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[en] Four operational techniques of monsoon monitoring the Australian monsoon at Darwin have been developed in the Darwin Regional Specialised Meteorological Centre. Two techniques used the rainfall only criteria and look into the onset of wet season rainfall/monsoon rainfall. The other two techniques are based purely on Darwin wind data. The data used for the study ranges from 14 to 21 years. The main purpose of the study is to develop near-real time monitoring tools for the Australian monsoon at Darwin. The average date of onset of the monsoon ranges from 19 December to 30 December. The average date of monsoon onset is 28 December. In eleven out of twenty-one years the onset date remained within three days range between the two rainfall techniques, whereas it is eleven out of fourteen years between the wind techniques. The median number of active monsoon spells in a wet season is 3 for the rainfall techniques and 6 for the wind techniques. The average length of each active monsoon spell is around 4 days for all of the techniques. The date of onset of the monsoon has shown negative correlation with the Southern Oscillation Index (SOI) that is late onset is found to occur in El Nino years while early onset is more likely in La Nina years.
[en] The North Maharashtra region comprises three districts, namely Jalgaon, Dhule, and Nandurbar. The region comprises 25 talukas, which are mostly covered by agricultural fields. Seasonality and trend analysis of rainfall over North Maharashtra region from 1901 to 2016 is conducted in the present work. The data were analyzed on the basis of season, i.e. winter, pre-monsoon, monsoon, and post monsoon. For trend detection, the Sen’s slope estimator and the Mann–Kendall test are used. The largest negative rainfall trends are found in the talukas Nandurbar and Jamner. In Akkalkuwa, the strongest positive trend is found. The rainfall trends in the recent years are discussed in the present work.
[en] The study found that deforestation causes more monsoon moisture to be retained in the mid-troposphere, thereby reducing the northward transport of moisture needed for rainfall over West Africa. Hence, deforestation has dynamical impacts on the West African monsoon and rainfall.
[en] The aim of this study was to investigate temporal variation in seasonal and annual rainfall trend over Ranchi district of Jharkhand, India for the period (1901–2014: 113 years). Mean monthly rainfall data series were used to determine the significance and magnitude of the trend using non-parametric Mann–Kendall and Sen’s slope estimator. The analysis showed a significant decreased in rainfall during annual, winter and southwest monsoon rainfall while increased in pre-monsoon and post-monsoon rainfall over the Ranchi district. A positive trend is detected in pre-monsoon and post-monsoon rainfall data series while annual, winter and southwest monsoon rainfall showed a negative trend. The maximum decrease in rainfall was found for monsoon (− 1.348 mm year−1) and minimum (− 0.098 mm year−1) during winter rainfall. The trend of post-monsoon rainfall was found upward (0.068 mm year−1). The positive and negative trends of annual and seasonal rainfall were found statistically non-significant except monsoon rainfall at 5% level of significance. Rainfall variability pattern was calculated using coefficient of variation CV, %. Post-monsoon rainfall showed the maximum value of CV (70.80%), whereas annual rainfall exhibited the minimum value of CV (17.09%), respectively. In general, high variation of CV was found which showed that the entire region is very vulnerable to droughts and floods.
[en] The primary focus of this study is the analysis of droughts in the Tons River Basin during the period 1969–2008. Precipitation data observed at four gauging stations are used to identify drought over the study area. The event of drought is derived from the standardized precipitation index (SPI) on a 3-month scale. Our results indicated that severe drought occurred in the Allahabad, Rewa, and Satna stations in the years 1973 and 1979. The droughts in this region had occurred mainly due to erratic behavior in monsoons, especially due to long breaks between monsoons. During the drought years, the deficiency of the annual rainfall in the analysis of annual rainfall departure had varied from −26% in 1976 to −60% in 1973 at Allahabad station in the basin. The maximum deficiency of annual and seasonal rainfall recorded in the basin is 60%. The maximum seasonal rainfall departure observed in the basin is in the order of −60% at Allahabad station in 1973, while maximum annual rainfall departure had been recorded as −60% during 1979 at the Satna station. Extreme dry events (z score <−2) were detected during July, August, and September. Moreover, severe dry events were observed in August, September, and October. The drought conditions in the Tons River Basin are dominantly driven by total rainfall throughout the period between June and November.
