Our Common Future Under Climate Change

International Scientific Conference 7-10 JULY 2015 Paris, France

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Wednesday 8 July - 15:00-16:30 UPMC Jussieu - ROOM 101 - Block 14/24

1117 - Climate variability and external forcings of the Common Era with special focus on the role of volcanic eruptions

Parallel Session

Lead Convener(s): D. Swingedouw (Universite de Bordeaux, Pessac, France)

Convener(s): C. Deser (National Center for Atmospheric Research, Boulder, United States of America)

15:00

Global and regional climate variability: a pause for thought

E. Hawkins (University of Reading, Reading, United Kingdom)

Abstract details
Global and regional climate variability: a pause for thought

E. Hawkins (1)
(1) University of Reading, Dept. of meteorology, Reading, United Kingdom

Abstract content

Our experience of how climate changes depends on the interaction between externally driven changes, such as those due to greenhouse gases, aerosols and volcanic eruptions, and internal climate variations, both globally and locally. The recent slowdown in global temperature rise has dramatically highlighted these interactions and challenged the climate community to understand, explain and communicate the complex issues involved.

This talk will focus on some of the history of our understanding of climate variability, the links between changes in global temperature and regional climate, and the limits on the precision with which we can make climate projections. The concept of 'climate emergence' will also be discussed as a useful framework to help understand the role of climate variability and to help communicate the many plausible possibilities for how regional climate may change over the coming decades.

15:15

Past and future European climate trends: uncertainty due to internal variability

L. Terray (CERFACS/CNRS, Toulouse, France), C. Deser (National Center for Atmospheric Research, Boulder, United States of America)

Abstract details
Past and future European climate trends: uncertainty due to internal variability

L. Terray (1) ; C. Deser (2)
(1) CERFACS/CNRS, Sciences de l'Univers au CERFACS, URA1875, Toulouse, France; (2) National Center for Atmospheric Research, Climate analysis section, Boulder, United States of America

Abstract content

This talk will highlight the relative importance of internally generated versus externally forced climate trends over the past and next fifty years at local and regional scales over Europe. The assessment is based on a large ensemble of climate simulations performed with the CESM1 NCAR model that cover the 1920-2100 period. The ensemble contains a large number of integrations (30), each of which is subject to identical natural and anthropogenic radiative forcing but initialized from a slightly different atmospheric state. The large ensemble shows that natural climate variability superimposed upon forced climate change results in a large range of possible trends for surface air temperature and precipitation over the past and next fifty years. A variant of the flow-analogue approach is used to partition the temperature  and precipitation changes due to internal variability into dynamical and thermodynamical components. Internal thermodynamical changes are shown to be more important in summer while dynamical ones are the dominant contributors in winter. Removing the unpredictable dynamical contribution results in a significant increase of the signal to noise ratio, particularly in winter. The thermodynamical changes are shown to be linked with the ocean, sea-ice and land surface conditions. While large ensembles are needed to fully characterize the forced response at global and regional scales, they also provide a robust estimation of the uncertainties due to internal variability that is needed to better characterize model structural differences. This is important for a wide range of stakeholders as the partial masking of human-induced climate change by internal multidecadal variability is an important consideration for policy and planning efforts.

15:30

The Pacific decadal oscillation, revisited

M. Newman (University of Colorado/CIRES and NOAA/ESRL/PSD, Boulder, CO, United States of America), M. Alexander (NOAA/ESRL/PSD, Boulder, United States of America), T. Ault, (Cornell University, Ithaca, United States of America), K. Cobb (Georgia Tech, Atlanta, United States of America), C. Deser (National Center for Atmospheric Research, Boulder, United States of America), E. Dilorenzo (Georgia Tech, Atlanta, United States of America), N. Mantua (NOAA/NMFS Southwest Fisheries Science Center, Santa Cruz, United States of America), A. Miller (Scripps Institute of Oceanography, La Jolla, United States of America), S. Minobe (Hokkaido University, Sapporo, Japan), H. Nakamura (University of Tokyo, Tokyo, Japan), N. Schneider (University of Hawaii, International Pacific Research Center, Honolulu, United States of America), D. Vimont (University of Wisconsin, Madison, United States of America)

