment and Sustainability, Ispra, Italy

Abstract content

Heat waves are defined as prolonged periods of extremely hot weather and their intensity, duration, and frequencies are expected to increase in the future under climate change.

Recently, a new Heat Wave Magnitude Index (HWMI) was developed that, by taking into account both heat wave duration and intensity, enables the quantification of the magnitude of heat waves across different time periods and regions of the world.

Here we first apply the HWMI to grade the observed heat waves occurred in Europe since 1950: in fact, although the worst event in the last decades occurred in Russia in 2010 (the strongest recorded globally in recent decades exceeding in amplitude and spatial extent the previous hottest European summer in 2003), many other heat waves, documented in literature and in newspapers, occurred in different European regions in the past decades.

Subsequently, we apply the HWMI to the predictions from several regional climate models (RCM) from the COordinated Regional climate Downscaling Experiment (CORDEX). RCMs have been used to downscale CMIP5 Global Circulation Models under different Representative Concentration Pathway (RCP), namely RCP4.5 and RCP8.5. We focus on two regions, Africa and Europe, for which a large ensemble of models’ results is available.

Results show that, by the end of this century, under the most severe emission scenario, events with magnitude even greater than the one in Russia in the summer of 2010 will become more frequent and are projected to occur as often as every 10 years for regions such as southern Europe and central Africa.

The Role of Teleconnection Patterns in Wave Climate and Storms Distribution: The SW Spanish and Wales Coasts Examples

R. Nelson Guillermo (Universidad del Atlantico, Barranquilla, Colombia), T. Thomas, (University of Wales Trinity Saint David (Swansea), Swansea, United Kingdom), A. Giorgio (Cadiz University, Cadiz, Spain), P. Michael (University of Wales Trinity Saint David (Swansea), Swansea, United Kingdom)

Abstract details
The Role of Teleconnection Patterns in Wave Climate and Storms Distribution: The SW Spanish and Wales Coasts Examples

R. Nelson Guillermo (1) ; T. Thomas, (2) ; A. Giorgio (3) ; P. Michael (2)
(1) Universidad del Atlantico, Basic Sciences, Barranquilla, Colombia; (2) University of Wales Trinity Saint David (Swansea), Swansea, United Kingdom; (3) Cadiz University, Faculty of marine sciences, Cadiz, Spain

Abstract content

Tele-connection patterns such as the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) are influenced by climate change, as models predict a weakening of the overturning circulation that may affect both regional and global climate, sea level and extreme waves. Occurrence and distribution of storms are important variables in the incidence of coastal erosion, deterioration and/or complete destruction of ecosystems. This work presents the characterization of wave climate and energy, coastal storms and their recurrence intervals, related to several regional cycles in Cadiz (SW Spanish Atlantic coast), Tenby and Swansea (S Wales, UK). At the former site, wave records include 22 years of data covering the period between 1987 and 2012. Storm characterization was carried out using the Storm Power Index and five classes were obtained, from class I (weak events) to V (extreme events). Storm occurrence probability was 96% for class I (i.e. almost one event per year) to 3% for class V. The return period for class V was 25 years and ranged from 6 to 8 years for classes III and IV storms, e.g. significant and severe events. Classes I and II showed a period of recurrence ranging from 1 to 3 years. Approximately 40% of the change in monthly wave data and storminess indices was related to several teleconnection patterns, being the Arctic Oscillation (AO), with 21.45%, and the North Atlantic Oscillation (NAO), with 19.65%, the most important drivers of change. 

Timescales of Change: Unraveling East Africa's Climate Paradox

B. Lyon (IRI, Columbia University, Palisades, NY, United States of America), A. Giannini (IRI, Columbia University, Palisades, NY, United States of America), N. Vigaud (IRI, Columbia University, Palisades, NY, United States of America)

Abstract details
Timescales of Change: Unraveling East Africa's Climate Paradox

B. Lyon (1) ; A. Giannini (1) ; N. Vigaud (1)
(1) IRI, Columbia University, Palisades, NY, United States of America

Abstract content

East Africa is currently facing something of a climate paradox.  Over roughly the past 15 years, the region has been experiencing an increased frequency of drought, particularly during the "long rains" season from March-May.  In a seeming contradiction, there is a general consensus among climate change projections that the region of East Africa will become wetter as a result of anthropogenic climate change by the end of the current century. One possibile explanation of this discrepency is that the climate models are not properly responding to increasing greenhouse gases and their influence on East African climate: The future climate of East Africa may in fact become drier, not wetter.  Another possibility is that the recent rainfall decline is associated with processes in the climate system operating on shorter time scales than long-term climate change.  In this case, the recent rainfall decline may be masking longer-term climate change.  A third possibilty is that there is an interaction of procceses operating on these different time scales.

