Circulation anomalies and their impacts on autumn temperature variations over China in association with different phase combinations of atmospheric mass migration between lands and oceans and inter-hemispheric oscillations

Using the NECP/NCAR monthly reanalysis for the period of 1961-2010, circulation anomalies and their possible influences on variations of surface air temperature in China during the boreal fall in association with different phase combinations of migration of atmospheric mass between lands and oceans (MAMLO) and interhemispheric oscillations (IHO) have been investigated. Two indices including MAMLO and IHO are defined to describe the sea-saw-like surface air pressure oscillations between Eurasia and North Pacific and between the southern and northern hemispheres. It is found that the MAMLO and IHO are almost independent of each other in a statistical sense. Four types of phase combination of MAMLO and IHO have been configured. These four types are respectively defined as type-Ⅰ for both MAMLO and IHO in positive phases, type-Ⅱ for both in negative, type-Ⅲ for the MAMLO in positive and IHO in negative, and type-Ⅳ for the MAMLO in negative and IHO in positive. The mean climatological surface air pressure distribution in the boreal fall can be affected by these four types of anomalous surface air pressure patterns, and hence the surface air temperature in China and even in Eurasia-North Pacific can be modulated significantly. When type-Ⅰ cases occur, IHO tends to weaken the low pressure in North Pacific and intensify the high pressure in Eurasia by piling more than normal atmospheric mass in these two places, inducing an east-warm-west-cold pattern in North China when the MAMLO is in its positive phase. In type-Ⅱ cases, IHO weakens the positive air mass anomaly in Eurasia and strengthens the negative air mass anomaly in North Pacific, which leads to a consistent warm pattern in China. In Type-Ⅲ cases, an east-cold-west-warm pattern contrary to that for type-Ⅰ forms in North China. In type-Ⅳ cases, the spatial pattern of temperature is similar to that in type-Ⅱ cases. The above results indicate that MAMLO plays a primary role in the distribution of temperature anomaly in Eurasia during the boreal fall, whereas IHO imposes a disturbing effect superimposed on the influence of MAMLO on the temperature anomaly in North China. Results of the present study are helpful for our better understanding of the mechanisms for atmospheric circulation variations and related temperature changes in North China and even in Eurasia.