Water and energy fluxes in the lower Mackenzie valley, 1994/95

Abstract

The 1994/95 water year in the lower Mackenzie Valley was an extraordinary year hydrologically, with the important winter to summer transition being the earliest on record. Unlike more temperate areas, the northern water year is dominated, to a great extent, by this onset of spring which results in the melting of nearly half of the annual precipitation over a period of a few weeks, initiates the thawing of the river and lake ice and the soil active layer, and marks the beginning of the evaporation season. An early winter to summer transition occurred at two small research basins in the Inuvik area and at the East Channel of the Mackenzie River Delta. At the research basins, for example, the spring of 1994/95 had the earliest onset of continuous above‐freezing air temperatures, removal of the snow cover, and initiation of runoff. Consideration of the entire water year at the research basins demonstrates that rain and snow were nearly equal in magnitude, evaporation exceeded runoff, and the annual change in storage was negative to near zero. This negative change in storage was related to the long, snow‐free evaporation season, above‐average air temperatures, and below‐normal precipitation. The unusual winter to summer transition on the Mackenzie River in the eastern portion of the Mackenzie Delta was, in many ways, even more remarkable than that in the research basins. Earlier work had suggested that the timing of the spring breakup was very consistent from year to year. An analysis of the timing of breakup from the early 1960s to the late 1990s, however, shows a trend towards earlier spring breakup, with the mean for the 1990s being nine days earlier than that for the 1960s, and with the 1995 breakup being the earliest on record. Such an early breakup is not only an indication of warm local conditions, but of warm temperatures and an early runoff event over the more southerly areas of the Mackenzie basin. A companion Mackenzie Global Energy and Water Cycle Experiment study illustrates the importance of a high pressure circulation pattern centred east of the basin to this early melt event.     

Atmospheric contribution to hydrologic variations in the Arctic

Abstract

High‐latitude rawinsonde data for 18 years (1973–1990) are used to compute the atmospheric moisture flux convergence over two regions: the Arctic Ocean and the Mackenzie River drainage basin. The primary objectives are to assess the interannual variability and to compare the macroscale hydrologie regimes of the two regions. The moisture flux convergence is positive in all months over the Arctic Ocean, but is occasionally negative during summer over the Mackenzie Basin. The climatological seasonal cycle of the moisture convergence contains a late‐summer (August‐September) maximum over the Arctic Ocean but a late‐summer minimum over the Mackenzie Basin. Evaporation, deduced from the moisture inflow and independent data on precipitation, makes a much greater contribution to the atmospheric moisture budget of the Mackenzie domain, especially during summer. The respective equivalent area averages of the 18‐year annual mean moisture flux convergence, precipitation and derived evaporation are 17.3, 19.5 and 2.2 cm a^‐1 for the Arctic Ocean and 24.9, 33.6 and 8.7 cm a^‐1 for the Mackenzie domain. However, the range of interannual variations of the flux convergence is about ±50% of the annual means and more than twice the monthly means. The annual totals of the flux convergence are correlated with station‐derived precipitation over the Mackenzie domain and with yearly variations of the Mackenzie discharge. The moisture flux convergence over the Mackenzie domain suggests that station reports underestimate precipitation during the winter months by amounts equivalent to several centimetres per annum.     

长江中下游1998年夏季梅雨期降水再循环研究

改进了Eltahir的降水再循环率计算模式,引入水汽变化量,使其可用于小于月际时间尺度的降水再循环评估。并利用1998年6~8月间长江中下游162个测站的旬蒸发和降水资料,结合NCEP/NCAR的高空逐日再分析资料,对长江中下游1998年梅雨期的降水再循环率做了计算。分析发现:1998年夏季暴雨时期长江中下游的降水平均约有三成来自当地的蒸发。区域蒸发的水汽在安徽南部和湖北东部对降水的贡献最大。区域平均再循环率的旬变化强烈,最高可达67.8%,最小只有0.8%。表明地表水文敏感,地-气相互作用不稳定。     