This paper will review recent and ongoing research that is helping to explain the recent variations in East African climate. A combination of observational and climate model experiments indicate that the recent rainfall decline in East Africa has been associated with decadal-scale variations of the climate system, specifically in the Pacific Ocean.  Evidence is presented that the rainfall decline in East Africa was manifest as an abrupt shift towards drier conditions that occurred in 1998-99 when the Pacific Decadal Oscillation (PDO) shifted from its warm to cold phase.  This shift in the PDO was associated with a colling of eastern equatorial Pacific while the western Pacific has remained anomalously warm. Observational evidence indicates that the recent rainfall decline in East Africa was part of a near-global scale shift in seasonal rainfall patterns, with similar shifts observed previously over the past century. Results from climate model experiments, using only observed sea surface temperatures in the tropical Pacific as forcing, indicate the models are able to reproduce the recent drying in East Africa and the associated shift in global precipitation patterns.  However, observations also indicate that the western Pacific warm pool region has continued to warm over the past several decades, an increase which almost certainly contains an anthropogenic component. Current work is investigating whether this combination of "natural" decadal variations and anthropogenic change, particularly in sea surface temperatures in the Pacific warm pool region, led to an exacerbation of recent East African droughts (we note that the devastating drought in 2010-11 was likely the most severe of the past 50 years). Understanding the physical processes associated with climate model projected increases in East African rainfall and how realistic they are in light of what is known about the behavior of the current climte system remains a critical next step in fully unraveling the East African climate paradox.

Physical insights on future European summer heat waves and record-breaking temperatures

M. Bador (CERFACS/CNRS, Toulouse, France), L. Terray (CERFACS/CNRS, Toulouse, France), J. Boé (CERFACS/CNRS, Toulouse, France)

Abstract details
Physical insights on future European summer heat waves and record-breaking temperatures

M. Bador (1) ; L. Terray (1) ; J. Boé (1)
(1) CERFACS/CNRS, Sciences de l'univers au cerfacs, ura1875, Toulouse, France

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Recent summer heat waves had strong socio-economic impacts in different parts of Europe. This highlights the need for improved understanding of key processes and feedbacks. We focus on the detection of an anthropogenic signal on record-breaking summer temperature using historical and 21st century simulations from a set of CMIP5 climate models. Results show that simulated and observed record evolutions follow the stationary climate theoretical record rate until the 1980s. They then diverge from the expected value, with an increase of the number of warm records and a decrease of the cold ones. These changes are shown to accentuate over the 21st century. The influence of internal variability based on control simulations is used to estimate an anthropogenic signal emergence time around 2030.

We then focus on a set of case studies of future heat waves. We analyze a high spatial resolution simulation (from 1950 up to 2100) of the ALADIN regional atmospheric model driven by the CNRM-CM5 model. Based on warm spell duration indices, we select a few intense events that occur in the second part of the 21st century. Heat waves are generally associated with quasi-stationary anticyclonic circulation anomalies that produce clear skies and warm air advection. They are often associated to anomalously dry land surface conditions. For each case study, we perform ALADIN sensitivity experiments by perturbing either the prescribed large-scale circulation and/or the initial soil moisture content. We then infer the dominant mechanisms and the feedbacks operating to amplify or mitigate the heat waves. We also perform a worst-case scenario where we try to generate an extreme heat wave in order to assess the associated temperature rise and its possible saturation due to negative feedback.

