Observed Linkage between Tibetan Plateau Soil Moisture and South Asian Summer Precipitation and the Possible Mechanism

Abstract

Understanding the Tibetan plateau (TP) thermal processes is of utmost significance in changing climate. This study investigates the effect of soil moisture in changing the TP thermal profile and consequently summer precipitation in South Asia (SA). Soil moisture from Special Sensor Microwave Imager (SSMI) developed from the F08, F11, and F13 fundamental climate data record and atmospheric reanalysis from ERA-Interim, MERRA-2, and NCEP/CFSR during 1988-2008 are used. A generalized linear method that assesses the reciprocal forcing between two connected fields named Coupled Manifold Technique (CMT) is applied to TP soil moisture and SA summer precipitation. It is revealed that interannual variations of SA precipitation are significantly (confidence level= 99%) impacted by TP soil moisture and explained ratio of variance in SA is 0.3-0.4. Composite analysis indicates that SA summer precipitation has positive anomalies in response to dry TP soil moisture in the previous spring and vice versa. For understanding the possible mechanism, thermal processes, relative humidity, wind components, and moisture flux anomalies were calculated for dry/wet TP soil moisture and summer precipitation. The results suggested that TP soil moisture is likely to regulate near-surface energy balance and diabatic heating profile over TP. As a result, the surrounding lower level westerlies/easterlies (850 hPa) converge/diverge, which are associated with divergence/convergence at the upper troposphere (200 hPa). The westerlies/easterlies are usually moisture-rich/deficient and thus cause more/less precipitation in western/eastern SA. It is thus suggested that the spring soil moisture may affect the thermal profile of TP, affecting the summer precipitation in SA as a consequence.

The Empirical Influences of Tibetan Plateau Soil Moisture on South Asian Monsoon Onset

“…In conclusion, the TP soil moisture can in uence the ascent and SAH intensity and modulation in zonal and meridional domains that exhibit teleconnections with downstream zonal and meridional wind and modulate the monsoon onset. The ndings agree with previous studies suggesting that TP thermal pro le has a strong in uence on the Asian monsoon by changing the circulation patterns and thus precipitation magnitude (Wu et al 2012;Rajagopalan and Molnar 2013;Ullah et al 2020). From the composite analysis, it can be deduced that spring soil moisture anomalies over TP in uence the near-surface energy balance, which can further affect TP energy uxes, the vertical pro le of diabatic heating aloft (Wu et al 2012;Wang et al 2014).…”

“…The monsoon onset across the region has been linked with several factors, including the sea surface temperature, westerlies progression, convection, and local continentalscale tipping elements. Less attention has been paid to the in uence of the TP soil moisture-induced thermal pro le in uence on the monsoon onset across time (Zhang et al 2002;Stolbova et al 2016;Ullah et al 2020Ullah et al , 2021. The following section shows a diagnostic relationship of the soil moisture pro le over the TP and its thermodynamics role in the SA monsoon onset.…”

“…4. The average soil moisture from April to June (AMJ) indicates springtime soil moisture over the TP (Ullah et al 2020). It appears that both XV (Fig.…”

“…Over the TP, both sensible and latent heat uxes are critically linked with the soil moisture's freeze-thaw stage during the transitional spring season (Cui and Wang 2009). They hence can actively in uence the vertical pro le of the diabatic heating over the plateau and produce divergent motion with lower/upper level cyclonic/anticyclonic pattern (Ullah et al 2020).…”

“…Soil moisture anomalies can persist for months, affecting the albedo and thermal uxes (Seneviratne et al 2010), and thus they can in uence the coupled interactions between the land, atmosphere, and oceans (Koster et al 2016). Such soil moisture effects have been studied for the African monsoon (Berg et al 2017), Indian summer monsoon (Asharaf et al 2012;Ullah et al 2020), and East Asian monsoon systems (Bao et al 2010;Zuo and Zhang 2016).…”