Changing Global Seasonal Onsets and Durations

In this study, observed and projected changes in onsets and lengths of climatological thermal seasons are examined across the globe. Spatio-temporal variations are defined from surface air temperature. Fixed thresholds of 11-day running mean surface air temperature from NCEP/NCAR reanalysis I and CMIP5 RCP4.5/8.5 climate change scenario data sets are employed to demarcate seasonal grid cell values annually from 1951-2100. Analyses of long-term trends show a reduction in winter duration since the late 1980s due to an earlier spring onset appeared in western North America, western Europe and East Asia. A recognized extension of summer duration along the Northern Hemisphere subtropical high-pressure belt is attributable to both earlier onset and delayed offset of summer zonally. Such changes are less obvious in the Southern Hemisphere. Ensembles of CMIP5 climate change scenario data suggest an intensification of these changes later in this century will expand the coverage of regions with reduced winter or expanded summer durations. Large-scale atmospheric circulation regimes associated with these recent and projected changes in seasonal cycles will be discussed.

Key terms: Climatological thermal seasons, seasonal onset, seasonal length, CMIP5, climate change