Global future changes of precipitation extremes

A. Toreti (European Commission, Joint Research Centre, Ispra, Italy), P. Naveau, (Laboratoire des Sciences du Climat et de l’Environnement, IPSL- CNRS, Gif-sur-Yvette, France), M. Zampieri, (Centro Euro-Mediterraneo sui Cambiamenti Climatici, Lecce , Italy), A. Schindler, (Federal Office of Meteorology and Climatology, MeteoSwiss, Zurich, Switzerland), E. Scoccimarro, (Centro Euro-Mediterraneo sui Cambiamenti Climatici, Lecce, Italy), E. Xoplaki (Justus-Liebig-University Giessen, Giessen, Germany), H. A. Dijkstra, (Utrecht University, Utrecht, Netherlands), S. Gualdi, (Centro Euro-Mediterraneo sui Cambiamenti Climatici, Lecce , Italy), J. Luterbacher (Justus-Liebig-University Giessen, Giessen, Germany)

Abstract details
Global future changes of precipitation extremes

A. Toreti (1) ; P. Naveau, (2) ; M. Zampieri, (3) ; A. Schindler, (4) ; E. Scoccimarro, (5) ; E. Xoplaki (6) ; HA. Dijkstra, (7) ; S. Gualdi, (3) ; J. Luterbacher (6)
(1) European Commission, Joint Research Centre, Ispra, Italy; (2) Laboratoire des Sciences du Climat et de l’Environnement, IPSL- CNRS, Gif-sur-Yvette, France; (3) Centro Euro-Mediterraneo sui Cambiamenti Climatici, Lecce , Italy; (4) Federal Office of Meteorology and Climatology, MeteoSwiss, Zurich, Switzerland; (5) Centro Euro-Mediterraneo sui Cambiamenti Climatici, Lecce, Italy; (6) Justus-Liebig-University Giessen, Geography: Climatology, Climate Dynamics and Climate Change, Giessen, Germany; (7) Utrecht University, Department of physics and astronomy, Utrecht, Netherlands

Abstract content

Precipitation extremes have a strong impact on ecosystems and societies especially in exposed and vulnerable areas. In a climate change context, where exposure and vulnerability are also expected to change, it is essential to achieve a better understanding and an improved characterisation of these events. By analysing the latest global climate model simulations from the Coupled Model Intercomparison Project Phase 5 and by using an innovative statistical approach, seasonal changes in daily precipitation extremes under the high emission (RCP8.5) and the mid-range mitigation emission (RCP4.5) scenarios are investigated. Two future time periods (2020-2059, 2060-2099) are compared with the historical time period 1966-2005 and the results presented in terms of very high risk events. Furthermore, global models are evaluated w.r.t. precipitation extremes by using the available (high-resolution) gridded observations during the selected time period of the historical run. At the European scale, complex changes in the tail behaviour are also assessed. Results show that in the historical period a reliable characterisation of daily extreme precipitation cannot be achieved for large areas of the world, where an estimation of the return levels cannot be obtained. This is the case, for instance, during boreal winter for a belt elongated over the subtropics and tropics of the Northern Hemisphere and the oceanic areas west of the three continents of the Southern Hemisphere. In the Euro-Mediterranean area, northern Eurasia, and North America, the simulations show lower intermodel variability and higher correlation with the observations in boreal winter. Conversely, for Australia, southern Asia, and the Middle East, all seasons are characterised by larger intermodel variability and lower correlation with the observations. Concerning the future projections, the main findings point to an intensification (more pronounced at the end of the century under the high-emission scenario RCP8.5) of precipitation extremes almost everywhere in the world and in all seasons. However, a lack of reliability and consistency affects the subtropics/tropics. The zonal means of the identified changes clearly show more pronounced increases over the high latitudes of both hemispheres in all seasons, with the exception of the Northern Hemisphere in the mid-century boreal summer, associated with larger intermodel variability. Over the Southern Hemisphere, a sharp decrease in the estimated positive changes from the high to the middle latitudes is evident in all seasons followed by (with the exception of the austral winter) a strong increase towards the low latitudes. Stronger hemispheric differences are also estimated over the high latitudes for RCP8.5 at the second half of the century that are most prominent in summer and autumn. At the regional level, models show a better agreement on the projected increase of return levels over land, although large variability affects the estimated seasonal changes over specific areas (e.g., eastern Asia in summer).

