Northeast Monsoon Rainfall Research Articles

Northeast monsoon rainfall variability over south peninsular India vis‐à‐vis the Indian Ocean dipole mode

AbstractWhereas the June through to September southwest summer monsoon season over the Indian subcontinent has been the main focus of research, the October through to December northeast monsoon season over south peninsular India has received less attention. Hence, this study focuses on the northeast monsoon rainfall (NEMR) variability using historical data for a 131 year period (1871–2001) and its relationship with the neighbouring Indian Ocean sea‐surface temperature (SST).Based on the observed available rainfall data, a time series of NEMR over the coherent south peninsular India has been developed. Similarly, an index to quantify the Indian Ocean dipole mode (IODM), exhibiting warm (cool) waters over the equatorial western (southeastern) Indian Ocean, has been developed using GISST data for the same period. The dynamics of the NEMR–IODM relationship are examined with the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data.Interannual and decadal variabilities in NEMR reveal alternate epochs of above‐ and below‐normal rainfall. The epochs tend to last for about a decade or two. Rainfall epochs near the equatorial Indian Ocean tend to last for about a decade. No long‐term trends in NEMR are noted. On the other hand, the decadal variability in the IODM reveals a dominance of the negative phase during the earlier decades (1880–1920) and a positive phase during recent decades (1960–2000), with suppressed activity in between. Long‐term variability in the IODM clearly reveals an increasing trend, which could be related to greenhouse warming. Further, the NEMR variability is enhanced during the decades when the IODM exhibits its active phase, and is suppressed during the decades when the IODM is inactive.The NEMR and the IODM are directly related, suggesting that the positive (negative) phase enhances (suppresses) the northeast monsoon activity. During the positive phase, the anomalous flow pattern shows winds converging and suggesting moisture transport from the southeast Indian Ocean and the Bay of Bengal towards south peninsular India. In contrast, the negative phase reveals winds diverging and transporting moisture away from the south Indian region. The anomalous SST and circulation features associated with the positive IOD/excess rainfall and negative IOD/deficient rainfall are consistent and clearly bring out the coupled ocean–atmosphere dynamics over the Indian Ocean. These results show the direct influence of the IODM phenomenon on the interannual and decadal NEMR variability over south India. Copyright © 2004 Royal Meteorological Society

Seasonal variation of 200 hPa upper tropospheric features over India in relation to performance of Indian southwest and northeast monsoons

The relation between 200 hPa upper tropospheric parameters such as temperature, zonal and meridional winds over India with the Indian southwest and northeast monsoons has been studied, based on monthly and seasonal upper air data of 14 well-distributed Indian radiosonde stations for the 36 year period 1963-98. It has been found that by and large, positive temperature/height anomalies, negative zonal wind anomalies and northerly position of the sub-tropical ridge during the preceding months/seasons are associated with good southwest/poor northeast monsoons and that complement of the above with poor southwest/good northeast monsoons. The profiles of the above relationship display a double peak in May and September/October. A multiple regression forecast scheme for seasonal forecasting of northeast monsoon rainfall, based on October data has been derived which when tested in an independent sample of 5 years yielded nearly 50% correct forecasts. The southwest monsoon rainfall of India and the northeast monsoon rainfall of Tamil Nadu/Southern parts of India have been shown to share a negative relationship, though the relationship is discordant rather than decisive, which gets defined better when years of deficient southwest monsoon rainfall get excluded. The feature of good Indian northeast monsoon getting preceded by colder upper tropospheric temperatures as shown in the study tie in well with the accepted mechanism of Asian winter monsoon getting intensified due to cold surges from the Siberian High.

A study on north-east monsoon rainfall over India

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.

North east monsoon rainfall of TamilNadu - retrospective analysis

Retrospective analysis of north east monsoon rainfall climatology in respect of Tamil Nadu indicated the following (1) Lesser than Tamil Nadu NEM average rainfall with higher CV's were recorded for North Arcot, Salem, Dharmapuri, Coimbatore, Periyar, Trichy, Pudukkottai and Madurai districts, whereas Chennai and Chengalput districts recorded above normal rainfall with lower CV values; (2) In all the districts of Tamil Nadu, 60-70% of years were under within one standard deviation (3) In 80% of El- Nino years, Thanjavur and Chengalput districts had shown above normal rainfall while Madurai, North Arcot, Dharmpuri and Coimbatore districts did exhibit such events in only 60% of El-Nino years.

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.

