The QBO, temperature and sunspots relationship

Abstract: The present study investigates the relationship between the quasi-biennial oscillation (QBO), the semi-annual oscillation (SAO), and sunspots using the ERA-5 dataset. The periodic characteristics of zonal mean zonal winds over the equator (QBO) are examined from 1957 to 2022 using Fourier analysis. In addition to minor changes in periodicity, a shift in the timing of SAO is observed: the onset of westward winds above 5 hPa during winter has moved from mid-September to early August, and its conclusion from mid-December to mid-November, along with an overall increase in wind speed averages during this period. Results reveal variations in harmonic numbers and small changes in periodicity compared to previous studies with respect to the broader dataset. Additionally, the correlation between the north pole temperatures (at 30 hPa) and sunspot numbers during different phases of the QBO (defined at 50 hPa) is revisited. While the QBO and the SAO exhibit consistency with prior research, the sunspot–temperature relationship differs from previous findings: whereas earlier studies reported strong positive correlations, our extended dataset yields a weaker correlation of approximately 0.23. We further study the QBO, the SAO, and their link to polar stratospheric temperature through frequency analysis using Fourier decomposition. Research Highlights: Analysis of the ERA-5 dataset reveals a statistically significant weakening of the QBO amplitude at 10 hPa over the past six decades. There is a notable phase shift in the SAO onset from mid-September to early August. Fourier decomposition confirms a stable QBO periodicity of approximately 28.29 months. The correlation between sunspots and wintertime polar stratospheric temperatures during the QBO westerly phase has diminished to an insignificant 0.23. Recent polar warming anomalies appear increasingly driven by internal atmospheric variability or climate change.