Evaluation of the subtropical westerly jet and its effects on the projected summer rainfall over central Asia using multi‐CMIP5 models

Abstract

ABSTRACTBased on historical and RCP8.5 experiments, along with 25 Coupled Model Intercomparison Project Phase 5 (CMIP5) models, we evaluate the skills of the CMIP5 models in simulating the subtropical westerly jet (SWJ) and its effect on the projected summer rainfall over central Asia (CA). The historical experiments show that 23 models can well simulate the changes in the position and strength of the SWJ. Further analysis finds that 21 models can capture the relationship between the SWJ position change and summer rainfall over the domain of 55–85°E and 35–50°N and 17 models can capture the relationship between the SWJ strength change and summer rainfall over the domain of 55–75°E and 45–55°N, where 15 models can capture the relationships between both SWJ strength and position change with summer rainfall. Based on the SWJ change among the 15 models, except for the HadGEM2‐CC model, the 500 hPa winds of the rest 14 models well match the summer rainfall over CA. So the 14 models are used to do next analysis. In the last 50 years of the 21st century, only the inmcm4 model presents the change in the SWJ strength, and the rest 13 models present the changes in the SWJ position. Among above 13 models, 10 models maintain the relationships between the SWJ position change and summer rainfall over the domain of 55–85°E and 35–50°N into the future. Under the background of global warming, the changes in the SWJ position are well related to the sea surface temperatures (SSTs) over the Indian Ocean in the future. The SST warming over the Indian Ocean results in the South Asian summer monsoon weakening, corresponding to an anomalous anticyclone over the low‐level troposphere of the monsoon regions; this situation results in an anomalous cyclone at the upper level of the troposphere of the northwestern flank of the monsoon heating regions due to Matsuno–Gill‐type responses, which favour the SWJ axis shifting further south. In addition, we also compare the current projected summer rainfall over CA from our selected models with other selected previous models using different methods. The ensemble results show that more summer rainfall occurring in most of Kazakhstan and Xinjiang Province of northwestern China is more likely in the future; in the rest of the countries of CA, there are uncertainties regarding future rainfall changes.