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1.
Using the Paleoclimate Modeling Inter-comparison Project Phase 2 and 3 (PMIP2 and PMIP3), we investigated the tropical Pacific climate state, annual cycle, and El Niño-Southern Oscillation (ENSO) during the mid-Holocene period (6,000 years before present; 6 ka run). When the 6 ka run was compared to the control run (0 ka run), the reduced sea surface temperature (SST) and the reduced precipitation due to the basin-wide cooling, and the intensified cross-equatorial surface winds due to the hemispheric discrepancy of the surface cooling over the tropical Pacific were commonly observed in both the PMIP2 and PMIP3, but changes were more dominant in the PMIP3. The annual cycle of SST was weaker over the equatorial eastern Pacific, because of the orbital forcing change and the deepening mixed layer, while it was stronger over the equatorial western pacific in both the PMIP2 and PMIP3. The stronger annual cycle of the equatorial western Pacific SST was accompanied by the intensified annual cycle of the zonal surface wind, which dominated in the PMIP3 in particular. The ENSO activity in the 6 ka run was significantly suppressed in the PMIP2, but marginally reduced in the PMIP3. In general, the weakened air-sea coupling associated with basin-wide cooling, reduced precipitation, and a hemispheric contrast in the climate state led to the suppression of ENSO activity, and the weakening of the annual cycle over the tropical eastern Pacific might lead to the intensification of ENSO through the frequency entrainment. Therefore, the two opposite effects are slightly compensated for by each other, which results in a small reduction in the ENSO activity during the 6 ka in the PMIP3. On the whole, in PMIP2/PMIP3, the variability of canonical (or conventional) El Niño tends to be reduced during 6 ka, while that of CP/Modoki El Niño tends to be intensified.  相似文献   

2.
Observational analysis and purposely designed coupled atmosphere–ocean (AOGCM) and atmosphere-only (AGCM) model simulations are used together to investigate a new mechanism describing how spring Arctic sea ice impacts the East Asian summer monsoon (EASM). Consistent with previous studies, analysis of observational data from 1979 to 2009 show that spring Arctic sea ice is significantly linked to the EASM on inter-annual timescales. Results of a multivariate Empirical Orthogonal Function analysis reveal that sea surface temperature (SST) changes in the North Pacific play a mediating role for the inter-seasonal connection between spring Arctic sea ice and the EASM. Large-scale atmospheric circulation and precipitation changes are consistent with the SST changes. The mechanism found in the observational data is confirmed by the numerical experiments and can be described as follows: spring Arctic sea ice anomalies cause atmospheric circulation anomalies, which, in turn, cause SST anomalies in the North Pacific. The SST anomalies can persist into summer and then impact the summer monsoon circulation and precipitation over East Asia. The mediating role of SST changes is highlighted by the result that only the AOGCM, but not the AGCM, reproduces the observed sea ice-EASM linkage.  相似文献   

3.
Studies of climate change 6,000 years before present using atmospheric general circulation models (AGCMs) suggest the enhancement and northward shift of the summer Asian and African monsoons in the Northern Hemisphere. Although enhancement of the African monsoonal precipitation by ocean coupling is a common and robust feature, contradictions exist between analyses of the role of the ocean in the strength of the Asian monsoon. We investigated the role of the ocean in the Asian monsoon and sought to clarify which oceanic mechanisms played an important role using three ocean coupling schemes: MIROC, an atmosphere–ocean coupled general circulation model [C]; an AGCM extracted from MIROC coupled with a mixed-layer ocean model [M]; and the same AGCM, but with prescribed sea surface temperatures [A]. The effect of “ocean dynamics” is quantified through differences between experiments [C] and [M]. The effect of “ocean thermodynamics” is quantified through differences between experiments [M] and [A]. The precipitation change for the African and Asian monsoon area suggested that the ocean thermodynamics played an important role. In particular, the enhancement of the Asian monsoonal precipitation was most vigorous in the AGCM simulations, but mitigated in early summer in ocean coupled cases, which were not significantly different from each other. The ocean feedbacks were not significant for the precipitation change in late summer. On the other hand, in Africa, ocean thermodynamics contributed to the further enhancement of the precipitation from spring to autumn, and the ocean dynamics had a modest impact in enhancing precipitation in late summer.  相似文献   

