| 1. |
- Berntell, Ellen, et al.
(author)
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Mid-Holocene West African monsoon rainfall enhanced in EC-Earth simulation with dynamic vegetation feedback
- 2024
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In: Climate Dynamics. - 0930-7575 .- 1432-0894.
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Journal article (peer-reviewed)abstract
- Proxy records have shown that the Mid-Holocene was a period of humid conditions across West Africa, with an enhanced West African Monsoon (WAM) and vegetated conditions in areas currently characterized by desert, often referred to as the Green Sahara. However, General Circulation Models regularly struggle with recreating this strengthened Mid-Holocene monsoon in West Africa. Vegetation feedbacks has long been viewed as an essential process modulating the monsoon variability in West Africa, and simulations using prescribed vegetation to recreate a Green Sahara have shown a strengthened WAM and increased rainfall. However, simulations with prescribed vegetation in Sahara represent an idealized vegetation cover and do not take any environmental heterogeneity into account. Furthermore, this only represents a one-directional forcing by the vegetation on the climate rather than the full vegetation feedback. To address this, we have simulated the Mid-Holocene (similar to 6 ka) climate using the Earth System Model EC-Earth3-Veg. The results show that coupled dynamic vegetation reproduces an apparent enhancement of the WAM, with the summer rainfall in the Sahel region increasing by 15% compared to simulations with a prescribed modern vegetation cover. Vegetation feedbacks enhance the warming of the Sahara region, deepens the Sahara Heat Low, results in increased rainfall and strengthens monsoonal flow across West Africa. However, the enhancement is still below what can be viewed in proxy reconstructions, highlighting the role of model limitation and biases and the importance of investigating other processes, such as the interactive aerosol-albedo feedback.
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| 2. |
- Bian, Jianpu, et al.
(author)
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Mechanisms for African easterly wave changes in simulations of the mid-Holocene
- 2023
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 61:7-8, s. 3165-3178
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Journal article (peer-reviewed)abstract
- The mid-Holocene was a warm period with significantly amplified precipitation in North Africa, and a northward shifted Western African Monsoon during boreal summer. We conduct simulations for the pre-industrial and mid-Holocene periods to investigate the connection between summer rainfall variability and changes of African easterly waves (AEWs) during the mid-Holocene. Summer rainfall increases and migrates northward during the mid-Holocene, but the magnitude of change fails to reconcile the discrepancy with mid-Holocene proxy evidence, possibly due to no prescribed vegetation change in our simulations. The spectrum of summer rainfall over the Sahel and West Africa reveals enhanced synoptic time scale (3-to-6 days) variability during the mid-Holocene, which is consistent with the enhanced AEW activity influence. Specifically, the southern AEW track strengthens and migrates poleward during the mid-Holocene period, which modulates summer rainfall over the Sahel and West Africa. By comparison, the northern AEW track changes less and produces a minor contribution to rainfall changes in those regions. We find enhanced baroclinic and barotropic instabilities to promote the AEW activity during the mid-Holocene, with a doubling of the eddy kinetic energy of the meridional wind from that in PI, and baroclinic energy conversion plays a more important role. Stronger low-level meridional thermal gradients increase moisture flux from the Atlantic Ocean to inland.The amplified AEW activity, together with promoted moist convection and increased precipitation, results in a northern shift of the summer rainfall band during the mid-Holocene.
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| 3. |
- Büntgen, Ulf, et al.
(author)
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Global tree-ring response and inferred climate variation following the mid-thirteenth century Samalas eruption
- 2022
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 59:1-2, s. 531-546
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Journal article (peer-reviewed)abstract
- The largest explosive volcanic eruption of the Common Era in terms of estimated sulphur yield to the stratosphere was identified in glaciochemical records 40 years ago, and dates to the mid-thirteenth century. Despite eventual attribution to the Samalas (Rinjani) volcano in Indonesia, the eruption date remains uncertain, and the climate response only partially understood. Seeking a more global perspective on summer surface temperature and hydroclimate change following the eruption, we present an analysis of 249 tree-ring chronologies spanning the thirteenth century and representing all continents except Antarctica. Of the 170 predominantly temperature sensitive high-frequency chronologies, the earliest hints of boreal summer cooling are the growth depressions found at sites in the western US and Canada in 1257 CE. If this response is a result of Samalas, it would be consistent with an eruption window of circa May-July 1257 CE. More widespread summer cooling across the mid-latitudes of North America and Eurasia is pronounced in 1258, while records from Scandinavia and Siberia reveal peak cooling in 1259. In contrast to the marked post-Samalas temperature response at high-elevation sites in the Northern Hemisphere, no strong hydroclimatic anomalies emerge from the 79 precipitation-sensitive chronologies. Although our findings remain spatially biased towards the western US and central Europe, and growth-climate response patterns are not always dominated by a single meteorological factor, this study offers a global proxy framework for the evaluation of paleoclimate model simulations.
