Summer Rainfall Variability in Low-Latitude Highlands of China and Subtropical Indian Ocean Dipole

Abstract

Based on reanalysis and observational datasets, this study proposes a reasonable mechanism for summer rainfall variations over the low-latitude highlands (LLH) of China, in which a subtropical Indian Ocean dipole (SIOD)-like pattern is the key external thermal forcing. In summers with a positive SIOD-like pattern, sea surface temperature (SST) anomalies may lead to lower-tropospheric divergence over the tropical Indian Ocean and convergence over the subtropical southwestern Indian Ocean and Arabian Sea. The convergence over the Arabian Sea can induce easterly anomalies of the divergent wind component off the eastern coast of the Bay of Bengal (BOB), while the divergence over the tropical Indian Ocean can change the interhemispheric vertical circulation and produce a descending motion over the same area and cyclonic anomalies in the rotational wind component over the Indian peninsula. The combined effect of the divergent and rotational wind anomalies and enhanced interhemispheric vertical circulation facilitates easterly anomalies and weakens climatological water vapor flux to the northern BOB. Therefore, anomalous water vapor divergence and less precipitation are observed over the LLH. In summers with a negative SIOD-like pattern, the situation is approximately the same but with opposite polarity and a weaker role of the divergent wind component. Further analyses indicate that the summertime SIOD-like pattern can be traced to preceding seasons, especially in positive SIOD-like years. The SST-wind-evaporation feedback mechanism could account for maintenance of the SIOD-like pattern. These results provide efficient prediction potential for summer rainfall variations over the LLH.

Remarkable Impacts of Indian Ocean Sea Surface Temperature on Interdecadal Variability of Summer Rainfall in Southwestern China

“…One branch of the corresponding upper-level outflows converges over the Indo-China peninsula, which is favorable for anomalous descending motion and lower-level northeasterly anomalies, both of which favor decreased rainfall in SWC. This is consistent with the results revealed by Cao et al [2], who showed that the summer rainfall in SWC is negatively connected with the subtropical Indian Ocean dipole on interannual timescale. The positive subtropical Indian Ocean dipole facilitated easterly anomalies off the eastern coast of BOB and weakened climatological water vapor flux to the northern BOB.…”

“…The climatological summer rainfall in SWC gradually decreases from southeast to northwest, with values ranging between 5 mm day −1 and 8 mm day −1 (Figure 1). Unlike flatlands, agriculture and ecosystems in this region are quite vulnerable to summer rainfall variability in low-latitude highlands [2,3].…”

“…Record-breaking droughts hit SWC in the summer of 2006 [6], the autumn of 2009 [7], and the late summer of 2011 [8], which have caused concern among the Chinese government, the public, and the academic sector. Most previous studies focused on the characteristics and the physical causes of SWCSR on interannual timescale [2,[7][8][9][10]. Therefore, understanding the primary features of the interdecadal variability of SWCSR and the associated mechanisms is both urgent and valuable.…”

“…This pattern of circulation anomalies resulted in additional moisture transport into Southeastern China and subsequent rainfall increase. Cao et al [2] revealed that the subtropical Indian Ocean dipole was the main driver of the interannual variability of SWCSR. To date, the impacts of the IO on the interdecadal variability of SECSR have been extensively studied, whereas the impacts of the IO on the interdecadal variability of SWCSR are less understood.…”

Interannual rainfall variability over China in the MetUM GA6 and GC2 configurations

“…While SST variability in other ocean basins has also been connected to IAV in China (e.g. Qiu et al, 2014;Cao et al, 2014), Stephan et al (2017a) did not find an influence of other basins on their EOT patterns. Therefore, we here focus on the teleconnection to ENSO.…”