Variation tendency and variability distribution of summer precipitation in China. The characteristic of precipitation variability is an important index that reflects the regional response of the QTP to global climate change. In the context of global warming, summer precipitation over the QTP is affected by SST increases and west winds36,37,27, monsoons38,39, and any structural changes in their circulation40,41,42,43. Meanwhile, the complicated megarelief and comprehensive land–air impact result in a nonuniformity of the spatial distribution of climate change. In this paper, monthly average precipitation statistics in 710 stations of China from 1991 to 2020 are used to calculate the summer precipitation linear trend in mainland China. The research results reveal that precipitation variability in the QTP areas demonstrates an opposite variation tendency in the south and north. At the same time, East China is featured with the variation tendency of increased precipitation in the north and decreased precipitation in the south, especially in western Jiangnan, southern China. (Fig. 1(a)). In summer, precipitation shows a reducing tendency in both the south and southeast of the QTP, but an increasing one in the north, forming a “dry south and wet north” pattern, which conforms with the conclusion when variations of the Tropical Rainfall Measuring Mission (TRMM) precipitation product after optimization are used to analyse the QTP’s precipitation variation tendency44. As shown in Fig. 1(a), from 1991 to 2020, precipitation in northern Xinjiang, central and eastern Inner Mongolia, southeastern Tibet and South China displays a reducing tendency while that in northern Tibet, Northwest China, and northern Northeast China in summer displays a notably increasing tendency. Based on the distribution characteristics of high-value areas of precipitation variability, the high-value areas of the QTP were divided into two parts in this paper, namely area A (northern Tibet) and area B (Qinghai and northern Sichuan). Notably, the two high-value areas of QTP summer precipitation variability (area A and area B) show salient variation scopes with an overall increasing tendency, obviously higher than that for mainland China (Figure S1), suggesting that the warming-wetting phenomenon in China mainly occurs in the northern part of the QTP, i.e., high-value areas of precipitation variability (area A and area B).
Correlation characteristics of summer precipitation in high variability areas of the QTP and SST changes in high-impact areas. On the temporal scale, the global average SST has increased in a nearly linear manner from the 1970s to the late 20th century, consistent with the increase of greenhouse gases emitted by human activities, such as carbon dioxide. During this time, the 1980s and 1990s were two periods when the SST increased fastest45. Taking the Pacific and the Indian Ocean as key research objects, the 1991–2020 global SST variabilities are analysed (Fig. 1(b)) in this paper, which show that four high-value areas of SST variability exist in the Pacific and the Indian Ocean, which are located in the western North Pacific (SST area 1), western Central Pacific (SST area 2), Southwest Pacific (SST area 3), and central Indian Ocean (SST area 4). Summer precipitation in the QTP’s key areas of warming-wetting shows a synchronous interannual increasing trend with the SSTs in the four high-value areas of SST variability. Their correlation coefficients reach 0.40, 0.49, 0.61, and 0.49 respectively (at 95% confidence) (Fig. 1(c)). Next, a correlation analysis was conducted on the summer precipitation of anomalous warm-wet key areas (area A and area B) on the QTP and the global SST in the corresponding period to identify the relevant high-value areas (Fig. 1(d)); the four key areas we identified agree with the spatial distribution of high-value areas of SST variability (Fig. 1(b)). Such analyses demonstrate a probable salient correlation between the significant areas of global SST increase and the warming-wetting phenomenon of the QTP, meaning the former are four SST high-impact areas for the warming-wetting on the QTP.
