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Study on the Trends of Rainfall and Different Temperatures During the Southwest Monsoon Season in Bangladesh

Haripada Sarker
Jahangirnagar University, Savar, Bangladesh

Abdul Mannan Chowdhury
Jahangirnagar University, Savar, Bangladesh

Samarendra Karmakar
Bangladesh Centre for Advanced Studies, Gulshan, Dhaka, Bangladesh

Abstract/ Executive Summary:

Attempts have been made to carry out the study on the trends in rainfall and different temperatures during the monsoon season in Bangladesh using the data for the period 1981-2010 at 34 stations. The study reveals that during the ongoing phases of the southwest monsoon, there exist negative trends of rainfall over the southern part of Bangladesh except in September when the negative trend of rainfall is in the northern part having the maximum negative trend in the northwest. It is found that the positive trend of dry-bulb temperature exists over the western and northeastern parts of Bangladesh and a relatively negative trend of dry-bulb temperature is found over the eastern and southeastern parts of the country. The higher increasing trend in minimum temperature is found over the northern part of Bangladesh during the southwest monsoon season. The trends in the country averaged minimum temperature over Bangladesh in June, July, August and September, are positive i.e. increasing. The increasing rates of average minimum temperature are +0.013⁰C/year, +0.022⁰C/year, +0.016⁰C/year and +0.014⁰C/year in June, July, August and September respectively. The trends in July and August are statistically significant at a 95% level of significance. The maximum temperature has increasing trends at most of the places in Bangladesh during the southwest monsoon season. The maximum increasing trend is 0.094⁰C/year at Syedpur in September and the maximum decreasing rate is -0.01⁰C/year at Srimangal in June. The rates of the increasing trend of country-averaged maximum temperature are +0.033⁰C/year, +0.036⁰C/year, +0.038⁰C/year and +0.036⁰C/year in June, July, August and September respectively. The trends in the months of July, August and September are statistically significant at a 100% level. The trend in the country averaged seasonal maximum temperature over Bangladesh has also increased trend at a rate of +0.036⁰C/year, which is also statistically significant at 100% level. The correlation between country-averaged mean monthly rainfall and country-averaged mean monthly different temperatures have also been studied; the monthly coefficients are found statistically significant.

Keywords: Trend, Southwest monsoon, Trend equation, Rainfall, Dry-bulb temperature, Maximum and minimum temperatures

Location: In Bangladesh

Start Date: 00-00-1981

End Date: 00-00-2010

Program Area: Risk Management in Agriculture

Commodity: Rainfall, Temperature

Objectives:

As Bangladesh is one of the most climate-vulnerable countries in the world in terms of monsoon temperature, rainfall, floods, heavy rainfall, erratic rainfall, etc., it is essential to study the trend in rainfall and temperature during the southwest monsoon season in Bangladesh. The present study has been undertaken to study climate change and its trends in Bangladesh with the recent data of the Bangladesh Meteorological Department (BMD) during the monsoon season of period 1981-2010.

Methodology:

Rainfall, dry-bulb temperature, maximum and minimum temperatures for the southwest monsoon season (June-September) during the period 1981-2010 at 34 stations (Figure 1) of BMD have been collected and utilized in the present study. The monthly mean data of Chittagong station for the years 2003 to 2007 are absent. For this reason, the same year’s data of the nearest station like Ambagan (Ctg.) are taken.

Trends of the parameters have been determined. The statistical significance of the trends has been studied with the help of the F-distribution test according to the formula:

F = {R² (n-k)} / {(k- 1) (1- R²)}

Where n is the number of pairs of data, (n-k) is the degrees of freedom and R2 is the coefficient of determination.

From the daily rainfall and temperature data, mean monthly and seasonal rainfall and temperatures have been computed for stations under study. These values of rainfall and temperatures for 34 stations are averaged to obtain country-averaged rainfall and temperatures and correlation among them has been studied.

Source of Information: Journal of Engineering Science 09(2), 2018, 41-54

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

On the basis of the present study, the following conclusions can be drawn: i. The monthly rainfall has increasing trends at most of the places in Bangladesh during the southwest monsoon season except August when there are decreasing trends in rainfall in most places. There exist negative trends of monthly rainfall over the southern part of Bangladesh during the monsoon season, except in September when there is the negative trend of rainfall in the northern part with the maximum negative trend in the northwest. There are no definite areas of positive or negative trends in rainfall over the country during the monsoon season; the areas vary from one month to another. ii. During the ongoing phases of the southwest monsoon, the positive trend of dry-bulb temperature exists over the western and northeastern parts of Bangladesh and a relatively negative trend of dry-bulb temperature is found over the eastern and southeastern part of the country. The highest positive and negative trends of dry-bulb temperature exist over Syedpur and Srimangal respectively. iii. The minimum temperature has increasing trends at most of the places in Bangladesh in all the months during the southwest monsoon season. The maximum increasing trend is +0.081⁰C/year at Dinajpur in July and the maximum decreasing rate is -0.035⁰C/year at Sandwip in September. The higher increasing trend in minimum temperature is found over the northern part of Bangladesh during the southwest monsoon season. iv. The maximum temperature has increasing trends at most of the places in Bangladesh during the southwest monsoon season. The maximum increasing trend is 0.094⁰C/year at Syedpur in the month of September and the maximum decreasing rate is -0.01⁰C/year at Srimangal in June. The maximum increasing trend in mean maximum temperature is found in the southeastern part in the month of June, M. Court to Cox's Bazar in July and August while in September, the maximum positive area is found over the Syedpur-Dinajpur-Tangail region. v. The country-averaged monthly and seasonal mean rainfall is negatively correlated with country averaged monthly and seasonal mean dry-bulb, maximum and minimum temperatures during the monsoon season and the correlation coefficients are statistically significant in case of monthly mean rainfall mainly with dry-bulb and maximum temperatures in June through September.

Knowledge Management: Journal