38CVC The Decadal Change of Summer Low Cloud over the Tibetan Plateau Lake Group and Associated Mechanisms

In recent years, the long-term change characteristics of the Tibetan Plateau Lake Group Low Cloud (TPLGLC) and its change mechanisms have not been fully investigated. In this study, we investigate the causes of the decadal variability of TPLGLC in summer by local and circulation factors and reveal the roles of thermal and dynamical processes on its decadal variability. Seven stations in the Tibetan Plateau Lakes Group region are selected and analyzed, and it is found that in 1979-2013, July and August have the highest low cloud in the whole year, and the decadal abrupt shift of low cloud occurred in 1998 in the summer. Reanalysis data showed that during 1979-2020, the summer TPLGLC had a decadal abrupt shift in 1995 with an increasing trend, and the summer precipitation over the lake group had a decadal abrupt shift in 1994. Positive anomalies of summer precipitation, evaporation, whole-layer specific humidity, and negative anomalies of clear-sky solar radiation in the lake group region correspond well with positive anomalies of low cloud with approximate decadal variability. The decadal increase in evaporation and whole-layer specific humidity leads to a decadal increase in water vapor in the lake group region. The decadal decrease in clear-sky solar radiation weakens upper-level water vapor evaporation, and water vapor is more likely to condense and form more low clouds. The anomalous convergence of water vapor flux over the lake group at 400hpa makes more increased water vapor converge over it. The significant increase in the total water vapor budget of the lake group indicates that the lake group becomes wetter on the decadal scale. The results of water vapor budget in different directions of the lake group show that the southern boundary has inflow of water vapor into the lake group from 1979 to 2020, and the decadal increase of water vapor is the most significant. The northern boundary has outflow of water vapor, and the outflow of water vapor increased after 1995. There is water vapor flowing into the lake group area on the west side, with a non-significant increase inflow after 1995. There is outflow of water vapor to the east of the lake group area as well as to the north, but much more than to the north, and the outflow of water vapor decreases after 1995. On the one hand, the circulation system on the south side of the lake group is analyzed because the decadal increase of water vapor on the south boundary is the most significant. The north-south oriented large-scale eddy modes in the southwestern part of the Tibetan Plateau resulted in an anomalous increase in the horizontal transport of water vapor to the lake group. The anomalous southerly and upward vertical winds on the southern side of the lake group favorably transport water vapor to the lake group region through the enhanced slope-climbing effect. On the other hand, the highest correlation coefficient between water vapor budget and TPLGLC was observed at the eastern boundary of the lake group. It was found that the decadal warming of the North Atlantic and Mediterranean-Black Sea SSTs inspired mid-latitude anomalous wave trains to transmit the Rossby waves eastward. The North Atlantic, the Black Sea and regions north of it, and the vicinity of Lake Baikal have wave activity flux propagating upward from the lower level to 200hpa, and there are significant anomalous wave activity flux propagating eastward from these three regions throughout the whole-layer. The anomalous propagation of wave activity flux maintains significant anticyclonic circulation anomalies over the Lake Baikal region, inhibiting water vapor outflow from the eastern boundary of the lake group region, which lead to a decadal increase of TPLGLC.