By Dr. Sebastian Lüning and Prof. Fritz Vahrenholt
(German text translated/edited by P Gosselin)
In September 2016 an exciting paper by a team led by Katinka Bellomo was published in the Geophysical Research Letters.
It described a climate amplifier for the Atlantic Multidecadal Oscillation (AMO) based on clouds. According to the study, the cloud effect accounts for up to one third of the AMO associated temperature change. The abstract:
New observational evidence for a positive cloud feedback that amplifies the Atlantic Multidecadal Oscillation
The Atlantic Multidecadal Oscillation (AMO) affects climate variability in the North Atlantic basin and adjacent continents with potential societal impacts. Previous studies based on model simulations and short-term satellite retrievals hypothesized an important role for cloud radiative forcing in modulating the persistence of the AMO in the tropics, but this mechanism remains to be tested with long-term observational records. Here we analyze data sets that span multiple decades and present new observational evidence for a positive feedback between total cloud amount, sea surface temperature (SST), and atmospheric circulation that can strengthen the persistence and amplitude of the tropical branch of the AMO. In addition, we estimate cloud amount feedback from observations and quantify its impact on SST with idealized modeling experiments. From these experiments we conclude that cloud feedbacks can account for 10% to 31% of the observed SST anomalies associated with the AMO over the tropics.”
In the same journal already in February 2016, an article by Tianle Yuan et al appeared on the same topic. The authors called out that the climate models were not able to reproduce the AMO ocean cycle in the tropics. They then described amplification mechanism through low-lying clouds that could incorporate the AMO into the models. The abstract follows:
Positive low cloud and dust feedbacks amplify tropical North Atlantic Multidecadal Oscillation
The Atlantic Multidecadal Oscillation (AMO) is characterized by a horseshoe pattern of sea surface temperature (SST) anomalies and has a wide range of climatic impacts. While the tropical arm of AMO is responsible for many of these impacts, it is either too weak or completely absent in many climate model simulations. Here we show, using both observational and model evidence, that the radiative effect of positive low cloud and dust feedbacks is strong enough to generate the tropical arm of AMO, with the low cloud feedback more dominant. The feedbacks can be understood in a consistent dynamical framework: weakened tropical trade wind speed in response to a warm middle latitude SST anomaly reduces dust loading and low cloud fraction over the tropical Atlantic, which warms the tropical North Atlantic SST. Together they contribute to the appearance of the tropical arm of AMO. Most current climate models miss both the critical wind speed response and two positive feedbacks though realistic simulations of them may be essential for many climatic studies related to the AMO.”
While on the subject of clouds, for German speakers the following 2 videos can be viewed until January 21, 2017 can be watched on ZDF German television:
Operation Cloud Lab: Cloud Chasers
A team of scientists flies over the USA inside the world’s largest aircraft and carry out a series of unusual experiments for understanding the atmospheric phenomena.
The second video:
Operation Cloud Lab: The sky lives
Scientists study the life of the earth’s atmosphere at an extreme elevation in their “flying lab” over the USA. What impact does man have on the atmosphere?
[Note: The originals are in English, and so with a little searching they likely can be found in the Internet.]