Distinguishing interannual variations and possible impacted factors for the northern and southern mode of South Asia High

Given the large meridional coverage of South Asian High (SAH), the interannual variabilities of SAH and the corresponding impacted factors may differ between mid- to high-latitude and low-latitude components of SAH. This study distinguishes the interannual variations of the mid- to high-latitude and low-latitude components of SAH to obtain further understanding of SAH variations. By means of composite analysis, this study investigates the characteristics of the northern and southern mode of SAH variation (N-SAH, S-SAH). When the SAH is regarded as an entirety (E-SAH) based on numerous previous studies, it strengthens distinctly and expands toward both meridional and zonal directions in strong E-SAH comparing to weak E-SAH. The geopotential height (HGT) and tropospheric temperature (TT) anomalies related to the E-SAH are concurrently observed over the low-latitude and mid- to high-latitude regions. In contrast, when N-SAH and S-SAH are defined in this study, the SAH is distinctly strengthened only over its northern (southern) side and expands northward (southward) in strong N-SAH (S-SAH) comparing to the weak group. The HGT and TT anomalies related to the N-SAH (S-SAH) cases are limited over the mid- to high-latitude (low-latitude) regions. Possible impacted factors related to E-SAH, N-SAH, and S-SAH are further analyzed. The N-SAH is closely associated with precipitation anomalies over Indian Peninsula, meridional circulation over Tibet Plateau (TP), land surface temperature (LST) anomalies over Indian Peninsula and northern TP, meridional LST gradient between Indian Peninsula and northern TP, and circumglobal teleconnection (CGT). For instance, positive rainfall anomalies together with obviously ascending motion over Indian Peninsula contributes to a decreased LST anomalies via cloud-radiation feedback here, which forms a north-to-south LST gradient. The LST gradient will reduce the climatological south-to-north LST gradient here, which in turn contributes to a northward meridional circulation over northern TP, and finally contribute to a strong N-SAH existing over north of 25°N. Furthermore, the positive rainfall anomalies over Indian Peninsula tends to inspire a strong CGT over mid-high latitude through atmospheric response to condensation latent heating source, which in turn contributes to a strong N-SAH. In contrast, the meridional circulation associated with S-SAH is exhibited by southward shifting over southern TP (south of 30°N) due to atmospheric response to heating source generated by significant SST anomalies over Indian Ocean. Obviously warm center is observed in low-upper troposphere from tropical region to southern TP, which finally result in a strong S-SAH, accompanied by shifting southward. However, variation of E-SAH is a result of the combined action of rainfall anomalies over Indian Peninsula and SST anomalies over Indian Ocean. These differences indicate that it is necessary and of scientific significance to distinguish the northern and southern mode of the SAH.