Tracing freshwater anomalies through the air‐land‐ocean system: A case study from the Mackenzie river basin and the Beaufort Gyre

Abstract

Mackenzie River discharge was at a record low in water year (WY) 1995 (October 1994 to September 1995), was near average in WY 1996, and was at a record high in WY 1997. The record high discharge in WY 1997, with above average flow each month, was followed by a record high flow in May 1998, then a sharp decline. Through diagnosing these changing flows and their expression in the Beaufort Sea via synthesis of observations and model output, this study provides insight into the nature of the Arctic's freshwater system. The low discharge in WY 1995 manifests negative anomalies in P‐E and precipitation, recycled summer precipitation, and dry surface conditions immediately prior to the water year. The complex hydrograph for WY 1996 reflects a combination of spring soil moisture recharge, buffering by rising lake levels, positive P‐E anomalies in summer, and a massive release of water held in storage by Bennett Dam. The record high discharge in WY 1997 manifests the dual effects of reduced buffering by lakes and positive P‐E anomalies for most of the year. With reduced buffering, only modest P‐E the following spring led to a record discharge in May 1998. As simulated with a coupled ice‐ocean model, the record low discharge in WY 1995 contributed to a negative freshwater anomaly on the Mackenzie shelf lasting throughout the winter of 1995/96. High discharge from July–October 1996 contributed approximately 20% to a positive freshwater anomaly forming in the Beaufort Sea in the autumn of that year. The remainder was associated with reduced autumn/winter ice growth, strong ice melt the previous summer, and positive P‐E anomalies over the ocean itself. Starting in autumn 1997 and throughout 1998, the upper ocean became more saline owing to sea‐ice growth.     

长江源流量对长江源流域气候年代际变化的响应

利用长江源流域气象站降水、气温资料和源区直门达水文站流量,建立了历年各月、季降水距平百分率和气温距平序列,分析了长江源流量与长江源流域降水、气温的年代际变化.结果表明,长江源流域气候演变存在非常明显的年代际变化.年降水量呈平缓下降趋势,60、80年代年降水量正常或偏多,70、90年代偏少,主要受夏季降水的影响;年气温明显呈上升趋势,60年代最冷,70年代开始回升,80年代暖在冬,90年代暖在秋,目前年、夏、秋、冬季已达到1961年以来的最暖期;年流量与年降水的年代际变化、突变年份对应,60、80年代偏多,70、90年代偏少,目前除春季流量外,夏、秋、冬季已转入上升趋势,1965、1979、1997年二者均发生了突变.     

On the interannual variability of precipitation and runoff in the Mackenzie drainage basin

An analysis is presented of extratropical cyclone frequency, the 500 hPa height standard deviation field and the monthly 700 hPa height field in the Northern Hemisphere, together with precipitation in the Mackenzie basin and the Mackenzie River runoff. Spatial and temporal variability in the data are examined for the period 1965 to 1989, and a cross-correlation analyses is performed to determine the relationship between the runoff and the precipitation variations, and between the precipitation and the atmospheric circulation anomalies. It is found that precipitation fluctuations in the Mackenzie River drainage basin are strongly linked to variations in the Mackenzie River runoff and in the North Pacific storm tracks, with the time scale of variability ranging from interannual to decadal. The results are discussed in relation to the interdecadal Arctic climate cycle proposed by Mysak, Manak and Marsden, and revised by Mysak and Power. In particular, the latter authors hypothesized that, as part of this cycle, air-sea interactions and synoptic scale processes over the northwestern North Atlantic influenced, via cyclone movements in the Labrador Sea and Baffin Bay, precipitation in northern Canada and hence river runoff into the Arctic. The results of this study indicate that such influences on the precipitation in the Mackenzie basin are small, and hence that the Mysak-Power feedback loop which describes this climate cycle needs further revision.     

