Pascal Yiou

Historical phenology: Grape ripening as a past climate indicator

French records of grape-harvest dates in Burgundy were used to reconstruct spring–summer temperatures from 1370 to 2003 using a process-based phenology model developed for the Pinot Noir grape. Our results reveal that temperatures as high as those reached in the 1990s have occurred several times in Burgundy since 1370. However, the summer of 2003 appears to have been extraordinary, with temperatures that were probably higher than in any other year since 1370.

Sedimentary record of rapid climatic variability in the North Atlantic Ocean during the Last Glacial Cycle

Comparisons between a Greenland ice core isotopic record and marine sediment grey level records of the North Atlantic Ocean show that rapid temperature variations witnessed by the ice core (Dansgaard-Oeschger events) during the last glacial period can also be detected in marine sediments. This shows that the resolution obtained in marine cores can be sufficiently high to record rapid climatic fluctuations. Several spectral analyses of those grey level marine records consistently indicate that significative fluctuations of the climatic response exist with periodicities of 5 to 1 kyr in addition to orbital forcing. These high frequencies are close to those predicted by various dynamic ocean models.

Atmospheric and oceanic evidences of El Niño-Southern Oscillation events in the south central Pacific Ocean from coral stable isotopic records over the last 137 years

We measured δ18O and δ13C in Porites lutea collected in the Moorea lagoon where the instrumental records show that an El Nino-Southern Oscillation (ENSO) event implies both a cloud cover decrease and a weak sea surface temperature (SST) increase. Two proxies allow an ENSO record to be reconstructed at Moorea: the annual δ13C anomaly, associated with the South Pacific Convergence Zone motion, and an annual δ18O anomaly showing increased SST. The frequency bands exhibited by the singular spectral analysis (SSA) and the multitaper method of the annual δ18O and δ13C records are centered on 2.5 and 5.2 years and 2.4 and 3.2 years, respectively, which are the main ENSO modes. We used SSA to reconstruct ENSO events at Moorea over the past 137 years. Results indicate that climate variability at this site is strongly affected by ENSO events.

Interhemispheric space-time attributes of the Dansgaard-Oeschger oscillations between 100 and 0 ka

The persistence, stability and interhemispheric phasing of the Dansgaard–Oeschger (DO) climate oscillation over the last glacial period (0–100 ka) has been evaluated in oxygen isotope records of three polar ice cores (GRIP, GISP2, and Byrd Station) and a midlatitude deep-sea core from the North Atlantic Ocean (MD95-2042). The results show that DO oscillations in atmospheric conditions occurred in both northern and southern polar ice records, although in the southern records the oscillations had at most only ca. one-tenth the power of those in the north. The DO oscillations first appeared duringMarine Isotope Stage (MIS) 4, and the average spectral power of the northern hemisphere DO oscillations increased markedly during MIS 3 (30–38 ka). The DO mode in the GISP2 record is confined to the frequency band 1/(1.59 kyr) to 1/(1.37 kyr), but in the GRIP record, the mode exhibits strong frequency splittingover a band that is wider by ca. 50%. Time-frequency analysis shows that in GRIP the DO mode underg oes a frequency modulation that is phase-locked with the Earth’s obliquity cycle; this modulation does not appear in nearby GISP2. In the North Atlantic marine record, DO oscillations behaved somewhat differently, appearingsporadically duringMIS 5 and 4. The planktonic DO oscillations increased in spectral power duringMIS 3, leadingpeak power in the GISP2 record by ca. 3 kyr. DO oscillations were relatively stable in all five records duringMIS 3; they could not be detected unequivocally in any of the records duringthe Holocene (0–11 ka). At other times, the DO mode in all of the records was amplitude-modulated by Earth’s orbital parameters. Finally, interhemispheric phasingof DO oscillations over 10–90 ka was assessed between the Byrd Station and GISP2 records, and between the benthic and planktonic records of Core MD95-2042. Coherency studies reveal an apparent time lead of Byrd DO oscillations over GISP2 by 384770 yr (2s level), and of the North Atlantic benthic over planktonic DO oscillations by 208733 yr. This apparent time lead of the southern over the northern temperature proxies is likely the consequence of the distinctive harmonic shapes affecting the northern (rectangular) vs. southern (triangular) DO oscillations; the actual northern–southern relationship, as suggested by modelling and other recent data studies, is most probably in simple antiphase (cross phase of 1801). r 2002 Published by Elsevier Science Ltd.

The North Atlantic Oscillation and its relationship with near surface temperature

A new NAO index is presented here using homogenized surface pressure data from Reykjavik and Gibraltar (for November to March) and Reykjavik and Ponta Delgada (for April to October). This index suggests that the positive trend in recent years is not unprecedented, as the early 20th century was also a period of persistently positive NAO values. The relationship between the secular warming trend and the influence of the NAO on near-surface temperatures over the North Atlantic region and surrounding land masses is examined on a seasonal basis using standardized temperature anomalies since 1900. The near-surface temperature field separates into two independent modes, which we designate a “warming” mode and dynamic (“NAO”) mode, with distinct seasonal cycles.