Weakened Flow, Persistent Circulation and Prolonged Heat Waves in Boreal Summer

D. Coumou (Potsdam Institute for Climate Impact Research, Potsdam, Germany), J. Lehmann, (Potsdam Institute for Climate Impact Research, Potsdam, Germany), K. Kornhuber, (Potsdam Institute for Climate Impact Research, Potsdam, Germany), V. Petoukhov, (Potsdam Institute for Climate Impact Research, Potsdam, Germany)

Abstract details
Weakened Flow, Persistent Circulation and Prolonged Heat Waves in Boreal Summer

D. Coumou (1) ; J. Lehmann, (1) ; K. Kornhuber, (1) ; V. Petoukhov, (1)
(1) Potsdam Institute for Climate Impact Research, Potsdam, Germany

Abstract content

Changes in atmospheric circulation can strongly alter the frequency and/or magnitude of high-impact extreme weather events. The Northern Hemisphere mid-latitudes have seen significant changes in the large-scale summer circulation over the last decades and this might have contributed to more prolonged heat waves (13). The zonal mean zonal wind (or “jet”) has weakened by about 5% over 1979-2014, likely driven by the much more rapid warming in the Arctic as compared to the rest of the Hemisphere. In conjunction with the summer jet, the kinetic energy associated with transient synoptic-scale weather systems (the Eddy Kinetic Energy, or EKE) has seen a significant weakening as well. The observed decline in EKE is more pronounced in relative terms than that of the jet (by about 10% over 1979-2014), which is consistent with theoretical arguments and climate model simulations. Transient eddies are both forced by the jet via vertical shear but can also accelerate it via the eddy-driven jet (4, 5). The observed summertime weakening of both jet and EKE is also a robust signal in future projections of CMIP5 climate models (5, 6). At the same time, for some wave numbers, we have seen an increased occurrence-frequency of high-amplitude quasi-stationary waves during recent boreal summers. We argue that this increase in frequency is associated with a recent cluster of resonance events which can create such high-amplitude waves.

The reduction in amplitude of fast-moving transient waves (as captured by EKE) and the more-frequent occurrence of high-amplitude quasi-stationary waves both favor more persistent weather conditions. It has been demonstrated that high-amplitude quasi-stationary waves in the atmosphere are statistically associated with extreme weather at the surface (7, 8). Especially regions at the western boundary of the continents show the strongest association between surface extremes and high-amplitude upper-level waves. In contrast, strong transient wave activity, i.e. large EKE, is linked to moderate surface temperatures and vice versa (3). Over most continental regions affected by storm tracks, there is a significant negative correlation between monthly EKE and surface temperature. Thus, the hottest summers are associated with extremely low EKE, while mild summers are associated with more pronounced EKE. Again the western boundaries of the continents are especially sensitive since these regions are most directly influenced by the storm tracks.

In conclusion, boreal summer circulation has seen pronounced changes over the last decades, trends which seem to have amplified since the onset of rapid Arctic Amplification around 2000. Especially the reduction in EKE, but also in zonal mean flow, have created conditions favorable for the buildup of heat and drought over the continents. Moreover, a cluster of resonance events is observed since 2000, which has increased the occurrence-frequency of high-amplitude quasi-stationary waves with wavenumbers close to 7. Thus, this generally implies a weakening of transient synoptic eddy activity and more-frequent states of quasi-stationary flow. These observed changes in large-scale flow point towards more persistent flow patterns and therefore more extreme surface weather. This is also consistent with the pronounced increase in heat extremes in Europe and other mid-latitude regions (9, 10).

