A paper published Friday in the Journal of Geophysical Research (GRL) finds that a known and natural atmospheric oscillation, the Southern Annular Mode or SAM, is correlated with observed increases in cloud cover resulting in regional cooling of approximately -2.5C. See for example this comparison from the paper with MODIS satellite data:
The Southern Annular Mode (SAM), which is defined by changes in the westerly winds that are driven by temperature contrasts between the tropics and southern polar areas. The annular modes generally take a circular pattern (‘annular’ means ring-shaped) and see-saw between positive and negative phases for weeks or months. In the SAM’s positive mode, the ring is stronger and further south, inhibiting Antarctic air outbreaks. In the negative mode, a weaker, more variable vortex allows Antarctic air to spill north more easily.
The Southern Annular Mode has steadily trended positive in recent decades. Computer models indicate this trend is related to ozone depletion above Antarctica and increases in greenhouse gases. (Image courtesy Jianping Li, China Institute for Atmospheric Physics.)
As presently programmed, climate models assume clouds result in net positive feedback and increased temperatures, however this new paper and several others that have recently been published show that clouds instead result in net negative feedback and cooling.
Key Points of the paper:
- Sudden regional increases in cloud cover are detected over S. America
- Changes linked to the Southern Annular Mode
- The cloud changes are associated with regional temperature reductions
The paper and abstract:
Understanding sudden changes in cloud amount: The Southern Annular Mode and South American weather fluctuations
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, D13103, 7 PP., 2012
Benjamin A. Laken Instituto de Astrofísica de Canarias, La Laguna, Spain, Department of Astrophysics, Faculty of Physics, Universidad de La Laguna, La Laguna, Spain
Enric Pallé Instituto de Astrofísica de Canarias, La Laguna, Spain, Department of Astrophysics, Faculty of Physics, Universidad de La Laguna, La Laguna, Spain
This work investigates the cause and effects of extreme changes in synoptic-scale cloud cover operating at daily timescales using a variety of satellite-based and reanalysis data sets. It is found that the largest sudden increases detected in globally averaged cloud cover over the last ten years of satellite-based observations occur following positively correlated shifts in the phase of the Southern Annular Mode (SAM) index. The associated pressure anomalies are found to generate frontal cloud formation over large areas of the South American continent, increasing regional cloud cover by up to 20%; these changes are correlated to statistically significant reductions in local temperatures of approximately −2.5°C with a +1 day time lag, indicating the SAM index is associated with large scale weather fluctuations over South America.