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FIG. 7. The distributions of monthly early spring SCE difference between 1999-2017 and 1979-98 obtained from (a)-(c) NOAA and (d)-(f) ERAS. Dots denote the difference passing the significance test of p < 0.1. The anomalous surface heating caused by the SCE reduc- tion in midlatitude Eurasia may lead to a change in the sub- tropical westerlies. To prove this hypothesis, Fig. 9 depicts the Snow cover, as a potential indicator for subseasonal to sea- sonal prediction (Jeong et al. 2013; F. Li et al. 2019; Orsolini et al. 2013), can alter the air thermal conditions through an in- stantaneous albedo effect (Cohen and Entekhabi 1999; Dickson 1984). This albedo effect depends on the intensity of the solar radiation and thus will be amplified from winter to spring. As Kim et al. (2013) indicated that SCE in eastern Europe has an abrupt shift in the late 1990s during early spring due to air warming, and Li et al. (2021, 2020, 2018) proved that SCE over the TP can affect the upper-level westerlies, SCE in midlatitude Eurasia thereby is analyzed to see its role in March. The dis- tribution of SCE difference between 1999-2017 and 1979-98 averaged over the (30°-80°E) is presented in Fig. 7. From February to April, the decadal reduction in SCE in midlatitude Eurasia is observed in both ERAS and NOAA. In February, SCE experienced a decrease in the south of 45°N. In March, SCE rapidly reduced in midlatitude Eurasia (north of 45°N).

Figure 7 The distributions of monthly early spring SCE difference between 1999-2017 and 1979-98 obtained from (a)-(c) NOAA and (d)-(f) ERAS. Dots denote the difference passing the significance test of p < 0.1. The anomalous surface heating caused by the SCE reduc- tion in midlatitude Eurasia may lead to a change in the sub- tropical westerlies. To prove this hypothesis, Fig. 9 depicts the Snow cover, as a potential indicator for subseasonal to sea- sonal prediction (Jeong et al. 2013; F. Li et al. 2019; Orsolini et al. 2013), can alter the air thermal conditions through an in- stantaneous albedo effect (Cohen and Entekhabi 1999; Dickson 1984). This albedo effect depends on the intensity of the solar radiation and thus will be amplified from winter to spring. As Kim et al. (2013) indicated that SCE in eastern Europe has an abrupt shift in the late 1990s during early spring due to air warming, and Li et al. (2021, 2020, 2018) proved that SCE over the TP can affect the upper-level westerlies, SCE in midlatitude Eurasia thereby is analyzed to see its role in March. The dis- tribution of SCE difference between 1999-2017 and 1979-98 averaged over the (30°-80°E) is presented in Fig. 7. From February to April, the decadal reduction in SCE in midlatitude Eurasia is observed in both ERAS and NOAA. In February, SCE experienced a decrease in the south of 45°N. In March, SCE rapidly reduced in midlatitude Eurasia (north of 45°N).

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FIG. 1. (a) The distribution of weather stations in the SETP (27°-30°N, 92°-100°E), with the background color indicating the altitude. (b) The seasonal variation of precipitation averaged over the 12 station observations (OBS) and the grids collocated with the stations and the area of the SETP obtained from ERAS. The climatological monthly precipitation (shaded; mm day ') and wind (vectors; m s_') at 700 hPa were obtained from ERAS during 1979 to 2017 in (c) February, (d) March, and (e) April. The black line is the contour of 3000 m above sea level. The red box represents the area of the SETP.
FIG. 1. (a) The distribution of weather stations in the SETP (27°-30°N, 92°-100°E), with the background color indicating the altitude. (b) The seasonal variation of precipitation averaged over the 12 station observations (OBS) and the grids collocated with the stations and the area of the SETP obtained from ERAS. The climatological monthly precipitation (shaded; mm day ') and wind (vectors; m s_') at 700 hPa were obtained from ERAS during 1979 to 2017 in (c) February, (d) March, and (e) April. The black line is the contour of 3000 m above sea level. The red box represents the area of the SETP.
FIG. 2. The time series of precipitation in the SETP during the period of 1979-2017 in (a) February, (b) March, and (c) April and (d)-(f) their corresponding MK test obtained from the station observations. The gray line represents the means of 1979-98 and 1999-2017. The black dashed lines in the MK test indicate the significance test of p < 0.05. UF (red line) and UB (blue line) in the MK test indicate the progressive and retrograde series.
FIG. 2. The time series of precipitation in the SETP during the period of 1979-2017 in (a) February, (b) March, and (c) April and (d)-(f) their corresponding MK test obtained from the station observations. The gray line represents the means of 1979-98 and 1999-2017. The black dashed lines in the MK test indicate the significance test of p < 0.05. UF (red line) and UB (blue line) in the MK test indicate the progressive and retrograde series.
