Interannual variation of South Asian high and its relationship with summer monsoon precipitation in India and East Asia

Time:2022-5-10

Summary:

Diagnostic analysis shows that the northwest southeast movement of SAH is the main interannual variation feature, which is closely related to the precipitation in India and East Asia. The southeast movement of SAH is closely related to the decrease of summer monsoon precipitation in India and the increase of summer monsoon precipitation in East Asia in the Yangtze River Basin (YRV). AGCM is used to study the influence of latent heat anomaly related to Asian summer monsoon precipitation on SAH. The negative latent heat anomaly in the northern part of the Indian peninsula is related to the weakening of the Indian summer monsoon. At the same time, the negative latent heat anomaly excited the abnormal cyclone over the Qinghai Tibet Plateau (northwest of the negative latent heat anomaly) and the abnormal anticyclone in the upper troposphere of eastern China (northeast of the negative latent heat anomaly), which also caused the East-West transfer of SAH and the increase of YRV rainfall. The release of positive latent heat associated with YRV precipitation anomaly in eastern China stimulated a southward anticyclone, produced feedback effect on SAH, and led to the southeast northwest movement of SAH.

introduction:

SAH is an important upper system of the Asian summer monsoon. As early as the 1950s, when the strongest and persistent anticyclone was found in the northern hemisphere in summer, its east-west oscillation had been revealed. The zonal variation of SAH can be characterized by the East-West movement of weather time scale and the bimodal of climate state (Tibet mode and Iran mode). This zonal variation is closely related to the changes of Indian summer monsoon (ISM) and summer rainfall in China.

On the interannual time scale, in addition to the impact of ism on China’s summer precipitation through low-level water vapor transport, the variability of ISM itself also changes the latent heat release of ISM, which leads to the zonal movement of SAH and further affects the Asian monsoon rainfall.

In addition to the zonal movement, SAH also changes along the meridional direction. The seasonal northward movement of SAH is very consistent with the northward movement of rain belt over China. On the interannual time scale, SAH located in the south is conducive to the maintenance of YRV rain belt, while located in the north is conducive to more precipitation in the north. Previous studies have revealed the close relationship between the meridional variation of SAH and the three pole model of summer precipitation anomaly in China on the interannual time scale. Affected by the continuous warming of sea surface temperature in the Indian Ocean, SAH shows an obvious long-term southward movement trend, which is consistent with the increase of precipitation in the Yangtze River Delta and the decrease of precipitation in northern China.

In the above research centers, the latitudinal and meridional changes of SAH are always discussed separately, but the accompanying changes between them are rarely mentioned. In fact, both zonal and meridional changes are related to the three-level model of China’s Precipitation Anomaly on the interannual time scale. However, we still do not know the relationship between the zonal and meridional changes of SAH and the interannual time scale, and the joint influence of the zonal and meridional changes on EASM rainfall. Condensation heating related to monsoon rainfall plays an important role in the formation and change of subtropical high. Strong ism leads to the release of more condensation latent heat, which will move SAH westward and reduce YRV rainfall; Weak ism leads to less latent heat release, which leads to the eastward movement of SAH and the increase of YRV rainfall. So, does the latent heat release on YRV also affect SAH? This study discusses the meridional and zonal changes of SAH and the double interaction between SAH and YRV summer precipitation.

The second paragraph briefly introduces the data and methods used in the study; The third paragraph discusses the main characteristics of SAH on interannual time scale; In the fourth section, the relationship between SAH and Asian summer monsoon precipitation is studied; In the fifth section, a general circulation anomaly model (AGCM) is used to discuss the feedback effect of China’s summer precipitation on SAH; The sixth paragraph is the conclusion and discussion.

Conclusion:

The interannual variation of SAH is mainly controlled by its southeast northwest movement, and the eastward movement is accompanied by the southward movement.

A SAH index is defined to predict the change of SAH and to diagnose the relationship between SAH and Asian summer monsoon precipitation. The results showed that there was a significant negative correlation between sah1 and air1, and a significant positive correlation between sah1 and pc1cn. When SAH is located in the southeast, the rainfall in ISM is less, but there is more rainfall in YRV of EASM. Ism precipitation and summer precipitation in China are more closely related to the southeast northwest movement of SAH than to the zonal or meridional change of SAH, which indicates that the southeast northwest change of SAH can better reflect the relationship among SAH, ism and EASM precipitation. The precipitation anomaly intensity related to the southeast northwest movement of SAH in the upper layer of YRV is similar to that in the northern part of the Indian peninsula. AGCM is used to study the feedback effect of latent heat release related to YRV precipitation anomaly on SAH. The results of idealized numerical experiments show that the latent heat generated by ISM precipitation anomaly leads to the East-West movement of SAH, which is conducive to the increase of precipitation on YRV. The latent heat generated by YRV precipitation anomaly will stimulate the southward anticyclone in the upper troposphere in eastern China, lead to the movement of SAH to the southeast northwest, and play an important role in the formation of abnormal anticyclone in Northeast Asia. The latent heat from YRV precipitation anomaly is not only conducive to the movement of SAH, but also to the global teleconnection (CGT) model in the upper troposphere of East Asia.

This paper focuses on the relationship between SAH variability and ism and EASM precipitation. Previous studies have shown that ism, EASM and SAH are affected by external forcing. For example, land surface processes and ocean thermal conditions significantly affect ism and EASM, Indian Ocean SST affects SAH, and the relationship between ism and EASM is regulated by ENSO. The relationship between SAH variability and ISM HESM EASM precipitation revealed in this study is an important issue for future research.

The second paragraph:

ERA-40 monthly data, GPCP monthly precipitation data set (0.5 ° * 0.5 °), monthly precipitation data of the longest instrumental precipitation series in India provided by Indian Institute of tropical Meteorological Sciences, and monthly precipitation data of 160 stations in China.