基于MTM-SVD方法的黄河流域夏季降水年际变化及其主要影响因子分析

应用中国气象台站积雪日数资料和NCEP/NCAR再分析资料以及多锥度—奇异值分解方法 (MTM-SVD),分析了近50年来黄河流域夏季降水的时空变化及其影响因子.发现黄河流域夏季降水存在显著的2～3年周期.在准3年周期上黄河流域夏季降水对前冬青藏高原东部积雪日数有很好的响应,当前冬高原积雪日数以正 (负) 异常为主时,接下来的夏季黄河流域降水偏少 (多).这种响应存在年代际变化,在1983年之前最为明显,1983~1993年是个调整时期,1993年以后又开始明显.在准2年周期上黄河流域夏季降水对前冬西太平洋暖池SST有很好的响应,当前冬西太平洋暖池SST偏高 (低) 时,接下来的夏季黄河流域降水表现为东多 (少)西少 (多) 型.这一响应同样存在年代际变化.前冬高原积雪和西太平洋暖池SST是影响黄河流域夏季降水的重要因子.     

我国北方地区降水再循环率的初步评估

利用1976~1995年的20年月平均NCEP/NCAR再分析资料, 对我国北方地区降水再循环率进行了评估, 结果表明:黄河流域总的降雨只有19%来自当地的水汽蒸发, 其他81%降水来自外部的水汽流；黄河上游降水主要由来自青藏高原较强的水汽所提供，因此这里的降水再循环率小于15%。但黄河上游的降水蒸发后，在西风的输送下, 为黄河下游提供了大量的水汽，通过汇合南北方向来的水汽，在黄河下游形成了降水再循环率达到30%以上的椭圆形区域。评估结果显示:降水再循环率随季节变化差异较大, 8月份的降水再循环率最高, 达31%, 而在11、12、1月份3个月的降水再循环率不到5%。结果还显示蒸发率和降水率的空间分布以及它们的年际变化都对应得很好，表明我国北方地区蒸发受降水影响较强。蒸发率、降水率和降水再循环率在20年中均有增加, 表明气候变暖对我国北方地区的水汽循环的影响已经显现。     

2012年我国夏季降水预测与异常成因分析

本文对2012年我国夏季降水的实况和预测进行简要回顾,发现2012年夏季降水大体呈北方涝、长江旱的分布,主雨带位于黄河流域及其以北,降水异常偏多的区域主要位于西北大部、内蒙古和环渤海湾,黄淮与江淮地区降水偏少,江汉至淮河上游一带干旱严重;预测的主雨带位于华北南部至淮河,较实况偏南。对我国北方降水异常偏多的成因分析表明:2012年夏季欧亚中高纬地区阻塞高压（简称阻高）强盛,同时东北冷涡活动频繁,中高纬500 hPa高度场从西至东呈“+-+”的分布,这种环流形势没有造成长江洪涝是因为东亚夏季风异常偏强,同时西太平洋副热带高压（副高）偏北,冷暖空气对峙于我国北方地区,导致北方降水异常偏多。分析还表明阻高、东北冷涡、东亚夏季风和副高这四个系统的不同配置影响着冷暖气流的对峙位置,进而形成我国夏季的主雨带。最后通过定量和定性判断相结合的方法,选取了2012年夏季降水的最佳相似年和最佳相反年,对比分析了2012年夏季降水与其最佳相似年和最佳相反年的海温演变与东亚夏季风环流系统主要成员的差异:1959年夏季降水作为2012年夏季降水的最佳相似年,虽然海温及东亚夏季风系统关键成员异常不明显,但是和2012年也呈近似相反的特征;而1980年夏季降水作为2012年夏季降水的最佳相反年,海温及东亚夏季风环流系统关键成员和2012年呈显著的反向特征,这些观测事实反映了我国夏季降水与海温及东亚夏季风环流系统关键成员这些主要影响因子之间关系的年代际变化。     

