Taiichi Hayashi

The effect of rainfall on the incidence of cholera in Bangladesh.

Background: The incidence of cholera in Bangladesh shows clear seasonality, suggesting that weather factors could play a role in its epidemiology. We estimated the effects of rainfall on the incidence of cholera in Dhaka, Bangladesh. Methods: We examined time-series patterns of the weekly number of hospital visits due to cholera in relation to weekly rainfall from 1996 to 2002. We used Poisson regression models, adjusted for seasonal variation, between-year variation, public holidays, and temperature. The role of river level on the rainfall-cholera relationship was also examined by incorporating river-level terms into the models. Results: The weekly number of cholera cases increased by 14% (95% confidence interval = 10.1%–18.9%) for each 10-mm increase above the threshold of 45 mm for the average rainfall, over lags 0 to 8 weeks. Conversely, the number of cholera cases increased by 24% (10.7%–38.6%) for a 10-mm decrease below the same threshold of average rainfall, over lags 0 to 16 weeks. River level partly explained the association between high rainfall and the number of cholera cases. Conclusions: The number of cholera cases increased with both high and low rainfall in the weeks preceding hospital visits. These results suggest that factors associated with river level are on the causal pathway between high rainfall and incidence of cholera.

Rotavirus infections and climate variability in Dhaka, Bangladesh: a time-series analysis.

Attempts to explain the clear seasonality of rotavirus infections have been made by relating disease incidence to climate factors; however, few studies have disentangled the effects of weather from other factors that might cause seasonality. We investigated the relationships between hospital visits for rotavirus diarrhoea and temperature, humidity and river level, in Dhaka, Bangladesh, using time-series analysis adjusting for other confounding seasonal factors. There was strong evidence for an increase in rotavirus diarrhoea at high temperatures, by 40.2% for each 1 degrees C increase above a threshold (29 degrees C). Relative humidity had a linear inverse relationship with the number of cases of rotavirus diarrhoea. River level, above a threshold (4.8 m), was associated with an increase in cases of rotavirus diarrhoea, by 5.5% per 10-cm river-level rise. Our findings provide evidence that factors associated with high temperature, low humidity and high river-level increase the incidence of rotavirus diarrhoea in Dhaka.

The effect of temperature on mortality in rural Bangladesh—a population-based time-series study

BACKGROUND Studies in urban cities have consistently shown evidence of increased mortality in association with hot and cold weather. However, few studies have examined temperature-mortality relationship in the rural areas of developing countries. In this study we therefore aimed to characterize the daily temperature-mortality relationships in rural Bangladesh. METHODS A generalized linear Poisson regression model was used to regress a time-series of daily mortality for all-cause and selected causes against temperature, controlling for seasonal and interannual variations, day of week and public holidays. A total of 13 270 all-cause deaths excluding external causes for residents under demographic surveillance in Matlab, Bangladesh were available between January 1994 and December 2002. RESULTS There was a marked increase in all-cause deaths and deaths due to cardiovascular, respiratory and perinatal causes at low temperatures over a lag of 0-13 days. Every 1 degrees C decrease in mean temperature was associated with a 3.2% (95% CI 0.9-5.5) increase in all-cause mortality. However, there was no clear heat effect on all-cause mortality for any of the lags examined. CONCLUSIONS This study found that daily mortality increased with low temperatures in the preceding weeks, while there was no association found between high temperatures and daily mortality in rural Bangladesh. Preventive measures during low temperatures should be considered especially for young infants.

Ecological significance of root tip rotation for seedling establishment of Oryza sativa L

How plant seeds secure root penetration into soil to obtain good seedling establishment is one of the basic ecological problems. In this study, seminal root growth was investigated to clarify the cause of varietal difference of seedling establishment in direct seeding of rice in flooded paddy fields, with special reference to root tip rotation. In a field experiment, seedling establishment percentage had a weak correlation with seminal root elongation rate but was not correlated with apparent seedling weight in water, which has been reported to be the cause of floating seedlings resulting in poor seedling establishment. Root tip rotation was analyzed for indoor-grown seedlings using spectrum analysis: the maximum entropy method (MEM) was used. Maximum entropy method power spectrum analysis clarified that maximum MEM power density (practically corresponds to spiral angle) detected in the frequency range above 0.1 cycles mm-1 was highly and positively correlated to seedling establishment percentage in the field experiment. Maximum MEM power density in high correlation with seedling establishment was mostly found around frequencies of 0.2 cycles mm–1, which corresponded to 2.0–3.4 cycles of root tip rotation per day. From these results, root tip rotation (circumnutation) with a larger spiral angle was suggested to play an important role in the establishment of rice seedlings on flooded and very soft soil. A possible explanation for why a larger spiral angle was advantageous for seedling establishment is that if buoyancy and seedling weight are constant, a larger pushing force of the seminal root is available without causing floating of a seedling, due to the upward force being a reaction of the seminal root pushing force.

