76 research outputs found
African monsoon teleconnections with tropical SSTs: validation and evolution in a set of IPCC4 simulations
A set of 12 state-of-the-art coupled oceanatmosphere general circulation models (OAGCMs) is explored to assess their ability to simulate the main teleconnections between the West African monsoon (WAM) and the tropical sea surface temperatures (SSTs) at the interannual to multi-decadal time scales. Such teleconnections are indeed responsible for the main modes of precipitation variability observed over West Africa and represent an interesting benchmark for the models that have contributed to the fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC4). The evaluation is based on a maximum covariance analysis (MCA) applied on tropical SSTs and WAM rainfall. To distinguish between interannual and multi-decadal variability, all datasets are partitioned into low-frequency (LF) and high-frequency (HF) components prior to analysis. First applied to HF observations, the MCA reveals two major teleconnections. The first mode highlights the strong influence of the El Niño Southern Oscillation (ENSO). The second mode reveals a relationship between the SST in the Gulf of Guinea and the northward migration of the monsoon rainbelt over the West African continent. When applied to HF outputs of the twentieth century IPCC4 simulations, the MCA provides heterogeneous results. Most simulations show a single dominant Pacific teleconnection, which is, however, of the wrong sign for half of the models. Only one model shows a significant second mode, emphasizing the OAGCMsâ difficulty in simulating the response of the African rainbelt to Atlantic SST anomalies that are not synchronous with Pacific anomalies. The LF modulation of these HF teleconnections is then explored through running correlations between expansion coefficients (ECs) for SSTs and precipitation. The observed time series indicate that both Pacific and Atlantic teleconnections get stronger during the twentieth century. The IPCC4 simulations of the twentieth and twenty-first centuries do not show any significant change in the pattern of the teleconnections, but the dominant ENSO teleconnection also exhibits a significant strengthening, thereby suggesting that the observed trend could be partly a response to the anthropogenic forcing. Finally, the MCA is also applied to the LF data. The first observed mode reveals a well-known inter-hemispheric SST pattern that is strongly related to the multi-decadal variability of the WAM rainfall dominated by the severe drying trend from the 1950s to the 1980s. Whereas recent studies suggest that this drying could be partly caused by anthropogenic forcings, only 5 among the 12 IPCC4 models capture some features of this LF coupled mode. This result suggests the need for a more detailed validation of the WAM variability, including a dynamical interpretation of the SSTârainfall relationships
Revisiting the ENSO Teleconnection to the Tropical North Atlantic
One of the most robust remote impacts of El NiñoâSouthern Oscillation (ENSO) is the teleconnection to tropical North Atlantic (TNA) sea surface temperature (SST) in boreal spring. However, important questions still remain open. In particular, the timing of the ENSOâTNA relationship lacks understanding. The three previously proposed mechanisms rely on teleconnection dynamics involving a time lag of one season with respect to the ENSO mature phase in winter, but recent results have shown that the persistence of ENSO into spring is necessary for the development of the TNA SST anomalies. Likewise, the identification of the effective atmospheric forcing in the deep TNA to drive the regional airâsea interaction is also lacking. In this manuscript a new dynamical framework to understand the ENSOâTNA teleconnection is proposed, in which a continuous atmospheric forcing is present throughout the ENSO decaying phase. Observational datasets in the satellite era, which include reliable estimates over the ocean, are used to illustrate the mechanism at play. The dynamics rely on the remote Gill-type response to the ENSO zonally compensated heat source over the Amazon basin, associated with perturbations in the Walker circulation. For El Niño conditions, the anomalous diabatic heating in the tropical Pacific is compensated by anomalous diabatic cooling, in association with negative rainfall anomalies and descending motion over northern South America. A pair of anomalous cyclonic circulations is established at upper-tropospheric