South Peninsular India Research Articles

Evidence of climate jump in annual temperature in India

Annual mean temperature of 50 well-distributed stations in India is analyzed for trend and climate jump. Four statistical tests are used to detect the climate jump. Table-look-up test, which is more appropriate than ‘t’ test for time series data, has been applied for the first time to detect the climate jump. Sudden changes in climate are noticed by getting discontinuities or jump in annual mean temperature of most of the stations. It was found that while the northern India received climate jump much earlier i.e. during the decade 1931-1940, extreme south Peninsular India received climate jump between 1961-63.

Forecasting the onset of monsoon over Kerala using the peak in pre-monsoon convective activity over south peninsular India

The role of low-frequency oscillations in the northward movement of the Equatorial Trough during pre-monsoon period over extreme south peninsular India is studied in relation to the associated enhancement of meso-scale convective activities over the region. Main objective of the present study is to bring out usefulness of this information in predicting the onset of summer monsoon over Kerala, well in advance. Thunderstorm data of selected stations over south peninsular India for the months March-June and for the years 1961-92 have been used in this analysis. A characteristic peak in weekly cumulative thunderstorm frequency is seen over these stations about 5-8 weeks prior to the onset of monsoon almost every year, suggesting the dominance of 30-60 day oscillation in the convection associated with these two events. A regression equation has been developed to predict the onset date. The relationship is found to be significant at 0.1% level. The results have been validated using an independent data set of five recent years. A characteristic fall in OLR values over the region, well in correspondence with the characteristic peak in thunderstorm activity suggests that OLR also can be used as a supporting tool in identifying such signal which is useful for the onset prediction.

Summer monsoon low pressure systems over the Indian region and their relationship with the sub-divisional rainfall

In the present paper performance of the monthly sub-divisional summer monsoon rainfall is studied in association with the position of the Low Pressure System (LPS) over the Indian region. Existence of the LPS over a particular location increases the rainfall activities in certain parts of the country while decreases in some other parts. For this study, the Indian region (5°-35° N and 60° -100° E) is divided into 5° Lat. ´ 5° Long. grids. The duration of LPS is taken in terms of LPS days with respect to the location of LPS in a particular grid. Monthly total number of LPS days in each of the grids are computed during the summer monsoon season, June to September for the period 1891 – 1990. Maximum number of LPS days (more than half of the total) are observed in the latitude belt between 20°-25°N. The percentages of total LPS days in this area are higher in July and August which are peak monsoon months as compared to June and September. When there is a LPS are in the area 20°-25° N and 80°-90° E, there is significant increase in the rainfall activities in the sub-divisions along mean monsoon trough while northeast India and southeast peninsular India experience significant decrease in rainfall in the months of July and August. Owing to the movement of LPS from east to west through central India, most parts of the country, excluding northeast India and south peninsular India get good rainfall activity. Correlation coefficients between monthly LPS days over the different grids and monthly sub-divisional rainfall are computed to study the relationships. The performance of sub-divisional rainfall mostly related with the occurrence of LPS in certain grid- locations. The correlation field maps may give some useful information about rainfall performance due to LPS in a particular grid locations.

The effect of ENSO / Anti ENSO on northeast monsoon rainfall

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.

On the variability and prediction of rainfall in the post‐monsoon season over India

Considerable rainfall occurs in India during the post-monsoon period from October to December, particularly over north-eastern, eastern and southern regions, and this is of great significance in agricultural and allied sectors. For the first time, we have attempted to provide detailed information on the variability and predictability of the post-monsoon rainfall (PMR) of the country. Details on the summer monsoon rainfall from June to September are extensively documented. This study comprises four sections: (i) examination of large-scale rainfall features; (ii) examination of small-scale (or regional) rainfall features; (iii) diagnostic study in order to identify possible regional forcings and global teleconnections; and (iv) modelling long-period rainfall series and extrapolation of future trends for the next 10 years. To understand large-scale features from year-to-year, expansion/contraction of the wet area (October–December rainfall ≥200 mm) was examined, involving data from 306 well-separated locations spanning 114 years (1871–1984). The post-monsoon wet area (PMWA) series (1871–1984) is homogeneous, but its interannual variations are quite large. In the low-frequency mode fluctuations however, some distinct wet/dry epochs can be identified that are broadly consistent with the wet/dry epochs of summer monsoon fluctuations, implying that the same forcing factors affect the large-scale rainfall activities of both periods. Variations in the regional rainfall were studied in order to provide some useful information for practical purposes. The longest-possible instrumental period from the October to December rainfall series for each of the six zones of the country was reconstructed by applying the objective optimization technique to the limited observations available. Rainfall series were produced for the following regions of India, the reference period of reconstruction being 1871–1980: northwest India, 1844–1996; North Central India, 1842–1996; northeast India, 1829–1996; West Peninsular India, 1841–1996; East Peninsular India, 1848–1996; and South Peninsular India, 1813–1996. The characteristics of different zonal series are documented; they do not possess a significant long-term trend, and are weakly correlated. The longest-possible all-India October–December rainfall series (1813–1996) has also been reconstructed, since it can be easily updated using the same optimum set of 116 India Meteorological Department (IMD) raingauge stations that are used for the six-zonal series; the PMWA series (1871–1984) would require the data from all 306 raingauges. The correlation between the PMWA and the all-India rainfall series is 0.92. To identify possible causes of rainfall variation, correlation of zonal, as well as all-India series, with 20 selected regional/global circulation parameters was examined. Correlation, albeit weak, of particularly large-scale features with the Southern Oscillation, El Nino, Quasi-Biennial Oscillation and surface air temperature was found to be of the same sign as that found with the large-scale features of the summer monsoon. This provided support for the previously reported theory that the sources of variation in the occurrence of rainfall during the two periods are the same. In the fourth section, different zonal and all-India series are subjected to time series analysis. A flexible approach, singular spectrum analysis (SSA), was applied first to smooth out the predictable portion of the low-frequency mode variation in the individual series. Variable harmonics of the smooth series were then estimated at a wavelength resolution of 0.1 year. A subset of a few variable harmonics was objectively identified whose combination was extrapolated to enable prediction of future trends of PMR patterns across India over a 10-year period. There are differences in the future rainfall trend over different parts of the country. Copyright © 1999 Royal Meteorological Society

