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Weakening of Indian summer monsoon rainfall (ISMR) is traditionally linked with large-scale perturbations and circulations. However, the impacts of local changes in land use and land cover (LULC) on ISMR have yet to be explored. Here, we analyzed this topic using the regional Weather Research and Forecasting model with European Center for Medium range Weather Forecast (ECMWF) reanalysis data for the years 2000–2010 as a boundary condition and with LULC data from 1987 and 2005. The differences in LULC between 1987 and 2005 showed deforestation with conversion of forest land to crop land, though the magnitude of such conversion is uncertain because of the coarse resolution of satellite images and use of differential sources and methods for data extraction. We performed a sensitivity analysis to understand the impacts of large-scale deforestation in India on monsoon precipitation and found such impacts are similar to the observed changes in terms of spatial patterns and magnitude. We found that deforestation results in weakening of the ISMR because of the decrease in evapotranspiration and subsequent decrease in the recycled component of precipitation.

Severe drought is arguably one of the greatest recurring natural disasters that strikes North America. A synthesis of multiproxy data shows that North America was in the grip of a severe centennial‐scale drought during medieval times (800–1300 AD). In this study, the Community Atmospheric Model (CAM) is used to investigate the role of sea surface temperature (SST) anomalies from the North Atlantic and the tropical Pacific Ocean on this megadrought. These anomalies are obtained from proxy reconstructions of SST. Four model experiments with prescribed SST anomalies in the tropical Pacific and/or North Atlantic Ocean were made. The CAM results captured the major dry features that occurred during medieval times in North America. The cold tropical Pacific alone can simulate essentially the drought intensity, while the warm North Atlantic alone can simulate the drought areal extent. The two working together can explain the severity and longevity of the drought. During the spring season, the cool tropical Pacific, or the warm North Atlantic, or both, results in less moisture transport to the High Plains, with a 15–40% decrease in rainfall. The importance of the Atlantic Ocean on medieval drought in North America suggests that attention should be paid not only to the tropical Pacific Ocean but also to the North Atlantic Ocean in understanding the North America drought variability and predictability, both at present and during the past. This is especially true because the Pacific Ocean SST anomalies in medieval times as recorded by proxy data are somewhat controversial, while the North Atlantic anomalies seem more certain.

Six high-resolution climatic reconstructions, based on diatom analyses from lake sediment cores from the northern prairies of North America, show that shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia. The predominant broad-scale pattern seen at all sites is a major shift in moisture regimes from wet to dry, or vice versa (depending on location), that occurred after a period of relative stability. These large-scale shifts at the different sites exhibit spatial coherence at regional scales. The three Canadian sites record this abrupt shift between anno Domini 500 and 800, and subsequently conditions become increasingly variable. All three U.S. sites underwent a pronounced change, but the timing of this change is between anno Domini 1000 and 1300, thus later than in all of the Canadian sites. The mechanisms behind these patterns are poorly understood, but they are likely related to changes in the shape and location of the jet stream and associated storm tracks. If the patterns seen at these sites are representative of the region, this observed pattern can have huge implications for future water availability in this region. Drought is a recurring natural feature of climate that has had dramatic environmental, economic, and social impacts on modern (1) and ancient (2) civilizations. Decade-to centuryscale episodes of prolonged drought or high rainfall have been recorded from North America in continental archives as diverse as tree rings (3), tree stumps (4), lake sediments (5), and river deposits (6, 7). The prairie region of North America is particularly susceptible to extreme droughts (8). However, few highresolution proxy records of climate exist from the prairies. Tree-ring records on the prairies typically are spatially limited to wooded areas at the periphery of the prairies (9) and temporally limited to, at best, Ϸ500 years (10). Sediments from closed-basin lakes can provide high-temporal-resolution paleoclimatic information from prairie regions for much longer periods.Here we provide evidence from high-resolution (subdecadal to decadal) sediment core records from six lakes on the Canadian and northern U.S. prairies (Fig. 1). All of the sites indicate that shifts in drought regimes have been a prevalent feature of this region, occurring on decadal through multicentennial scales. Inferred changes in climatic conditions over the past two millennia are based on analysis of diatom assemblages preserved in sediment cores, a commonly used technique for tracking past climatic conditions (11). Our focus here is on long-term dynamics and broad-scale similarities among the lake records. This approach is in part undertaken because of the inherent difficulties of comparing short-term dynamics across records constrained by carbon-dated chronologies. Materials and MethodsDiatom Inferences. Diatom remains in sediment cores from six lakes were used to reconstruct variables influenced by climatic conditions. Diatom-inferred salinity estimat...

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The ability of both the ECMWF-TOGA analyses and regional model RegCM2 to simulate the climate of the Middle East is examined. The climate of the region displays high spatial, seasonal and interannual variability, providing a strong test of a climate model's abilities. The higher resolution of RegCM2, compared with the ECMWF analyses, allows it to capture the spatial variability of temperature and precipitation better despite model biases being present. Both RegCM2 and the analyses have a cold bias, exacerbated in RegCM2 by a bias present in the prescribed sea-surface-temperature forcing. RegCM2 does not capture the annual cycle of precipitation on the Black and Caspian Sea coasts, where very steep topography exists, nor on the eastern Mediterranean coast, where the coastal mountains are not resolved. RegCM2 does capture the seasonal cycle in the Fertile Crescent and Zagros Mountains, where it is strongly influenced by a plateau circulation above the Iranian plateau. It is shown that accurate simulation of precipitation in these regions requires the correct simulation of storm tracks, topographic interactions and atmospheric stability. RegCM2 is better able to simulate the interannual variability averaged over the entire domain compared with the ECMWF analyses; however, they both have difficulty reproducing the interannual variability in particular subregions. It is also shown that processes controlling the seasonality of precipitation differ in different subregions and are often different from the processes controlling interannual variability. This suggests that, in order to model precipitation successfully, a horizontal scale that allows differentiation of precipitation zones dominated by different precipitation processes, both seasonally and interannually, is required.