Fic. 3. As in Fig. 2, but precipitation obtained from ERAS averaged over the SETP (27°-30°N, 92°E-100°E). As the decadal change in precipitation in February and April is relatively weak, we focus on the track of moisture sources of The decadal change in precipitation in the SETP in early spring can also be reproduced by ERAS, with a significant and abrupt decrease in March (Fig. 3). Actually, it has been proven that although the amount of precipitation in ERAS is
Fic. 3. As in Fig. 2, but precipitation obtained from ERAS averaged over the SETP (27°-30°N, 92°E-100°E). As the decadal change in precipitation in February and April is relatively weak, we focus on the track of moisture sources of The decadal change in precipitation in the SETP in early spring can also be reproduced by ERAS, with a significant and abrupt decrease in March (Fig. 3). Actually, it has been proven that although the amount of precipitation in ERAS is
FiG. 4. The distribution of climatological moisture contribution (unit: mm month7') to (a) the March precipitation in the SETP and (b) the difference between 1999-2017 and 1979-98. The gray solid line delineates the areas with a moisture contribution greater than 1 mm month !. The black dashed line in (b) denotes 70°E. The red box represents the SETP, and the dots in (b) denotes that the difference pass the significance test of p < 0.1.
FiG. 4. The distribution of climatological moisture contribution (unit: mm month7') to (a) the March precipitation in the SETP and (b) the difference between 1999-2017 and 1979-98. The gray solid line delineates the areas with a moisture contribution greater than 1 mm month !. The black dashed line in (b) denotes 70°E. The red box represents the SETP, and the dots in (b) denotes that the difference pass the significance test of p < 0.1.
Fic. 5. (a) The differences in the mean moisture transport [vectors; unit: g (cm s hPa) '] and zonal wind (shaded; unit: m s~') at 700 hPa and (b) the differences in surface evaporation (unit: mm month™') between 1999-2017 and 1979-98 in March. The brown box represents the SETP, and the black dashed line in (b) delineates 70°E. TP is masked at 700 hPa, and only the differences passing the significance test of p < 0.1 are presented. The data source is ERAS. Since the change of moisture contribution is affected by the capability of moisture carrier and the amount of evaporative sources (Keys et al. 2012; Zhang et al. 2019), the variation of To see the decrease of the westerlies clearly, we defined a westerlies index (WI) as the standardized zonal wind at 700 hPa (the subtropical westerlies) and 200 hPa (the subtropical jet) av- eraged in the core area of (25°-30°N, 55°-65°E) to verify the de- cadal change in the westerlies. Here, the subtropical jet and the subtropical westerlies share the same area due to their same lo- cation along (20°-30°N) in March (Schiemann et al. 2009). Since 1999, both the subtropical jet and the subtropical westerlies have experienced a decrease, which can be detected in their MK test (Figs. 6b,c). These weakened westerlies brought less moisture from the far-field sources to the SETP. In the follow- ing analysis, we only use the subtropical WI (700 hPa) due to much more moisture transport in the lower layers.
Fic. 5. (a) The differences in the mean moisture transport [vectors; unit: g (cm s hPa) '] and zonal wind (shaded; unit: m s~') at 700 hPa and (b) the differences in surface evaporation (unit: mm month™') between 1999-2017 and 1979-98 in March. The brown box represents the SETP, and the black dashed line in (b) delineates 70°E. TP is masked at 700 hPa, and only the differences passing the significance test of p < 0.1 are presented. The data source is ERAS. Since the change of moisture contribution is affected by the capability of moisture carrier and the amount of evaporative sources (Keys et al. 2012; Zhang et al. 2019), the variation of To see the decrease of the westerlies clearly, we defined a westerlies index (WI) as the standardized zonal wind at 700 hPa (the subtropical westerlies) and 200 hPa (the subtropical jet) av- eraged in the core area of (25°-30°N, 55°-65°E) to verify the de- cadal change in the westerlies. Here, the subtropical jet and the subtropical westerlies share the same area due to their same lo- cation along (20°-30°N) in March (Schiemann et al. 2009). Since 1999, both the subtropical jet and the subtropical westerlies have experienced a decrease, which can be detected in their MK test (Figs. 6b,c). These weakened westerlies brought less moisture from the far-field sources to the SETP. In the follow- ing analysis, we only use the subtropical WI (700 hPa) due to much more moisture transport in the lower layers.
FIG. 6. (a) Time series of the WI at 700 (red line) and 200 hPa (blue line) in March and (b),(c) their MK test. The black line indicate the year 1999. The black dashed lines in the MK test indicate the significance test of p < 0.05. UF (red line) and UB (blue line) in the Mk test indicate the progressive and retrograde series.
FIG. 6. (a) Time series of the WI at 700 (red line) and 200 hPa (blue line) in March and (b),(c) their MK test. The black line indicate the year 1999. The black dashed lines in the MK test indicate the significance test of p < 0.05. UF (red line) and UB (blue line) in the Mk test indicate the progressive and retrograde series.
