Seasonality of Precipitation over Himalayan Watersheds in CORDEX South Asia and their Driving CMIP5 Experiments

Abstract

Abstract:Since the Coupled Model Intercomparison Project Phase 5 (CMIP5) experiments exhibit limited skill in reproducing the statistical properties of prevailing precipitation regimes over the major Himalayan watersheds (Indus, Ganges, Brahmaputra and Mekong), this study evaluates the anticipated added skill of their dynamically refined simulations performed under the framework of Coordinated Regional Climate Downscaling Experiments for South Asia (CX-SA). For this, the fidelity of eight CX-SA experiments against their six driving CMIP5 experiments is assessed for the historical period in terms of time-dependent statistical properties (onset/retreat timings and rapid fractional accumulation-RFA) of the dominant summer monsoonal precipitation regime (MPR). Further, a self-defining seasonality index (SI), which is a product of precipitation and the distance of its actual distribution relative to its uniform distribution (relative entropy-RE), has been computed for MPR, westerly precipitation regime (WPR) and annual precipitation. The time evolution of precipitation, RE and SI has also been analyzed. Results suggest that CX-SA experiments simulate even higher wet biases than their driving CMIP5 experiments over all study basins, mainly due to higher wet biases simulated over the Himalayas and Tibetan Plateau. Most of the CX-SA experiments suggest unrealistic timings of the monsoon onset that are far earlier than their driving CMIP5 experiments for all basins. Generally, CX-SA experiments feature higher underestimation of RFA slope, RE and SI, distancing their driving CMIP5 experiments farther from observations. Interestingly, regardless of the diverse skill of CMIP5 experiments, their fine scale CX-SA experiments exhibit quite a similar skill when downscaled by the same regional climate model (RCM), indicating RCM's ability to considerably alter the driving datasets. These findings emphasize on improving the fidelity of simulated precipitation regimes over the Himalayan watersheds by exploiting the potential of RCMs in term of microphysics, resolutions and convective closures, and preferably, on resolving the crucial fine scale processes further down to their representative (meso-to-local) scales.

Prevailing climatic trends and runoff response from Hindukush–Karakoram–Himalaya, upper Indus Basin

“…On the other hand, observed winter precipitation increase for relatively high-latitudinal subregions is more consistent with the observed more frequent incursions of the westerly disturbances therein (Cannon et al, 2015;Madhura et al, 2015). Nevertheless, in view of the enhanced influence of prevailing weather systems and certain changes expected in their seasonality/intermittency under changing climate (Hasson et al, 2016a;Hasson, 2016a), we speculate significant changes in the timings of the meltwater availability from the UIB. Such a hypothesis can be tested by assessing changes in the seasonality of observed precipitation and runoff.…”

“…The field-significant spring drying (except for Karakoram) is mainly consistent with the weakening and northward shift of the midlatitude storm track (Bengtsson et al, 2006) and also with increasing number of spring dry days Hasson, 2016a). On the other hand, observed winter precipitation increase for relatively high-latitudinal subregions is more consistent with the observed more frequent incursions of the westerly disturbances therein (Cannon et al, 2015;Madhura et al, 2015).…”

“…The hydrology of the UIB is dominated by the precipitation regime associated with the year-round midlatitude western disturbances that intermittently transport moisture mainly during winter and spring and mostly in solid form (Wake, 1989;Ali et al, 2009;Hewitt, 2011;Hasson et al, 2016a;Hasson, 2016a). Such moisture contribution is anomalously higher during the positive phase of the North Atlantic oscillation (NAO), when the southern flank of the western disturbances intensifies over Iran and Afghanistan because of heat low there, causing additional moisture input from the Arabian Sea (Syed et al, 2006).…”

“…Consistent with the findings of Sheikh et al (2009) and Río et al (2013), warming dominates in the spring months, where it is fieldsignificant in March over almost all identified subregions of the UIB. Under the drying spring scenario, less cloudy conditions associated with increasing number of dry days for the westerly precipitation regime Hasson, 2016a) together with the snow-albedo feedback can partly explain spring warming. Contrary to long-term warming trends analyzed here or to those previously reported, a field-significant cooling is found for winter, which is consistently observed over the eastern United States, southern Canada and much of northern Eurasia (Cohen et al, 2012).…”

Future Water Availability from Hindukush-Karakoram-Himalaya upper Indus Basin under Conflicting Climate Change Scenarios

“…It is pertinent to mention here that though CORDEX-SA experiments provide high resolution present and future climate simulations, however, their ability to reproduce the observed hydro-climatology over the steep HKH topography is by hardly any means better than their forcing CMIP5 GCMs, which being consistent to their older versions and downscaled experiments, substantially over-(under-) estimate precipitation (temperatures) and fail to reproduce summer cooling [7,15,16,[82][83][84][85][86][87]. Such cold and wet biases in the CORDEX-SA and their CMIP5 forcing experiments are larger than their projected end of 21st century climatic changes under RCP8.5, indicating huge climatic uncertainty over the UIB [16].…”