Changes in lengths of the four seasons in China and the relationship with changing climate during 1961-2020

The spatial-temporal characteristics of changes in lengths and onsets of the four seasons in China during 1961-2020 are investigated using daily temperature observations from weather stations. The relationship between changes in season lengths and climate change is also explored based on annual temperature variation, the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO). The results show that lengths of spring and autumn are shorter in the plateau region but longer in the southwest compared with other regions. In the past 60 years, the onset was generally advanced for spring and summer, whereas it was delayed for autumn and winter. Lengths of summer (winter) showed an overall increase (decrease) as 10.78 days (11.74 days) from the 1960s to the 2010s, which is positively (negatively) correlated with the temperature variation. Comparatively, changes in lengths of spring and autumn are much smaller and weakly correlated with the temperature variation. However, lengths of spring and autumn significantly increased in the plateau region and parts of the southwest and decreased in the south, which is positively and negatively correlated with the temperature trend, respectively. Changes in season lengths in China are also related to the AMO and modulated by the PDO phases. At the similar to 55-year scale, summer lengths and winter lengths are simultaneously affected by the AMO and the PDO. The impact of the AMO on changes in lengths of spring and autumn is mainly due to its effect on the nonlinear trend associated with long-term climate change.