Intraseasonal variation of monsoon activities associated with the rainfall over Bangladesh during the 1995 summer monsoon season

The rainfall over Bangladesh during the 1995 summer monsoon season has been investigated in terms of the intraseasonal variation of monsoon activities. The rainfall over Bangladesh is basically dominated by the north-south oscillation of the monsoon trough. The rainfall increases when the monsoon trough is located at the foot of the Himalayas, because synoptic-scale convective activity is much more vigorous to the south of the monsoon trough axis than to the north of it. In addition, the strong southwesterly wind to the south of the monsoon trough intensifies local convective activity owing to the effects of the orography to the north and east of the country. It is also found that the monsoon rainfall over Bangladesh in 1995 varies with a periodicity of ∼20 days, and this rainfall variation is closely associated with synoptic-scale monsoon activities spreading over South and Southeast Asia. The active/break cycle of the rainfall variation during the 1995 summer monsoon season can be mostly explained by the northward propagation of what is called the 10–20 day variation of monsoon activities.

Cholera in Bangladesh: climatic components of seasonal variation.

Background: The mechanisms underlying the seasonality of cholera are still not fully understood, despite long-standing recognition of clear bimodal seasonality in Bangladesh. We aimed to quantify the contribution of climatic factors to seasonal variations in cholera incidence. Methods: We investigated the association of seasonal and weather factors with the weekly number of cholera patients in Dhaka, Bangladesh, using Poisson regression models. The contribution of each weather factor (temperature and high and low rainfall) to seasonal variation was estimated as the mean over the study period (1983–2008) for each week of the year of each weather term. Fractions of the number of cholera patients attributed to each weather factor, assuming all values were constant at their minimum risk levels throughout the year, were estimated for spring and monsoon seasons separately. Results: Lower temperature predicted a lower incidence of cholera in the first 15 weeks of the year. Low rainfall predicted a peak in spring, and high rainfall predicted a peak at the end of the monsoon. The risk predicted from all the weather factors combined showed a broadly bi-modal pattern, as observed in the raw data. Low rainfall explained 18% of the spring peak, and high rainfall explained 25% of the peak at the end of the monsoon. Conclusions: Seasonal variation in temperature and rainfall contribute to cholera incidence in complex ways, presumably in interaction with unmeasured environmental or behavioral factors.

Prediction of epidemic cholera due to Vibrio cholerae O1 in children younger than 10 years using climate data in Bangladesh

To determine if a prediction of epidemic cholera using climate data can be made, we performed autoregression analysis using the data recorded in Dhaka City, Bangladesh over a 20-year period (1983-2002) comparing the number of children aged <10 years who were infected with Vibrio cholerae O1 to the maximum and minimum temperatures and rainfall. We formulated a simple autoregression model that predicts the monthly number of patients using earlier climate variables. The monthly number of patients predicted by this model agreed well with the actual monthly number of patients where the Pearsons correlation coefficient was 0.95. Arbitrarily defined, 39.4% of the predicted numbers during the study period were within 0.8-1.2 times the observed numbers. This prediction model uses the climate data recorded 2-4 months before. Therefore, our approach may be a good basis for establishing a practical early warning system for epidemic cholera.

Onset and withdrawal of Indian summer monsoon

The onset of the Indian summer monsoon (ISM) is defined in terms of zonal asymmetric temperature anomaly and withdrawal is defined in terms of vertical wind shear between 850 hPa and 200 hPa. The onset and withdrawal dates determined by these two indexes are also consistent with the onset/withdrawal timing of the real monsoon circulation. The onset and withdrawal day values are also reasonably correlated with the strength of the monsoon and to some of its boundary forcings such as SST anomalies over the Nino-3 and Bay of Bengal regions. Thus the proposed criteria are both physically and practically viable to study the interannual variability of the Indian summer monsoon.

An analysis of wind velocity fluctuations in the atmospheric surface layer using an orthonormal wavelet transform

A wavelet analysis can supply information of both the location (time) and the scale of fluctuations. This method is applied to the fluctuations of the natural wind and the turbulent transport of momentum in the atmospheric surface layer. The shapes of both the wavelet spectra and the Fourier spectra of the three components of the wind velocity fluctuations are similar to each other. The quadrant representation of momentum transport shows the scale difference of the transport. The large-scale fluctuations mainly contribute to the downward transport of momentum.

Gust and downward momentum transport in the atmospheric surface layer

The intermittent structure of the turbulent wind field in the atmospheric surface layer is investigated by conditional sampling methods. The results show that downward momentum can be transported efficiently during a peak gust. The horizontal structure of the gust is also revealed by observing the spatial structure of wind speed fluctuations from a network of 28 anemometers. The high wind region consists of a gust front with a sudden increase of wind speed at the front and a slow decrease to its rear. The development of the gust front is related closely to the momentum flux during the short time of passage of the gust.