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| 4. |
- Chai, J., et al.
(author)
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A robust equatorial Pacific westerly response to tropical volcanism in multiple models
- 2020
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 55, s. 3413-3429
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Journal article (peer-reviewed)abstract
- After each of the 1963 Agung, 1982 El Chichon, and 1991 Pinatubo eruptions, an El Nino was observed. The increased likelihood of an El Nino after a tropical eruption has also been found in long-term reconstructed proxy data. Through examining simulations over the last millennium by 11 different models, we show that a tropical volcano eruption can robustly excite a western-to-central equatorial Pacific (WCEP) westerly anomaly at 850 hPa in eight out of the 11 models; such a westerly anomaly is favorable for El Nino development. Under the volcanic forcing, there are significant extratropical continent surface cooling and tropical drying with negative precipitation anomalies over the South-South East Asia (SSEA), West African monsoon, and Intertropical Convergence Zone (ITCZ) regions. This common precipitation suppression response occurs in most of the models. Sensitivity experiments show that a WCEP westerly anomaly can be excited by the tropical land cooling, especially the SSEA cooling induced precipitation suppression rather than by the extratropical land surface cooling. Theoretical results show that a WCEP westerly anomaly is excited due to a Gill response to reduced precipitation over the SSEA and West African monsoon regions; and the SSEA contributes more than the West African monsoon does. The ITCZ weakening, however, excites an easterly wind anomaly. The models with more sensitive convective feedback tend to simulate an El Nino more easily, while a failed simulation of an El Nino after a robust westerly anomaly in some models calls for further studies on these models' delayed responses to radiative forcing induced by volcano eruptions.
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| 5. |
- Cheung, Ho-Nam, et al.
(author)
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Assessing the influence of sea surface temperature and arctic sea ice cover on the uncertainty in the boreal winter future climate projections
- 2022
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 59:1-2, s. 433-454
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Journal article (peer-reviewed)abstract
- We investigate the uncertainty (i.e., inter-model spread) in future projections of the boreal winter climate, based on the forced response of ten models from the CMIP5 following the RCP8.5 scenario. The uncertainty in the forced response of sea level pressure (SLP) is large in the North Pacific, the North Atlantic, and the Arctic. A major part of these uncertainties (31%) is marked by a pattern with a center in the northeastern Pacific and a dipole over the northeastern Atlantic that we label as the Pacific–Atlantic SLP uncertainty pattern (PA∆SLP). The PA∆SLP is associated with distinct global sea surface temperature (SST) and Arctic sea ice cover (SIC) perturbation patterns. To better understand the nature of the PA∆SLP, these SST and SIC perturbation patterns are prescribed in experiments with two atmospheric models (AGCMs): CAM4 and IFS. The AGCM responses suggest that the SST uncertainty contributes to the North Pacific SLP uncertainty in CMIP5 models, through tropical–midlatitude interactions and a forced Rossby wavetrain. The North Atlantic SLP uncertainty in CMIP5 models is better explained by the combined effect of SST and SIC uncertainties, partly related to a Rossby wavetrain from the Pacific and air-sea interaction over the North Atlantic. Major discrepancies between the CMIP5 and AGCM forced responses over northern high-latitudes and continental regions are indicative of uncertainties arising from the AGCMs. We analyze the possible dynamic mechanisms of these responses, and discuss the limitations of this work.
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| 6. |
- Christensen, Ole B., et al.