Characteristics of the synchronous variation trends between SST increase and sea-surface water vapor in high-impact areas in summer. The sea-surface temperature anomalies in the Pacific and the Indian Ocean have a significant impact on the water vapor budget of the QTP46,47,48,49. Multiscale modulations of monsoons and atmospheric circulations due to the sea–land thermal contrast in summer constitute the anomalous water vapor transport structure of the ocean water vapor sources, generating a climate teleconnection with the atmospheric water cycle over the QTP or the land. Therefore, SST increases in the high-impact areas of the Pacific and the Indian Ocean in summer might result in local water vapor anomalies, and inter-hemispheric teleconnection water vapor transport might be one of the vital factors that regulates precipitation on the QTP. In light of the above considerations, the global sea-surface specific humidity variability from 1991 to 2020 (Fig. 2(a)) was calculated. By comparing the four SST high-impact areas indicated in Fig. 1(d), namely, the western North Pacific (the correlated area 1), the western Central Pacific (the correlated area 2), the Southwest Pacific (the correlated area 3), and the central Indian Ocean (the correlated area 4), it is found that all of them show high sea-surface specific humidity variability. Moreover, the sea-surface specific humidity variabilities in the four high-impact areas all demonstrate synchronous upward interannual trends (Figures S2(a–d)). The correlation coefficients of the SST and the sea-surface specific humidity in the four high-impact areas are 0.90, 0.52, 0.85, and 0.77 respectively (at 99% confidence) (Figs. 2(b–e)). This result reveals that SST increases in the SST high-impact areas of the Pacific and the Indian Ocean in summer might lead to a tendency of anomalous high specific humidity in the local sea surface.
How does the anomalous water vapor flow pattern caused by the SST increase in the high-impact areas affect the warming-wetting of the Plateau? The QTP can capture anomalous warm and humid gas flows from the Indian Ocean, the South China Sea, the low-latitude western Pacific and other areas in the south through the “hollow heat island” effect and its continuous heat source22. The water vapor over the QTP has experienced dramatic changes under the effects of global warming, thus affecting the precipitation, lake water storage, and water vapor budget of the Plateau. According to the study by Zhang et al.27, increased water vapor in the west of the QTP in recent years has been the culprit for the precipitation growth in the central and western QTP. The stronger water vapor transport from the Indian Ocean to the Plateau boosts the water cycle on the QTP, which is the main process of water vapor for the “wetting” of the QTP. In this paper, the correlation vector method for water vapor transport is used as a tracer for the water vapor source to reveal any anomalous changes in the water vapor transport path due to SST anomalies in the high-impact areas. The aim here is to explore the regulating effect of the high-impact areas of SST increase on the water vapor transport in the cloud precipitation process on the QTP and further ascertain the structural characteristics of the water vapor transport channel correlated to the SST anomalies of the Pacific and Indian Ocean. According to Fig. 3(a–d), the SST in area 1 and the whole-layer water vapor flux of East Asia over the same period demonstrate an anticyclonic circulation pattern, where a correlated water vapor flow A comes from the Pacific Ocean to the west and then turns northward to a high-value area of precipitation variability in North China and the northern QTP (Fig. 3(a)). The SST in area 2 and the whole-layer water vapor flux of the East Asia over the same period demonstrate an anticyclonic circulation pattern in the Pacific, and the related water vapor flow B in the southwest is transported northward from the equatorial Pacific and turns westward to the high-value area of precipitation variability in the northern QTP (Fig. 3(b)). The SST in area 3 and the whole-layer water vapor flux of East Asia over the same period also demonstrate an anticyclonic circulation pattern in the Pacific Ocean, and the related water vapor flow C in the south crosses the equator, turns northward, and is transported to a high-value area of precipitation variability in the northern QTP (Fig. 3(c)). Again, the SST in area 4 and the water vapor flux of East Asia over the same period demonstrate an anticyclone circulation pattern in the central Indian Ocean, which is just on the southwest edge of the Pacific anticyclonic westward-extending circulation. After the related water vapor flow D crosses the equator, it is transported from the western QTP to a high-value area of precipitation variability in the northern Plateau (Fig. 3(d)).