levels in the tropical Atlantic straddling the equator, displaying a characteristic baroclinic structure with height. In the TNA region, the mirrored anomalous anticyclonic circulation at lower-tropospheric levels weakens the northeasterly trade winds, leading to a reduction in evaporation and of the ocean mixed layer depth, hence to positive SST anomalies. Apart from the dominance of latent heat flux anomalies in the remote response, sensible heat flux and shortwave radiation anomalies also appear to contribute. The âlaggedâ relationship between mature ENSO in winter and peaking TNA SSTs in spring seems to be phase locked with the seasonal cycle in both the location of the mechanismâs centers of action and regional SST variance.This work has been supported by
the EU/H2020-funded MSCA-IF-EF DPETNA project (GA 655339) and JG-S was partially supported by the Spanish MINECO-funded DANAE Project (CGL2015-68342-R). Technical support at BSC (Computational
Earth Sciences group) is sincerely acknowledged. The authors are grateful to the anonymous reviewers for their comments, which helped to improve the scope of
the manuscript.Peer ReviewedPostprint (published version
An updated assessment of past and future warming over France based on a regional observational constraint
Building on CMIP6 climate simulations, updated global and regional observations, and recently introduced statistical methods, we provide an updated assessment of past and future warming over France. Following the IPCC AR6 and recent global-scale studies, we combine model results with observations to constrain climate change at the regional scale. Over mainland France, the forced warming in 2020 with respect to 1900â1930 is assessed to be 1.66 [1.41 to 1.90]ââC, i.e., in the upper range of the CMIP6 estimates, and is almost entirely human-induced. A refined view of the seasonality of this past warming is provided through updated daily climate normals. Projected warming in response to an intermediate emission scenario is assessed to be 3.8ââC (2.9 to 4.8ââC) in 2100 and rises up to 6.7 [5.2 to 8.2]ââC in a very high emission scenario, i.e., substantially higher than in previous ensembles of global and regional simulations. Winter warming and summer warming are expected to be about 15â% lower than and 30â% higher than the annual mean warming, respectively, for all scenarios and time periods. This work highlights the importance of combining various lines of evidence, including model and observed data, to deliver the most reliable climate information. This refined regional assessment can feed adaptation planning for a range of activities and provides additional rationale for urgent climate action. Code is made available to facilitate replication over other areas or political entities.</p
Recommended from our members
Advances in understanding large-scale responses of the water cycle to climate change
Globally, thermodynamics explains an increase in atmospheric water vapor with warming of around 7%/°C near to the surface. In contrast, global precipitation and evaporation are constrained by the Earth's energy balance to increase at âŒ2â3%/°C. However, this rate of increase is suppressed by rapid atmospheric adjustments in response to greenhouse gases and absorbing aerosols that directly alter the atmospheric energy budget. Rapid adjustments to forcings, cooling effects from scattering aerosol, and observational uncertainty can explain why observed global precipitation responses are currently difficult to detect but are expected to emerge and accelerate as warming increases and aerosol forcing diminishes. Precipitation increases with warming are expected to be smaller over land than ocean due to limitations on moisture convergence, exacerbated by feedbacks and affected by rapid adjustments. Thermodynamic increases in atmospheric moisture fluxes amplify wet and dry events, driving an intensification of precipitation extremes. The rate of intensification can deviate from a simple thermodynamic response due to inâstorm and largerâscale feedback processes, while changes in largeâscale dynamics and catchment characteristics further modulate the frequency of flooding in response to precipitation increases. Changes in atmospheric circulation in response to radiative forcing and evolving surface temperature patterns are capable of dominating water cycle changes in some regions. Moreover, the direct impact of human activities on the water cycle through water abstraction, irrigation, and land use change is already a significant component of regional water cycle change and is expected to further increase in importance as water demand grows with global population