4.
This work focuses on the Late Saalian (140?ka) Eurasian ice sheets?? surface mass balance (SMB) sensitivity to changes in sea surface temperatures (SST). An Atmospheric General Circulation Model (AGCM), forced with two preexisting Last Glacial Maximum (LGM, 21?ka) SST reconstructions, is used to compute climate at 140 and 21?ka (reference glaciation). Contrary to the LGM, the ablation almost stopped at 140?ka due to the climatic cooling effect from the large ice sheet topography. Late Saalian SST are simulated using an AGCM coupled with a mixed layer ocean. Compared to the LGM, these 140?ka SST show an inter-hemispheric asymmetry caused by the larger ice-albedo feedback, cooling climate. The resulting Late Saalian ice sheet SMB is smaller due to the extensive simulated sea ice reducing the precipitation. In conclusion, SST are important for the stability and growth of the Late Saalian Eurasian ice sheet.  相似文献   

5.
Modulation of a monsoon under glacial forcing is examined using an atmosphere?Cocean coupled general circulation model (AOGCM) following the specifications established by Paleoclimate Modelling Intercomparison Project phase 2 (PMIP2) to understand the air?Csea?Cland interaction under different climate forcing. Several sensitivity experiments are performed in response to individual changes in the continental ice sheet, orbital parameters, and sea surface temperature (SST) in the Last Glacial Maximum (LGM: 21?ka) to evaluate the driving mechanisms for the anomalous seasonal evolution of the monsoon. Comparison of the model results in the LGM with the pre-industrial (PI) simulation shows that the Arabian Sea and Bay of Bengal are characterized by enhancement of pre-monsoon convection despite a drop in the SST encompassing the globe, while the rainfall is considerably suppressed in the subsequent monsoon period. In the LGM winter relative to the PI, anomalies in the meridional temperature gradient (MTG) between the Asian continents minus the tropical oceans become positive and are consistent with the intensified pre-monsoon circulation. The enhanced MTG anomalies can be explained by a decrease in the condensation heating relevant to the suppressed tropical convection as well as positive insolation anomalies in the higher latitude, showing an opposing view to a warmer future climate. It is also evident that a latitudinal gradient in the SST across the equator plays an important role in the enhancement of pre-monsoon rainfall. As for the summer, the sensitivity experiments imply that two ice sheets over the northern hemisphere cools the air temperature over the Asian continent, which is consistent with the reduction of MTG involved in the attenuated monsoon. The surplus pre-monsoon convection causes a decrease in the SST through increased heat loss from the ocean surface; in other words, negative ocean feedback is also responsible for the subsequent weakening of summer convection.  相似文献   

6.
Vasubandhu Misra  H. Li 《Climate Dynamics》2014,42(9-10):2491-2507
An extensive set of boreal summer seasonal hindcasts from a two tier system is compared with corresponding seasonal hindcasts from two other coupled ocean–atmosphere models for their seasonal prediction skill (for precipitation and surface temperature) of the Asian summer monsoon. The unique aspect of the two-tier system is that it is at relatively high resolution and the SST forcing is uniquely bias corrected from the multi-model averaged forecasted SST from the two coupled ocean–atmosphere models. Our analysis reveals: (a) The two-tier forecast system has seasonal prediction skill for precipitation that is comparable (over the Southeast Asian monsoon) or even higher (over the South Asian monsoon) than the coupled ocean–atmosphere. For seasonal anomalies of the surface temperature the results are more comparable across models, with all of them showing higher skill than that for precipitation. (b) Despite the improvement from the uncoupled AGCM all models in this study display a deterministic skill for seasonal precipitation anomalies over the Asian summer monsoon region to be weak. But there is useful probabilistic skill for tercile anomalies of precipitation and surface temperature that could be harvested from both the coupled and the uncoupled climate models. (c) Seasonal predictability of the South Asian summer monsoon (rainfall and temperature) does seem to stem from the remote ENSO forcing especially over the Indian monsoon region and the relatively weaker seasonal predictability in the Southeast Asian summer monsoon could be related to the comparatively weaker teleconnection with ENSO. The uncoupled AGCM with the bias corrected SST is able to leverage this teleconnection for improved seasonal prediction skill of the South Asian monsoon relative to the coupled models which display large systematic errors of the tropical SST’s.  相似文献   