Abstract details
The Pacific decadal oscillation, revisited

M. Newman (1) ; M. Alexander (2) ; T. Ault, (3) ; K. Cobb (4) ; C. Deser (5) ; E. Dilorenzo (4) ; N. Mantua (6) ; A. Miller (7) ; S. Minobe (8) ; H. Nakamura (9) ; N. Schneider (10) ; D. Vimont (11)
(1) University of Colorado/CIRES and NOAA/ESRL/PSD, Boulder, CO, United States of America; (2) NOAA/ESRL/PSD, Boulder, United States of America; (3) Cornell University, Ithaca, United States of America; (4) Georgia Tech, Atlanta, United States of America; (5) National Center for Atmospheric Research, Climate analysis section, Boulder, United States of America; (6) NOAA/NMFS Southwest Fisheries Science Center, Santa Cruz, United States of America; (7) Scripps Institute of Oceanography, La Jolla, United States of America; (8) Hokkaido University, Sapporo, Japan; (9) University of Tokyo, Tokyo, Japan; (10) University of Hawaii, International Pacific Research Center, Honolulu, United States of America; (11) University of Wisconsin, Madison, United States of America

Abstract content

Since its identification in the late 1990’s as the dominant pattern of North Pacific sea surface temperature (SST) variability, the Pacific decadal oscillation (PDO) has been connected both to other parts of the climate system and to impacts on natural resources and marine and terrestrial ecosystems. Variability associated with the PDO has often been confused iwth externally forced climate change including anthropogenic effects. Subsequent research, however, has found that the PDO is not a single physical mode of climate variability but instead largely represents the combination of three groups of processes: (1) changes in ocean surface heat fluxes and Ekman (wind-driven) transport related to the Aleutian low, due to both local, rapidly decorrelating, unpredictable weather “noise” and to remote forcing from interannual to decadal tropical variability (largely El Nino) via the “atmospheric bridge”; (2) ocean memory, or processes determining oceanic thermal inertia including “re-emergence” and oceanic Rossby waves, that act to integrate this forcing and thus generate added PDO variability on decadal time scales; and (3) decadal changes in the Kuroshio-Oyashio current system forced by the multi-year history of basin-wide Ekman pumping, manifested as SST anomalies along the subarctic front at about 40ºN in the western Pacific ocean. Thus, the PDO represents the effects of different processes operating on different timescales, with few of its apparent impacts due to extratropical SST anomalies. This talk presents a synthesis of this current view of the PDO, and discusses corresponding implications for climate diagnosis, including of PDO climate impacts and predictability (both oceanographic and atmospheric); potential decadal "regime"-like behavior; simulations of the PDO in climate models; the interpretation of paleoclimate multicentennial reconstructions of the PDO; and its impacts on marine ecosystems. We conclude with some suggested “best practices” for future PDO diagnosis and forecasts including investigating the potential role of the PDO in the global temperature hiatus.

15:42

Importance of Atlantic decadal variability for near-term assessment and predictability of western Amazon dry-season dry and wet events

K. Fernandes (Columbia University, Palisades, NY, United States of America), W. Baethgen, (Columbia University, Palisades, NY, United States of America), L. Verchot (CIFOR, Bogor, Indonesia), A. Giannini (Columbia University, Palisades, NY, United States of America), M. Pinedo-Vasquez, (CIFOR, Bogor, Indonesia)

Abstract details
Importance of Atlantic decadal variability for near-term assessment and predictability of western Amazon dry-season dry and wet events

K. Fernandes (1) ; W. Baethgen, (2) ; L. Verchot (3) ; A. Giannini (1) ; M. Pinedo-Vasquez, (3)
(1) Columbia University, International Research Institute for Climate and Society (IRI), Palisades, NY, United States of America; (2) Columbia University, International institute for climate and society (iri), Palisades, NY, United States of America; (3) CIFOR, Bogor, Indonesia

Abstract content

The drought of 2005 was a “1 in 100 years” event in western Amazon resulting in fire damage to over 300,000 hectares of rainforest in the Brazilian state of Acre and 22,000 hectares in the province of Coronel Portillo in Peru. In 2010 another severe drought resulted in the isolation of entire communities as the Negro River, a major northwestern Amazon tributary, registered its lowest water lever in over 100 years. The Amazon ecosystem is sensitive to repeated occurrence of droughts, which interferes with the forest’s natural ability to recover from stress and undermine climate change mitigation efforts to reduce CO2 emissions from deforestation and forest degradation. Whether this apparent increase in drought severity and frequency is related to natural low-frequency modes of climate variability, to anthropogenic influence on climate, or to a combination of both is explored here.