作物生长对流域水文过程与区域气候的影响

利用区域气候模式RegCM3以及考虑作物生长过程的耦合模式RegCM3_CERES对东亚区域进行20年模拟,研究作物生长对流域水文过程与区域气候的影响。结果表明：考虑作物生长过程的耦合模式模拟海河流域、松花江流域、珠江流域多年平均降水效果明显改进,在除黑河流域外的各流域模拟的温度负偏差有所减小,其中在海河流域、淮河流域的夏季改进尤为明显。各流域夏季（6、7、8月）月蒸散量最高,其中长江流域、海河流域、淮河流域、珠江流域的夏季月蒸散量基本上在100 mm左右,并且七大流域蒸散发的季节变化趋势跟总降水基本一致。多数流域考虑作物生长过程的耦合模式模拟得出蒸散发减少且进入的水汽增加,导致局地水循环率减小;黑河流域与黄河流域降水有所增加,其他流域均有不同程度的减小。针对长江流域,比较耦合模式RegCM3_CERES与模式RegCM3模拟结果显示,叶面积指数减少1.20 m²/m²,根区土壤湿度增加0.01 m³/m³,进而导致潜热通量下降1.34 W/m²（其中在四川盆地地区减少16.00 W/m²左右）,感热通量增加2.04 W/m²,从而影响到降水和气温。     

Forecasts of hydrological parameters over the Mackenzie River Basin: Sensitivity to initial conditions,horizontal resolution and forecast range

Abstract

As part of the Global Energy and Water Cycle Experiment, Canadian global spectral forecast model predictions of surface water and energy fluxes over the Mackenzie River basin are examined. Two nine‐member ensemble forecasts of one month duration are produced with the operational model, for a spring and a summer case, at a horizontal resolution of about 100 km (T95). The sensitivity to initial conditions is measured by the degree to which the individual forecasts in the ensembles vary one from another. The evolution in time of this estimated error (ensemble standard deviation) is determined for the surface energy and water accumulations, averaged over the basin. For comparison the calculations are repeated for the Mississippi basin and over North America. The greatest sensitivity is found for the net accumulation of precipitation minus evaporation. The spring ensemble is redone at a coarser horizontal resolution (T47), and the results are similar. The forecast uncertainty (ensemble standard deviation) of the area‐averages over the basin appear to be unaffected by this change, although the ensemble mean values are sensitive to the change in resolution. The ensemble standard deviation makes a significant, abrupt increase toward the end of the second week into the forecasts. This investigation suggests a need for an improved model, if the forecasts’ useful range is to extend to one month. Available upgrades to the land‐surface, precipitation and evaporation schemes will be used in subsequent work, and the forecasts reported here will serve as a baseline for comparison.     

Understanding precipitation recycling over the Tibetan Plateau using tracer analysis with WRF

The precipitation recycling (PR) ratio is an important indicator that quantifies the land-atmosphere interaction strength in the Earth system’s water cycle. To better understand how the heterogeneous land surface in the Tibetan Plateau (TP) contributes to precipitation, we used the water-vapor tracer (WVT) method coupled with the Weather Research and Forecasting (WRF) regional climate model. The goals were to quantify the PR ratio, in terms of annual mean, seasonal variability and diurnal cycle, and to address the relationships of the PR ratio with lake treatments and precipitation amount. Simulations showed that the PR ratio increases from 0.1 in winter to 0.4 in summer when averaged over the TP with the maxima centered at the headwaters of three major rivers (Yangtze, Yellow and Mekong). For the central TP, the highest PR ratio rose to over 0.8 in August, indicating that most of the precipitation was recycled via local evapotranspiration in summer. The larger daily mean and standard deviation of the PR ratio in summer suggested a stronger effect of land-atmosphere interactions on precipitation in summer than in winter. Despite the relatively small spatial extent of inland lakes, the treatment of lakes in WRF significantly impacted the calculation of the PR ratio over the TP, and correcting lake temperature substantially improved both precipitation and PR ratio simulations. There was no clear relationship between PR ratio and precipitation amount; however, a significant positive correlation between PR and convective precipitation was revealed. This study is beneficial for the understanding of land-atmosphere interaction over high mountain regions.