7.
Coupled atmosphere–ocean general circulation models (AOGCMs) commonly fail to simulate the eastern equatorial Atlantic boreal summer cold tongue and produce a westerly equatorial trade wind bias. This tropical Atlantic bias problem is investigated with a high-resolution (27-km atmosphere represented by the Weather Research and Forecasting Model, 9-km ocean represented by the Regional Ocean Modeling System) coupled regional climate model. Uncoupled atmospheric simulations test climate sensitivity to cumulus, land-surface, planetary boundary layer, microphysics, and radiation parameterizations and reveal that the radiation scheme has a pronounced impact in the tropical Atlantic. The CAM radiation simulates a dry precipitation (up to ?90%) and cold land-surface temperature (up to ?8?K) bias over the Amazon related to an over-representation of low-level clouds and almost basin-wide westerly trade wind bias. The Rapid Radiative Transfer Model and Goddard radiation simulates doubled Amazon and Congo Basin precipitation rates and a weak eastern Atlantic trade wind bias. Season-long high-resolution coupled regional model experiments indicate that the initiation of the warm eastern equatorial Atlantic sea surface temperature (SST) bias is more sensitive to the local rather than basin-wide trade wind bias and to a wet Congo Basin instead of dry Amazon—which differs from AOGCM simulations. Comparisons between coupled and uncoupled simulations suggest a regional Bjerknes feedback confined to the eastern equatorial Atlantic amplifies the initial SST, wind, and deepened thermocline bias, while barrier layer feedbacks are relatively unimportant. The SST bias in some CRCM simulations resembles the typical AOGCM bias indicating that increasing resolution is unlikely a simple solution to this problem.  相似文献   

8.
We examine the Florida Climate Institute–Florida State University Seasonal Hindcast (FISH50) skill at a relatively high (50 km grid) resolution two tiered Atmospheric General Circulation Model (AGCM) for boreal winter and spring seasons at zero and one season lead respectively. The AGCM in FISH50 is forced with bias corrected forecast sea surface temperature averaged from two dynamical coupled ocean–atmosphere models. The comparison of the hindcast skills of precipitation and surface temperature from FISH50 with the coupled ocean–atmosphere models reveals that the probabilistic skill is nearly comparable in the two types of forecast systems (with some improvements in FISH50 outside of the global tropics). Furthermore the drop in skill in going from zero lead (boreal winter) to one season lead (boreal spring) is also similar in FISH50 and the coupled ocean–atmosphere models. Both the forecast systems also show that surface temperature hindcasts have more skill than the precipitation hindcasts and that land based precipitation hindcasts have slightly lower skill than the corresponding hindcasts over the ocean.  相似文献   

9.
The ability of state-of-the-art climate models to capture the mean spatial and temporal characteristics of daily intense rainfall events over Africa is evaluated by analyzing regional climate model (RCM) simulations at 90- and 30-km along with output from four atmospheric general circulation models (AGCMs) and coupled atmosphere–ocean general circulation models (AOGCMs) of the Climate Model Intercomparison Project 5. Daily intense rainfall events are extracted at grid point scale using a 95th percentile threshold approach applied to all rainy days (i.e., daily rainfall ≥1 mm day?1) over the 1998–2008 period for which two satellite-derived precipitation products are available. Both RCM simulations provide similar results. They accurately capture the spatial and temporal characteristics of intense events, while they tend to overestimate their number and underestimate their intensity. The skill of AGCMs and AOGCMs is generally similar over the African continent and similar to previous global climate model generations. The majority of the AGCMs and AOGCMs greatly overestimate the frequency of intense events, particularly in the tropics, generally fail at simulating the observed intensity, and systematically overestimate their spatial coverage. The RCM performs at least as well as the most accurate global climate model, demonstrating a clear added value to general circulation model simulations and the usefulness of regional modeling for investigating the physics leading to intense events and their change under global warming.  相似文献   