A modest negative trend in dry-season precipitation is observed in western Amazon over the period 1935-2012, which along with a multi-decadal pattern of reduced moisture transport from the tropical Atlantic worked to enhance the severity of the recent droughts. Most of the western Amazon dry-season precipitation decadal variability is attributable to decadal fluctuations of the north-south gradient (NSG) in Atlantic sea surface temperature (SST). The observed western Amazon and NSG decadal co-variability is well reproduced in Global Climate Models (GCMs) pre-industrial control (PIC) and historical (HIST) experiments that were part of the Intergovernmental Panel on Climate Change fifth assessment report (IPCC-AR5). This suggests that unforced or natural climate variability, characteristic of the PIC simulations, determines the nature of this coupling, as the results from HIST simulations (forced with greenhouse gases (GHG) and natural and anthropogenic aerosols) are comparable in magnitude and spatial distribution. Decadal fluctuation in the NSG also determines shifts in the probability of repeated dry and wet events in western Amazon, as there is a 66% chance of 3 or more years of dry events per decade when NSG>0 compared to 19% when NSG<0. The HIST and PIC model simulations also reproduce the observed shifts in probability distribution of dry and wet events as a function of the NSG decadal phase, suggesting there is great potential for decadal predictability based on GCMs. Persistence of the current NSG positive phase may lead to continuing above normal frequencies of western Amazon dry-season droughts. 

15:54

Stochastic low-frequency variability in the eddying ocean: mid-latitude imprints, possible atmospheric impacts

T. Penduff (LGGE, Grenoble Cedex 9, France), L. Terray (CERFACS/CNRS, Toulouse, France), G. Sérazin (LGGE, Grenoble Cedex 9, France), S. Leroux (LGGE, Grenoble Cedex 9, France), L. Bessières (CERFACS/CNRS, Toulouse, France), J.-M. Molines (LGGE, Grenoble Cedex 9, France), B. Barnier (LGGE, Grenoble Cedex 9, France)

Abstract details
Stochastic low-frequency variability in the eddying ocean: mid-latitude imprints, possible atmospheric impacts

T. Penduff (1) ; L. Terray (2) ; S. Leroux (1) ; G. Sérazin (1) ; B. Barnier (1) ; JM. Molines (1) ; L. Bessières (2)
(1) LGGE, MEOM, Grenoble Cedex 9, France; (2) CERFACS/CNRS, Sciences de l'Univers au CERFACS, URA1875, Toulouse, France

Abstract content

Laminar Ocean General Circulation Models (2° to 1° resolution) used in recent climate projections are being progressively replaced by turbulent ocean models (about 1/4° resolution) in the perspective of the next CMIP exercise. Atmospherically-forced ocean experiments show that this resolution increase improves the physical consistency of simulations, but also allows the ocean to spontaneously generate a substantial variability up to interannual-to-multidecadal timescales. Consistently with idealized studies, this low-frequency intrinsic variability (LFIV) is negligeable when mesoscale oceanic eddies are not explicitly resolved, and spontaneously emerges in the turbulent regime. 

This non-linearly driven oceanic LFIV has a stochastic character, and a marked signature on the upper ocean temperatures in mid-latitude regions where air-sea fluxes are maximum in Nature. Seasonally-driven global eddying ocean simulations exhibit the strong, large-scale imprints of this stochastic LFIV on several climate-relevant oceanic indices: sea-surface height (SSH) and temperature (SST) in western boundary current systems and the Antarctic Circumpolar Current, Meridional Overturning Circulation (AMOC) throughout the Atlantic Ocean, etc.

