Contributions of internal variability and external forcing to winter-season snow cover over Tibetan Plateau

Abstract:

This study investigates the controlling factors of the interannual variability of Tibetan Plateau snow cover (TPSC) in winter. Since snow observation in Tibetan Plateau is limited in space and time, a high-resolution multi-satellite data for TPSC were analyzed during 1982-2016. In addition, a large ensemble AGCM experiment, called d4PDF, driven using observed SST were analyzed from 1951-2010 to compare the effects of internal variability and external forcings including the change in greenhouse gases (GHGs) concentration on TPSC variation. In this study TPSC fraction (hereafter, TPSCF) is defined as the percentage of the snow-covered area over the Tibetan Plateau (higher than 3000 m ASL). For both observation and the model high and low TPSCF years determined by the standardized January-March TPSCF were analyzed. The range of interannual TPSCF variation (i.e., TPSCF difference between high and low TPSCF years) is about 10% in both observation and the model, suggesting the AGCM well reproduced the TPSCF variability in the interannual timescale.

We found that high TPSCF is linked to a positive-AO-like pattern. The interannual variation of the observed AO index and TPSCF are significantly correlated. In d4PDF high TPSCF more likely appears with a higher (positive) AO index and vice versa. In high TPSCF years, the subtropical jet is strengthened, which significantly enhances zonal water vapor flux reaching the plateau supporting more precipitation. Another interesting result is that the interannual variation of the ensemble mean AO index in d4PDF is negatively correlated with TPSCF. We speculate that ENSO regulates both AO and TPSCF as an external forcing. Although internal variability is dominant, the likelihood of negative-AO-like pattern increases in El Niño years in both observation and AGCM, which slightly enhanced the water vapor transport to TP.