The semi-permanent systems such as Seasonal Heat Low (HL), Monsoon Trough (MT), Tibetan Anticyclone (TA), Tropical Easterly Jet (TEJ) and Low Level Jet (LLJ) or Somali jet are observed over Indian region during Indian summer monsoon season (June through September). These systems play a vital role in defining the strength of the Indian summer monsoon rainfall as a whole. Here we evaluate the ability of Consortium for Small-Scale Modeling (COSMO) regional Climate Model (COSMO-CLM), a high resolution regional climate model within the Coordinated Regional Climate Downscaling Experiment for South Asia (CORDEX-SA) framework, to simulate these systems of Indian summer monsoon. The historical runs of the COSMO-CLM for the period 1951-2000 are analysed. Overall the COSMO-CLM is able to simulate these components reasonably well. Possible changes in the position and the strength of these systems and their role in changing rainfall pattern over India are examined to assess the impact of global warming, under the RCP 4.5 simulations towards the end of the century (2051-2100). The analysis shows that the semi permanent systems may not strengthen in the future as compared to the present climate. The summer monsoon rainfall does not show uniform changes over the region. It is likely to enhance over the southern parts of the country, south of 20 ?S while it is projected to decrease in the northern parts under the global warming scenario.
Southwest monsoon dominates the climate over the Indian sub-continent. Even now more than 60% of the population is engaged in the agricultural activities for their livelihood. The agriculture mainly depends on the monsoon rainfall received during the four months June through September. Performance of the monsoon and the spatio-temporal variation in the quantum of seasonal rainfall depends on the so called semi-permanent systems that are seen to be present over the Indian sub continent during summer monsoon season of June through September every year. These systems include Seasonal Heat Low (HL), Monsoon Trough (MT), Tibetan Anticyclone (TA), Tropical Easterly Jet (TEJ) and Low Level Jet (LLJ) or Somali Jet. Krishnamurti and Bhalme (1976) [
Findlater (1966) [
The development of a Heat Low (HL) over Indian subcontinent and its location over central regions of Pakistan in July is perhaps the most important causative factor of the monsoon. Heat Low is characterized by anomalously low pressures (3 - 10 hPa lower than the surrounding regions) and is formed due to intense heating of land surface from solar radiation. The low extends only up to 1.5 km above sea level and is overlain by a well marked ridge above. The low pressure in the HL region and the above normal pressures in the peninsula are regarded as favorable for very good monsoon activity over the country. It develops in April-May along with the low-level southwesterly wind regime over the Arabian Sea, almost a month before monsoon arrives over western India (e.g., Ramage, 1966) [
Monsoon Trough (MT) is regarded as the part of the equatorial trough of the northern summer season over the Indian latitudes. It extends from the seasonal HL over northwest India and neighborhood into the Bay of Bengal. The monsoon disturbances forming over Bay of Bengal region are usually associated with the strengthening of MT over India and in most of the situations herald the active phase of the Indian summer monsoon. It extends up to 6 km above sea level (500-hPa) sloping southwards with height. The position of the trough axis varies on daily scale, which affects rainfall activity over India. The mean position of monsoon trough is along 14˚N. The northward (southward) shift of trough axis, from its mean position, reduces (enhances) the rainfall activity over Indian landmass.
A narrow belt of strong easterly winds is observed between 100 and 200 hPa over southern periphery of India, around 13˚N latitude, throughout the southwest monsoon season (June through September). This is known as Tropical Easterly Jet (TEJ) first observed by Koteswaram (1958) [
The upper tropospheric (500 hPa) subtropical anticyclone of Asia centered over SE Tibet is known as Tibetan Anticyclone (TA). This high is more marked at 300-hPa and its extent is 70˚ - 110˚E. The heating of the elevated Tibetan Plateau in boreal summer is responsible for the development of the anticyclonic cell over Tibet. This heating accentuates the north-south temperature gradient causing upper tropospheric easterlies to strengthen. Westward shift in the location of Tibetan high, with respect to its normal position, is favorable for the rainfall activity over Indian subcontinent. The equator ward outflow from this anticyclone gains easterly angular momentum and therefore it appears as an easterly jet stream over SE Asia south of 20˚N between 15 hPa and 100 hPa. The TEJ is found in the southern flank of the Tibetan high circulation stretching between South China Sea and Africa, south of 20˚N.