How these low-frequency intrinsic variability modes are impacted, and may or not be paced, by the interannually-varying atmosphere is an important question about climate uncertainty. The ongoing OCCIPUT project aims at investigating these questions probabilistically through a 50-member ensemble of 1/4° global ocean/sea-ice 57-year hindcasts, driven by the same 1958-present atmospheric forcing. Present results demonstrate that initial state perturbations spontaneously grow, cascade toward long space and time scales and non-linearly saturate. The resulting ensemble spread describes the atmospherically-paced stochastic LFIV (uncertainty), with marked imprints on oceanic variables at large space and time scales both at the surface (SST, SSH) and below (AMOC, mode/intermediate/deep water mass properties and depths, etc).

This ensemble experiment will provide the first probabilistic description of the global ocean state and evolution over the last decades, and a measure of the actual constraint exerted by the atmosphere on low-frequency ocean variability. The imprint of this stochastic LFIV on the upper-ocean thermal fields and AMOC will then provide insights into how this eddy-driven low-frequency oceanic “noise” might ultimately impact the atmosphere and climate predictability in future coupled climate projections.

16:06

Contribution of sulfate anthropogenic aerosols to the Euro-Mediterranean climate trends since 1980 using a regional coupled modelling approach

P. Nabat (Météo-France / CNRM-GAME, Toulouse, France), S. Somot (Météo-France / CNRM-GAME, Toulouse, France), M. Mallet (Laboratoire d'Aerologie, Toulouse, France), A. Sanchez-Lorenzo (University of Girona, Girona, Spain), M. Wild (ETH, Zurich, Zurich, Switzerland)

Abstract details
Contribution of sulfate anthropogenic aerosols to the Euro-Mediterranean climate trends since 1980 using a regional coupled modelling approach

P. Nabat (1) ; S. Somot (1) ; M. Mallet (2) ; A. Sanchez-Lorenzo (3) ; M. Wild (4)
(1) Météo-France / CNRM-GAME, Toulouse, France; (2) Laboratoire d'Aerologie, Toulouse, France; (3) University of Girona, Department of physics, Girona, Spain; (4) ETH, Zurich, Institute for atmospheric and climate science, Zurich, Switzerland

Abstract content

Since the 1980s, sulfur emissions have considerably been reduced in industrialized countries, notably in Europe, thus leading to a decrease of sulfate aerosol concentration over the Euro-Mediterranean region. Meanwhile, an increase of incoming solar radiation reaching the surface, known as the brightening effect, has been observed over the same period but global and regional climate models still have trouble in reproducing the all-sky surface solar radiation trends and their consequences on climate.  

In order to investigate the consequences of this aerosol trend on regional climate and its role in the observed changes during the last three decades, the present study introduces an original approach, through the use of a fully coupled regional climate system model (CNRM-RCSM). The latter includes the different components of the regional climate system, namely the atmosphere (with ALADIN-Climate), the ocean (with NEMOMED8), the land surface (with ISBA) and the rivers (with TRIP). This approach enables us to take into account the high-frequency feedback of the sea surface temperature (SST) on the atmosphere, as well as the river-ocean-atmosphere feedback. Aerosols are included in ALADIN-Climate through monthly interannual climatologies, coming from a combination of satellite-derived and model-simulated products, and considered as the best possible relevant estimation of the atmospheric aerosol content for the five most relevant species (sea salt, desert dust, sulfates, black and organic carbon aerosols). Simulations using the lateral boundary forcing of the ERA-INTERIM reanalysis have been carried out over the period 1980-2012 with and without the trend in sulfate aerosols.The scattering of the incoming solar radiation by sulfate aerosols leads to important changes in the Euro-Mediterranean climate. Comparisons between both simulations and homogenized surface observations reveal that our model is able to reproduce the all-sky surface shortwave radiation trends only when the aerosol trend is included. This improvement is particularly visible in regions where aerosols have been strongly reduced (i.e., Central Europe, Po Valley). Aerosol changes explain 81 ± 16 per cent of the simulated brightening over the 1980-2012 period, while the direct effect has been found to be the main cause of the simulated brightening.  

As a result of this brightening effect, when including the aerosol decrease, the surface temperature trend is higher and closer to homogenized surface observations, indicating that aerosols explain 23 ± 5 per cent of the observed warming between 1980 and 2012. The use of an atmosphere-ocean coupled model enables us to show that Mediterranean sea surface temperature changes are also better reproduced using the aerosol trend. Air-sea fluxes have consequently been modified by this evolution in the sulfate aerosol content, as well as river flow.

