Association of the pre-monsoon thermal field over north India and the western Tibetan Plateau with summer monsoon rainfall over India

In this paper, interannual variability of tropospheric air temperatures over the Asian summer monsoon region during the pre-monsoon months is examined in relation to Indian summer monsoon rainfall (ISMR; June to September total rainfall). For this purpose, monthly grid-point temperatures in the entire troposphere over the Asian summer monsoon region and ISMR data for the period 1949–2012 have been used. Spatial correlation patterns are investigated between the temperature field in the lower tropospheric levels during May over the Asian summer monsoon region and ISMR. The results indicate a strong and significant northwest–southeast dipole structure in the spatial correlations over the Indian region, with highly significant positive (negative) correlations over the regions of north India and the western Tibetan Plateau region – region R1 (north Bay of Bengal: region R2). The observed dipole is seen significantly up to a level of 850 hPa and eventually disappears at 700 hPa. Thermal indices evaluated at 850 hPa level, based on average air temperatures over the north India and western Tibetan Plateau region (TI1) and the north Bay of Bengal region (TI2) during May, show a strong, significant relationship with the ISMR. The results are found to be consistent and robust, especially in the case of TI1 during the period of analysis. A physical mechanism for the relationship between these indices and ISMR is proposed. Finally the composite annual cycle of tropospheric air temperature over R1 during flood/drought years of ISMR is examined. The study brings out the importance of the TI1 in the prediction of flood/drought conditions over the Indian subcontinent

Forecasting Indian summer monsoon rainfall by outgoing longwave radiation over the Indian Ocean

The satellite derived outgoing longwave radiation (OLR) over the Indian Ocean (30°N-30°S and 40°E-100°E) from 1974 to 1996 has been analysed for the relationship with the Indian summer monsoon total (June-September) rainfall. The OLR of two regions appears to be related to summer monsoon rainfall. One of the regions is located over the Head Bay of Bengal (near 22.5°N and 92.5°E) during May and the other one over the south Indian Ocean (near 30°S and 97.5°E) during April. The average OLR (index) for these two regions shows a strong and stable relationship with the Indian summer monsoon rainfall and they are found to be independent. A multiple linear regression equation is developed to predict the Indian summer monsoon rainfall using these indexes and the empirical relations are verified on independent data. Good results were obtained in forecasting the summer monsoon rainfall for the whole of India. The forecast of summer monsoon rainfall for west-central India and all-India rainfall for July also appears to be encouraging. The indexes, thus, seem to be useful in long-range forecasting of the Indian summer monsoon rainfall

Assessing Hydrological Response to Changing Climate in the Krishna Basin of India

Impact of climate change on water balance components in the Krishna river basin are investigated using a semidistributed hydrological model namely Soil and Water Assessment Tool (SWAT). The model is calibrated and validated using the measured stream flow and meteorological data for the period (1970-1990) at a single guage outlet. The model has been used further for hydrologic parameter simulations. Daily climate simulations from regional climate model PRECIS (Providing Regional Climates for Impacts Studies) is used as input for running SWAT and monthly hydrologic parameters such as precipitation, surface flow, water yield, Evapotranspiration (ET) and Potential Evapotranspiration (PET) are generated under the assumption of no change in Land Use and Land Cover (LULC) pattern over time. Simulations at 23 sub-basins of the Krishna basin have been obtained for the control runs (1961-1990) and the for two time slices of future scenarios (2011-2040) and (2041-2070). Model projections indicate increase in the annual discharge, surface runoff and base flow in the basin in mid-century