Relative contribution of the summertime SST increase in the high-impact areas to the warming-wetting of the QTP and an image of its comprehensive effect. Based on the previous discussions, the western North Pacific, the western Central Pacific, the Southwest Pacific, and the central Indian Ocean were identified to be the key areas affecting the summer warming-wetting of the northern QTP in China. Under the combined effects of the SST high-impact areas, it is worth noting which areas have the most significant impact on the warming-wetting of the Plateau and what the contribution of each SST high-impact area to the warming-wetting is. Therefore, the relative contribution of the SST in these high-impact areas in summer to the warming-wetting of the QTP during the same period is further quantified in this section.
With the standardized SST in the four high-impact areas as the independent variable and the standardized precipitation in the warming-wetting sensitive areas of the QTP as the dependent variable, a standardized multiple linear regression equation was established and then the standardized regression coefficients can directly explain the share of contribution of the SST in the high-impact areas to the warming-wetting of the QTP under their combined effects. It can be learned from the relative contribution rates of the SST that the Southwest Pacific (SST area 3) is the region with the most significant influence on the “warming and wetting” of the QTP, with a relative contribution rate of 51%, followed by the central Indian Ocean (SST area 4) with a relative contribution rate of 24%. Both the western North Pacific (SST area 1) and the western Central Pacific (SST area 2) have less influence, with relative contribution rates of merely 12% and 13%, respectively. In summary, the trans-hemispheric SST increase with energy and water vapor transport can be determined to be a crucial and nonnegligible factor affecting the warming-wetting of the QTP.
According to the Fifth Assessment Report by the IPCC, the global oceans are experiencing remarkable warming, with the fastest SST increase in the near-surface layer yet recorded50. Although global ocean warming is largely certain, there are great regional differences in the rate and magnitude of SST increase in time and space51. By comparing the anomaly field differences of the whole-layer water vapor fluxes from 1961 to 1990 and from 1991 to 2020, the differences in the trans-hemispheric ocean energy and water vapor transport circulation on interdecadal temporal scales under the influence of global warming are discussed in this paper to identify the large-scale circulation background for the formation of the warming-wetting of the QTP. It can be observed from the comparison between Figures 3(e) and 3(f) that the circulation of the water vapor transport on the QTP affected by the anomaly field of the whole-layer water vapor fluxes from 1991 to 2020 was opposite to that from 1961 to 1990. That is, from 1961 to 1990, the water vapor transport fluxes from the Indian Ocean to the QTP and its East Asian region was shown as a north–south axial cyclonic circulation, with the southerly water vapor flow on its eastern side transported to the southern Plateau and the eastern region of China. In contrast, from 1991 to 2020, the water vapor transport fluxes of the Indian Ocean to the QTP and the East Asian region was exhibited as a north–south axial anticyclonic circulation. In this way, the southerly water vapor flow on its western side was transported to the western and central Plateau, while the northerly water vapor flow on its eastern side transported the Northwest Pacific water vapor flow from the east to the northern Plateau. Moreover, from 1961 to 1990, the anomaly field of the water vapor transport fluxes in the western Pacific was opposite to that from 1991 to 2020. Specifically, from 1991 to 2020, the anticyclonic circulation extended to the southern hemisphere, the trans-equatorial easterly water vapor flow on its southern edge passed through the Indian Ocean to the easterly flow of the southern edge of the anticyclonic circulation of the Plateau, and then was transported to the western and central part of the Plateau.
It reveals that the trans-equatorial water vapor transport in the Southern Hemisphere was significantly enhanced from 1991 to 2020, further indicating that the SST increase in the Southwest Pacific and the central Indian Ocean is the key reason for the warming-wetting of the QTP. In addition, the characteristics of the 500 hPa circulation situation were observed to be similar to those of the anomaly field of the whole-layer water vapor transport fluxes (Figures S3(a) and 3(b)). The anticyclonic water vapor transport circulation in the Pacific and the Indian Ocean from 1991 to 2020 was favorable for the transport of warm-wet ocean water vapor to the western and central Plateau. Thereby, an image of the comprehensive effect of the water vapor transport structures in the four high-impact areas on the warming-wetting of the QTP is proposed, with the anomaly field of water vapor transport from 1991 to 2020 as the background (Fig. 3(g)).