From the UNIVERSITY OF MIAMI ROSENSTIEL SCHOOL OF MARINE & ATMOSPHERIC SCIENCE and the “correlation is not causation department” comes this paper that suggests GHG’s decrease the Walker Circulation. Only one problem. They don’t give a clear cut mechanism, only tenuous linkage. I suspect Willis will have a thing or two to say about this paper.
Study of cloud cover in tropical Pacific reveals future climate changes
UM Rosenstiel School researchers find new evidence for weakening of Walker circulation
MIAMI – A new analysis using changes in cloud cover over the tropical Indo-Pacific Ocean showed that a weakening of a major atmospheric circulation system over the last century is due, in part, to increased greenhouse gas emissions. The findings from researchers at the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science provide new evidence that climate change in the tropical Pacific will result in changes in rainfall patterns in the region and amplify warming near the equator in the future.
“Our findings show that an increasing concentration of greenhouse gases leads to significant changes in atmospheric circulation and tropical rainfall patterns,” said Katinka Bellomo, an alumna of the UM Rosenstiel School. “This study demonstrates that we can predict these changes in the Walker circulation from changes in cloud cover.”
The UM Rosenstiel School researchers used historical observations of cloud cover as a proxy for wind velocity in climate models to analyze the Walker circulation, the atmospheric air flow and heat distribution in the tropic Pacific region that affects patterns of tropical rainfall. Their findings revealed a weakening and eastward shift of the Walker circulation over the last century due to greenhouse gas emissions. The analysis showed that changes in cloud cover can serve as a proxy in climate models for wind velocity in the atmosphere, which cannot be directly measured.
“This study makes innovative use of a decades old-dataset,” said Amy Clement, professor of atmospheric science at the UM Rosenstiel School. “It is impressive that visual observations from the decks of ships transiting the Pacific Ocean over a half-century can tell us something so fundamental about climate change.”
This new information can be incorporated into current climate models to predict future changes in the magnitude and pattern of the Walker Circulation due to increased greenhouse gas emissions. The study suggests that rainfall will decrease over Indonesia and in the western Pacific and increase over the central Pacific Ocean.
The study, titled “Evidence for weakening of the Walker circulation from cloud observations,” was published in the journal Geophysical Research Letters. The study’s authors include: Katinka Bellomo and Amy C. Clement of the UM Rosenstiel School. The work was supported by grants from National Science Foundation Climate and Large Scaled Dynamics, National Oceanic and Atmospheric Administration’s Climate Program Office, grant # NA10OAR4310204 and Department of Energy Biological and Environmental Research, grant #DESC0004897.
The publication can be accessed here: http://onlinelibrary.wiley.com/doi/10.1002/2015GL065463/abstract (Paywalled)
Climate models simulate a weakening of the Walker circulation in response to increased greenhouse gases, but it has not been possible to detect this weakening with observations because there are not direct measurements of atmospheric circulation strength. Indirect measurements, such as equatorial gradients in sea level pressure (SLP), exhibit trends of inconsistent sign. In this study we estimate the change in midtropospheric velocity (ω500) from observed change in cloud cover, which we argue is more closely tied to the overturning circulation than indirect measurements of SLP at the surface. Our estimates suggest a weakening and eastward shift of the Walker circulation over the last century. Because changes in cloud cover in Atmospheric Model Intercomparison Project simulations forced with increased sea surface temperature are remarkably similar in pattern, sign, and magnitude, we assert that the observed changes in cloud cover and the associated weakening of Walker circulation are at least in part externally forced.