Interannual and long-term variability of Indian summer monsoon rainfall

Analysis of summer monsoon (June to September) rainfall series of 29 subdivisions based on a fixed number of raingauges (306 stations) has been made for the 108-year period 1871–1978 for interannual and long-term variability of the rainfall. Statistical tests show that the rainfall series of 29 sub-divisions are homogeneous, Gaussian-distributed and do not contain any persistence. The highest and the lowest normal rainfall of 284 and 26 cm are observed over coastal Karnataka and west Rajasthan sub-divisions respectively. The interannual variability (range) varies over different sub-divisions, the lowest being 55 and the highest 231% of the normal rainfall, for south Assam and Saurashtra and Kutch sub-divisions respectively. High spatial coherency is observed between neighbouring sub-divisions; northeast region and northern west and peninsular Indian sub-divisions show oppositic correlation tendency. Significant change in mean rainfall of six sub-divisions is noticed. Correlogram and spectrum analysis show the presence of 14-year and QBO cycles in a few sub-divisional rainfall series.     

Large-scale interannual variation of the summer monsoon over India and its empirical prediction

Large-scale interannual variability of the northern summer southwest monsoon over India is studied by examining its variation in the dry area during the period 1871–1984. On the mean summer monsoon rainfall (June to September total) chart the 800 mm isohyet divides the country into two nearly equal halves, named as dry area (monsoon rainfall less than 800 mm) and wet area (monsoon rainfall greater than 800 mm). The dry area/wet area shows large variations from one year to another, and is considered as an index for assessing the large-scale performance of the Indian summer monsoon. Statistical and fluctuation characteristics of the summer monsoon dry area (SMDA) are reported. To identify possible causes of variation in the Indian summer monsoon, the correlation between the summer monsoon dry area and eleven regional/global circulation parameters is examined. The northern hemisphere surface air temperature, zonal/hemispheric/global surface air and upper air temperatures, Southern Oscillation, Quasi-biennial oscillation of the equatorial lower stratosphere, April 500-mb ridge along 75°E over India, the Indian surface air temperature and the Bombay sea level pressure showed significant correlation. A new predictor parameter that is preceding year mean monsoon rainfall of a few selected stations over India has been suggested in the present study. The stations have been selected by applying the objective technique ‘selecting a subset of few gauges whose mean monsoon rainfall of the preceding year has shown the highest correlation coefficient (CC) with the SMDA’. Bankura (Gangetic West Bengal), Cuddalore (Tamil Nadu) and Anupgarh (West Rajasthan) entered the selection showing a CC of 0.724. Using a dependent sample of 1951–1980 a predictive model (multiple CC = 0.745) has also been developed for the SMDA with preceding year mean monsoon rainfall of the three selected stations and the sea level pressure tendency at Darwin from Jan–Feb to Mar–May as independent parameters.     

春季热带海温与北疆夏季极端降水的关系研究

基于1961—2017年北疆47站夏季逐日降水资料、NOAA海温资料和NCEP/NCAR再分析资料,利用90%分位确定北疆夏季单站降水阈值,得出极端降水量,讨论了春季热带海温与北疆夏季极端降水的关系。结果表明：北疆夏季极端降水和春季热带印度洋（20° S~15° N,50°~110° E）及赤道东太平洋（15° S~15° N,90°~180° W）海温呈正相关,两个关键区春季热带海温异常偏暖时,北疆夏季极端降水偏多,仅春季热带印度洋关键区海温异常偏暖时,北疆西北地区夏季极端降水偏多。当春季热带印度洋和赤道东太平洋关键区海温同时异常偏暖时,200 hPa西风急流轴明显偏南,500 hPa西西伯利亚和中亚地区低值系统活跃,南方路径输送的水汽增加,有利于北疆夏季极端降水的发生;仅春季热带印度洋关键区海温异常偏暖时,200 hPa西风急流强度增强,500 hPa西西伯利亚地区低值系统活跃,配合偏东路径的水汽输送,北疆西北地区夏季极端降水偏多。     

Evolution of salinity field in the upper layers of the east central Arabian Sea and northern Bay of Bengal during summer monsoon experiments

The intra-seasonal variability observed in the salinity field of the upper layers at a few locations in the east central Arabian Sea and the northern Bay of Bengal during the summer monsoon seasons of 1977 and 1979 is documented with the aid of short time series (1–2 weeks) of salinity measurements made from USSR and Indian ships deployed during MONSOON-77 (1977) and MONEX-79 (1979) field experiments. In the Arabian Sea a typical subsurface maxima observed beneath the mixed layer base either disappeared or considerably weakened due to strong vertical mixing caused by the monsoonal forcing. In the northern Bay of Bengal the salinity variability in the top 30 m water column was rapid and appeared to be influenced by large amounts of fresh water from rain and probably from the major adjoining rivers. Some simple diagnostic calculations are presented to assess the relative importance of various processes which control the observed salinity variability.     

黄河源区气温变化特征及预估分析

利用黄河源区青海段9个代表性站点1961-2017年逐日气温资料和未来RCP4.5排放情景下的预估数据,分析和预估了黄河源区年平均、年平均最高、年平均最低和极端气温变化特征。结果表明：近57年来年平均最高、年平均、年平均最低气温均呈显著上升趋势且倾向率依次增大。年平均气温和年平均最高气温在1997年存在显著突变。通过分析1961-1997年、1998-2007年以及2008-2017年阶段性变化可知,年平均气温持续上升,年平均最高气温先上升后趋于稳定,而年平均最低气温升温速率在1998-2007年最大,2008-2017年升温速率较1998-2007年有所降低。暖昼日数持续增多,霜冻日数和冰封日数持续减少,冷夜日数在1998-2007年减少速率最低,近10年来减少速率增大。未来33年黄河源区年平均、年平均最高、年平均最低气温和极端暖事件均呈明显的增加趋势,极端冷事件呈减少趋势。对黄河源区过去和未来气温变化规律进行了探讨,将为该区域气温变化对策的制定与实施提供理论依据。