Robust and perfectible constraints on human-induced Arctic amplification
Abstract The Arctic near-surface warming is much faster than its global counterpart. Yet, this Arctic amplification occurs a rate that is season, model and forcing-dependent. The present study aims at using temperature observations and reanalyses to constrain the projections of Arctic climate during the November-to-March season. Results show that the recently observed four-fold warming ratio is not entirely due to a human influence, and will decrease with increasing radiative forcings. Global versus regional temperature observations lead to complementary constraints on the projections. When Arctic amplification is defined as the additional polar warming relative to global warming, model uncertainties are narrowed by 30% after constraint. Similar results are obtained for projected changes in the Arctic sea ice extent (40%) and when using sea ice concentration and polar temperature observations to constrain the projected polar warming (37%), thereby confirming the key role of sea ice as a positive but model-dependent surface feedback
Influence des surfaces continentales sur la variabilité du cycle hydrologique des échelles inter-annuelle à multi-décennale.: Une brÚve histoire de la « dérive des continents » dans les modÚles de climat
La contribution des surfaces continentales Ă la variabilitĂ© du systĂšme climatique et du cycle de lâeau a fait lâobjet de nombreux travaux au cours des deux derniĂšres dĂ©cennies. En lâabsence de climatologies suffisamment prĂ©cises et/ou Ă©tendues des paramĂštres continentaux (humiditĂ© du sol, neige ou vĂ©gĂ©tation), la plupart des Ă©tudes reposent cependant sur des simulations consistant Ă tester la rĂ©ponse dâun modĂšle atmosphĂ©rique Ă une perturbation plus ou moins rĂ©aliste des conditions aux limites et/ou des conditions initiales continentales. Ainsi, sâil apparaĂźt que les continents reprĂ©sentent une source de prĂ©visibilitĂ© atmosphĂ©rique significative aux Ă©chelles mensuelles Ă Ăšme saisonniĂšres et une source importante de rĂ©troactions et/ou dâincertitudes dans les scĂ©narios climatiques du 21siĂšcle, les limites de ces expĂ©riences devraient parfois nous inciter Ă un peu plus de prudence. Sans remettre en cause lâintĂ©rĂȘt de ces travaux, elles soulignent notamment la nĂ©cessitĂ© de poursuivre lâamĂ©lioration des modĂšles de surface et de leur couplage avec les modĂšles atmosphĂ©riques, de produire des analyses de surface en assimilant de nouvelles observations dans les modĂšles de surface et/ou les modĂšles mĂ©tĂ©orologiques, dâutiliser ces analyses pour valider et initialiser les modĂšles climatiques et proposer des tests de sensibilitĂ© plus rĂ©alistes, enfin et surtout de relativiser le rĂŽle des surfaces continentales au regard des autres sources de prĂ©visibilitĂ© atmosphĂ©rique et des autres sources dâincertitudes dans les scĂ©narios climatiques. En rĂ©sumĂ©, aprĂšs sâĂȘtre persuadĂ©s quâils avaient une partition Ă jouer dans le grand orchestre de la modĂ©lisation climatique, il est temps pour les spĂ©cialistes des surfaces continentales (et pour moi) de trouver leur juste place plutĂŽt que de chercher Ă capter lâattention du public au risque de favoriser la cacophonie
Revisiting the Potential to Narrow Model Uncertainty in the Projections of Arctic Runoff
International audienceAbstract Despite multiple advances in the understanding of the water cycle intensification in a warmer climate, climate models still diverge in their hydrological projections. Here we constrain annual runoff projections over individual and aggregated Arctic river basins. For this purpose, we use two ensembles of global climate models and two statistical methods: a regression scheme assuming similar runoff sensitivities at interannual versus climate change timescales, and a Bayesian method where models are used to derive a posterior runoff response conditioned on historical observations. While both techniques are shown to narrow model uncertainties, more or less substantially depending on rivers, the Bayesian method is less sensitive to the choice of the model ensemble and is more skillful when tested with synthetic observations. It has also been applied over the whole Arctic watershed, showing so far a limited narrowing of the interâmodel spread, but its skill will further improve with increasing climate change
- âŠ