Trends in cyclonic disturbances for the period 1891-1997 were studied over Bay of Bengal and Arabian Sea. It is noticed that there is a significant decreasing trend at 99% level of confidence in the frequency of storms. The slopes of decreasing trend in cyclonic activity over Bay of Bengal and that over Arabian Sea were found to be maximum during last four decades. Weakening of Hadley circulation due to upper tropospheric warming may be one of the cause of this decreasing trend. There appears to be decrease in intensification of cyclonic disturbances in recent period.
The North-East (NE) monsoon is one of the important components of Indian climate system. The aerial extent of NE monsoon generally extends over south-eastern peninsular India. The following five meteorological sub-divisions of India viz. coastal Andhra Pradesh, Rayalaseema, south interior Karnataka, Kerala and Tamil Nadu receive significant amount of rainfall during the NE monsoon. In the present study, using a long period data series 1875-1997, the variability of NE monsoon rainfall over south-eastern peninsula and whole India, is studied. The association of NE monsoon rainfall with El-Nino and La-Nina years is also analysed. It is observed that, the NE monsoon rainfall, over the south-eastern peninsular India is higher (lower) than normal, during El-Nino (La-Nina) years. Also, the probability of NE monsoon rainfall over this region being excess or normal during El-Nino years is very high, 0.93.
Northeast monsoon precipitation data of 5 meteorological sub-divisions in India, spanning the period 1901-97, were analysed to identify the effect of ENSO/Anti ENSO events on the rainfall over southern peninsular India. ENSO/Anti ENSO years were selected on the basis of seasonal Southern Oscillation Index (SOI). The analysis revealed that ENSO years were generally associated with enhanced northeast monsoon precipitation while there was reduced precipitation during Anti ENSO years, the reduction in Anti ENSO years being significant for Tamil Nadu (at 0.1% level), for Kerala (at 1% level) and for South Peninsular India (at 1% level). Of 22 ENSO years, 18 years were found to be either flood or wet years, while 11 years out of 15 Anti ENSO years were found to be either drought or dry years. During ENSO years, the Sea Surface Temperature (SST) anomalies both over the Arabian Sea and the Bay of Bengal were positive during the months October to December, while the reverse was the case during Anti ENSO years. A concurrent significant positive correlation was noted between SST over east central Arabian Sea and the north central Bay regions and northeast monsoon rainfall. The cyclonic systems were observed to form relatively at lower latitudes during ENSO years as compared to those during Anti ENSO years. These systems were also found to move in a more westerly direction, hit Tamil Nadu and south Andhra coast, thus giving more rain over peninsula during ENSO years. The ridge line at 200 hPa level during ENSO years was located 3° south as compared to its location during Anti ENSO years.
Abstract. The study examines whether maximum/minimum temperature is responsible for a warming/ cooling trend. In order to accomplish this, linear regression was used in temperature series such as mean (Tmean), maximum (Tmax), minimum (Tmin) and diurnal temperature range (DTR). A detailed analysis indicates that 11 out of 13 stations over the West Coast region (WCR) show a significant increase in annual Tmean as a result of an increase in annual Tmax. However, the Northeast region (NER) shows a mixed trend, with 6 stations displaying significant increases in annual Tmean and 2 showing significant decreases. Both these patterns can be ascribed to a decrease or increase either in Tmin or Tmax. In DTR, 85% of the stations over the WCR show a significant increasing trend, while 60% of stations in the NER display a significant decreasing trend. Analysis of meteorological parameters reveals that low/medium cloud, calm days, winds, forest cover and population growth influence Tmax/Tmin for the NER, whereas low cloud cover and wind direction have an influence over the WCR.
In order to study the evolution of flow patterns associated with 'strong' and 'break' monsoon we have studied six cases each of such epochs during the period 1965 to 1973. As we are interested in the departure of the flow patterns from the normal, we have subtracted long term climatological pentad means from the daily data and constructed anomaly wind, height and thickness charts at standard levels. The several contrasting features during the epochs as well as prior to the epochs. A few days prior to the beginning of the strong monsoon epoch (usually in association with a depression), an eastwest anomaly trough is seen extending from SE Asia to peninsular India in the lower and, middle tropospheres. The trough over peninsular India shifts northwards with the advance of the epoch. Westward movement of a cyclonic anomaly from across Southeast Asia IS also seen. In the upper troposphere a warm anomaly ridge isseen to the northwest of India. Prior to the beginning of breaks, however, an anomaly ridge is seen extending from peninsular India to Malaysia in the lower and middle tropospheres. This feature also shows some northward shift with the advance of the epoch. In the upper troposphere, an anomaly trough is seen to the northwest of India.
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