[en] In the present study, surface water samples were collected during three seasons (summer, monsoon and winter) from four different study sites (T-dam, Padma Garden, I-dam and Talaimari point) of Padma River at Rajshahi, Bangladesh, and various physicochemical and bacterial parameters were analyzed based on standard methods. Significant differences (p < 0.05) in physicochemical parameters were observed among the seasons and sites except for water temperature. However, except for fecal coliform, other bacterial parameters such as total heterotrophic bacteria, total coliform and Vibrio cholerae counts showed significant differences (p < 0.05) among the seasons, while difference among the sites was insignificant (p < 0.05). The result also showed that all the bacterial parameters were maximum during summer and minimum during monsoon season. Untreated sewage and industrial effluents together with reduced water flow and water level were found to increase bacterial counts during summer at Site 2 (Padma Garden). Although the present situation is not serious and alarming enough, the river water requires intensive monitoring to improve its quality for better and sustainable management.
[en] This study investigates the transition from East Asian winter monsoon (EAWM) to following summer monsoon (EASM) under two types of El Niño and La Niña events. A robust out-of-phase transition from weak EAWM to strong EASM is related to El Niño events, which is more distinct in eastern Pacific (EP) El Niño than that in central Pacific (CP) El Niño due to the stronger and wider western North Pacific (WNP) anticyclone (WNPAC) as a persistent atmospheric bridge. The WNPAC differences result from the combined impacts of the warming over northern Indian Ocean (NIO) remotely, the dipolar sea surface temperature (SST) anomalies and the anomalous sinking motion over WNP locally. In terms of La Niña, the out-of-phase strong EAWM to weak EASM transition exists only for CP La Niña. Moreover, this connection is weaker compared to that for El Niño events because of a weaker WNP cyclone (WNPC). Conversely, when EP La Niña occurs, an in-phase transition is detected with a strong EAWM evolving into a strong EASM due to the emergence of WNPAC in summer. For CP and EP La Niña, the cooling SST anomalies over NIO and WNP play opposite roles in affecting WNP summertime circulation anomalies. Observational and model results suggest that the WNPC (WNPAC) is dominated by remote (local) cooling in NIO (WNP) in the summer following CP (EP) La Niña. In addition, the local rising (sinking) flow also contributes to the WNPC (WNPAC) associated with CP (EP) La Niña.
[en] Partial trend method (PTM) is an innovative and efficient trend analysis method, especially for sub-trends in different magnitudes. However, it depends on graphs and identifies trends subjectively, which limit the application for a large mass of data and cannot reveal significant variations from natural variability and randomness. To remedy these shortcomings, this paper applies a trend index derived from the PTM plot, and develops a nonparametric bootstrap approach to identify statistically significant trends. The improved PTM is used to detect trends of annual rainfall, monthly rainfall, monsoon rainfall, annual number of rainy days, and average intensity of daily rainfall in Hainan Island from 1950s to 2014. The Mann–Kendall test is also employed for a comparison purpose. The PTM reveals that, except at the Sanya station, annual rainfall and monsoon rainfall generally show an insignificant change in various magnitudes. Rainy days and daily rainfall intensity show a prevailing decreasing and increasing trend, respectively. The opposite trends explain the no trend in annual rainfall. The trends revealed by the PTM and the Mann–Kendall test show high consistency. This demonstrates that the PTM index combined with the nonparametric bootstrap method is a reliable technique. The improved PTM makes it flexible to detect overall and partial trends by graphical analysis, or detect significant trends for a mass of data by an index. Not limited to rainfall variables, this method is hoped to analyze trends in other fields.
[en] Here we develop a numerical model to investigate the hypothesis proposed by a companion paper [Eltahir, this issue], which describes a soil moisture-rainfall feedback mechanism. The model is designed to describe the seasonal evolution of the West African monsoon rainfall and is used to perform numerical experiments that elucidate the mechanisms of the response of rainfall to soil moisture anomalies. A significant rainfall anomaly is simulated by the model in response to a hypothetical soil moisture anomaly that has been imposed during early summer. However, the magnitude of this anomaly almost vanishes when the net radiation at the surface is not allowed to respond to the soil moisture anomaly. Hence the results of the numerical experiments support the proposed hypothesis and highlight the crucial importance of the radiative and dynamical feedbacks in regulating the rainfall anomalies that result from the soil moisture anomalies