53 Journal of Agrometeorology 16 (Special Issue-I) : 53-58 (October 2014)
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Future rainfall change scenarios simulated through AR4 and AR5 GCMs over the Western Himalayan Region JITENDRA KUMAR MEHER*, LALU DAS and JAVED AKHTER Department of Agricultural Meteorology and Physics, Bidhan Chandra Krishi Viswavidyalaya, Mohanur, Nadia, West Bengal, Pin 741252. E-mail:
[email protected]*,
[email protected],
[email protected] ABSTRACT Future (2006-2099) precipitation projection over Western Himalayan Region were prepared from the monthly simulations of 17 Global Circulation Models (GCMs) of Intergovernmental Panel on Climate Change (IPCC) fourth assessment report (AR4) and 50 GCMs of IPCC fifth assessment report (AR5). AR4 GCMs analysis reveals that numbers of GCMs namely 8, 11, 6, 7 and 7 projected a decreasing precipitation trend in annual, monsoon, post-monsoon, pre-monsoon and winter seasons respectively. The Multi Model Ensemble (MME) generated by taking the mean of all GCMs in the individual season shows that the magnitude of precipitation trends are very less in all the seasons. In AR5, numbers of GCMs namely 13, 8, 10, 14 and 16 projected a decreasing precipitation trend in annual, monsoon, post-monsoon , premonsoon and winter seasons respectively out of 28 GCMs in Representative Concentration Pathways (RCP) 4.5 while 5, 5, 5, 6, 12 GCMs in RCP6.0 showed similar decreasing precipitation out of 16 GCMs. Majority of the GCMs showed winter precipitation will decrease at the end of the 21st century but for other seasons, some models projected increasing trends while other simulated decreasing precipitation. In general, the MME indicates an increasing trend of annual, monsoon and post-monsoon precipitation while a decreasing trend was simulated in the winter and pre-monsoon season for all RCPs.
Key words: WHR, AR4, AR5, GCM, Precipitation, Trend, RCP INTRODUCTION In the context of global warming, how much the precipitation and temperature is going to be changed is a global issue to be investigated critically for future water resources management (Maurer, 2007). Over the last few decades, GCMs have been developed to emulate the present climate system and to project future climate scenarios (Wheater, 2002). The results related to future projection from GCMs put more confidence after third assessment report of IPCC in 2001. GCMs simulations for the fifth assessment report (AR5) of the IPCC have recently become available and it is expected that some of the scientific questions that occur during the preparation of the IPCC AR4 will be addressed in the AR5 (Taylor et al., 2012). Comparing to the IPCC AR4, the GCMs in AR5 include a more diverse set of model types (Liu et al., 2013). IPCC AR5 models emerged with a new set of scenarios called Representative Concentration Pathways (RCPs). The RCPs span a large range of stabilization, mitigation and nonmitigation pathways. Based on various GCM experiments under the
combined influence of Green House Gases (GHGs) and sulphate aerosols, the third assessment report of IPCC, shows that the projected increased in precipitation will be limited to 2±1% in the decade 2020s, 3±1% in 2050s and 7±3% in the 2080s. According to Climate Models from the European Centre Hamburg Models namely the ECHAM3 and ECHAM4 projected that future monsoon rainfall over India for the period 19802039 with respect to 19021979 will increase by 10% and 13% respectively while model HadCM2 from Hadley centre indicated a reduction by 6% (Rupa Kumar et al., 2002). Model simulations have attempted to address issues related to future climatic change in mountain regions, primarily because the current spatial resolution of models is too crude to adequately represent the topographic and land use details (Beniston et al., 2003). Despite notable development, GCMs do not provide perfect simulations of reality and cannot provide the details on very small spatial scales due to incomplete scientific understanding and limitations of available observations (Solomon et al., 2007). Precipitation projection maps from the PRECIS (Providing Regional Climates for Impact Studies) regional simulation study (Rupa Kumar et al., 2006) reveal that