1. J. E. Overland, J. A. Francis, E. Hanna, M. Wang, Geophys. Res. Lett. 39, L19804 (2012).

2. J. a Francis, S. J. Vavrus, Environ. Res. Lett. 10 (2015), doi:10.1088/1748-9326/10/1/014005.

3. D. Coumou, J. Lehmann, J. Beckmann, Science (80-. ). (accepted) (2015).

4. T. Woollings, M. Blackburn, J. Clim. 25, 886–902 (2012).

5. J. Lehmann, D. Coumou, K. Frieler, A. V Eliseev, A. Levermann, Environ. Res. Lett. 9, 084002 (2014).

6. P. A. O’Gorman, Proc. Natl. Acad. Sci. USA 107, 19176–19180 (2010).

7. J. A. Screen, I. Simmonds, Nat. Clim. Chang. 4, 704–709 (2014).

8. D. Coumou, V. Petoukhov, S. Rahmstorf, S. Petri, H. J. Schellnhuber, Proc. Natl. Acad. Sci. U. S. A. 111, 12331–12336 (2014).

9. S. Russo et al., J. Geophys. Res. Atmos. , n/a–n/a (2014).

10. N. Christidis, G. S. Jones, P. a. Stott, Nat. Clim. Chang. , 3–7 (2014).

Climate Model Simulation of Present and Future Extreme Events in Latin America and the Caribbean: What Spatial Resolution is Required?

R. Oglesby (University of Nebraska, Lincoln, Lincoln NE, United States of America), C. Rowe, (University of Nebraska, Lincoln, Lincoln NE, United States of America), R. Mawalagedara, (University of Nebraska, Lincoln, Lincoln, United States of America)

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Climate Model Simulation of Present and Future Extreme Events in Latin America and the Caribbean: What Spatial Resolution is Required?

R. Oglesby (1) ; C. Rowe, (2) ; R. Mawalagedara, (3)
(1) University of Nebraska, Lincoln, Earth and atmospheric sciences and daugherty water for food institute, Lincoln NE, United States of America; (2) University of Nebraska, Lincoln, Earth and atmospheric sciences, Lincoln NE, United States of America; (3) University of Nebraska, Lincoln, Earth and atmospheric sciences and daugherty water for food institute, Lincoln, United States of America

Abstract content

Latin America and the Caribbean are regions presently at grave risk to a variety of extreme climate events. These include flooding rains, damaging winds, drought, heat waves, and in high elevation mountainous regions, excessive snowfalls. The causes of these events are numerous. For example, flooding rains and damaging winds are often associated with tropical cyclones, but also can occur, either separately or in tandem, due to smaller, more localized storms. Similarly, heat waves and droughts can be large scale or localized, and frequently occur together (as excessive drying can lead to enhanced heating, while enhanced heating in turn promotes additional drying). Even in the tropics, extreme snow and ice events can have severe consequences due to avalanches, and also on water resources. Understanding and modeling the climate controls behind these extreme events requires consideration of a range of time and space scales. A common strategy is to use a global climate model (GCM) to simulate the large-scale (~100 km) daily atmospheric controls on extreme events. A limited area, high resolution regional climate model (RCM) is then employed to dynamically downscale the results, so as to better incorporate the influence of topography and, secondarily, the nature of the land cover. But what resolution is required to provide the necessary results, i.e., minimize biases due to improper resolution? In conjunction with our partners from participating Latin American and Caribbean nations, we have made an extensive series of simulations, both region-wide and for individual countries, using the WRF regional climate model to downscale output from a variety of GCMs, as well as Reanalyses (as a proxy for observations).  The simulations driven by the Reanalyses are used for robust model verification against actual weather station observations. The simulations driven by GCMs are designed to provide projections of future climate, including importantly how the nature and number of extreme events may change through coming decades. Our results suggest that for proper simulation of both mean climate, and importantly extreme events, a spatial resolution of 4 km is required in regions of complex mountainous topography. A somewhat coarser resolution of 12 km is adequate in regions without much topographic relief, and where differing land covers account for most of the spatial heterogeneity. 

Should we expect more extreme rainfall from Atlantic tropical cyclones as a response to global warming?

F. Chauvin (Météo-France, Toulouse, France), H. Douville (Météo-France, Toulouse, France)

Abstract details
Should we expect more extreme rainfall from Atlantic tropical cyclones as a response to global warming?

