The role of semiannual cycle in modulating seasonality changes of surface air temperature over China and its mechanism

Abstract

Taking the semiannual cycle of surface air temperature (SAT) into account in tting the ner structure of the seasonal cycle, this study reveals the speci c role of the semiannual cycle in modulating variations of the season onsets and lengths for the rst time. The time-varying amplitudes and phases of the annual and semiannual SAT cycle for the period 1964 to 2017 in China are extracted from daily observations based on harmonic analysis. The results show that the phase differences between the two harmonics remain essentially unchanged, while the amplitude ratio of the semiannual to annual harmonics tends to increase after 1988. This enhanced semiannual cycle together with the global warming and annual cycle changes jointly led to a much longer summer with an average speed of 7.45 days/10 years and shorter spring, autumn, and winter with speeds of -2.43, -1.63, and − 4.26 days/10 years, respectively. Such obvious asymmetry cannot be found in tting seasonal cycles by only annual harmonics. The average absolute percentages of the linear trends of season onsets and lengths caused by the intensi cation of the semiannual cycle are 52.63% and 78.66%. It indicates that season onsets and length not only depend on the phase of annual and semiannual harmonic, but also highly related to the strength of semiannual cycle. The time evolutions of semiannual cycle strength in China are found to be signi cantly correlated to the meridional temperature advection. The anomalous stronger southward wind in December can result in colder temperature and hence intensify the semiannual component.

Understanding seasonal cycle of daily extreme temperatures based on generalized additive model for location, scale and shape with smoothing spline

“…A relatively adaptive method called ensemble empirical mode decomposition has been applied to extract smooth SCs in long-term daily temperature series Qian, Yan, et al, 2011;Wu et al, 2008). A similar method named nonlinear mode decomposition is also adopted to construct smooth SC of annual daily temperatures (Deng et al, 2023;Iatsenko et al, 2015).…”