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western Nepal, Uttarakhand, Himachal Pradesh, and Bhutan will receive higher monsoon precipitation in 2071 2100 compared to base precipitation. Moderate increases are simulated in the rest of the Himalayas. Seasonal precipitation scenarios show variations in winter precipitation with reduced precipitation in lowland and hill areas of the Nepalese and Indian Himalayas, increased precipitation in the region’s high mountainous belt. In general, extreme precipitation was projected to increase substantially over a large area in the Himalayan region (with the exception of Jammu & Kashmir) with heavy maximum daily rainfall in monsoon season. Increase moderately in Himachal, Jammu & Kashmir, and Arunachal. In Uttarakhand, daily precipitation extremes in the premonsoon season are projected to increase over the whole region with the largest increase in Arunachal Pradesh. The simulation indicated increase in frequency of heavy precipitation (days with >10 mm rain) events towards the end of the 21st century mostly in monsoon season over the whole region (Revadekar et al., 2011). The climate change in Indian Western Himalayas is a topic of great interest and the amount of research associated with the GCMs is still undeveloped in this region. However, the studies that exist are primarily focused on the early experiments of the IPCC. An evaluation and application of the updated generation of the AR5 GCMs in Western Himalayan Region is missing. In this study, we focus to estimate the future rainfall status over the Western Himalayan region using the stateof theart models that have been made publicly available through the fourth and fifth assessment report (AR4 and AR5). This study is aimed at answering what changes in climate mean may be expected in the future. Our results potentially provide inputs for climate change impact assessments that explore the probability of climaterelated risks in Indian Western Himalaya Region. MATERIALS AND METHODS Western Himalayan region (WHR), extending from 73.8 to 81.10 E and 28.7 to 35.40 N in Northern India, is represented by three states namely Uttarakhand, Jammu & Kashmir and Himachal Pradesh (Fig. 1). The high mountains regions form the watershed for most of the rivers flowing in northern India. These regions are influenced by the variations in topographical features along three dimensional frame (i.e., latitudinal: SouthNorth; longitudinal: EastWest; altitudinal: LowHigh) causing
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diversity in climate of the place. Last two generation GCMs namely AR4 GCMs (also known as CMIP3 or Coupled Model Intercomparison Project Phase three) and AR5 GCMs (CMIP5) are used in the study. In case of AR4 (https:// esgcet.llnl.gov:8443/home/publicHomePage.do) the future scenario runs or Special Report on Emissions Scenarios (SRES A1B, run1) of 17 GCMs (Meehl et al., 2007) having monthly precipitation is used in this study. The reason for selecting SRES A1B scenario is mainly due to present scenario is available for all the GCMs in AR4. In case of the AR5 (http://esgfindex1.ceda.ac.uk/ esgfwebfe/live) the future simulations of the monthly precipitation data sets for 50 GCM is taken. Out of 50 GCMs, 28 GCMs from RCP 4.5, 16 GCMs from RCP 6.0 and 6 GCMs from the RCP 8.5 are taken for the present study. For the present study we have used the GCMs simulations for the year 2001 to 2099. For comparison purpose, all GCM outputs are regridded to the same resolution of the minimum resolution GCM i.e. Model for Interdisciplinary Research on Climate (MIROC4h) (0.56° × 0.56° grid) by bilinear interpolation technique. Each of the GCMs is then spatially averaged to get an individual time series of monthly precipitation. From the monthly time series we have calculated annual and four seasonal time series. Seasonal series are calculated for winter (DecemberJanuaryFebruary), premonsoon (MarchAprilMay), monsoon (JuneJulyAugust September) and postmonsoon (OctoberNovember). Absolute precipitation anomalies (Das et al., 2012) are calculated for annual and seasonal time series for each GCM. This method is mostly adopted as the mean subtraction makes the anomaly time series to have a zero mean. More formally, Apij = pij – Pj, where pij is the mean monthly rainfall for the ith year and jth month while Pj represents the long term mean monthly rainfall for the jth month. In general it is known that taking mean would minimize the variance of the resulting series. Further, instead of using the entire data for base computation, a short reference interval can be chosen. In this study the base period is chosen as 20062035. In general the magnitude of trend in a time series is determined either using regression analysis (parametric test) or using Sen’s estimator method (nonparametric method). Both these methods assume a linear trend in the time series. Regression analysis is conducted with time as the