Relationship between Mid-latitude Circulation Indices and Indian Northeast Monsoon Rainfall

—The study presents the results of the statistical relationship between seasonal northeast monsoon rainfall over Tamil Nadu state of India (TNR) and southeast India (SER) and mid-latitude circulation indices viz., zonal index (ZON) meridional index (MER) and the ratio of meridional to zonal index (M/Z) between the geographical area 35°N to 70°N at 500 hPa level over three sectors and hemisphere, based on 19 years (1971–1989) of data. The results indicate that northeast monsoon rainfall over India shows a strong antecedent relationship with the strength of ZON over all the sectors and hemisphere. The best association is observed during antecedent March over sector I (45°W–90°E) where direct and strong correlation coefficients of 0.69 and 0.64 are obtained with TNR and SER, respectively. Antecedent MAM (spring) season over sector I also shows a significant positive correlation with TNR/SER. Thus, the mid-latitude zonal circulation index may have possible use for the long-range forecasting of northeast monsoon rainfall over India.

Influence of some circulation anomalies on Indian northeast monsoon rainfall

The relationship between Indian northeast monsoon rainfall over Tamil Nadu (TNR) and southeast India (SER) as well as two indices of southern oscillation (SOI), and sea surface temperature (SST) anomalies over different Nino regions of equatorial Pacific Ocean and seven tropical circulation indices (TCI), have been studied for different periods. The study indicates that northeast monsoon rainfall (TNR) shows significant inverse relationship with SOI (I-D) during previous MAM (March- April-May) season. significant direct relationship with SST anomalies over Nino-4 region during previous JJA (June-July- August) and significant direct relationship with TCI (C-N) during previous DJF, The SOI (I-D), MAM correlates I significantly and negatively with both the northeast monsoon rainfall series, the TNR rainfall series displaying the better correlation. The strongest correlation is observed during 1961-90. For SSTA, the strongest correlation is during 1964-85 and for TCI, the highest correlation is observed during 1958-82.

A scheme for advance prediction of northeast monsoon rainfall of Tamil Nadu

Monthly means of winds. contour height and temperature of seven standard isobaric levels of ten well distributed Indian upper air stations for the months of April, June. July, August and September were subjected to correlation analysis to detect parameters that have predictive value to forecast in advance the northeast monsoon rainfall of Tamil Nadu. The period 1965-87 was taken as developmental period and 1988-94 as test period. Six predictors, out of which three were completely new, were identified. The final forecast of rainfall was obtained as the weighted average of the individual forecasts based on the six predictors by employing a screening technique different from the conventional ones. The system explained between 65-77% variation of the predict and with standard error of 13-18% and provided reasonably correct forecasts during the test period. The physical significance of the predictors has been explained based on the intensity of the subtropical anticyclone over India. The possibility of extending the study to include global parameters in the context of proven negative and significant relationship between Southern Oscillation Index (SOl) and the Indian northeast monsoon has been discussed. Scope for further studies on the topic has been spelt out.

DAKSHIN BHARTIYA PADHAR PAR HONE WALI MONSOON WARSHA KA PRADHAN AVYAV VISHLESHAN

The result of the Principal Component Analysis of southwest and northeast monsoon rainfall on the southern India plateau have been discussed. Monsoon rainfall data of five meteorological sub-divisions, i.e., Coastal Andhra Pradesh, Rayalseema, Tamilnadu, Interior parts of South Karnataka & Kerala, for a period of 33 years (1960-92), have been utilized. The results indicate that the rainfall of Coastal Andhra Pradesh and Rayalseema has maximum impact on first principal component of southwest monsoon rainfall of five meteorological sub-divisions. The study of only first principal component is sufficient in order to understand the 49% of total variability of southwest monsoon rainfall. Analysis of first three principal components is important to understand 85% of total variability of the rainfall of this season.
 
 On the first principal component of northeast monsoon rainfall of aforesaid five meteorological sub-divisions the impact of the rainfall of Kerala and south interior Karnataka has been found maximum. In order to understand the 56% of total variability the analysis of first principal component is sufficient.
 
 The special negative relation is found between northeast monsoon rainfall on the Coastal Andhra Pradesh and southwest monsoon rainfall of previous year on this very sub-division and Rayalseema. The principal components of southwest monsoon rainfall may prove useful for forecasting the northeast monsoon rainfall of southern Indian plateau.

On some aspects of southwest monsoon rainfall over Tamilnadu

The rainfall during southwest monsoon season over Tamilnadu is quite significant from the point of view of water storage in major reservoirs as northeast monsoon rainfall, which is about half of the annual rainfall, is not stable enough due to its large interannual variability. The southwest monsoon rainfall, on the other hand, is more stable. The north-south oriented trough over Tamilnadu and adjoining Bay togetherwith upper air cyclonic circulation/trough in lower tropospheric levels account for three fourths of significant rainfall occurrence during southwest monsoon season. Rainfall during southwest monsoon and northeast monsoon seasons was found to be independent with a small negative correlation of -0.18. This shows that the southwest monsoon rainfall may not be of much use to predict the pattern of northeast monoon rainfall over Tamilnadu.

