The Basic Climatic Features of Stratospheric Circulation Transition in Northern Hemisphere

The basic climatic features of stratospheric circulation in Northern Hemisphere demonstrate different forms in winter and summer. In winter, the cold cyclone system and westerly winds prevail in high latitudes, while in summer the situation is the opposite. In terms of inversion of geopotential height gradient and zonal wind direction, a transition date index (TDI) indicating the change dates from summer to winter circulations in the stratosphere in Northern Hemisphere is defined by using NCEP/NCAR reanalysis daily data. Some statistic methods such as linear tendency, wavelet analysis, binomial coefficient smooth and Mann-Kendall are applied to analyze the inter-annual and inter-decadal features of the transition dates at all main levels in the stratosphere. Results indicate that in the stratosphere, with the height rising, the transition date becomes earlier and the summer circulation lasts longer. For instance, the earliest circulation transition in the stratosphere occurs at the height of 10 hPa and 20 hPa, and it shifts to 30 hPa in a short period. However, it takes longer for the transition to shift from 30 hPa to 50 hPa than that from 10 hPa to 30 hPa, which takes almost one month. The average onset date of the South China Sea Summer Monsoon (SCSSM) is one of the earliest dates in Asia Summer Monsoon (ASM) system and it is much later than the transition dates in stratosphere. Therefore, TDI can be used as a pre-signal for monitoring and predicting ASM. Furthermore, there exists an obvious regional difference in the circulation transition, among which the transition dates at each level in Siberia is the earliest and that is relatively later in Bering Sea and Greenland. The inter-annual and inter-decadal features of the circulation transition dates in Northern Hemisphere and the aforementioned three different regions are quite apparent, turning from late to early and then to late again in the past 62 years. Particularly the circulation transition date in Northern Hemisphere and in Siberia shares some similarities in inter-annual and inter-decadal variations, for example, the time variation shows significant fluctuations, and both have a transition peak in 1975. The transition dates in Bering Sea and Greenland also have the similar features, for example, the time fluctuation is relatively small. Moreover, circulation transition dates vary with the height and region, but they all have a quasi-2-year, a quasi-3-to-6-year, a quasi-9-to-12-year or a quasi-21-to-24-year cycle which may have close connections with other members of the climate system.