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independent variable and precipitation as the dependent variable. The regression analysis can be carried out directly on the time series or on the anomalies. A linear equation, y = mt + c, defined by c (the intercept) and trend m (the slope), can be fitted by regression. The linear trend value represented by the slope of the simple least square regression line provided the rate of rise/fall in the variable. Studies by Naidu et al. (1999) used this method to find the trend in rainfall dataset over India for the long term period of 1871 to 1994. The seasonal multi model ensemble is calculated by simply averaging the trends obtained from all GCMs for a particular season. RESULTS AND DISCUSSION Projected change in precipitation during 2001-2099 SRES A1B: The GCMs showing decreasing trend in annual precipitation varies from a minimum of 2.7 mm/ decade to a maximum of 12.2 mm/decade. The increasing trend among the GCMs varies from a minimum of 3.1 mm/decade to 26.35 mm/decade. In case of monsoon season 11 out of 17 GCMs are showing decreasing trend with a variation from 0.5mm/decade to 6.8 mm/decade (Fig 2). RCP 4.5: Analysis from RCP 4.5 GCMs (Fig 3) shows that 13 out of 28 GCMs are showing decreasing trend in annual precipitation and 15 GCMs are showing increasing trend. The decreasing trend of annual rainfall varies from 0.2 mm/decade to 34.7 mm/decade and the increasing trend varies from 9 mm/decade to 28.0 mm/ decade. It is clear that the magnitude of decreasing precipitation trend is higher than the increasing trend. More than 70% of the total GCMs are showing increasing trend in monsoon and postmonsoon. In premonsoon season equal numbers of GCMs are showing increasing and decreasing trend of precipitation. The most noticeable season is the winter season where 57% of the total GCMs are showing decreasing trends of precipitation. RCP 6.0: In RCP 6.0, the decreasing trends of precipitation varies from 4.73 mm/decade to 23.74 mm/ decade in annual, 2.27 mm/decade to 16.0 mm/decade in monsoon, 1.73 mm/decade to 6.83 mm/decade in post monsoon, 0.44 mm/decade to 7 mm/decade in pre monsoon, and 0.63 mm/decade to 11 mm/decade in winter season. Similarly, the increasing trend of precipitation varies from 0.1 mm/decade to 26 mm/decade in annual, 0.9 mm/decade to 14.25 mm/decade in monsoon, 0.1 mm/
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decade to 6 mm/decade in postmonsoon, 0.1 mm/decade to 28 mm/decade in premonsoon and 1 mm/decade to 2.16 mm/decade in winter (Fig 4). RCP 8.5: Most of the GCM in RCP 8.5 are showing increasing trends of precipitation in Annual, Monsoon, Premonsoon and Winter Season (Fig 5). In case of postmonsoon season, 5 out of 6 GCMs are showing decreasing trends of precipitation but the magnitude of these decreasing trends is very less. The increasing trend varies from 6mm/decade to 14.7 mm/ decade in annual, 3.6 mm/decade to 10.5 mm/decade in monsoon, 2 mm/decade to 8.42 mm/decade in pre monsoon and 1 mm/decade to 9 mm/decade in winter season. Multi Model Ensemble (MME): The results from different simulations are giving a mixed response of both increasing and decreasing trend of precipitation in different seasons. MME gives a quantified variation of precipitation trend in different season irrespective of GCMs. MME17 of AR4SRES A1B simulation shows the precipitation trend is projected to increase in both annual and seasonal scale. It is worth to note that the magnitude of this trend is very less and it varies from 0.16 mm/decade in postmonsoon season to 1.60 mm/ decade annually. Similar analysis of MME28 from AR5
Fig 1. Western Himalayan Region of India showing gradient of altitude in meter.
