Changes in the Impact of the Autumn Tibetan Plateau Snow Cover on the Winter Temperature Over North America in the mid-1990s

摘要

This study investigates the changes in the impact of the interannual variation of autumn snow cover over the eastern Tibetan Plateau (TP; ASCETP) on winter surface air temperature (SAT) over North America (NA) in the mid-1990s using both observational data and a linear baroclinic numerical model. The index of ASCETP shows an abrupt change with a predominant negative phase before and a predominant positive phase after 1994; thus, the analysis is performed for two subperiods: 1979-1994 (P1) and 1995-2014 (P2). An ASCETP-related energy budget analysis shows that the ASCETP variation has a more pronounced cooling effect on the above atmosphere in P1 than in P2. The snow-related anomalous negative geopotential height above the TP is more pronounced and extends farther northeastward in P1 than in P2. Further analysis shows that the relationship between the ASCETP and NA winter SAT is not steady. During P1, associated with an anomalous positive ASCETP, negative geopotential height anomalies emerge over the northeastern TP. These negative anomalies impose an anomalous vorticity perturbation near the East Asian westerly jet core that propagates eastward, forming a wave train-like pattern. It crosses the North Pacific Ocean and reaches the NA region, leading to positive and negative winter SAT anomalies over western and eastern NA, respectively. In contrast, during P2, the impact of anomalous ASCETP on the variation in the winter SAT over NA is weak, probably due to the relatively weak local cooling effect of the ASCETP and the weak ASCETP-related vorticity forcing around the East Asian westerly jet.