El Niño Southern Oscillation (ENSO) and Indian Ocean dipole (IOD) play an important role in determining the weather pattern over the Indian subcontinent region. The role of ENSO and IOD on the occurrence of tropical cyclone activity during post‐monsoon season (October–December) over the Bay of Bengal (BoB) has been investigated through an objective analysis of observed data for the period of 1990–2015. Accumulated cyclone energy (ACE) index is an important measure of tropical cyclone activity over a basin for a defined period of time. Sea‐surface temperature (SST) anomalies in the Niño 3.4 region (oceanic Niño index) is negatively correlated at 95% confidence level of significance with ACE over the BoB during post‐monsoon season. Positive phase of IOD has negative impact on the tropical cyclone formation over the BoB. The ACE has large inter‐annual variability with coefficient of variation 124% for tropical cyclone activity over BoB during post‐monsoon season. Frequency distribution of annual ACE values for post‐monsoon season over the BoB indicated that cold phases of ENSO along with negative IOD index values are the most favourable for development of tropical cyclone over the BoB. Principal component regression (PCR) model developed by cross‐verification method based on training period data for 1990–2013 and ONI and DMI values of different lag periods as predictors is found to be functional for both deterministic and probabilistic prediction of ACE values of post‐monsoon season. Above‐ and below‐normal TC activities were observed in 2014 and 2015, respectively, which have been well predicted by the PCR model.
The thunderstorm activity over Kolkata during pre-monsoon months (March, April and May) has been studied using the radiosonde data of Kolkata (Dumdum). The objective of the study is to examine the utility of Total-Totals Index (TTI) in forecasting occurrence/non-occurrence of thunderstorm over Kolkata. The investigation reveals that Total-Totals Index can be preliminarily used as a predictor to differentiate thunderstorm and non-thunderstorm days. The probability of occurrence of thunderstorm is higher when Total-Totals Index value is higher. While the exact prediction of thunderstorm in all the cases, using the TTI alone, is obviously not possible as TTI alone does not describe the total atmospheric conditions. However, a significant forecast can be furnished by using a threshold value of Total-Totals Index.
Premonsoon season in north-east India is known for thunderstorm activity with moderate to heavy rain, gusty wind, squall and sometimes with the occurrences of hail fall. In this paper an attempt has been made to find out the climatological and synoptical aspects of hailstorms as well as squall over Guwahati Airport (26.18° N, 91.75° E) during Premonsoon season. 20 years data from 1987 to 2006 have been analyzed. The frequency of hail and squall during premonsoon season is observed to be 1.35 and 3.65 respectively. The most favourable time of occurrence of hail is during afternoon to late evening and that for the occurrence of squall is during evening to early morning over Guwahati Airport. The significant synoptic situations associated with occurrence of hail are Sea level trough from East Uttar Pradesh/Bihar to North-East India and low level cycir over Bihar and neighbourhood.
The summer monsoon rainfall over northeast India shows characteristic spatial and temporal variability due to the interaction of basic monsoon flow with orography and the synoptic scale systems developing over Indian region. The aim of this study is to find out the main features of spatial variability of daily monsoon rainfall over northeast India and associated synoptic systems. The principal component analysis (PCA) is a good tool to filter out the main components from the noise and this is applied to daily monsoon rainfall (June-September) data of 50 almost uniformly distributed stations of northeast India over a period of 10 years (1991-2000). The association of synoptic systems with significant principal components (PC) has been confirmed by analyzing daily synoptic systems over northeast India and neighbourhood during the same period. Eight PCs explaining about 46% of the total variance of daily monsoon rainfall over northeast India are significant and can be attributed to different physical processes. The first PC associated with good monsoon condition over northeast India may be attributed to low over Sub Himalayan West Bengal (SHWB). Similarly, the second PC may be attributed to good monsoon condition due to low level cyclonic circulation (cycir) over East Madhya Pradesh and Chattisgarh (EMPC), third PC to good monsoon condition due to depression over Bihar, fourth PC to good monsoon condition due to low level cycir over Arunachal Pradesh, fifth and sixth PCs to weak monsoon condition due to low over west central Bay and cycir extending upto mid tropospheric level (MTL) over North Coastal Andhra Pradesh respectively, seventh and eighth PCs to rainfall activity due to cycir extending upto MTL over Bangladesh and northwest Bay respectively. According to rotation of these significant PCs in S-mode, northeast India consists of eight homogeneous regions with the rainfall over each region being dominantly associated with one significant PC.
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