Large shift of the Pacific Walker Circulation across the Cenozoic
Fluctuations in the Pacific Walker circulation (PWC), a zonally-oriented overturning cell across the tropical Pacific, can cause widespread climatic and biogeochemical perturbations. It remains unknown how the PWC developed during the Cenozoic era, with its substantial changes in greenhouse gases and continental positions.
Yan and colleagues examined the evolution of the PWC across the Cenozoic using a suite of coupled model simulations on tectonic timescales. During the Early Eocene (ca. 54-48 Ma), when the Pacific was larger in size, the western edge of the PWC was ~18° west of its present position, in tandem with a 20° eastward expansion of the eastern edge. This leads to a significant broadening of the PWC by ~38°. As the climate cooled from the Early Eocene to the Late Miocene, the width of the PWC shrank, accompanied by an increase in intensity that was tied to the enhanced Pacific zonal temperature gradient.
However, the locations of the western and eastern branches behave differently from the Early Eocene to the Late Miocene, with the western edge remained steady with time due to the relatively stable geography of the western tropical Pacific; the eastern edge migrates westward with time as the South American continent moves northwest. A transition occurs in the PWC between the Late Miocene and Late Pliocene, manifested by an eastward shift (both the western and eastern edges migrate eastward by >12V) and weakening (by ~22%), which they show here is linked with the closure of the tropical seaways.
Further sensitivity experiments that separate the influences of CO2 and land-sea configurations illustrate that rising CO2 alone leads to a weaker PWC, a robust feature across the large spread of Cenozoic climates considered here and therefore in a warmer future. The results also highlight that, at least on tectonic timescales, the location of the PWC is largely controlled by plate movements, with CO2 concentrations playing a secondary role impacting solely the intensity.
Although there are uncertainties to be considered, these results provide a testable relationship between the tectonic/CO2-induced climate change and the behavior of the PWC. The substantial changes in the PWC simulated here serve as a potential factor responsible for the reconstructed hydrological changes across the globe during the Cenozoic era. Moreover, a comprehensive understanding of the controls on the PWC could help advance its predictive skill and translate into better forecast of extreme weather conditions.