     

The synoptic‐ and planetary‐scale signatures of precipitating systems over the Mackenzie River Basin

Abstract

The synoptic‐ and planetary‐scale signatures of precipitating systems over the Mackenzie River Basin (MRB) are elucidated using composites based on a 28‐year sample of widespread precipitation events. These wet events are defined as days on which 5 or more of 12 surface stations in the MRB receive at least 2.5 mm of precipitation. Seasonal composites based on a total of 600 wet events reveal a sequence of statistically significant flow anomalies. Examination of individual wet events motivates stratification of the seasonal samples according to sea‐level pressure distribution. One evolution that is particularly common during fall, winter and spring involves lee cyclogenesis over the southern MRB in association with a strong cyclone over the Gulf of Alaska; such events are dubbed Gulf Redevelopment (GR) cases. A composite based on 59 wintertime GR events indicates upslope flow north of the lee cyclone and warm advection along an east‐west oriented warm front during the precipitation event. Composites of the Q‐vector and the divergence of this field confirm the presence of quasigeostrophic (QG) forcing for ascent over the MRB during this period. A thermally indirect “topographic tilting” mechanism, involving downs‐lope warming over the southern MRB and upslope cooling to the north, is hypothesized to increase warm‐frontal baroclinicity over the MRB. The GR composite 500 hPa geopotential height anomaly pattern is characterized by a series of anomalies extending from the Bering Sea to the Gulf of Mexico. The western (eastern) anomalies tend to decay (amplify) with time. The composite exhibits a positive anomaly over the Bering Sea, a negative anomaly over the Gulf of Alaska that moves eastward into the MRB during the precipitation event, and a positive anomaly that moves eastward over western and central North America. The presence of large, slow‐moving flow anomalies and an extended period of enhanced southwesterly geostrophic flow over the MRB in the composite suggests that a persistent influx of Pacific moisture is required to moisten the atmosphere over the MRB sufficiently for widespread precipitation. An independent composite of dry MRB cyclone events exhibits substantially weaker southwesterly geostrophic flow into the MRB relative to the wet GR composite.     

Sensitivity Analysis of Discharge in the Arctic Usa Basin, East-European Russia

The high sensitivity of the Arctic implies that impact of climate change and related environmental changes on river discharge can be considerable. Sensitivity of discharge to changes in precipitation, temperature, permafrost and vegetation, was studied in the Usa basin, Northeast-European Russia. For this purpose, a distributed hydrological model (RHINEFLOW) was adapted. Furthermore, the effect of climate change simulated by a GCM (HADCM2S750 integration) on runoff was assessed, including indirect effects of permafrost thawing and changes in vegetation distribution. The study shows that discharge in the Usa basin is highly sensitive to changes in precipitation and temperature. The effect of precipitation change is present throughout the year, while temperature changes affect discharge only in seasons when temperature fluctuates around the freezing point (April and October). Discharge is rather sensitive to changes in vegetation. Sensitivity to permafrost occurrence is high in winter, because infiltration and consequently base flow increases if permafrost melts. The effect of climate change simulated by the scenario on discharge was significant. Peak flow can both decrease (by 22%) and increase (by 19%) comparedwith present-day, depending on the amount of winter precipitation. Also, runoff peaks earlier in the season. These results can have implications for the magnitude and timing of the runoff peak, break-up and water-levels.     

