The Period and Phase Speed of Upper-tropospheric Planetary- and Synoptic-scale Waves during the Solstice Seasons: Climatology and Trends during 1979-2023
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Graphical Abstract
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Abstract
By applying convolution-based Hilbert transform in zonal direction on 6-hourly streamfunction fields at 200 hPa, we present the climatology and trends of local wave period, zonal and meridional phase speeds of Rossby waves over the globe during the solstice seasons from 1979 to 2023. While partly similar to and inspired by Fragkoulidis and Wirth (2020), our method differs in its ability of covering both planetary-scale and synoptic-scale waves over not only the extratropics, but also the tropics and subtropics. Based on a physically reasonable global distribution of wave periods, our key new finding is a robust prolonging of wave periods over most regions of the tropics and subtropics during both solstice seasons of 1979-2023, except for the tropic Atlantic where experiences shortened wave period during June-July-August of 1979-2022. Both prolonging and shortening of wave periods are mainly associated with the changes in planetary-scale waves. Regionally varying trends of zonal phase speed (C<sub>px</sub>) of synoptic waves are consistent in sign with, but smaller in magnitude than, the trends of local zonal wind, confirming Wu and Lu (2023)’s conclusion on the opposite effects of zonal wind and the meridional gradient of potential vorticity on C<sub>px</sub>. Meanwhile, the C<sub>px</sub> trends of planetary-scale waves are relatively weak, and do not exhibit a robust relation with the trend of zonal wind. The new results can be helpful to understand the change of atmospheric waves and extreme events under global warming.
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