Overall, our results demonstrate the importance of changes in aerosol loads for the understanding of regional climate variability.

16:18

Volcanic Forcing: new initiatives to establish its impacts on climates of the Southern Hemisphere

P. Harvey (University of Witwatersrand, Johannesburg, South Africa), S. Grab (University of Witwatersrand, Johannesburg, South Africa), F. Engelbrecht, (Centre for Scientific and Industrial Research, Pretoria, South Africa)

Abstract details
Volcanic Forcing: new initiatives to establish its impacts on climates of the Southern Hemisphere

P. Harvey (1) ; S. Grab (1) ; F. Engelbrecht, (2)
(1) University of Witwatersrand, Geography, archaeology and environmental studies, Johannesburg, South Africa; (2) Centre for Scientific and Industrial Research, Natural resources and the environment, Pretoria, South Africa

Abstract content

In the northern hemisphere (NH), 1816 is well known as ‘the year without a summer’. But what happened in the southern hemisphere (SH) in 1816, or for that matter following other major tropical or SH volcanic eruptions? Whilst recent work has taken on an increasingly global focus on volcanic forcing as a driving mechanism influencing climate, the emphasis has largely been on the NH where most of the world’s landmass and people have been impacted by large scale volcanic eruptions. To this end, the SH continues to remain in the shadows; hence our initiative to investigate: a) past impacts of major volcanic eruptions on SH climate and environment/society, and b) to model the climatic impacts of such eruptions on specific SH landmasses such as southern Africa, South America, Australia and New Zealand.

Several numerically based studies and experiments have confirmed global mean temperature declines during years immediately following major volcanic eruptions, and in particular, this correlation seems to be driven by considerably cooler than normal summers which more than compensate for warm winter anomalies over the NH (e.g. Briffa et al., 1998; Fischer et al., 2007; Miller et al., 2012). Such work has additionally been supported with longer-term tree ring density and frost ring data for North America and Europe, demonstrating a clear environmental response and signature to such events (e.g. D’Arrigo and Jacoby, 1999; Breitenmoser et al., 2012). Recent modeling approaches have demonstrated the association between high northern latitude volcanic eruptions and the dispersion of aerosols, which remain mostly confined north of 30°N (Oman et al., 2006), and the connection to Arctic/sub-Arctic ice growth development and consequent sea-ice/ocean feedbacks causing hemispheric cooling (Miller et al., 2012).  Thus, much is known and much has been learnt from volcanic forcing events on NH climates and environments, but to what extent are these patterns mirrored in the SH?

Given the scarcity of SH instrumental records covering the Tambora and other major historical period volcanic events, little is known about their impact in southerly latitudes. Here we triangulate evidence from a variety of proxies including historical documentary sources, and tree ring and speleothem data, to demonstrate that major tropical or SH volcanic eruptions are usually followed by extreme climate events across much of the SH.  For instance, in southern Africa, such events may be followed by exceptionally hot summers and severe winters (with unusually early and/or late frosts and heavy snowfall events) (Grab and Nash, 2010). Whilst this temporally mirrors austral winters and summers in the NH (which are warmer and cooler respectively), the cooling anomalies are seasonally reversed between the hemispheres. In addition, data thus far suggest that one to two years following such a major eruption has [have] a very high probability of experiencing very wet [floods] or very dry [drought] conditions, but that the relationship is not linear owing to other climate drivers such as ENSO cycles.  This work is ongoing and seeks further evidence to substantiate these new findings.

Finally, we present our current and future aims to model the climatic impacts (all major climate parameters) of major volcanic eruptions across various spatial and temporal scales in the SH using the Variable-Resolution Earth System Model (VRESM) and its atmospheric component the Conformal Cubic Atmospheric Model (CCAM) (see Engelbrecht et al., 2011). The models will be integrated to simulate volcanic events ranging in size from Toba to Pinatubo (or smaller), and will test the associations between geographic location (longitude/latitude) and event magnitude to establish spatial, temporal and type/magnitude of climatic impacts in the SH.