Historical perspective on the dust bowl drought in the central United States

Three new 159-year long reconstructions of spring, summer, and growing season precipitation totals were developed for northeastern Kansas and northwestern Missouri from five station clusters (Lawrence, Leavenworth, and Manhattan, Kansas; Miami and Oregon, Missouri). Nonstandard observation practices are inherent in the early meteorological data, which can induce an undercount in precipitation measurements, particularly during the cool season. Threshold analyses of these five station clusters indicated undercount can be lessened for daily precipitation totals of 0.50 in. and greater during the warm season (“half-inch threshold”). Therefore, “adjusted reconstructions” of total precipitation for the spring (AMJ), summer (JA), and growing season (AMJJA) were derived using the “half-inch threshold” totals and an estimate of the missing amount between 0.00 and 0.50 in. based on an average of the modern observations at each station (or the nearest available station). The new precipitation reconstructions suggest that the most severe spring drought may have occurred during the mid-19th century, although the potential for undercount is likely highest during the spring season. The most severe summer precipitation deficit is estimated during the 1930s Dust Bowl drought, followed by the summer drought of the 1910s. When precipitation is totaled for the entire growing season, the mid-19th century and Dust Bowl droughts were of approximately equal magnitude and duration in this reconstruction. However, the integration of precipitation and temperature into seasonal measures of effective moisture, using a new 19th century temperature reconstruction for northeastern Kansas, indicates that the 1930s growing season moisture deficit was the most severe and sustained since 1855, highlighting the extraordinarily high temperatures recorded during the 1930s Dust Bowl drought.     

不同气候情景下华北平原蒸发与径流时空变化分析

基于中国气象局国家气候中心生成的IPCC第四次评估报告中23种气候模式的情景集成数据,采用Schreiber公式和Thornthwaite方法计算实际蒸发和径流,分析了2001-2060年SRES A1B、A2和 B1这3种情景下,华北平原气温、降水、蒸发与径流的时空变化。结果表明：未来华北平原气温呈升高趋势,且冬半年升温幅度大于夏半年;降水亦呈增加趋势,而冬半年降水增加幅度小于夏半年;与此相应,华北平原蒸发和年径流呈增加趋势,增幅和空间差异随时间推移而增大,到2041-2060年蒸发将上升7.1%～9.4%,径流将增加8.7%～10.7%。     

Hydroclimatic trends during 1950–2018 over global land

Global hydroclimatic changes from 1950 to 2018 are analyzed using updated data of land precipitation, streamflow, and an improved form of the Palmer Drought Severity Index. The historical changes are then compared with climate model-simulated response to external forcing to determine how much of the recent change is forced response. It is found that precipitation has increased from 1950 to 2018 over mid-high latitude Eurasia, most North America, Southeast South America, and Northwest Australia, while it has decreased over most Africa, eastern Australia, the Mediterranean region, the Middle East, and parts of East Asia, central South America, and the Pacific coasts of Canada. Streamflow records largely confirm these precipitation changes. The wetting trend over Northwest Australia and Southeast South America is most pronounced in austral summer while the drying over Africa and wetting trend over mid-high latitude Eurasia are seen in all seasons. Coupled with the drying caused by rising surface temperatures, these precipitation changes have greatly increased the risk of drought over Africa, southern Europe, East Asia, eastern Australia, Northwest Canada, and southern Brazil. Global land precipitation and continental freshwater discharge show large interannual and inter-decadal variations, with negative anomalies during El Niño and following major volcanic eruptions in 1963, 1982, and 1991; whereas their decadal variations are correlated with the Interdecadal Pacific Oscillation (IPO) with IPO’s warm phase associated with low land precipitation and continental discharge. The IPO and Atlantic multidecadal variability also dominate multidecadal variations in land aridity, accounting for 90 % of the multidecadal variance. CMIP5 multi-model ensemble mean shows decreased precipitation and runoff and increased risk of drought during 1950–2018 over Southwest North America, Central America, northern and central South America (including the Amazon), southern and West Africa, the Mediterranean region, and Southeast Asia; while the northern mid-high latitudes, Southeast South America, and Northwest Australia see increased precipitation and runoff. The consistent spatial patterns between the observed changes and the model-simulated response suggest that many of the observed drying and wetting trends since 1950 may have resulted at least partly from historical external forcing. However, the drying over Southeast Asia and wetting over Northwest Australia are absent in the 21st century projections.

