The mechanisms of El Niño–Southern Oscillation (ENSO) variability change during the mid-Holocene (MH) were investigated through analyzing the model simulations from the Paleoclimate Modelling ...Intercomparison Project Phases (PMIP) phase-2 and phase-3. The majority of PMIP2 and PMIP3 model simulations show a lower level of ENSO activity in the MH simulation compared to the pre-industrial (PI) simulation, which is qualitatively consistent with that inferred from the paleoclimate proxies. Through employing the Bjerknes stability index, we quantified the dynamic and thermodynamic air-sea feedbacks in the PI and MH simulations. The quantitative analyses showed that the reduced ENSO variability in MH arose from the weakening in the thermocline (TH), zonal-advection (ZA) and Ekman (EK) feedback terms. We found that all the weakened TH, ZA, and EK terms are associated with the reduction in the response of anomalous thermocline depth (
Dʹ
) to the zonal wind stress anomaly (
τ
x
′
) in MH compared to PI. The reduced
Dʹ
response is attributed to the flattened meridional structure of ENSO-related
τ
x
′
field in MH, which is linked to the enhanced surface poleward mean meridional current in MH. Among many aspects of the mean state changes under the MH orbital forcing, this study identified that the surface mean meridional current change might be a key factor behind the suppressed ENSO variability in MH. Lastly, through comparing our findings with the ENSO future projection studies, we found that the wind-thermocline feedback is susceptible in a changing climate, which implies that minimizing the uncertainty in the wind-thermocline feedback change may help constrain future ENSO response.
Shortwave forcing and feedbacks in Last Glacial Maximum and Mid-Holocene PMIP3 simulations Braconnot, Pascale; Kageyama, Masa
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
11/2015, Volume:
373, Issue:
2054
Journal Article
Peer reviewed
Open access
Simulations of the climates of the Last Glacial Maximum (LGM), 21 000 years ago, and of the Mid-Holocene (MH), 6000 years ago, allow an analysis of climate feedbacks in climate states that are ...radically different from today. The analyses of cloud and surface albedo feedbacks show that the shortwave cloud feedback is a major driver of differences between model results. Similar behaviours appear when comparing the LGM and MH simulated changes, highlighting the fingerprint of model physics. Even though the different feedbacks show similarities between the different climate periods, the fact that their relative strength differs from one climate to the other prevents a direct comparison of past and future climate sensitivity. The land-surface feedback also shows large disparities among models even though they all produce positive sea-ice and snow feedbacks. Models have very different sensitivities when considering the vegetation feedback. This feedback has a regional pattern that differs significantly between models and depends on their level of complexity and model biases. Analyses of the MH climate in two versions of the IPSL model provide further indication on the possibilities to assess the role of model biases and model physics on simulated climate changes using past climates for which observations can be used to assess the model results.
UNDERSTANDING ENSO DIVERSITY Capotondi, Antonietta; Wittenberg, Andrew T.; Newmaman, Matthew ...
Bulletin of the American Meteorological Society,
06/2015, Volume:
96, Issue:
6
Journal Article
Peer reviewed
Open access
El Niño–Southern Oscillation (ENSO) is a naturally occurring mode of tropical Pacific variability, with global impacts on society and natural ecosystems. While it has long been known that El Niño ...events display a diverse range of amplitudes, triggers, spatial patterns, and life cycles, the realization that ENSO’s impacts can be highly sensitive to this event-to-event diversity is driving a renewed interest in the subject. This paper surveys our current state of knowledge of ENSO diversity, identifies key gaps in understanding, and outlines some promising future research directions.
The examination of more than 1500 paleohydrological dated records collected between 10 and 28°N during the last 50 years have been used to improve our knowledge and understanding of the Sahara and ...Sahel vulnerability to the Atlantic monsoon changes in the long-term. We have analyzed the distribution of water bodies (mainly lakes and wetlands) over time and space: the central Saharan massifs played a major role in favoring water supply to the lowlands throughout the whole African Humid Period. In addition, distinct East–West dynamics is recorded with humidity starting – and stopping – several millennia earlier to the east than to the west of the Sahara.
A series of time lags are discussed: (1) between the maximum of deep (fresh water) lake formation during the early Holocene and the maximum of water body extensions during the mid-Holocene which highlight the primary role of aquifer water level in lake response to climate change (2) between the hydrological history of the Sahara and the Sahel and the forcings – mainly insolation changes – during the early and mid-Holocene which involves complex interactions between remnant ice sheet in the Northern Hemisphere, open water bodies in the Sahara and Sahel and the Atlantic monsoon system.
► A record of millennial scale hydrological changes in Sahara/Sahel during the Holocene. ► A time lag between the hydrological optimum and the maximum of NH insolation. ► Role of the groundwaters in the distribution and importance of water bodies. ► Role of the central massifs in supplying fresh waters to the lowlands. ► Evidence for a distinct E–W dynamics at the onset and the termination of the Holocene.