Thermal field over Tibetan Plateau and Indian summer monsson rainfall

The interannual variability of the temperature anomalies over the Tibetan Plateau (25-45 °N, 75-105 °E) is examined in relation to the Indian summer monsoon rainfall (ISMR: June to September total rainfall). For this purpose, the temperature anomaly data of the central-eastern Tibetan Plateau is divided into three regions using principal component analysis and the ISMR data for the period 1957-89 have been used. It is found that the January temperature anomaly of Region 2 has a significant negative relationship (r = -0.67) with the ISMR of the subsequent season. This region is located over the northeastern part of the Tibetan Plateau, mostly in Qinghai province, including the Bayan Harr Mountain range and the Qaidam Basin. This relationship is consistent and robust during the period of analysis and can be used to predict the strength of the Indian summer monsoon in the subsequent season. It was found that the January temperature anomaly in this region was associated with a persistent winter circulation pattern over the Eurasian continent during January through to March. Finally, the variation patterns of the temperature anomalies in all three regions over the central-eastern Tibetan Plateau during extreme years of the ISMR are examined. It is concluded that the January temperature anomaly over the northeastern Tibetan Plateau can be useful in forecasting the drought and flood conditions over India, especially in predicting the monsoon rainfall over the areas lying along the monsoon trough

Outgoing long-wave radiation over the Tropical Pacific and Atlantic Ocean and Indian summer monsoon rainfall

In this paper, the interannual variability of satellite derived outgoing longwave radiation (OLR) is examined in relation to the Indian summer monsoon rainfall (June to September total rainfall; ISMR). Monthly grid point OLR field over the domain i.e. the tropical Pacific and Atlantic region (30° N to 30° S, 110° E to 10° W) and the ISMR for the period 1974-2001 are used for the study. A strong and significant north-south dipole structure in the correlation pattern is found between the ISMR and the OLR field over the domain during January. This dipole is located over the west Pacific region with highly significant negative (positive) correlations over the South China Sea and surrounding region (around north-east Australia). The dipole weakens and moves northwestward during February and disappears in March. During the month of May, the OLR over the central Atlantic Ocean shows a significant positive relationship with the ISMR. These relationships are found to be consistent and robust during the period of analysis and can be used in the prediction of the ISMR. A multiple regression equation is developed, using the above results, for prediction of the ISMR and the empirical relationships are verified using an independent data set. The results are encouraging for the prediction of the ISMR. The composite annual cycle of the OLR, over the west Pacific regions during extreme ISMR is found to be useful in the prediction of extreme summer monsoon rainfall conditions over the Indian subcontinent

Lag-relationship between mid-tropospheric geopotential heights over the Northern Hemisphere and the Indian summer monsoon rainfall: Implications for forecasting

In this study the relationship between mid-tropospheric geopotential heights over the Northern Hemisphere (20° N to 90° N, around the globe) and Indian summer monsoon rainfall (ISMR: June to September total rainfall) have been examined. For this purpose, the monthly 500 hPa geopotential heights in a 2.5° lat./lon. grid over the Northern Hemisphere and the ISMR data for the period 1958 to 2003 have been used. The analysis demonstrates a dipole structure in the correlation patte rn over the East Pacific Ocean in the month of January which intensifies in February and weakens in March. The average 500 hPa geopotential height over the eastern tropical Pacific Ocean during February (index one), has a significant positive relationship (r = 0.72) with the ISMR. In addition, the surface air temperature (SAT) anomaly over North-west Eurasia during January (index two) is found to be strongly related with the subsequent summer monsoon rainfall. These relationships are found to be consistent and robust during the period of analysis and these indices are found to be independent of each other. Hence, using index one and index two, a multiple linear regression model is developed for the prediction of the ISMR and the empirical relationships are verified on independent data. The forecast of the ISMR, using the above model, is found to be satisfactory. The dipole structure in the correlation pattern over the East Pacific region during February weakens once the ENSO (El-Nino and Southern Oscillation) events are excluded from the analysis. This suggests that the dipole type relationship between mid-tropospheric geopotential heights over the East Pacific Ocean and the ISMR may be a manifestation of the ENSO cycle

Pre-monsoon surface pressure and summer monsoon rainfall over India

The relationship between the surface air pressure field during the pre-monsoon months and the Indian summer monsoon rainfall is analysed using climate data from 105 stations situated in Eurasia between 0°-60° N and 20°-100° E. Moreover, grid-point data for the whole northern hemisphere are used. Pressure during April over an area around 50° N and 35° E is found to show a significant negative correlation with the subsequent monsoon rainfall. During May the pressure over a large part of the study area south of 40° N shows a significant correlation with its highest value in the heat low region over Pakistan. It is assumed that monitoring of pressure variations over this region may be useful in predicting monsoon rainfall, particularly the rainfall during the first half of the season. Certain limitations of the climate data in this region are also discussed

