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# Change in Precipitation over the Tibetan Plateau Projected by Weighted CMIP6 Models

• Precipitation over the Tibetan Plateau (TP) is important to local and downstream ecosystems. Based on a weighting method considering model skill and independence, changes in the TP precipitation for near-term (2021–40), mid-term (2041–60) and long-term (2081–2100) under shared socio-economic pathways (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSSP3-7.0, SSP5-8.5) are projected with 27 models from the latest Sixth Phase of the Couple Model Intercomparison Project. The annual mean precipitation is projected to increase by 7.4%–21.6% under five SSPs with a stronger change in the northern TP by the end of the 21st century relative to the present climatology. Changes in the TP precipitation at seasonal scales show a similar moistening trend to that of annual mean precipitation, except for the drying trend in winter precipitation along the southern edges of the TP. Weighting generally suggests a slightly stronger increase in TP precipitation with reduced model uncertainty compared to equally-weighted projections. The effect of weighting exhibits spatial and seasonal differences. Seasonally, weighting leads to a prevailing enhancement of increase in spring precipitation over the TP. Spatially, the influence of weighting is more remarkable over the northwestern TP regarding the annual, summer and autumn precipitation. Differences between weighted and original MMEs can give us more confidence in a stronger increase in precipitation over the TP, especially for the season of spring and the region of the northwestern TP, which requires additional attention in decision making.
摘要: 青藏高原的降水对局地及下游生态具有重要影响。基于CMIP6最新释出的27个模式，及综合考虑了模式性能和独立性的加权方法，研究了5种共享社会经济路径(SSP1-1.9、SSP1-2.6、SSP2-4.5、SSSP3-7.0、SSP5-8.5)下青藏高原降水的近期(2021-2040年)、中期(2041-2060年)和长期(2081-2100年)变化。结果表明，21世纪末青藏高原的年平均降水相较于当前气候态将增加7.4% — 21.6%，其中高原北部的变化更为强烈。除冬季降水在高原南部的减少趋势外，其他季节的降水呈现和年平均类似的增加趋势。与等权重的预估结果相比，加权后的青藏高原降水增加趋势略有增强，同时模式不确定性减小。此外，加权的影响存在空间和季节差异，季节上，加权对降水增加的加强效应在春季更明显，空间上，高原西北部的降水变化受加权的影响更为显著。预估结果在加权前后的差异提升了未来青藏高原降水趋湿的可信度，尤其是春季和高原西北部的降水，相关结果可为决策者提供参考
• Figure 1.  Evaluation of precipitation over the Tibetan Plateau (TP) simulated by the 27 CMIP6 models in terms of equally-weighted multi-model ensemble mean (MME; middle column) and weighted MME (right column) compared to the observation (left column). (a–c) Annual mean precipitation over the TP (mm d–1). (d–f) The contribution of summer to the annual total precipitation over the TP (%). (g–i) Interannual variability of annual mean precipitation over the TP (mm d–1). R values given above panels of the rightmost two columns are pattern correlation coefficient between MME and observation. The TP is outlined by the black curves, where the topography is above 2500 m.

Figure 2.  Distance matrix and weights. (a) Distance matrix between models (each column, sorted by unified skill weights in parentheses) and the observation in terms of climatology of annual mean, spring (March to May), summer (June to August), autumn (September to November) and winter (December to February) precipitation, the contribution of summer to the annual total precipitation , interannual variability of annual mean, spring, summer, autumn and winter precipitation, as well as their combined average (each row from top to bottom). (b) Distance matrix in terms of combined metrics between models, with unified uniqueness weights in parentheses along the x-axis and unified final weights in parentheses of the y-axis.

Figure 3.  Change in the annual mean precipitation for long-term projection relative to 1985–2014 under (a) SSP1-1.9, (b) SSP1-2.6, (c) SSP2-4.5, (d) SSP3-7.0 and (e) SSP5-8.5 in %. Slashes mean more than two thirds of the models agree with the sign. (f–j) the same as (a–e), but for the difference between weighted and equally-weighted MME.

Figure 4.  Spatial patterns of the standard deviation among models in the long-term projection of annual mean precipitation over the TP (%) relative to historical period (1985–2014) under SSP5-8.5: (a) equally-weighted MME, (b) weighted MME, (c) difference between weighted and equally-weighted MMEs.

Figure 5.  The same as Fig. 3, but for spring precipitation (March−April−May mean).

Figure 6.  The same as Fig. 3, but for summer precipitation (June−July−August mean).

Figure 7.  The same as Fig. 3, but for autumn precipitation (September−October−November mean).

Figure 8.  The same as Fig. 3, but for winter precipitation (December−January−February mean).

Figure 9.  The scatter plots of the change in the global mean surface temperature (units: K) and TP precipitation (units: %) in the (a) near-term (2021–40), (b) mid-term (2041–60), (c) long-term (2081–2100) projection. Dots indicate different models and black lines are linear regression lines, with regression values indicated by r (% K−1).

Figure 10.  The response of the annual mean precipitation over the TP to the global-mean surface air temperature increase under five scenarios (% K−1). Models are sorted according to the weights in the parentheses. The black solid (dashed) lines are weighted (original) MMEs.

Figure 11.  Ratios of weighted MME to equally-weighted MME for long-term projections under SSP5-8.5 varying with different values of Du or Ds. (a) The case of Ds = 0.9. (b) The case of Du = 1.1. The blue, skyblue and green lines represent relative change in the annual mean TP precipitation, standard deviation among models and the signal-to-noise ratio, respectively.

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## Manuscript History

Manuscript revised: 22 February 2022
Manuscript accepted: 25 March 2022
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Change in Precipitation over the Tibetan Plateau Projected by Weighted CMIP6 Models

###### Corresponding author: Tianjun ZHOU, zhoutj@lasg.iap.ac.cn;
• 1. LASG, Institute of Atmospheric Physics, Chinese Academy of Science, Beijing 100029, China
• 2. University of Chinese Academy of Sciences, Beijing 100049, China
• 3. State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing 100049, China

Abstract: Precipitation over the Tibetan Plateau (TP) is important to local and downstream ecosystems. Based on a weighting method considering model skill and independence, changes in the TP precipitation for near-term (2021–40), mid-term (2041–60) and long-term (2081–2100) under shared socio-economic pathways (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSSP3-7.0, SSP5-8.5) are projected with 27 models from the latest Sixth Phase of the Couple Model Intercomparison Project. The annual mean precipitation is projected to increase by 7.4%–21.6% under five SSPs with a stronger change in the northern TP by the end of the 21st century relative to the present climatology. Changes in the TP precipitation at seasonal scales show a similar moistening trend to that of annual mean precipitation, except for the drying trend in winter precipitation along the southern edges of the TP. Weighting generally suggests a slightly stronger increase in TP precipitation with reduced model uncertainty compared to equally-weighted projections. The effect of weighting exhibits spatial and seasonal differences. Seasonally, weighting leads to a prevailing enhancement of increase in spring precipitation over the TP. Spatially, the influence of weighting is more remarkable over the northwestern TP regarding the annual, summer and autumn precipitation. Differences between weighted and original MMEs can give us more confidence in a stronger increase in precipitation over the TP, especially for the season of spring and the region of the northwestern TP, which requires additional attention in decision making.

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