These abovementioned components of summer monsoon over India exist over Indian subcontinent during the four monsoon months of June through September. They play important role in the spatio-temporal variability of monsoon rainfall. Hence to assess their simulation in the regional climate model and the possible changes in these systems under global warming scenario is very crucial. Bucchignani et al. (2014) [
CORDEX aims to foster international collaboration for generating an ensemble of high-resolution historical and future climate projections on regional scale, by downscaling different Global Climate Models (GCMs) participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) (Taylor et al., 2012) [
We have used model simulation from the suite of CORDEX-SA models as this is the most advanced dynamically downscaled data set for the Indian Monsoon region. There are various modeling groups participating in the CORDEX-SA domain, however at present all the parameters required for the present study are available for only one Regional Climate Model (RCM) viz. COSMO-CLM. Hence this study is based on only one RCM.
The CLM (Climate Limited-Area Modeling) Community extended the COSMO-Model to be able to run long- term simulations, the so-called climate mode. The resulting modeling system is called the COSMO-CLM (or CCLM). The COSMO model in Climate Mode (COSMO-CLM or CCLM) is a nonhydrostatic regional climate model developed from the Local Model (LM) of the German Meteorological Service by the CLM-Community. The LM has been developed by the German Weather Service (DWD) for operational weather forecast. Meanwhile it is used and further developed by several other weather services organized in the consortium for small- scale modelling (COSMO). The general goal of COSMO is to develop, improve and maintain a non-hydrostatic limited-area atmospheric model, to be used both for operational and for research applications by the members of the consortium (www.clm.community.eu)
Daily simulations of the Historical run of CCLM, for the period of 1951-2000 have been used for model validation. The atmospheric variables used are precipitation, mean sea level pressure, specific humidity as well as the zonal and meridional winds at two levels 850 hPa and 200 hPa. To examine the future changes in monsoonal climate, the RCP4.5 simulations archived at Indian Institute of Tropical Meteorology, Pune, India (http://cccr.tropmet.res.in/cordex/index.jsp) have been used. The model simulations are available at the horizontal resolution of 0.44˚ longitude × 0.44˚ latitude. grid (~50 km × 50 km).
Zonal and Meridional winds and Mean sea level pressure (MSLP)
The National Centre for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalysis project data sets (Kalnay and Coauthors, 1996) [
Daily precipitation data from APHRODITE project
The recently introduced Asian Precipitation―Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources (APHRODITE) project (Yatagai et al., 2009) [
To examine the ability of COSMO-CLM model to simulate summer monsoon rainfall and semi permanent systems during the monsoon season, the historical model simulations are evaluated over fifty years period 1951- 2000. Unless the model represents the present climate reasonably well, one can hardly have any confidence in the future projections. For evaluation, all the observed and model simulated parameters have been interpolated on a uniform grid of 0.25 × 0.25 long/lat. The projected changes in the position and the strength of semi-perm- anent systems towards the end of the century (2051-2100) with respect to 1951-2000 have also been studied.
The rainfall received over Indian landmass during the four summer monsoon months of June to September are vital for the agriculture as well as water resources of the country. Not only quantum, but the spatial as well as temporal distribution of rainfall within the season also plays an important role for the Indian economy. West coast as well as the Northeastern region receives highest rainfall where as northwest India receives the lowest amount of rainfall. The Indo-Gangetic plain and the east central India also receives good amount of rainfall (
RCP4.5 simulations for the period 2051-2100 are examined to assess the possible change in the quantum and pattern of precipitation under global warming scenario. The model projects 10% - 30% decrease in the summer monsoon rainfall over east central India, Northern states of Northeast India and Northwest India except west Rajasthan (
The specific humidity at surface as well as 850 hPa over Arabian Sea has been analysed to investigate the likely future changes in the moisture content. Model has shown reasonable skill in simulating the specific humidity as compared to Ratna et al., 2014 [
Several indices have been proposed to define the strength of the Indian summer monsoon. Parthasarathy et al. (1992) [
warming scenario. Webster and Yang (1992) [
Observed | Historical | RCP4.5 | |
---|---|---|---|
Goswami Wind Index | 2.26 | 2.36 | 1.81 |
Trend | Y = 2.66 − 0.016 X | Y = 2.055 + 0.012 X | Y = 1.57 + 0.0094 X |
Webster Yang Index | 24.29 | 21.78 | 20.00 |
Trend | Y = 25.299 − 0.041 X | Y = 21.343 + 0.017 X | Y = 19.778 + 0.009 X |
not intensify under the future scenarios.