10.
Recent studies indicate a weakening of the Walker Circulation during the twentieth century. Here, we present evidence from an atmospheric general circulation model (AGCM) forced by the history of observed sea surface temperature (SST) that the Walker Circulation may have intensified rather than weakened. Observed Equatorial Indo-Pacific Sector SST since 1870 exhibited a zonally asymmetric evolution: While the eastern part of the Equatorial Pacific showed only a weak warming, or even cooling in one SST dataset, the western part and the Equatorial Indian Ocean exhibited a rather strong warming. This has resulted in an increase of the SST gradient between the Maritime Continent and the eastern part of the Equatorial Pacific, one driving force of the Walker Circulation. The ensemble experiments with the AGCM, with and without time-varying external forcing, suggest that the enhancement of the SST gradient drove an anomalous atmospheric circulation, with an enhancement of both Walker and Hadley Circulation. Anomalously strong precipitation is simulated over the Indian Ocean and anomalously weak precipitation over the western Pacific, with corresponding changes in the surface wind pattern. Some sensitivity to the forcing SST, however, is noticed. The analysis of twentieth century integrations with global climate models driven with observed radiative forcing obtained from the Coupled Model Intercomparison Project (CMIP) database support the link between the SST gradient and Walker Circulation strength. Furthermore, control integrations with the CMIP models indicate the existence of strong internal variability on centennial timescales. The results suggest that a radiatively forced signal in the Walker Circulation during the twentieth century may have been too weak to be detectable.  相似文献   

11.
The impact of ocean–atmosphere coupling on the simulation and prediction of the boreal summer intraseasonal oscillation (ISO) has been investigated by diagnosing 22-year retrospective forecasts using the Seoul National University coupled general circulation model (CGCM) and its atmospheric GCM (AGCM) forced with SSTs derived from the CGCM. Numerous studies have shown that the ocean–atmosphere coupling has a significant effect on the improvement of ISO simulation and prediction. Contrary to previous studies, this study shows similar results between CGCM and AGCM, not only in regard to the ISO simulation characteristics but also the predictability. The similarities between CGCM and AGCM include (1) the ISO intensity over the entire Asian-monsoon region; (2) the spatiotemporal evolution of the northward propagating ISO (NPISO); and (3) the potential and practical predictability. A notable difference between CGCM and AGCM is the phase relationship between precipitation and SST anomalies. The CGCM and observation exhibits a near-quadrature relationship between precipitation and SST, with the former lagging about two pentads. The AGCM shows a less realistic phase relationship. The similar structure and propagation characteristics of ISO between the CGCM and AGCM suggest that the internal atmospheric dynamics could be more essential to the ISO than the ocean–atmosphere interaction over the Indian monsoon region.  相似文献   

12.
The sensitivity of the representation of the global monsoon annual cycle to horizontal resolution is compared in three AGCMs:the Met Office Unified Model-Global Atmosphere 3.0;the Meteorological Research Institute AGCM3;and the Global High Resolution AGCM from the Geophysical Fluid Dynamics Laboratory.For each model,we use two horizontal resolution configurations for the period 1998–2008.Increasing resolution consistently improves simulated precipitation and low-level circulation of the annual mean and the first two annual cycle modes,as measured by the pattern correlation coefficient and equitable threat score.Improvements in simulating the summer monsoon onset and withdrawal are region-dependent.No consistent response to resolution is found in simulating summer monsoon retreat.Regionally,increased resolution reduces the positive bias in simulated annual mean precipitation,the two annual-cycle modes over the West African monsoon and Northwestern Pacific monsoon.An overestimation of the solstitial mode and an underestimation of the equinoctial asymmetric mode of the East Asian monsoon are reduced in all high-resolution configurations.Systematic errors exist in lower-resolution models for simulating the onset and withdrawal of the summer monsoon.Higher resolution models consistently improve the early summer monsoon onset over East Asia and West Africa,but substantial differences exist in the responses over the Indian monsoon region,where biases differ across the three low-resolution AGCMs.This study demonstrates the importance of a multi-model comparison when examining the added value of resolution and the importance of model physical parameterizations for simulation of the Indian monsoon.  相似文献   

