The Relationship Between ENSO And Thermal Condition Over The Tibetan Plateau And Its Effects On Downstream Flow

Both the preceding El Ni?o events and snow anomalies over the Tibetan Plateau play important roles in summer precipitation in China, and it is of positive significance to explore the relationship between them for climate prediction, summer rainfall forecasting, disaster prevention and mitigation. Therefore, according to snow depth and snow water equivalent over the plateau, combined with the observations of surface temperature for 85 stations, the extended reconstructed sea surface temperature(SST) in the Pacific for earlier periods and so on, this paper analyzes the influence of ENSO on the subsequent surface thermal conditions over the plateau for nearly 20 years, which are represented by snow and surface temperature. And the corresponding atmospheric circulation anomalies and possible physical processes are also studied. In addition, the effect of the plateau anomalous heating on downstream flow from May to August is explored through numerical simulation. The present study consists of the following several parts:1、Using the monthly snow depth and snow water equivalent over the plateau, which are derived from satellite data, the first part analyzes the correlations between ENSO and snow anomalies over the eastern and western plateau and their continuity for nearly 20 years. And the results are as follows. The correlations are greatly different between SST of previous November in the eastern equatorial Pacific and snow depth over the eastern and western plateau. The positive correlation is over 95% confidence level between snow depth over the western plateau and SST in the equatorial Pacific to the east of 130oW of previous November, which persists to April; however, there is almost no significant positive correlation zone in the eastern equatorial Pacific corresponding to the eastern plateau. The correlations between snow water equivalent over the eastern and western plateau and SST in the eastern equatorial Pacific of previous November are not so different, especially embodying in the significant positive correlation zone in April for the eastern plateau in the equatorial Pacific extending to 160°W. But the autocorrelation coefficients of snow water equivalent averaged over the eastern plateau between previous months and April are less than 0.46, which is critical value of 95% confidence level. And the analysis of atmospheric circulation suggests that, from the mature phase of El Ni?o in winter and before April, the extratropical westerly jet at 200 hPa is very active, which benefits the propagation along the jet to the plateau of the Rossby wave excited by SST anomalies in the eastern equatorial Pacific in the upper troposphere and then results in the enhancement of local convective activities and increased rainfall and snowfall. With weakening of ENSO signal and the westerly jet as waveguide, the Rossby wave tends to disappear and the snow anomalies over the plateau caused by ENSO also become not important.2、The correlations between ENSO and the subsequent surface temperature anomalies over the eastern plateau are further analyzed for nearly 20 years with observations of surface temperature from 85 stations. The results are as follows. During April and May after the occurrence of El Ni?o, surface temperature in the eastern plateau is significantly higher than normal, and the correlation coefficients with previous Ni?o3 index are respectively 0.44 and 0.52, which is over 95% confidence level, especially the significant positive correlation zone for May in the equatorial Pacific can reach the International Date Line. And on longer time scales, the correlation coefficient between surface temperature averaged over the eastern plateau from 75 stations in May and the previous Ni?o3 index is 0.30, which is over 95% confidence level. During warmer years, from April to June, air temperature departure and geopotential height anomaly at 200 hPa and 500 hPa are both positive over the Plateau, with stronger centers in April and May, but it is not completely opposite in colder years. Additionally, from March to April during the decaying phase of El Ni?o, great changes take place in the atmospheric circulation over the plateau and surrounding regions. The geopotential height departure at 200 hPa and 500 hPa changes from negative to positive, and the air temperature anomaly at 500 hPa does the same. And the significant warming over the eastern plateau in late spring is very likely to relate with the anomalous warm advection from the north of the Caspian Sea to the Tibetan Plateau at 500 hPa in March. But there is no similar transition after the occurrence of La Ni?a.3、By using the Community Atmospheric Model v3.1, the snow depth and surface temperature anomalies over the Tibetan Plateau after the occurrence of El Ni?o are simulated. A case study for 1997/1998 and the sensitive experiment with observed SST suggest that this model is not able to reproduce the positive anomalies of snow depth over the plateau in winter and spring during the decaying phase of El Ni?o, even completely opposite to the previous statistical diagnosis. But the case study for 1997/1998 simulates the Rossby wave train of relative vorticity at 200 hPa in January and February very well. According to several sensitive experiments with different intensities of SST anomaly, with the strengthening of El Ni?o, the snow depth anomalies in the plateau still remain negative and there are no corresponding linear variations, while surface temperature anomalies over the eastern plateau oscillate near zero.4、Using the Regional Climate Model and setting three different kinds of anomalous heating rates in the Tibetan Plateau, this study discusses the effect of the plateau anomalous heating on the atmospheric circulation from May to August. The results are as follows. After 12 days when the atmosphere over the plateau is heated with the rate of 4K/day in May, the geopotential height anomaly at 500 hPa is positive near the Okhotsk Sea and negative in Northeast China and to the south of the Kamchatka Peninsula, forming a suspected wave train; however, it turns to be a dipole pattern at 200 hPa, with positive in the west and negative in the east. When it comes to June, the construction of this wave train becomes more obvious, with a complete wave like the Okhotsk-Japan(OKJ) pattern appearing at 200 hPa. There are no similar wave structures among other months. In July, the geopotential height anomaly is always negative from Mongolia to the Okhotsk Sea and to the east of Japan at both 200 hPa and 500 hPa. And in august, it turns to be positive off the east coast of Japan, and the negative values near the Okhotsk Sea remain to be unchanged. With the strengthening of anomalous heating rate, the geopotential height anomalies at each point tend to be almost proportionally increasing along the wave track at 200 hPa in June. The analysis of ray tracing shows that, the ray paths in May are similar to that in June, and the simulation route lies between the ray paths with zonal wavenumber 3 and 4.