Yongli He, Jianping Huang, Dongdong Li, Yongkun Xie, Guolong Zhang, Yulei Qi, Shanshan Wang, Sonja Juliana Totz

• The influence of winter and summer land-sea surface thermal contrast on blocking for 1948-2013 is investigated using observations and the coupled model intercomparison project outputs. The land-sea index (LSI) is defined to measure the changes of zonal asymmetric thermal forcing under global warming. The summer LSI shows a slower increasing trend than winter during this period. For the positive of summer LSI, the EP flux convergence induced by the land-sea thermal forcing in the high latitude becomes weaker than normal, which induces positive anomaly of zonal-mean westerly and double-jet structure. Based on the quasiresonance amplification mechanism, the narrow and reduced westerly tunnel between two jet centers provides a favor environment for more frequent blocking. Composite analysis demonstrates that summer blocking shows an increasing trend of event numbers and a decreasing trend of durations. The numbers of the short-lived blocking persisting for 5-9 days significantly increases and the numbers of the long-lived blockingThe influence of winter and summer land-sea surface thermal contrast on blocking for 1948-2013 is investigated using observations and the coupled model intercomparison project outputs. The land-sea index (LSI) is defined to measure the changes of zonal asymmetric thermal forcing under global warming. The summer LSI shows a slower increasing trend than winter during this period. For the positive of summer LSI, the EP flux convergence induced by the land-sea thermal forcing in the high latitude becomes weaker than normal, which induces positive anomaly of zonal-mean westerly and double-jet structure. Based on the quasiresonance amplification mechanism, the narrow and reduced westerly tunnel between two jet centers provides a favor environment for more frequent blocking. Composite analysis demonstrates that summer blocking shows an increasing trend of event numbers and a decreasing trend of durations. The numbers of the short-lived blocking persisting for 5-9 days significantly increases and the numbers of the long-lived blocking persisting for longer than 10 days has a weak increase than that in negative phase of summer LSI. The increasing transient wave activities induced by summer LSI is responsible for the decreasing duration of blockings. The increasing blocking due to summer LSI can further strengthen the continent warming and increase the summer LSI, which forms a positive feedback. The opposite dynamical effect of LSI on summer and winter blocking are discussed and found that the LSI-blocking negative feedback partially reduces the influence of the above positive feedback and induce the weak summer warming rate.…