Seasonal variability in mean sea level pressure extremes over the Indian region

The extreme values of maximum and minimum daily atmospheric mean sea level pressure (MSLP) and its seasonal variability over the 11 zones of India, Arabian Sea, Bay of Bengal and all-India as a whole during the period 1951-2007 using daily-averaged NCEP/NCAR reanalysis dataset have been analyzed. Trends in the frequencies of low MSLP days are found to be decreasing and for high MSLP days are increasing during winter, pre-monsoon, summer monsoon and post-monsoon seasons. Decrease in the extreme low MSLP days observed in the study may affect the seasonal rainfall distribution and its characteristics across India. High MSLP days are found to be negatively correlated with the All-India Summer Monsoon Rainfall (AISMR). Both the extreme frequencies of the MSLP days show a different nature in the recent decades. The high MSLP days shows a sharp increasing tendency after 1975 with a large variability whereas least variability was observed in case of the low MSLP days. Low/high MSLP days are found to be negatively/positively correlated with Nino4 SSTs for all the seasons. The possible causes for the large-scale changes in the characteristics and its seasonal variabilities of extreme low/high MSLP days might be the increased frequency of strong El Niño events during the period 1975-2007. This might have triggered the Indian summer monsoon circulation causing the reduction in the low MSLP days that resulted in decreasing tendency of the rainfall activities across India

Trends in extreme mean sea level pressure and their characteristics during the summer monsoon season over the Indian region

Using daily National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) (1948-2009) and European Centre for Medium-Range Weather Forecasts (ECMWF) ReAnalyses (ERA)-40 (1958-2001) reanalysis mean sea level pressure (MSLP) data, the frequencies in the extremes of low/high MSLP days were computed with the 10th and 90th percentiles during the summer monsoon season (June-September) over 11 zones of India, the Arabian Sea, the Bay of Bengal and all-India as a whole. We observed that the trends in the frequencies of high (low) MSLP days are increasing (decreasing), indicating a increase/decrease in anticyclonic/cyclonic activities, respectively, which affect the monsoon performance over the Indian region. The decreasing (increasing) tendency of the frequency of the low (high) MSLP days suggests a consistency between the extreme MSLP and its associated rainfall activities. The frequency of the high MSLP days over India is highly correlated negatively with the Indian summer monsoon rainfall (ISMR). The linear trends in the frequencies of the extreme low/high MSLP days for different zones and all-India are found to be highly significant (at the 0.1 level and above). After 1978, the trends in the series of low/high MSLP days are found to be more towards decreasing/increasing tendencies, respectively, whereas the frequencies of low MSLP days are much higher than those of high MSLP days during the late 1970s. The same characteristics were also evidenced from the analysis based on ECMWF ERA-40 data for the period 1958-2001. The possible causes of this may be El Niño events, greenhouse gases (GHGs), unprecedented surface warming and also tropospheric cooling trends during recent decades over the northern hemisphere as well as over the Indian region and major volcanic eruptions that occurred during the period 1978-2009

The relationship of monsoon onset with subsequent rainfall over India

Utilizing 40 (1941–1980) years of daily monsoon rainfall data averaged over 52, 2.5° × 2.5° latitude and longitude contiguous blocks covering the whole of India, we have investigated the relationship between monsoon onset at nine locations in India with the monsoon rainfall during the first 10, 15, 20, and 30 days of June and during the whole monsoon season. The onset is found to have a significant association only with rainfall observed during the subperiod of June, no significant correlations are found for the seasonal monsoon rainfall for any station. Interrelationships among onsets at these nine locations have also been studied. It is found that the arrival of the monsoon over the northern parts of the country has no relationship with the onset over Kerala. The relationship between the onset of the Baiu rains over 11 stations in Japan and the monsoon onset over India also has been studied by using 35 (1946–1980) years of data. Even though the onset of the Baiu over a few stations shows significant association with the monsoon onset over some stations of India, the relationship between the Baiu and monsoon onset is not very encouraging for linear predictio