13.
Mathew Roxy 《Climate Dynamics》2014,43(5-6):1159-1169
Over the tropical oceans, higher sea surface temperatures (SST, above 26 °C) in summer are generally accompanied by increased precipitation. However, it has been argued for the last three decades that, any monotonic increase in precipitation with respect to SST is limited to an upper threshold of 28–29.5 °C, and beyond this, the relationship fails. Based on this assessment it has often been presumed that, since the mean SSTs over the Asian monsoon basins (Indian Ocean and north-west Pacific) are mostly above the threshold, SST does not play an active role on the summer monsoon variability. It also implies that increasing SSTs due to a changing climate need not result in increasing monsoon precipitation. The current study shows that the response of precipitation to SST has a time lag, that too with a spatial variability over the monsoon basins. Taking this lag into account, the results here show that enhanced convection occurs even up to the SST maxima of 31 °C averaged over these basins, challenging any claim of an upper threshold for the SST-convection variability. The study provides us with a novel method to quantify the SST-precipitation relationship. The rate of increase is similar across the basins, with precipitation increasing at ~2 mm day?1 for an increase of 1 °C in SST. This means that even the high SSTs over the monsoon basins do play an active role on the monsoon variability, challenging previous assumptions. Since the response of precipitation to SST variability is visible in a few days, it would also imply that including realistic ocean–atmosphere coupling is crucial even for short term monsoon weather forecasts. Though recent studies suggest a weakening of the monsoon circulation over the last few decades, results here suggest an increased precipitation over the tropical monsoon regions, in a global warming environment with increased SSTs. Thus the signature of SST is found to be significant for the Asian summer monsoon, in a quantifiable manner, seamlessly through all the timescales—from short-term intraseasonal to long-term climate scales.  相似文献   

14.
利用中国科学院大气物理研究所(IAP,Institute of Atmospheric Physics)3个大气环流模式(AGCM,Atmospheric General Circulation Model)和欧洲多模式集合预报计划(DEMETER,Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction)中7个海气耦合模式(CGCM,Coupled General Circulation Model)的1981—2000年共20 a集合回报结果,比较了不同模式在热带地区,尤其是热带西太平洋地区夏季平均降水的可预测性差异。结果表明:所有模式都能够较好地再现这20 a平均降水的空间分布特征;IAP 9层AGCM最好地再现了热带西太平洋地区,尤其是140°E以西地区降水异常的主要空间特征,而CGCM则对海洋上空的降水异常特征有较好的回报能力。回报的降水异常量值偏弱和系统偏移使得IAP的AGCM原始回报技巧评分明显低于CGCM,但是经过统计订正后AGCM对热带夏季降水表现出与CGCM相当的可预测性。统计订正方法对部分CGCM模式的预报评分也有改进效果,但是当模式原始预报评分较好时订正方法的效果并不明显。对于IAP模式,随着IAP大气模式的不断改进,模式对热带西太平洋降水预测改进最为明显,但是在太平洋东部地区,IAP大气模式依然存在降水异常偏弱的不足。  相似文献   

15.
用一个耦合的全球格点大气环流模式－植被模式模拟中全新世的气候变化，模拟试验中考虑了地球轨道参数的变化，而其他强迫条件均取成现今值。结果表明，耦合的模式能够模拟出较今强的大尺度夏季风，特别是亚洲－非洲季风，而其他季节和区域的变化值一般都比较小。季风环流和季风降水都大幅度地增大了。结果还显示，耦合模式模拟的大尺度季风系统的变化同单纯大气环流模式模拟的结果非常相似，但是，在非洲北部季风区耦合模式模拟的降水和温度变化较单纯大气模式模拟的值要大，而且，耦合模式模拟的冬季降温值要比单纯大气模式模拟的结果小。  相似文献   