FIG. 8. The distributions of monthly early spring (a)-(c) surface net solar radiation (SSR) and (d)-(f) sensible heat flux (SHF) difference between 1999-2017 and 1979-98 obtained from ERAS. Dots denote the difference passing the significance test of p < 0.1.
FIG. 8. The distributions of monthly early spring (a)-(c) surface net solar radiation (SSR) and (d)-(f) sensible heat flux (SHF) difference between 1999-2017 and 1979-98 obtained from ERAS. Dots denote the difference passing the significance test of p < 0.1.
Fic. 9. Latitude-height profiles of air temperature (units: K), geopotential height (units: m), and zonal wind (units: m s~') difference averaged over the (30°-80°E) between 1999-2017 and 1979-98 in (a)-(c) February, (d)-(f) March and (g)-(i) April. Shaded areas are the difference passing significance test of p < 0.1. The data source is ERAS.
Fic. 9. Latitude-height profiles of air temperature (units: K), geopotential height (units: m), and zonal wind (units: m s~') difference averaged over the (30°-80°E) between 1999-2017 and 1979-98 in (a)-(c) February, (d)-(f) March and (g)-(i) April. Shaded areas are the difference passing significance test of p < 0.1. The data source is ERAS.
Fic. 10. Schematic diagram of the influence of the midlatitude SCE on the decadal decrease of March precipitation in the SETP. The dark blue shaded base is the distribution of SCE difference between 1999-2017 and 1979-98. The gray and cyan shaded bases indicate continent and ocean, respectively.
Fic. 10. Schematic diagram of the influence of the midlatitude SCE on the decadal decrease of March precipitation in the SETP. The dark blue shaded base is the distribution of SCE difference between 1999-2017 and 1979-98. The gray and cyan shaded bases indicate continent and ocean, respectively.
Fic. 11. Distribution of the precipitation difference in March between 1999-2017 and 1979-98 obtained from ERAS and GPCP. Dots indicate the difference passing the significance test of p < 0.1. The decadal decrease in March precipitation in the SETP may have reduced springtime snow accumulation in this re- gion, contributing to the glacial shrinkage. If the reduction trend of the SCE in midlatitude Eurasia cannot be reversed, March precipitation may remain low. In addition, previous studies (Bollasina et al. 2013; Kajikawa et al. 2012; Liu et al. 2019; Xiang and Wang 2013; W. Zhang et al. 2017) have shown that May precipitation in the SETP experienced an in- crease due to the advanced onset of the South Asian summer monsoon. Therefore, there has been a distinct intraseasonal variation in spring precipitation in the SETP, which may lead to the disappearance of spring precipitation peak in this re- gion in the future. The declined far-field moisture contribution in March is probably attributed to the weakened subtropical westerlies induced by the reduced SCE in midlatitude Eurasia. SCE has suffered a remarkable reduction in midlatitude Eurasia in early spring, causing anomalous air warming through snow albedo-radiation feedback. According to the theory of thermal adaption, an anomalous high in the mid- to upper troposphere was formed. This anomalous high led to an anomalous easterly wind in its south through geostrophic adjustment. As a result, the subtropical westerlies were weakened, causing less moisture transport upstream. With the advance of spring (and thus increased solar radiation), the weakening belt of the subtropical westerlies shifted from south to north. In March, the weakening belt moved to the latitude of about 30°N, leading to a decadal decrease in precipitation on mountains’ windward slopes along the
Fic. 11. Distribution of the precipitation difference in March between 1999-2017 and 1979-98 obtained from ERAS and GPCP. Dots indicate the difference passing the significance test of p < 0.1. The decadal decrease in March precipitation in the SETP may have reduced springtime snow accumulation in this re- gion, contributing to the glacial shrinkage. If the reduction trend of the SCE in midlatitude Eurasia cannot be reversed, March precipitation may remain low. In addition, previous studies (Bollasina et al. 2013; Kajikawa et al. 2012; Liu et al. 2019; Xiang and Wang 2013; W. Zhang et al. 2017) have shown that May precipitation in the SETP experienced an in- crease due to the advanced onset of the South Asian summer monsoon. Therefore, there has been a distinct intraseasonal variation in spring precipitation in the SETP, which may lead to the disappearance of spring precipitation peak in this re- gion in the future. The declined far-field moisture contribution in March is probably attributed to the weakened subtropical westerlies induced by the reduced SCE in midlatitude Eurasia. SCE has suffered a remarkable reduction in midlatitude Eurasia in early spring, causing anomalous air warming through snow albedo-radiation feedback. According to the theory of thermal adaption, an anomalous high in the mid- to upper troposphere was formed. This anomalous high led to an anomalous easterly wind in its south through geostrophic adjustment. As a result, the subtropical westerlies were weakened, causing less moisture transport upstream. With the advance of spring (and thus increased solar radiation), the weakening belt of the subtropical westerlies shifted from south to north. In March, the weakening belt moved to the latitude of about 30°N, leading to a decadal decrease in precipitation on mountains’ windward slopes along the
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