The COSMO-CLM model simulates strong positive bias in the meridional monsoon index indicating stronger Hadley circulation in model as compared to observed. However model simulates weak bias in zonal circulation index. Based on 50 years of observed data, both the indices depict significant decreasing trend. The model simulated average indices are comparable with observed, however both indices depict decreasing tendency over fifty years though not statistically significant. Thus the observed significant decreasing trend in both the indices could not be captured by the model (
Low pressure area over northwestern part of India exists from May to September. The normal position of the HL is centred around Jacobabad (Pakistan) and its isobaric value is 998 hPa. Bollasina and Nigam (2011) [
Monsoon trough is an extended low pressure area from the seasonal heat low over northwest India and adjoining area up to the North Bay of Bengal. The position of monsoon trough varies on day-to-day scale and plays an important role in the spatial distribution of seasonal rainfall. When the monsoon trough shifts to the south of its normal position, active phase of monsoon prevails, whereas its northward movement to the foot hills of Himalaya marks the weak phase or break phase of the monsoon (Rao, 1976) [
indicating that the monsoon trough towards the end of the century may not be as deep as in the historical simulations, under climate change scenario. However, there may not be any shift in the position of MT in future.
The cross equatorial flow and the low level jet (LLJ) are very vital components of the summer monsoon at the onset phase and later. The strengthening of the lower tropospheric westerlies in the Arabian Sea is a sign of increase of monsoon rains along the west coast of the Indian Peninsula (south of 20˚N) both at the time of onset and later.
COSMO-CLM historical simulations compared with the observed NCEP/NCAR vector winds show that the LLJ is very well represented in the model as seen from the
Upper level winds at 200 hPa are representative of TEJ though the strongest winds are attained at 150 - 100 hPa level. The winds at 100 and 150 hPa level are not available for model simulations. The position as well as the wind speeds at 200 hPa is well represented in the model (
Tibetan anticyclone is a large anticyclone that has greatest amplitude near 200 hPa during the southwest monsoon season. During monsoon season anticyclone occupies very large area of Tibetan plateau and neighborhood. The normal position of TA at 200 hPa is centered around Lat. 30˚N/Long. 88˚E (Rao, 1976) [
south southeastwards in September. Change in the position of the TA affects the rainfall associated with the monsoon disturbances along monsoon trough region. Observed TA for the period 1951-2000 using NCEP/ NCAR winds at 200hPa compared with the model simulated TA for Jun-August are shown in
The regional climate model COSMO-CLM under the CORDEX-SA framework is assessed for its ability in simulating the semi-permanent systems of the summer monsoon over India. The historical simulations for the period 1951-2000 are analysed for the validation. The model shows reasonable skill in simulating the semi-perm- anent systems of the summer monsoon over India. The RCP4.5 simulations for the period 2051-2100 are examined to study the likely changes in the strength and position of these components under the global warming scenario RCP4.5.
The seasonal monsoon rainfall is likely to decrease during 2051-2100, with increase in precipitation over some pockets over Indian landmass. Overall, the semi-permanent systems are likely to be less intense in future as compared to the model historical simulations of 1951-2000. The central pressure over HL as well as over MT region might be more during 2051-2100 under the RCP4.5 scenario. The lower as well as upper tropospheric circulation may not strengthen in the future. In effect the monsoon is projected to decrease over major parts of the country. These results are based on the single model from CORDEX-SA and hence involve a large uncertainty. More models will be analysed when the data is made available.
The authors are thankful to the Director, IITM, Pune and Dr. Krishnan, Exe. Director, CCCR, IITM for their support and encouragement. Thanks are due to CORDEX-SA team for providing the simulations for model COSMO-CLM. The graphics have been prepared using GrADS software, the authors are thankful to their team.
Savita Patwardhan,Ashwini Kulkarni,Sudhir Sabade, (2016) Projected Changes in Semi Permanent Systems of Indian Summer Monsoon in CORDEX-SA Framework. American Journal of Climate Change,05,133-146. doi: 10.4236/ajcc.2016.52013