16.
江洁  周天军  吴波  邹立维 《大气科学》2019,43(3):467-482
观测发现,西北太平洋区域夏季降水—SST存在显著的负相关,主要是由于El Ni?o衰减年西北太平洋异常反气旋持续至夏季,该过程是检验耦合模式性能的重要参照标准。本文利用中国科学院大气物理研究所近期气候预测系统IAP-DecPreS,通过海洋同化试验、大气模式AMIP试验与观测结果的比较,评估海洋同化试验对西北太平洋夏季局地海气相互作用特征的模拟影响。结果表明,海洋同化试验能够模拟出西北太平洋区域夏季降水—SST负相关,但负相关区域范围偏小。其与观测之间的最大差异出现在8月,西北太平洋负降水异常及异常反气旋位置偏东,强度偏弱。这是由于其模拟的El Ni?o衰减年夏季赤道东印度洋正降水异常偏弱且移动至赤道南侧,对流层增温偏弱,对西太平洋的遥相关作用偏弱。AMIP试验未考虑大气对海洋的反馈作用,不能再现西北太平洋降水—SST负相关,无法模拟出El Ni?o衰减年夏季西北太平洋异常反气旋。研究表明,海洋同化试验对西北太平洋区域局地海气相互作用特征的模拟能力较AMIP试验有所提升,其对8月西北太平洋降水与环流场的模拟偏差与东赤道印度洋降水模拟偏差有关。  相似文献   

17.
The SST-precipitation relationship in the intraseasonal variability (ISV) over the Asian monsoon region is examined using recent high quality satellite data and simulations from a state of the art coupled model, the climate forecast system version 2 (CFSv2). CFSv2 demonstrates high skill in reproducing the spatial distribution of the observed climatological mean summer monsoon precipitation along with its interannual variability, a task which has been a conundrum for many recent climate coupled models. The model also exhibits reasonable skill in simulating coherent northward propagating monsoon intraseasonal anomalies including SST and precipitation, which are generally consistent with observed ISV characteristics. Results from the observations and the model establish the existence of spatial variability in the atmospheric convective response to SST anomalies, over the Asian monsoon domain on intraseasonal timescales. The response is fast over the Arabian Sea, where precipitation lags SST by ~5 days; whereas it is slow over the Bay of Bengal and South China Sea, with a lag of ~12 days. The intraseasonal SST anomalies result in a similar atmospheric response across the basins, which consists of a destabilization of the bottom of the atmospheric column, as observed from the equivalent potential temperature anomalies near the surface. However, the presence of a relatively strong surface convergence over the Arabian Sea, due to the presence of a strong zonal gradient in SST, which accelerates the upward motion of the moist air, results in a relatively faster response in terms of the local precipitation anomalies over the Arabian Sea than over the Bay of Bengal and South China Sea. With respect to the observations, the ocean–atmosphere coupling is well simulated in the model, though with an overestimation of the intraseasonal SST anomalies, leading to an exaggerated SST-precipitation relationship. A detailed examination points to a systematic bias in the thickness of the mixed layer of the ocean model, which needs to be rectified. A too shallow (deep) mixed layer enhances (suppress) the amplitude of the intraseasonal SST anomalies, thereby amplifying (lessening) the ISV and the active-break phases of the monsoon in the model.  相似文献   