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Fig 2. Bar plots showing the trend values of different GCMs of AR4SRESA1B for all the seasons
Fig 3. Bar plots showing the trend values of different GCMs of AR5 RCP 4.5 for all the seasons
RCP 4.5 shows that the precipitation trends is decreasing in annual, premonsoon, and winter season whereas the trend is increasing in case of postmonsoon and monsoon with a lesser magnitude in monsoon of 1.4 mm/decade. Here the annual and winter season shows a noticeably decreasing trend of precipitation of 4 mm/decade and 7.1 mm/decade in winter. The MME16 in RCP 6.0 shows an increasing trend of precipitation in annual and all season except the winter. The increasing trend of precipitation varies from 0.22 mm/decade in postmonsoon to 2.43 mm/ decade in annual. The winter season is showing a decreasing trend of 1.5 mm/decade. The result obtain from RCP 8.5 shows that there is an increasing trend of precipitation in annual and all seasons except post monsoon. The increasing trend varies from 0.9 mm/
decade in winter to 7 mm/decade in annual. The post monsoon season is showing a decreasing trend of 1.5 mm/ decade. Overall study of GCM precipitation simulations in AR4 and AR5 suggest that the monsoon and annual precipitation is projected to rise towards the end of 21st century, these results agree with the climate model projections (Palazzi et al., 2013) giving an increase in precipitation in the monsoon season over the Western Himalayan Region by the end of the 21st century, the result may lead to increased temperature as due to the projected atmospheric moisture buildup due to increased greenhouse gases. Results from all the RCPs indicate that winter rainfall will decrease similarly to the study by
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Fig 4. Bar plots showing the trend values of different GCMs of AR5 RCP 6.0 for all the seasons
Fig 5. Bar plots showing the trend values of different GCMs of AR5 RCP 8.5 for all the seasons
Palazzi et al. (2013) while increasing trends were reported by Archer and Fowler (2004) which contradicts with the present study. These decreasing trends of winter rainfall may cause the increasing tendency of snow cover and size of the glacier which has been reported by Gardelle et al. (2012). On the other hand, there is an increasing trend of postmonsoon rainfall. This may lead to increase intensity of western disturbances according to the study by Gautam et al. (2013). CONCLUSION A total of 67 GCMs were taken for this study from both the assessment reports i.e. 17 GCMs from AR4 and 50 GCMs from AR5. It is found that 42 (> 64%) number of GCMs are projected either a decreasing trend of precipitation in winter season or have a very less magnitude of increasing precipitation trend that varies from 0.02 mm/ decade to 0.5 mm/decade. As winter rainfall in WHR is mainly attributed to passage of weather systems called western disturbances (Dimri and Das, 2011), hence GCM simulated decreasing winter rainfall indicates weakening
of the Western Disturbances over WHR of India. Similarly, in case of premonsoon 35 (>52%) number of GCMs projected either a decreasing trend of precipitation or have a very less magnitude of increasing precipitation trend that varies from 0.09 mm/decade to 0.9 mm/decade. Hence the GCMs are projecting a warmer summer and decrease of convective storms over the WHR. In monsoon season a total of 42 (>62%) number of GCMs are showing increasing trend of precipitation which is consistent with the results obtained by precipitation projection maps from the PRECIS regional simulation study (Rapa Kumar et al., 2006). Except the winter and premonsoon all other seasons and annual precipitation trends are showing an increasing trend. It is concluded that precipitation projections by GCMs are less consistent, reflecting the greater uncertainty associated with precipitation trends. ACKNOWLEDGEMENTS The work is based on INDONORWAY
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international research project ‘INDICE’. Authors thankfully acknowledge the assistance and fund provided by the Norway Research Council, through NVE. The authors are also gratefully acknowledge the contributions of the knowledge partners Dr. Rasmus E. Benestad and Dr. Abdelkader Mezghani, Met. No. (Oslo, Norway). The constructive suggestions received from the anonymous reviewer are thankfully acknowledged. REFERENCES Archer, D. R., and Fowler, H. J. (2004). Spatial and temporal variations in precipitation in the Upper Indus Basin, global teleconnections and hydrological implications. Hydrology and Earth System Sciences Discussions, 8(1): 4761. Beniston, M. (2003). Climatic change in mountain regions: a review of possible impacts. Clim. Chang., (59): 531. Das, L., Nagaraj, K., Khan, S. A., and Sarkar, S. (2012). Performance of three generation IPCC climate models to simulate monsoonal rain over Gangetic West Bengal and its neighbourhood. J. Agrometeorol.,14: 312319 Dimri, A. P. and Dash, S. K. (2011). Wintertime climate trends in the western Himalayas. Climate Change: doi: 10.1007/A105840110201y. Gardelle, J., Berthier, E., & Arnaud, Y. (2012). Slight mass gain of Karakoram glaciers in the early twenty first century. Nature geoscience, 5(5): 322325. Gautam, M. R., Timilsina, G. R., & Acharya, K. (2013). Climate change in the Himalayas: current state of knowledge. World Bank Policy Research Working Paper, (6516). Meehl, G. A., Covey, C., Taylor, K. E., Delworth, T., Stouffer, R. J., Latif, M., McAvaney, B. and Mitchell, J. F. (2007). The WCRP CMIP3 multimodel dataset: A new era in climate change research. Bulletin of the American Meteorological Society, 88(9):13831394. Maurer, E. (2007). Uncertainty in hydrologic impacts of climate change in the Sierra Nevada, California under two emissions scenarios. Clim. Chang. 82: 309325.
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