On the Evolution of the Annual Cycle of Surface Air Temperature in the Northern Hemisphere

The annual cycle is a large scale climate process which influences smaller scale climate and meteorological processes. In this research, the annual cycle is defined by the climates response to Earth's orbit about the sun. In this definition, the annual cycle can be modulated and therefore change over time (Wu, Z. et al. 2008). The goal of this research is to diagnose changes to the Northern Hemisphere annual cycle from 1949 to 2017 using multi-dimensional ensemble empirical mode decomposition (MEEMD). MEEMD is a newer decomposition method that is built on empirical mode decomposition (EMD) and ensemble empirical mode decomposition (EEMD; Wu et al. 2009). Unlike EMD and EEMD, MEEMD can be used to decompose spatiotemporal data sets making climate research easier, which often relies heavily on gridded data sets (Wu et al. 2016). In this study, MEEMD is applied to 2-meter pentad temperature data from the National Centers for Environmental Prediction (NCEP)/National Centers for Atmospheric Research (NCAR) Reanalysis 1 (Kalnay et al. 1996) to obtain the amplitude trend of the annual cycle. The amplitude trend of the annual cycle for all points are normalized to compare the trends between polar and tropical latitudes, as the polar latitudes are changing at a more rapid rate than the tropical latitudes, likely due to anthropogenic forcing (IPCC 2019). Results show that the annual cycle is decreasing in polar latitudes and increasing in tropical latitudes. In mid-latitudes, the annual cycle appears to change depending on topography where, in general, the annual cycle is decreasing in highland regions and increasing in lowland regions. Changes in climate modes may also play a role in mid-latitude results but this remains to be studied in depth. Identifying changes to the annual cycle will prove useful for understanding the spatially differing effects of climate change on weather and climate in the Northern Hemisphere. The the results of the evolution of the annual cycle amplitude trend are plotted as a movie and can be found as a supplemental file. Similarly, the evolution of the normalized results are plotted as a movie and can be found as a supplemental file to this thesis. Both movies show how the annual cycle of surface air temperature as changed yearly from 1949 through 2017, with the first movie showing the change in temperature and the latter showing the change in percent. The results of this research serve solely as a diagnosis and open the opportunity for future research to help explain the results of this paper.