18.
This paper examines an issue concerning the simulation of anomalously wet Indian summer monsoons like 1994 which co-occurred with strong positive Indian Ocean Dipole (IOD) conditions in the tropical Indian Ocean. Contrary to observations it has been noticed that standalone atmospheric general circulation models (AGCM) forced with observed SST boundary condition, consistently depicted a decrease of the summer monsoon rainfall during 1994 over the Indian region. Given the ocean?Catmosphere coupling during IOD events, we have examined whether the failure of standalone AGCM simulations in capturing wet Indian monsoons like 1994 can be remedied by including a simple form of coupling that allows the monsoon circulation to dynamically interact with the IOD anomalies. With this view, we have performed a suite of simulations by coupling an AGCM to a slab-ocean model with spatially varying mixed-layer-depth (MLD) specified from observations for the 1994 IOD; as well as four other cases (1983, 1997, 2006, 2007). The specification of spatially varying MLD from observations allows us to constrain the model to observed IOD conditions. It is seen that the inclusion of coupling significantly improves the large-scale circulation response by strengthening the monsoon cross-equatorial flow; leading to precipitation enhancement over the subcontinent and rainfall decrease over south-eastern tropical Indian Ocean??in a manner broadly consistent with observations. A plausible physical mechanism is suggested to explain the monsoonal response in the coupled frame-work. These results warrant the need for improved monsoon simulations with fully coupled models to be able to better capture the observed monsoon interannual variability.  相似文献   

19.
We present an assessment of climate change impacts on the hydrologic regime of the 600,000 km2 Upper Paraguay River basin, located in central South America based on predictions of 20 Atmospheric/Ocean General Circulation Models (AOGCMs). We considered two climate change scenarios from the Intergovernmental Panel on Climate Change (IPCC) and two 30-years time intervals centered at 2030 and 2070. Projected temperature and precipitation anomalies estimated by the AOGCMs for the study site are spatially downscaled. Time series of projected temperature and precipitation were estimated using the delta change approach. These time series were used as input to a detailed coupled hydrologic-hydraulic model aiming to estimate projected streamflow in climate change scenarios at several control points in the basin. Results show that impacts on streamflow are highly dependent on the AOGCM used to obtain the climate predictions. Patterns of temperature increase persist over the entire year for almost all AOGCMs resulting in an increase in the evapotranspiration rate of the hydrological model. The precipitation anomalies show large dispersion, being projected as either an increase or decrease in precipitation rates. Based on these inputs, results from the coupled hydrologic-hydraulic model show nearly one half of projections as increasing river discharge, and other half as decreasing river discharge. If the mean or median of the predictions is considered, no discernible change in river discharge should be expected, despite the dispersion among results of the AOGCMs that reached +/?10 % in the short horizon and +/? 20 % in the long horizon, at several control points.  相似文献   

20.
Results from multiple model simulations are used to understand the tropical sea surface temperature (SST) response to the reduced greenhouse gas concentrations and large continental ice sheets of the last glacial maximum (LGM). We present LGM simulations from the Paleoclimate Modelling Intercomparison Project, Phase 2 (PMIP2) and compare these simulations to proxy data collated and harmonized within the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface Project (MARGO). Five atmosphere–ocean coupled climate models (AOGCMs) and one coupled model of intermediate complexity have PMIP2 ocean results available for LGM. The models give a range of tropical (defined for this paper as 15°S–15°N) SST cooling of 1.0–2.4°C, comparable to the MARGO estimate of annual cooling of 1.7 ± 1°C. The models simulate greater SST cooling in the tropical Atlantic than tropical Pacific, but interbasin and intrabasin variations of cooling are much smaller than those found in the MARGO reconstruction. The simulated tropical coolings are relatively insensitive to season, a feature also present in the MARGO transferred-based estimates calculated from planktonic foraminiferal assemblages for the Indian and Pacific Oceans. These assemblages indicate seasonality in cooling in the Atlantic basin, with greater cooling in northern summer than northern winter, not captured by the model simulations. Biases in the simulations of the tropical upwelling and thermocline found in the preindustrial control simulations remain for the LGM simulations and are partly responsible for the more homogeneous spatial and temporal LGM tropical cooling simulated by the models. The PMIP2 LGM simulations give estimates for the climate sensitivity parameter of 0.67°–0.83°C per Wm−2, which translates to equilibrium climate sensitivity for doubling of atmospheric CO2 of 2.6–3.1°C.  相似文献   

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