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Abstract
Alterations of annual temperature cycles have profound implications on how the planet responds to global climate change. In this study, a high resolution global analysis of temperature cycle shifts and their development over time is presented. We show that over the last 63 years, phase shifts in the annual near surface temperature cycle exhibit large spatiotemporal variability. The calculated phase shifts comprise earlier onsets of seasons as well as delays with similar frequencies, depending on location. From 1978 to 2010 Eastern Europe experienced an advanced annual cycle of near-surface temperature of 3.2 days while Eastern Australia shows an opposite shift towards later seasons of 3.5 days in comparison to the preceding 30-year period from 1948 to 1977. The largest phase shifts of –5.5 days toward earlier seasons over land were found in Belarus and Northwest Russia. For the first time the developments of seasonal temperature shifts were generalized for large areas by using self-organizing feature map neural networks resulting into 4 significant global trends. The temperature phase shifts are also shown to have strong correlations with the timing of shrub foliation observed at 57 phenological stations across the USA. The findings have far-reaching, yet regionally distinct consequences on agriculture, animal life cycles, plant phenology, and regional weather phenomena that change with annual temperature cycles.
Highlights
Climate change is associated with momentous consequences on vital aspects of human life
In this study we focus on the phase shifts between temperature series of different periods using a crossspectrum analysis approach [21,22] and a combination of a self-organizing feature map (SOFM) neural network [23,24] and a subsequent k-means clustering
After excluding outliers as well as the vectors from areas that showed spectra with the highest variance attributed to sub-yearly temperature cycles, the remaining input data consisting of 461,312 phase shift values (8704 grid points × 53 temporal moving-window steps) was presented to a new SOFM with the same topology for the second SOFM clustering run
Summary
Climate change is associated with momentous consequences on vital aspects of human life. An increased frequency of wildfires is associated with corresponding earlier onsets of spring in Western USA [20] These examples underline the importance of the phase shift and its effects on complex interactions between the phase of the temperature and dependent processes in our environment, while the extensive and complex reactions of ecosystems to changes in the seasonal cycle are still far from being understood or predictable. The used NCEP/NCAR Reanalysis 1 temperature dataset with a 2.5 ̊ resolution covers the last six decades and is spatially comprehensive This high resolution enables us to assess the phase shift over vast areas on the southern hemisphere and apply a pattern recognition algorithm on the global distribution of the phase shifts of the annual temperature cycle for the first time. Due to the Nyquist frequency restrictions, a second corresponding dataset with a 6-hourly temporal resolution was used to safely exclude time series whose biggest part of the spectral variance was not caused by
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