Past Monsoons Hold Key To Current Changes

Palaeoclimatic records help us to better understand the internal feedback processes within the climate system which are believed to influence the monsoon circulation more severely than external factors such as the increase in greenhouse gases in the atmosphere. This graphic is from the Nature paper and shows the basic components of a summer monsoon and its driving forces. Summer solar radiation heats the surface. A land–ocean thermal contrast, resulting from a lower surface heat capacity of the land compared to the ocean, causes the low-level inflow of moist air from ocean to land during summer and the rising of air over the landmass. Courtesy: authors and Nature.Palaeoclimatic records help us to better understand the internal feedback processes within the climate system which are believed to influence the monsoon circulation more severely than external factors such as the increase in greenhouse gases in the atmosphere. This graphic is from the Nature paper and shows the basic components of a summer monsoon and its driving forces. Summer solar radiation heats the surface. A land–ocean thermal contrast, resulting from a lower surface heat capacity of the land compared to the ocean, causes the low-level inflow of moist air from ocean to land during summer and the rising of air over the landmass. Courtesy: authors and Nature.

A new Nature paper discusses the main influences on monsoon variability and their uncertainties, and argues that a coordinated effort to quantify what happened in the past will help improve our understanding of changes in monsoons today.

Palaeoclimatic evidence shows that monsoon dynamics are strongly shaped by large-scale meridional temperature gradients and the related position of the intertropical convergence zone, according to the paper, entitled “Palaeoclimatic insights into forcing and response of monsoon rainfall”.

However, study of past monsoons also reveals that these temperature gradients are sensitive to many types of forcing, the influence of which seems to vary in time and space. Different forcings perturb different components of the monsoon system, the authors say. These then affect the entire system via feedback mechanisms, and act on different timescales. They include the Atlantic Multidecadal Oscillation (AMO); Atlantic meridional overturning circulation (AMOC); El Niño–Southern Oscillation (ENSO); and the Pacific Decadal Oscillation (PDO).

The authors point out that until climate models converge on the simulation of monsoon forcings, a robust projection of future monsoons will remain elusive. They conclude that the “various aspects of changes in monsoon rainfall require a deeper understanding of the mechanisms behind palaeo-monsoon variability, which may be achieved by means of palaeo- climatic monsoon reconstructions that use multiple proxy records, ideally from the same location”.

Abstract

Monsoons are the dominant seasonal mode of climate variability in the tropics and are critically important conveyors of atmospheric moisture and energy at a global scale. Predicting monsoons, which have profound impacts on regions that are collectively home to more than 70 per cent of Earth’s population, is a challenge that is difficult to overcome by relying on instrumental data from only the past few decades. Palaeoclimatic evidence of monsoon rainfall dynamics across different regions and timescales could help us to understand and predict the sensitivity and response of monsoons to various forcing mechanisms. This evidence suggests that monsoon systems exhibit substantial regional character.

Citation

Mahyar Mohtadi, Matthias Prange and Stephan Steinke; Palaeoclimatic insights into forcing and response of monsoon rainfall; Nature, doi:10.1038/nature17450.

Source

Nature.

 

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