Assessment of quality and impact of full resolution TOVS temperature profile data on the operational global data assimilation-forecast system of India

TOVS temperature profile data (SATEM) at its full resolution (85 km) has now become available in India on experimental basis. An attempt is made in this study to examine the quality and impact of this on the medium range forecasts over India and neighbourhood. For this purpose, a seven day period starting from 15 March 1996 is chosen to study the impact of the data on the global analysis-forecasting system operational in India. Though one week data is utilized for the impact study, the complete data of march 1996 is used for examining quality, representativeness and consistency of the retrievals. In the operational system of the National Centre for Medium Range Weather Forecasting (NCMRWF), all types of data, including coarse resolution (500 km) global TOVS retrievals-coarse grid SATEM (CGS) data, received on GTS at hourly intervals are used in the assimilation cycle. For the present study the assimilation cycle is repeated for the above period by including high resolution data over the geographical regions covered by the New Delhi's high resolution picture transmission (HRPT) station and simultaneously removing coarse resolution SATEM data. The analysis and forecast fields thus generated are compared with the corresponding operational archives. The impact of the data is examined in terms of various objective scores and through circulation characteristics. The study reveals that the quality of high resolution SATEM (HRS) data is satisfactory and is such that it can be utilized in the global data assimilation system on real-time basis. A general improvement in the RMSE and ACC scores of the medium range forecasts is found over the data void equatorial sectors of the Indian Ocean after the incorporation of the HRS data fields in the assimilation cycle. With regard to a typical easterly wave activity of moderate intensity during the period of experimentation a marginal modulation in low level vorticity and divergence forecasts is found to be improving the precipitation magnitudes over the south peninsular India as well.

Longest instrumental regional and all-India summer monsoon rainfall series using optimum observations: reconstruction and update

Long period June-to-September total rainfall series are vital for the study of summer monsoon/climatic variability over the Indian region. Owing to high spatial variability in rainfall, the representa tiveness of the summer monsoon rainfall series for the entire country is limited. In order to develop an effective system for monsoon rainfall studies, the country has been divided into six zones, named as North West India (NWI), North Central India (NCI), North East India (NEI), West Peninsular India (WPI), East Peninsular India (EPI) and South Peninsular India (SPI). Fluctuation, empirical orthogonal function (EOF) and cluster analyses of the summer monsoon rainfall of the period AD 1871-1984 of 306 raingauges are carried out for this classifi cation. Updating the different zonal rainfall series on a real-time basis from all the gauges is a difficult task. In the present study, an objective technique is applied to select a subset of gauges whose mean showed the highest correlation with the all-gauges mean series; a total of 116 raingauge stations (19 for NWI, 27 for NCI, 15 for NEI, 18 for WPI, 14 for EPI and 23 for SPI) maintained by the India Meteorological Department (IMD) have been identified for updating the different zonal series. The different zonal series are extended as far as possible backward (prior to AD 1871) by applying an objective technique to the available gauges. The NEI and the EPI series are extended back to 1848, the NWI to 1844, the NCI to 1842, the WPI to 1817 and the SPI to 1813. Though of limited practical value, the all-India series is widely used in studies of long-range monsoon forecasting, teleconnections and large-scale climate dynamics. The all-India monsoon rainfall series from AD 1871 onwards is therefore calculated from the area-weighted mean of the six zones. The series is also extended back to AD 1813 by applying the objective technique on the available gauges. The different recon structed series for the longest instrumental period are reported.

On the activity of the Arabian Sea monsoon

In connection with the activity of the Arabian Sea monsoon, Ramage's postulate regarding the relationship between the intensity of the heat low over West Pakistan and the monsoon rains was examined and it was found that the heat low does not contribute to the strengthening. If the low level monsoon flow. The effects of monsoon perturbations are found to extend from sea level and are therefore considered directly responsible for strengthening the low level monsoon flow. A method of Identifying the effects of perturbations of active as well as weak Arabian Sea monsoon is postulated. This brought out that active Arabian Sea monsoon is associated with two cyclonic vertices which in the mean are lacated over the Gulf of Cambay (Gujarat low) and the northern parts of east coast 'eastern low' respectively and extend from sea level upwards affecting the wind flow practically throughout the entire troposphere. The reason for the cyclonic circulation associated with the Gujarat low not being obvious in lower tropospheric levels is discussed. The importance .of contribution to northerly meridional flow along the west coast at upper tropospheric levels by the perturbations of strong monsoon is highlighted. The strength of upper level east wind maximum over south Peninsular India is found to be less during strong than in weak monsoon. This goes against the hypothesis regarding the role of release of latent heat in causing the summer time easterly jet. The effects of perturbations of weak Arabian Sea monsoon are found to be just the reverse of those of strong monsoon. Certain aspects of the transformation of airmass over the east Arabian Sea are also discussed.