1958-2002 “JJA”

Airi (All India precipitation index): used to measure the intensity of ISM.

It is considered that 100 ° e is the boundary of the East Asian monsoon region, and only the precipitation to the east of 100 ° E in China is recognized as EASM precipitation.

AGCM is used to study the response of high-level circulation anomaly to heat source anomaly (the method is to apply 1 day / 1 strong power and thermal damping in the momentum and temperature equations outside 40 ° N and 40 ° s to weaken the influence of high latitude region. The research area in this paper is concentrated in the middle latitude region, so as to expand the latitude to 50 ° s and 50 ° n) σ Coordinate system, dividing the atmosphere into five vertical layers, Rayleigh friction in momentum equation, damping rate from 1 day / 1( σ= 0.9) linear attenuation to 0.1 days / 1( σ= 0.7)。

Previous scholars have applied abnormal AGCM to examine the response of the atmosphere over the Kuroshio extension area and the northern Indian subcontinent to heating.

Paragraph 3: changes in SAH and definition of SAH index

In summer in the northern hemisphere, the oval SAH is located in the subtropical South Asian continent. (Fig.1)

Interannual variation of South Asian high and its relationship with summer monsoon precipitation in India and East Asia

200hPa climatic height field, 1958-2002 JJA red dotted line is zonal wind zero line, which can also refer to SAH ridge line.

On the interannual time scale, its east-west movement can be expressed by index

The East-West movement index iew is 22.5-32.5 ° n, and the potential height of 85-105 ° e minus the potential height of 22.5-32.5 ° N and 55-75 ° E;

The North-South movement index INS is 27.5-32.5 ° n, the potential height of 50-100 ° e minus the potential height of 22.5-27.5 ° n, 50-100 ° E.

After removing the decadal variation, it is observed that there is a significant negative correlation between the two indexes. The correlation between the two indexes is -0.44 (through the 99% significance test, Fig. 2), which shows that there is a good correlation between the zonal and meridional changes of SAH on the interannual time scale. The eastward movement of SAH is often accompanied by southward movement, which means that SAH often moves in the southeast northwest direction on the interannual time scale.

Interannual variation of South Asian high and its relationship with summer monsoon precipitation in India and East Asia

Fig.2 time series of iew index (solid line) and INS index (dotted line)

In addition, the latitudinal and meridional displacements are closely related to the tropical cyclone pattern in eastern China. Fig. 3 shows the regression relationship between precipitation anomalies in eastern China and iew and INS respectively. SAH located in the East and South will lead to more precipitation in the Yangtze River Basin and less precipitation in southern and Northern China. SAH in the West and north is opposite to the three polar precipitation. Therefore, the changes of SAH latitude and longitude may have a synergistic impact on China’s precipitation.

Interannual variation of South Asian high and its relationship with summer monsoon precipitation in India and East Asia

Fig. 3 for JJA precipitation anomaly regressed by iew and INS index, the area with more than 95% confidence is marked with isoline.

Further prove the existence of the southeast northwest variation of SAH. According to the 200hPa potential height of summer climate state (Fig. 1), SAH is divided into four regions according to its ridge line (about 27.5 ° n), and then the standardized time series with an average of 200hPa in these four regions are calculated respectively, and the partial correlation coefficients between the four time series in each region are calculated by partial correlation analysis. In partial correlation analysis, in partial correlation analysis, we eliminate the synchronous change of height in all four regions by eliminating the influence of SAH coverage change. SAH coverage is defined by the number of grids covered by 12500 GPM contour lines. As shown in Table 1, it is obvious that the correlation between SAH changes in Southeast (SE) and Northwest (NW) is the most significant, with a correlation coefficient of -0.41 (passing the 99% significance test), while the correlation coefficients between other directions are not significant. Therefore, the southeast northwest variation is the most significant feature of the interannual variation of SAH.

Interannual variation of South Asian high and its relationship with summer monsoon precipitation in India and East Asia

Table 1 partial correlation coefficient between four regions of SAH activity.

After de trending the data and eliminating the underestimation of SAH ten-year change by ERA-40 (minus the linear regression of the average 200hPa of 15-40 ° N and 25-130 ° e, and eliminating the linear influence of SAH intensity), the main characteristics of SAH southeast northwest change can also be proved by decomposing the 200hPa height field by EOF.

As shown in EOF1 (Fig. 4), the contour zero line from southwest to northeast passes through the SAH area and divides the SAH into southeast to northwest. EOF1 mode shows that the change of SAH in the southeast is opposite to that in the northwest, indicating that the southeast northwest change of SAH is the main feature on the interannual time scale.

Interannual variation of South Asian high and its relationship with summer monsoon precipitation in India and East Asia

Fig.4 the first mode obtained by EOF of 200hPa potential height field, and the red line indicates the SAH region.

The SE-NW variation of SAH is briefly quantified. The SAH index (Sahi) is obtained by taking the potential height of 20-27.5 ° n, 85-115 ° E region (SE region in Fig1) minus the potential height of 27.5-35 ° n, 50-80 ° e (NW region in Fig1). Fig. 5 shows the time series of Sahi. The high Sahi represents the movement of SAH to the southeast. The interannual variation of Sahi can be clearly seen. It has a good correlation with PC1 of EOF1 at 200hPa potential height, and the correlation coefficient can reach 0.75 (through 99% significance level). The correlation coefficients of Sahi, iew and ins are 0.72 and -0.78 respectively, which means that Sahi can well describe the zonal and meridional cooperative motion of SAH.

Interannual variation of South Asian high and its relationship with summer monsoon precipitation in India and East Asia

Fig. 5 time series of Sahi (solid line) and pc1sah (dotted line).