Summer temperatures in Europe and land heat fluxes in observation-based data and regional climate model simulations

The occurrence and intensity of heatwaves is expected to increase with climate change. Early warnings of hot summers have therefore a great socio-economical value. Previous studies have shown that hot summers are preceded by a Southern European rainfall deficit during winter, and higher spring temperatures. Changes in the surface energy budget are believed to drive this evolution, in particular changes in the latent and sensible heat fluxes. However these have rarely been investigated due to the lack of long-term reliable observation data. In this study, we analyzed several data-derived gridded products of latent and sensible heat fluxes, based on flux tower observations, together with re-analyses and regional climate model simulations over Europe. We find that warm summers are preceded by an increase in latent heat flux in early spring. During warm summers, an increase in available energy results in an excess of both latent and sensible heat fluxes over most of Europe, but a latent heat flux decrease over the Iberian Peninsula. This indicates that, on average, a summertime soil-moisture limited evapotranspiration regime only prevails in the Iberian Peninsula. In general, the models that we analyzed overestimate latent heat and underestimate sensible heat as compared to the flux tower derived data-product. Most models show considerable drying during warm seasons, leading to the establishment of a soil-moisture limited regime across Europe in summer. This over-estimation by the current generation of models of latent heat and hence of soil moisture deficit over Europe in summer has potential consequences for future summertime climate projections and the projected frequency of heat waves. We also show that a northward propagation of drought during warm summers is found in model results, a phenomenon which is also seen in the flux tower data-product. Our results lead to a better understanding of the role of latent and sensible heat flux in summer heatwaves, and provide a framework for benchmark of modeling studies.

Grape harvest dates for checking NAO paleoreconstructions

Dates of the grape harvests in northeastern France and Switzerland exhibit significant correlations with the North Atlantic Oscillation (NAO), a large-scale mode of climate variability over North Atlantic. These correlations, established for the monthly values of the NAO index over the last 175 years, reveal characteristic signatures as a function of frequency. Conversely, these signatures may be used to test the robustness of existing NAO paleoreconstructions, thanks to the availability of harvest dates for northwestern Europe since the end of the fifteenth century. Climatic signal appears prevailing over possible human factors in these series. The results demonstrate the importance of including old instrumental data and of providing monthly values rather than annual means in the NAO reconstructions. They suggest that the available long harvest dates series could constitute an interesting proxy to constrain such reconstructions.

Dynamics of future seasonal temperature trends and extremes in Europe: a multi-model analysis from CMIP3

European surface temperatures have increased during the past decades. According to climate projections, this warming is expected to continue in future years under enhanced radiative forcings. In addition to the mean increase, changes in temperature variability are likely to occur, with more frequent extreme seasons such as those recently observed in the past decade (e.g. summer 2003, autumn 2006). Yet, most of the processes driving such long-term tendencies remain unidentified and unexplored. A particularly important issue is how changes in the atmospheric dynamics over the North-Atlantic and Europe (NAE) contribute to trends in both mean and extreme temperatures. The high occurrence of the positive phase of the North Atlantic Oscillation (NAO) in the 1980s–1990s suggested that the European warming could result from a re-organization in the main structures of the NAE dynamics. However, the 2000s have rather revealed an inconsistency between atmospheric circulation conditions and European temperatures. Here we investigate the relationship between sea-level pressure (SLP) and 2-m temperature (T2m) using a flow-analogues method applied over both observations and future projections. We use 13 models of the Third phase of the Coupled Model Intercomparison Project (CMIP3) over 1961–2000, 2046–2065 and 2081–2100, from which we extract seasonal subensembles with respect to their representation of observed SLP-T2m seasonal relationships. First we show that the distribution of SLP does not undergo major changes in future climate according to these seasonal model-ensembles, albeit a general decline of the variability is observed for all seasons. Then, using the flow-analogues, we conclude that the projected European warming appears disconnected from changes in the NAE dynamics. Only in winter a slight shift towards positive NAO conditions could partially contribute to the future temperature increase. Finally, a focus over unusually warm/cold seasons reveals that future temperature extremes should likely to be associated with similar atmospheric circulations as observed during recent ones.

An open database of grape harvest dates for climate research: Data description and quality assessment

V. Daux, I. Garcia de Cortazar-Atauri, P. Yiou, I. Chuine, E. Garnier, E. Le Roy Ladurie, O. Mestre, and J. Tardaguila LSCE/IPSL, laboratoire CEA/CNRS/UVSQ, UMR8212, Gif/Yvette, France Université Versailles-Saint Quentin en Yvelines, Versailles, France INRA-Agroclim, Avignon, France CEFE, UMR5175, Montpellier, France CRHQ UMR CNRS Université de Caen, Caen, France Institut Universitaire de France, Paris, France Collège de France, Paris, France Météo-France, Toulouse, France Universidad de la Rioja, Logroño, Spain

Climate reconstructions from grape harvest dates: Methodology and uncertainties

Several studies have used grape harvest date (GHD) as a proxy for temperature variations of the last centuries in Europe. However, the use of grape harvest dates to reconstruct climate is not straightforward, with four possible causes of major flaws. In this study we identify and evaluate the accuracy of GHD as a proxy to past temperature anomalies, uncertainties in the model used to relate temperature to GHD, identity of the grape varieties cultivated in the past, type of wine produced in the past and cultural practices used in the past. Our analyses are based on several phenological and crop models, and on the most complete data set on grape vine phenology and harvest quality. We show that the two methodologies currently used — linear regression models and process-based phenological models — can be accurate, but process-based phenological models ascertain robustness to be applied confidently in different vineyards and different periods. However, we show that several factors can induce a bias in temperature reconstructions using process-based models. We demonstrate the importance of historical information on the studied areas such as the varieties cultivated, the style of wine produced, the quality sought, the agricultural practices, in order to build the most robust model.