(author)
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Filling the matrix : an ANOVA-based method to emulate regional climate model simulations for equally-weighted properties of ensembles of opportunity
- 2022
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 58:9-10, s. 2371-2385
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Journal article (peer-reviewed)abstract
- Collections of large ensembles of regional climate model (RCM) downscaled climate data for particular regions and scenarios can be organized in a usually incomplete matrix consisting of GCM (global climate model) x RCM combinations. When simple ensemble averages are calculated, each GCM will effectively be weighted by the number of times it has been downscaled. In order to facilitate more equal and less arbitrary weighting among downscaled GCM results, we present a method to emulate the missing combinations in such a matrix, enabling equal weighting among participating GCMs and hence among regional consequences of large-scale climate change simulated by each GCM. This method is based on a traditional Analysis of Variance (ANOVA) approach. The method is applied and studied for fields of seasonal average temperature, precipitation and surface wind and for the 10-year return value of daily precipitation and of 10-m wind speed for a completely filled matrix consisting of 5 GCMs and 4 RCMs. We quantify the skill of the two averaging methods for different numbers of missing simulations and show that ensembles where lacking members have been emulated by the ANOVA technique are better at representing the full ensemble than corresponding simple ensemble averages, particularly in cases where only a few model combinations are absent. The technique breaks down when the number of missing simulations reaches the sum of the numbers of GCMs and RCMs. Also, the method is only useful when inter-simulation variability is limited. This is the case for the average fields that have been studied, but not for the extremes. We have developed analytical expressions for the degree of improvement obtained with the present method, which quantify this conclusion.
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| 7. |
- Christensen, Ole B., et al.
(author)
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Partitioning uncertainty components of mean climate and climate change in a large ensemble of European regional climate model projections
- 2020
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 54:9-10, s. 4293-4308
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Journal article (peer-reviewed)abstract
- A study of seasonal mean temperature, precipitation, and wind speed has been performed for a set of 19 global climate model (GCM) driven high-resolution regional climate model (RCM) simulations forming a complete 5 x 4 GCM x RCM matrix with only one missing simulation. Differences between single simulations and between groups of simulations forced by a specific GCM or a specific RCM are identified. With the help of an analysis of variance (ANOVA) we split the ensemble variance into linear GCM and RCM contributions and cross terms for both mean climate and climate change for the end of the current century according to the RCP8.5 emission scenario. The results document that the choice of GCM generally has a larger influence on the climate change signal than the choice of RCM, having a significant influence for roughly twice as many points in the area for the fields investigated (temperature, precipitation and wind speed). It is also clear that the RCM influence is generally concentrated close to the eastern and northern boundaries and in mountainous areas, i.e., in areas where the added surface detail of e.g. orography, snow and ice seen by the RCM is expected to have considerable influence on the climate, and in areas where the air in general has spent the most time within the regional domain. The analysis results in estimates of areas where the specific identity of either GCM or RCM is formally significant, hence obtaining an indication about regions, seasons, and fields where linear superpositions of GCM and RCM effects are good approximations to an actual simulation for both the mean fields analysed and their changes. In cases where linear superposition works well, the frequently encountered sparse GCM-RCM matrices may be filled with emulated results, leading to the possibility of giving more fair relative weight between model simulations than simple averaging of existing simulations. An important result of the present study is that properties of the specific GCM-RCM combination are generally important for the mean climate, but negligible for climate change for the seasonal-mean surface fields investigated here.
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| 8. |
- Collier, Emily, et al.
(author)
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The first ensemble of kilometer-scale simulations of a hydrological year over the third pole
- 2024
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In: Climate Dynamics. - 0930-7575 .- 1432-0894.