F. Chauvin (1) ; H. Douville (1)
(1) Météo-France, CNRM-GAME, Toulouse, France

Abstract content

Daily precipitation extremes increase in intensity over many regions of the globe in simulations of a warming climate, including the tropics where observational constraints suggest that this sensitivity could be higher than expectations for the extratropics and from the Clausius-Clapeyron (CC) relationship (e.g. O’Gorman 2012).  Here, the focus is on heavy precipitation related to Atlantic tropical cyclones (TCs). The objective is to investigate whether the sensitivity to global warming of TC rainfall is mainly controlled by the atmospheric water holding capacity, following the CC relationship, or could be even higher due to a dynamical cyclone amplification associated with an increased release of latent heat in the atmosphere. This amplification may lead to rates of change reaching twice the CC rate (Trenberth 2007). Our investigation is based on TRMM satellite precipitation observations and atmospheric ERA-Interim reanalysis in association with the IBTracs observed TC dataset on the one hand, and on coupled ocean-atmosphere simulations performed with a stretched version of the CNRM-CM5 global climate model on the other hand. For the CNRM-CM5 simulation an objective TC-tracking has been performed.A water budget analysis has been conducted along the cyclone tracks for both observation and model. It was found that moisture convergence is the dominant contribution to TC rainfall, as expected from previous case studies. Results also show that our climate model captures quite well the water budget derived from ERA-Interim, although the latter shows a systematic underestimation of TC rainfall, as compared with TRMM, possibly due to an underestimation of TC winds. Moreover, the analysis of our climate scenario suggests that the increase of spatially aggregated TC mean rainfall follows the CC relationship, while super CC rates are only reached in the very inner core of the TCs.

Temperature and precipitation extreme compound events in Southeastern South America and the associated atmospheric circulation?

M. Rusticucci, B. Tencer (School of Earth and Ocean Sciences, University of Victoria, Victoria, Canada), M. L. Bettolli, (University of Buenos Aires, Buenos Aires, Argentina)

Abstract details
Temperature and precipitation extreme compound events in Southeastern South America and the associated atmospheric circulation?

M. Rusticucci () ; B. Tencer (1) ; ML. Bettolli, (2)
(1) School of Earth and Ocean Sciences, University of Victoria, Victoria, Canada; (2) University of Buenos Aires, Atmosphere and ocean sciences, Buenos Aires, Argentina

Abstract content

Compound events consist of the simultaneous or successive occurrence of two or more extreme events, the combination of extreme events with conditions that amplify the impact of the events, or the combination of events that are not individually extreme but can lead to extreme impacts when occurring together (IPCC, 2012). In this paper we analyse the joint occurrence of extreme temperature and heavy precipitation events (simultaneous or lagged by one day) in southern South America during 1961-2000.

The study is based on a comprehensive dataset of daily precipitation and daily minimum and maximum temperature observed at meteorological stations of the region and compiled during the CLARIS LPB project. Four different extreme temperature events were defined: warm nights (days) correspond to days with minimum (maximum) temperature exceeding the 90th percentile of the daily distribution; cold nights (days) are days with minimum (maximum) temperature below the 10th percentile. Heavy precipitation events are events with daily rainfall above the 75th percentile of the empirical distribution of rainy days. A compound event is defined when one of the above temperature extremes occurs simultaneously with, preceded or followed by a heavy precipitation event.

The existence of a significant statistical relation between these extremes could help to better characterize the uncertainties associated with projections of extreme precipitation events for a future warmer climate. Results show that the probability of occurrence of an intense precipitation increases during or after a warm night, but decreases during a cold night, compared to the expected likelihood of occurrence of this type of events in the absence of a relation between temperature and precipitation extremes. Warm days are usually associated to the occurrence of heavy precipitation events on the same day or the day before, but they rarely occur afterwards. On the contrary, cold days happen more often after an intense rain.

In order to characterize the atmospheric circulation during the occurrence of a compound event, we use a synoptic classification developed by Barrucand et al (2014) and based on daily mean fields of geopotential height at 500hPa from the NCEP2 reanalysis. The associated circulation during a compound event of warm nights or warm days and heavy precipitation shows a trough over the Pacific Ocean and a cold front over the continent that lead to warm and wet air advected to the east of the region of study. Cold days and heavy precipitation events in the southwestern part of the domain of study are usually characterised by a positive anomaly of geopotential height at the southern part of the continent associated with an eastern anomaly over the region.