Monsoon Systems

 

Monsoons are a climate phenomenon that seasonally produces the majority of rainfall in tropical regions.  In Chapter 14, monsoons are assessed on both a global and regional scale.  The major monsoon systems covered in this chapter are the Asian-Australian monsoon (AAM), North American monsoon (NAMS), South American monsoon (SAMS), and African monsoon (Fig. 14.3).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 14.3 | Regional land monsoon domain based on 26 CMIP5 multi-model mean precipitation with a common 2.5° × 2.5° grid in the present-day (1986–2005). For regional divisions, the equator separates the northern monsoon domains (North America Monsoon System (NAMS), North Africa (NAF), Southern Asia (SAS) and East Asian summer (EAS)) from the southern monsoon domains (South America Monsoon System (SAMS), South Africa (SAF), and Australian-Maritime Continent (AUSMC)), 60°E separates NAF from SAS, and 20°N and 100°E separates SAS from EAS. All the regional domains are within 40°S to 40°N.

 

Global Overview

 

Global monsoon precipitation (land+ocean) has intensified during the 1979-

2008 period. The historical record is generally simulated fairly well by CMIP5

models, but a large gapexists between the best and worst models. 

CMIP5 models of future changes in the global monsoon domain show a

projected increase in global monsoon area, total precipitation, and

precipitation intensity by 2100 (Fig. 14.2).  Projected precipitation changes

range from 5-15% depending on the emissions scenario used in the models. 

Additionally, CMIP5 models predict that monsoon onset dates will become

earlier, leading to an overall increase in the duration of the monsoon season.

 

 

 

 

 

 

 

 

 

 

 

 

Asian-Australian Monsoon

 

The powerful Asian-Australian monsoon system (AAM) is driven by a seasonal variation in temperature contrast between the Eurasian landmass and the Indo-Pacific Ocean.  AAM is comprised of five sub-systems: South Asian, East Asian, Maritime Continent, Australian, and Western North Pacific monsoons.  Fig. 14.5 summarizes projected trends in these different sub-systems.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

For the South Asian monsoon, CMIP5 models project an increase in mean precipitation, interannual variability, and extremes.  As is the case for the global monsoon, this projected in crease in precipitation is due to the increased moisture flux from the ocean in a warmer climate.  The East Asian monsoon is projected to have increases in both rainfall and circulation, the latter of which is unique amongst Asian monsoon systems, most of which are projected to have a weakening monsoon circulation.  The other monsoon systems are smaller in extent and have similar projected trends. 

 

Models have lower confidence in projected changes in the American and African monsoons, but the trends generally follow those of the global monsoon.          

 

Summary

 

From the IPCC report: “It is projected that global monsoon precipitation will likely strengthen in the 21st century with increase in its area and intensity while the monsoon circulation weakens. Precipitation extremes including precipitation intensity and consecutive dry days are likely to increase at higher rates than those of mean precipitation. Overall, CMIP5 models project that the monsoon onset will be earlier or not change much and the monsoon retreat dates will delay, resulting in a lengthening of the monsoon season. Such features are likely to occur in most of Asian-Australian Monsoon regions.”

 

In general, the Asian monsoon systems are modeled better than others, leading to a higher confidence in projected changes for those sub-systems.  There is not much specific discussion on what limitations are preventing higher confidence models for certain regions or monsoon systems.