OUTLOOK ON NORTHEAST MONSOON RAINFALL OF TAMIL NADU

The mean monthly wind vectors at 850, 500 and 150 hPa levels over Thiruvananthapuram (TRV) and Madras (MDS) for August. and September have been subjected to stepwise screening. The objective is to develop a scheme capable of providing an outlook of .northeast monsoon rainfall of Tamil Nu in the beginning of October. A multiple regression scheme of S1K predictors has been identified. The scheme developed from 23•year data performed well when te.ted in an independent five-year period.

Statistical relations between winter monsoon Rainfall and the preceding summer monsoon

The mean monthly wind vectors at 7 standard isobaric levels and surface parameters of several stations of the summer (southwest) monsoon months of June to September over India have been examined. The objective was to select predictors for the northeast monsoon rainfall of the Indian Peninsular subdivisions of TamilNadu, Kerala, coastal Andhra Pradesh and Rayal3seema. The data for a 33-year period (1951-83) were used in this study. A sequence of screening tests were employed to select reliable predictors for these subdivisions. It has been shown that the August-September 150 mb zonal mean wind of Trivandrum is capable of giving satisfactory forecasts for the following northeast monsoon rainfall of Tamilnadu. The constraints in using multiple predictors have been discussed.

Foreshadowing Northeast Monsoon Rainfall Over Tamil Nadu, India

Abstract The association between the southwest and northeast monsoon (October-December) rainfall over Tamil Nadu has been examined for the 100-year period from 1877 to 1976. A correlation analysis between the two rainfall series revealed that the southwest monsoon (June-September) rainfall is negatively correlated with that of the northeast monsoon rainfall. That is, an excess or deficit of southwest monsoon rainfall over this region is generally followed by an opposite tendency in the northeast monsoon rainfall. A χ2 test of dependence further indicated that the rainfalls in the two monsoons are not independent of each other. The negative rainfall relationship can be a useful tool in foreshadowing the northeast monsoon rainfall over Tamil Nadu, which is of considerable economic importance for this region.

Fluctuations in northeast monsoon rainfall of Tamil Nadu

AbstractThe average rainfall series of Tamil Nadu for the northeast monsoon months of October to December and the season as a whole were analysed for trends, periodicities and variability using standard statistical methods. The trend analysis showed that there are no long‐term trends of increasing or decreasing rainfall in the individual months or the season as a whole. The power spectrum analysis of rainfall series showed peaks at frequencies corresponding to the time periods of 2 to 2–44 years and 3–66 to 4–40 years.

A study of the Indian Northeast Monsoon season

The northeast monsoon period in India, as is well known, is from October to December. This is the main rainy season for most of the districts in the Madras State(Tamilnad). In the year 1946, Tamilnad(Fig. 1) received a rainfall of 31.2'' which was 73 per cent above the normal amount of 18.5'', while in the year 1949, the rainfall over this area was only 10.5'' which was 43 per cent below normal. The northeast monsoon rainfall for the years 1946-50 and the normal rainfall for this season are given in Fig. 2.

A study of the Northeast Monsoon Rainfall of Tamilnadu

A mainly statistical study of the northeast monsoon (October to December) rainfall of Tamilnadu has been made utilising the rainfall data for the 80 years, 1870 to 1949. A grouping of the districts on the basis of average rainfall has been indicated. The distribution of the rainfall has been studied and the probability of occurrence of rainfall within certain specified limits has been given. The normality of distribution of the district rainfalls has been tested. The occurrence of abnormalities has been discussed with reference to the incidence depressions and cyclonic storms in the south Bay of Bengal. It is found that large excesses of rainfall occur when depressions and cyclonic storms strike or come very near to the Coromandel coast and large defects occur when such depressions and storms do not strike the coast although they may occur in the south Bay of Bengal.
 
 An examination has also been made as to how far the influence of "World Factors " on the northeast monsoon rainfall of Tamilnadu shown by Doraiswamy Iyer could account for the abnormalities, The secular variation in the seasonal rainfall in the Tamilnadu districts has, been examined but no significant changes during the past 80 years could be found. The inter-district correlation co-efficients for the northeast monsoon rainfall have been calculated and it is found that all the correlation co-efficients are positive and except for a few all are significant by indicating that the whole of Tamilnad is a fairly homogeneous region as far as northeast monsoon rainfall is concerned.