R. Bhatla

Zonal circulation indices, 500 hPa April ridge position along 75° E and Indian summer monsoon rainfall: Statistical relationships

SummaryThis paper presents an examination of the statistical relationship between summer monsoon rainfall over all India, and two sub-regions (north west India and peninsular India) and the indices of mid-latitude (35° to 70° N) zonal circulation at 500 hPa level, over different sectors of the hemisphere, based on 19 years (1971–1989) data. The results indicate that summer monsoon rainfall (June–September) over India shows; (i) a significant and direct relationship with the strength of the zonal circulation index during concurrent July over the sector 90° E to 160° E. (ii) a significant inverse relationship with the strength of the zonal index during the previous April over the sector 160° E to 45° W and a similar relationship with the whole northern hemisphere and (iii) a significant and direct relationship with the frequency of the zonal index during the previous January over the sector 45° W to 90° E.Significant relationships are also observed between the zonal circulation indices of the above mentioned months and sectors with the 500 hPa ridge location in April at 75° E over India.

Performance of Regional Climate Model in Simulating Monsoon Onset Over Indian Subcontinent

The performance of various Convective Parameterization Schemes (CPSs) of Regional Climate Model version 4.3 (RegCM-4.3) for simulation of onset phase of Indian summer monsoon (ISM) over Kerala was studied for the period of 2001–2010. The onset date and its associated spatial variation were simulated using RegCM-4.3 four core CPS, namely Kuo, Tiedtke, Emanuel and Grell; and with two mixed convection schemes Mix98 (Emanuel over land and Grell over ocean) and Mix99 (Grell over land and Emanuel over ocean) on the basis of criteria given by the India Meteorological Department (IMD) (Pai and Rajeevan in Indian summer monsoon onset: variability and prediction. National Climate Centre, India Meteorological Department, 2007). It has been found that out of six CPS, two schemes, namely Tiedtke and Mix99 simulated the onset date properly. The onset phase is characterized with several transition phases of atmosphere. Therefore, to study the thermal response or the effect of different sea surface temperature (SST), namely ERA interim (ERSST) and weekly optimal interpolation (OI_WK SST) on Indian summer monsoon, the role of two different types of SST has been used to investigate the simulated onset date. In addition, spatial atmospheric circulation pattern during onset phase were analyzed using reanalyze dataset of ERA Interim (EIN15) and National Oceanic and Atmospheric Administration (NOAA), respectively, for wind and outgoing long-wave radiation (OLR) pattern. Among the six convective schemes of RegCM-4.3 model, Tiedtke is in good agreement with actual onset dates and OI_WK SST forcing is better for simulating onset of ISM over Kerala.

Role of Madden–Julian Oscillation in Modulating Monsoon Retreat

The Madden–Julian oscillation (MJO) is the major fluctuation in tropical weather on a seasonal scale. The impact of MJO on different epochs, viz., onset, advance and active break is well known. There can be several MJO events in a season and it may enhance/suppress the retreat process. The present study aims to find the MJO-modulated retreat of monsoon. The results suggest that the fastest retreat of monsoon occurred in the years 2007 and 2008, while slowest retreat of monsoon occurred in the year 1979. The retreat features of the Indian summer monsoon (ISM) are investigated with the MJO phase and amplitude variations. The daily MJO indices for the retreat period 1979–2016 are used. The results reveal that the MJO strength decreases during the transition phase (i.e., summer monsoon to winter monsoon transition). The monsoon retreat is most favored by strong MJO phase 4 and phase 5. The fastest retreat of monsoon occurred in the years 2007 and 2008, while the slowest retreat of monsoon occurred in the year 1979. There exists a weak positive correlation between the MJO amplitude and the retreat period of monsoon. The monsoon retreat is most favored by strong MJO phase 4 and phase 5. The MJO amplitude variations during MJO phases 1–8 suggest that the MJO amplitude decreases with increase in retreat period. The MJO-modulated retreat results in slow retreat of monsoon, whereas fast and normal retreat of monsoon is seen on rare occasions. Weak MJO events lead to normal retreat of monsoon.

