Evaluation of the performance of the CMIP5 General Circulation Models in predicting the Indian Ocean Monsoon precipitation over south Sistan and Baluchestan, using the past hydrological changes in the region

Climate change refers to any significant change in the existing mean climatic conditions within a certain time period (Jana and Majumder, 2010; Giorgi, 2006). Earth's climate change through history has happened (Nakicenovic et al., 2000; Bytnerowicz et al., 2007). 

In this study, daily precipitation and daily maximum (Tmax) and daily minimum (Tmin) temperatures in the Iranshar synoptic station, southeastern Iran were predicted for the future (2061-2080) by statistical downscaling outputs of 5 GCM models (EC-EARTH, GFDL-CM3, HadGEM2-ES, MIROC5, MPI-ESM-MR) under RCP 4.5 and RCP 8.5 emission scenarios. Figure 1: Major climate systems over West Asia (Gurjazkaite et al., 2018; Sharifi et al., 2015; Vaezi et al., 2019). and location of the Iranshahr station (marked as a red box). A) . Dotted lines indicate the approximate current location of the Intertropical Convergence Zone (ITCZ), Mediterranean winter precipitation zone lying between the dashed lines, and the Siberian Anticyclone; IOSM refers to the Indian Ocean Summer Monsoon, and a 30 years average of minimum and maximum monthly mean air temperature (°C) and mean monthly rainfall (mm) as recorded at Iranshahr.

The results of statistical downscaling outputs of 5 GCM models by LARS-WG model under RCP 8.5 emission scenarios (the business-as-usual) in Iranshahr station was modeled from 2061–2080. During this period the mean temperature will increase between 3.2 to 4.6 °C compared to the base period.At The Paris Agreement, the average increase in temperature was set at below 2 °C by the end of the 21st century, which includes the 0.9 °C increase since the industrial revolution, to avoid adverse and unpredictable weather effects (IPCC, 2021; Millar et al., 2017; Rogelj et al., 2018). However, the results of this study show that southeastern Iran is firmly set on the path by increasing the current temperature by several degrees more by 2080. Based on the two modeled scenarios, the change in temperature will have an increasing trend in the coming years and is in good agreement with the recent assessments of future temperature changes in southwest Asia (Babar et al., 2016; Hamidianpour et al., 2016; Pal et al., 2016; Evans, 2009). Therefore, if greenhouse gas emissions continue at their present rate, earth's surface temperature in this region will pass the temperature threshold of 2 °C.One of the mechanisms contributing to the poor monsoon rainfall simulation in CMIP5 and HadGEM3 models may be the Arabian Sea cold sea surface temperature biases that persist until summer and reduce moisture fluxes over the Arabian Sea (Levine et al. 2013; Levine and Turner 2012). In modern times, IOSM induced upwelling of cold water leads to reduce sea surface temperatures in western Arabian Sea in summer that in turn cause reduced evaporation over a cooler Sea and less moisture in the low-level monsoon jet (Levine et al. 2013; Saher et al. 2007). In order to evaluate this hypothesis, paleoclimate changes discussed in the southeastern Iran is useful. The multi-proxy climate record from southeastern Iran reveals that the regional hydrology of southeastern Iran since ca. 14.7 cal kyr BP is primarily governed by IOSM strength, which is linked to the position of the ITCZ in response to the orbital-scale changes in summer insolation (Fleitmann et al., 2007; Gupta et al., 2003; Overpeck et al., 1996).

As the first comprehensive (both future and paleo) climatic change study in the arid region of southeastern Iran on the north most border of IOSM, we compare simulated future precipitation based on different scenarios of global warming with real paleoclimatic changes that happened since ca. 14.7 cal kyr BP in the region. In this respect, the maximum and minimum temperatures and precipitation projection in the southeastern Iran is derived from the downscaling of the CMIP5 GCMs (EC-EARTH, GFDL-CM3, HadGEM2-ES, MIROC5, and MPI-ESM-MR) under RCP 4.5 and RCP 8.5 using the LARS- WG model. Paleoenvironmental records since 14.7 cal kyr BP from southeastern Iran is used to examine whether the predicted changes in precipitation (variability in IOSM and MLW output) based on the two scenarios of global warming are valid.we postulate that results of statistical downscaling outputs of the GCMs by LARS-WG model in Iranshar synoptic station did not have a sensitivity to simulate monsoon precipitation in this complicated region with various factors impacting climate change. Because the results dose not match the paleohydrological changes and Intensify of IOSM during past warm periods. We suggest, in the new generation of climate models, the effect of a consistent increase in seasonal mean precipitation during the summer monsoon under warming scenarios must be considered more for north most monsoonal domain area like southeastern Iran.