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Journal article (peer-reviewed)abstract
- An accurate understanding of the current and future water cycle over the Third Pole is of great societal importance, given the role this region plays as a water tower for densely populated areas downstream. An emerging and promising approach for skillful climate assessments over regions of complex terrain is kilometer-scale climate modeling. As a foundational step towards such simulations over the Third Pole, we present a multi-model and multi-physics ensemble of kilometer-scale regional simulations for the hydrological year of October 2019 to September 2020. The ensemble consists of 13 simulations performed by an international consortium of 10 research groups, configured with a horizontal grid spacing ranging from 2.2 to 4km covering all of the Third Pole region. These simulations are driven by ERA5 and are part of a Coordinated Regional Climate Downscaling EXperiment Flagship Pilot Study on Convection-Permitting Third Pole. The simulations are compared against available gridded and in-situ observations and remote-sensing data, to assess the performance and spread of the model ensemble compared to the driving reanalysis during the cold and warm seasons. Although ensemble evaluation is hindered by large differences between the gridded precipitation datasets used as a reference over this region, we show that the ensemble improves on many warm-season precipitation metrics compared with ERA5, including most wet-day and hour statistics, and also adds value in the representation of wet spells in both seasons. As such, the ensemble will provide an invaluable resource for future improvements in the process understanding of the hydroclimate of this remote but important region.
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| 9. |
- Deng, Kaiqiang, et al.
(author)
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The offshore wind speed changes in China: an insight into CMIP6 model simulation and future projections
- 2024
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In: CLIMATE DYNAMICS. - 0930-7575 .- 1432-0894.
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Journal article (peer-reviewed)abstract
- Offshore wind speed in China plays a key role in affecting air-sea interactions, coastal tides, and wind energy, but its changes in a warming climate and the associated causes remain unclear. Based on the ERA5 reanalysis and the Coupled Model Intercomparison Project Phase 6 (CMIP6) models, this study evaluates the past and future variations of wind speed at 10 m (WS10) over China's offshore seas in summer and winter. The results show that the CMIP6 multi-model mean performs well in simulating the climatological patterns (1981-2010) of WS10 for both seasons. The trends and leading variabilities in WS10 are also reasonably reproduced in the South China Sea (SCS). In the northern SCS, WS10 has strengthened during both seasons in the recent decades. In contrast, in the East China Sea (ECS), WS10 has increased (decreased) during summer (winter). Further attribution analysis suggests that the forcing of greenhouse gasses (aerosols) may make WS10 stronger (weaker) in the two seas and for both seasons, while natural variability tends to slow down (speed up) WS10 in the SCS and ECS during summer (winter). In addition, according to the CMIP6 model projections under various warming scenarios, WS10 is likely to increase over both the northern SCS and the ECS in summer, while WS10 will increase over the northern SCS but decrease over the ECS in winter. Differences in the projected WS10 changes in the ECS during summer and winter are attributed to the projected intensification (weakening) of the East Asian summer (winter) monsoon circulation.
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| 10. |
- Du, W. T., et al.
(author)
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Can summer monsoon moisture invade the Jade Pass in Northwestern China?
- 2020
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 55, s. 3101-3115
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Journal article (peer-reviewed)abstract
- Heavy precipitation events are increasingly concerned because their significant contribution to annual precipitation in the Northwestern China, which might be related to invasion of summer monsoon moisture. It is interest whether or not the same is Jade Pass as being outside the control of the Asian summer monsoon. In this work, six heavy precipitation events were selected based on the 95 percentiles of the daily precipitation at the 12 weather stations around the Jade Pass from 1970-2000, with consideration of the influences of elevation. The event on June 19th, 2013 was chosen for a detailed examination due to the fact that the day has a large-scale precipitation as revealed by a gridded precipitation dataset over a large region. Using a Weather Research and Forecasting Model (WRF) simulation with high spatiotemporal resolution and in situ isotopic tracing (delta O-18, delta D), under a large-scale heavy precipitation event, this study provides ambitious view at the synoptic scale. A dramatic decrease in the delta O-18, delta D and deuterium (d)-excess of precipitation, very high relative humidity (98%), and reduced air temperature indicate that the precipitation was a result of long-distance-transported monsoon vapor. In addition, the slope of the local water meteoric line (LWML) of the precipitation for this event was very close to that of the global meteoric water line (GWML), indicating the source of moisture was from the ocean. Meanwhile, the WRF simulation confirms that the precipitation at the Jade Pass was not caused by local convection, but by summer monsoon. Both WRF simulation and isotopic tracing support the view that the monsoon moisture could invade Jade Pass at the synoptic scale and impact on precipitation, which need be further investigated.
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