Au cours de son histoire, notre planete a connu des conditions climatiques tres diverses. Comment fonctionne la machine climatique ? Qu'est-ce que le climat global ? Pourquoi et comment a-t-il change ...au cours des temps geologiques ? Comment se situe le changement climatique en cours dans ce contexte geologique et historique ? Le rechauffement en cours est marque par des phases d'acceleration et de ralentissement, et des amplitudes differentes selon les regions. Comment faire la part des facteurs naturels et des activites humaines dans ce rechauffement recent ? A quels risques climatiques serons-nous amenes a faire face, pour les prochaines decennies, et les prochains siecles ? Les sciences du climat ont construit des methodes d'observation, d'analyse, de modelisation et de reconstitution des climats passes qui permettent de repondre a ces questions. Cet ouvrage, clair et precis, presente ainsi les acteurs du climat, leurs interactions, leurs echanges avec toutes les composantes de l'environnement terrestre, cet environnement qui est a la fois conditionne par le climat et affecte son evolution. Il en resulte un systeme qui genere sa propre variabilite, et peut produire des variations abruptes, comme en temoignent les climats passes. C'est egalement un systeme qui va reagir a des perturbations, qu'elles soient naturelles ou liees a l'influence humaine, avec des mecanismes complexes de retroactions. La modelisation du climat est donc centrale pour decrypter les causes des changements recents ou passes, et pour anticiper les futurs possibles.
Understanding the response of the El Niño–Southern Oscillation (ENSO) to global warming requires quantitative data on ENSO under different climate regimes. Here, we present a reconstruction of ENSO ...in the eastern tropical Pacific spanning the past 10,000 years derived from oxygen isotopes in fossil mollusk shells from Peru.We found that ENSO variance was close to the modern level in the early Holocene and severely damped ~4000 to 5000 years ago. In addition, ENSO variability was skewed toward cold events along coastal Peru 6700 to 7500 years ago owing to a shift of warm anomalies toward the Central Pacific. The modern ENSO regime was established ~3000 to 4500 years ago. We conclude that ENSO was sensitive to changes in climate boundary conditions during the Holocene, including but not limited to insolation.
This study investigates the role of insolation in controlling the Indian and African monsoon evolutions during the Holocene using coupled ocean-atmosphere simulations of 0, 6, 9.5 kyr BP climates, ...for which only the variations of Earth's orbital configuration are considered. The two monsoon systems are enhanced at 6 and 9.5 kyr BP, compared to 0 kyr BP, as a result of the intensified seasonal cycle of insolation in the Northern Hemisphere. The analysis of daily climatologies indicates that even though the length of the “celestial” summer season is shorter at 9.5 kyr BP, the rainy season is longer than at present. Emphasis is put on the impact of the precession on the seasonality, which partly explains why the relative amplification of the Indian and African monsoon varies between 9.5 and 6 kyr BP. Moreover, the changes in snow cover over the Tibetan Plateau play a critical role in reinforcing the 9.5 kyr BP monsoon in India during spring. The results suggest that the teleconnection between convection over India and subsidence over the Mediterranean regions, through the Rodwell and Hoskins mechanism, has an impact on the development of the African monsoon at 9.5 kyr BP.
El Niño‐Southern Oscillation (ENSO) flavors have been defined to characterize ENSO events and their teleconnections. Studying El Niño flavor evolution during the Holocene period can provide valuable ...insights into changes over long time scales. We investigated ENSO flavor evolution using simulations spanning the last 6,000 years and present‐day observations. Two approaches to computing ENSO flavors, in agreement in the present, lead to opposite trends in the last 6,000 years. The methods also differ significantly in their representation of ENSO flavor patterns. However, incorporating the sensitivity of the methods to calibration periods and mean state changes yields similar interpretations of ENSO variability changes. Both methods suggest an increase in El Niño variability spreading to the west and east tropical Pacific over the past 6,000 years. Standardizing El Niño flavor definitions is necessary for meaningful comparisons between studies and robust climate variability analysis.
Plain Language Summary
El Niño events are the dominant mode of interannual variability. Looking at El Niño events in the past, during the Holocene period, could shed light on the linkages between the characteristics of El Niño patterns and changes in the climate mean state. Different indicators are used to characterize El Niño pattern diversity depending on the available data. While equivalent under present‐day conditions, these indicators lead to opposite conclusions on the evolution of El Niño patterns over the last 6,000 years. An in‐depth analysis indicates that all the methods suggest that El Niño‐related variance has spread to the west and east tropical Pacific over time when accounting for sensitivity to calibration periods and changes in the mean state during the Holocene. Agreeing on a standard definition for El Niño flavors is essential for accurate comparison between studies and for avoiding misleading conclusions.
Key Points
Two equivalent indicators of El Niño‐Southern Oscillation (ENSO) flavors in the present give opposite evolution during the Holocene
Results show that the conflict between methods comes from the representation of ENSO flavor patterns on short and long timescales
A proper assessment must account for the indicator's patterns and calibration, as well as the climate mean‐state change over time
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