Variability of summer monsoon rainfall over Indo-Gangetic plains in relation to El-Nino/La-Nina

The Indo-Gangetic plains (IGPs) incorporate seven meteorological subdivisions, viz. Punjab, West Uttar Pradesh and East Uttar Pradesh, Bihar, Jharkhand, and Gangetic West Bengal. IGPs are generally characterised by fertile soils, favourable climate and an abundant supply of water. They are considered as the ‘breadbasket’ for much of South Asia. The main source of rainfall is the southwest monsoon which is normally sufficient for agriculture. Based on observed available rainfall data, a time series of summer monsoon rainfall (SMR) (1871–2012) over the IGPs has been developed. Decadal and 30-year variability in SMR reveal alternate epochs of above and below normal rainfall. These epochs tend to last for a decade or two. Although small in magnitude, the long-term trend (1871–2012) in SMR shows a decreasing nature. On the other hand, the decadal variability in the SMR reveal a dominance of a positive phase in the past (during the period 1913–1922) and a negative phase during the recent decade (1993–2002). Recent climatology (1983–2012) shows a downward trend of the rainfall anomalies with the turning point around 1997 (associated with century’s strongest El-Nino, in 1997). The period before 1997 depicts subdued above normal rainfall, while the period after 1997 depicts below normal rainfall activity. Furthermore, the study reveals that SMR variability is enhanced during the El-Nino years and is suppressed during the La-Nina years, but the presence of El-Nino (La-Nina) does not guarantee droughts (floods) over the Gangetic plains. The spatiotemporal changes in rainfall activity are attributed to global warming and associated changes in the Indian summer monsoon circulations and the general atmospheric circulation.

Study of Energy Fluxes over the Indian Ocean Prior and During the Summer Monsoon

There is an overall reduction in the incoming shortwave radiation and latent heat flux during surplus monsoon years for the month of May and monsoon season both for NCEPR (energy fluxes taken directly from reanalysis data of NCEP/NCAR) and NCEPC (energy fluxes computed with semi-empirical relations using basic fields of the reanalysis data) as compared to deficient monsoon years. The distribution of net heat flux is predominantly negative over the eastern Arabian Sea, Bay of Bengal, and Indian Ocean. The comparison of shortwave radiation flux and latent heat flux for NCEPR and NCEPC shows that NCEPC overestimates in the North Arabian Sea and North Bay of Bengal and underestimates in the Indian Ocean as compared to NCEPR. However, the pattern of energy fluxes is almost similar both for NCEPR and NCEPC.

Variability of Monsoon Over Homogeneous Regions of India Using Regional Climate Model and Impact on Crop Production

Any alteration in climatic parameter (such as rainfall) governs crop growth and has had a direct impact on quantity of food production. On the complex topographical terrain of Indian subcontinent this work represents the impact of seasonal monsoon rainfall variability on major food crop production over five homogeneous regions of India. The major Rabi crops, wheat (Triticum aestivum), sorghum (Sorghum vulgare), pulses and kharif crops rice (Oryza sativa), maize (Zea mays) and groundnut (Arachis hypogea), have a sharp dependency on Indian summer monsoon rainfall (ISMR) over the regions. Trend analysis in production of major food crops has been analyzed along with the dependency on seasonal monsoon rainfall of IMD as well as regional climate model version 4.3. Yearly crop production of Rabi and kharif has shown a clear decreasing trend with ISMR distribution. This study also shows the worse affected homogeneous regions in agriculture crop production due to rainfall variability. Along with rice–sorghum–maize, wheat and groundnut production is sharply affected by the decreasing trend of monsoon rainfall over the North Central India which is also known as Gangetic plain. The post-monsoonal crop production is also influenced by seasonal monsoon rainfall variability, and the fluctuation in monsoonal and post-monsoonal crop production is indicating alarming situation for food security and becoming the current issue to feed the huge population of India.