     

El Niño Modoki connection to extremely-low streamflow of the Paranaíba River in Brazil

Extremely-low discharge events of the Paranaíba River basin during the austral summer season (December–February, DJF), are found to be associated with the Pacific sea surface temperature anomalies resembling the recently identified El Niño Modoki phenomenon. Extreme discharge events are identified based on their persistent flow for 7 days and more after taking retention time into consideration. Ninety percent of the extremely low discharge events during peak streamflow seasons of DJF, are found to occur during the El Niño Modoki years. A diagnostics study of atmospheric anomalies has shown a clear connection between the modified Walker circulation, associated with the El Niño Modoki, and the precipitation anomalies over the Paranaíba River basin. The climate variations have direct relationship with the rainfall. Streamflow variations are considered as the surrogates to rainfalls. Thus, El Niño Modoki phase is important component to understand and predict the streamflow variations in the Paranaíba River basin.     

Changes of time mean state and variability of hydrology in response to a doubling and quadrupling of CO2

This paper examines the subject of hydrologic variability and its changes in two separate integrations of a coupled ocean-atmosphere general circulation model developed at the Geophysical Fluid Dynamics Laboratory/NOAA assuming a 1% per year increase to a doubling and quadrupling of CO₂, respectively. Changes in time mean state and variability of precipitation, runoff and soil moisture are evaluated using monthly and seasonal mean data derived from these integrations. Various statistical tests are then performed on the resulting time mean and variability changes. The patterns of hydrologic change for these three quantities are similar to those obtained from previous studies. In northern middle to higher latitudes for the time means, the changes include increases in monthly mean precipitation, increases in monthly mean runoff during the fall, winter and spring seasons and decreases of monthly mean soil moisture during summer. Many of these changes are found to be statistically significant at the 5% significance level for both the time mean and variability especially for the results where CO₂ is quadrupled such as monthly mean precipitation. Significant changes also include increases of runoff variability during spring, winter and spring and increases of soil moisture variability during the summer season. These results support statements made in previous IPCC reports that increasing greenhouse gases can lead to more severe and frequent floods and droughts depending upon season and latitude. This study also indicates that the approaches to equilibrium of these two integrations, and the resulting hydrologic changes, take place over time scales of hundreds of years in agreement with several previous investigations.     

Indirect vs. Direct Effects of Anthropogenic Sulfate on the Climate of East Asia as Simulated with a Regional Coupled Climate-Chemistry/Aerosol Model

We intercompare a series of multi-year simulations with a coupled regionalchemistry-climate model for east Asia to assess the relative importance ofdirect and indirect (Type I) effects of anthropogenic sulfate on the climateof the region. Both direct and indirect aerosol effects induce a negativeradiative forcing that results in a cooling of the surface and in a decrease of precipitation. Under present day sulfur emissions,the direct aerosol effects prevail during the cold season, while the indirecteffects dominate in the warm season (when cloudiness is maximum over the region). When both the direct and indirect effects are included, the surface cooling varies in the range of –0.1 to over –1 K throughout the region and extended areas ofstatistically significant cooling are found in all seasons except winter.The indirect effects largely dominate in inhibiting precipitation, especiallyduring the summer. When doubling the sulfur emissions, the direct effects aresubstantially strengthened, while the indirect effects are only marginally affected. This indicates that the indirect effects over the region might be asymptotically approaching their maximum efficiency. Overall, the indirect effects appear necessary to explain theobserved temperature record over some regions of China, at least in the warm season.A number of uncertainties need to be addressed, such as due to Type IIindirect effects, modeling of the relationship between aerosol concentration and cloud optical properties, and contribution of aerosols other than anthropogenic sulfate.