Summer monsoons provide the majority of annual rainfall to countries throughout the tropics and subtropics, many of which have growing populations and densely populated regions. The rains are vital to both local economies and natural ecosystems. A recent article published in Reviews of Geophysics explores how our understanding of what drives these important climate systems is evolving. Here, some of the authors give an overview of what we know from the latest observational and modeling research.
What has been the traditional explanation for the annual pattern of monsoons in the tropics and subtropics?
In addition to the torrential summer rains, monsoons are characterized by a change in the prevailing wind direction over the continents from equatorward/easterly in winter to poleward/westerly in summer. Based on this, the summer monsoons have been described as huge “sea breezes.” This picture can be traced back to at least 1686, when Halley (of comet fame) presented his observations of the seasonal changes in the tropical winds. He proposed these were driven by the faster warming of land compared with ocean in summer, which would draw humid air over the continent. Land-sea contrast has widely been held as the primary driver of the monsoon circulations ever since. However, recent work suggests this is not the whole story.
How do recent observations challenge this understanding?
As our observational record has developed, the simple picture of warming of land forcing the monsoons has been shown to have limitations. For example, the warmest temperatures over land in India are actually seen before the monsoon begins, while drier summers with weaker monsoon rainfall are associated with hotter land temperatures.
In addition, the paleoclimate record has revealed a surprising millennial-timescale relationship between changes in Arctic sea ice extent recorded in ice cores and variations in monsoon rain estimated from mineral deposits in caves (speleothems). These records reveal that a warmer high-latitude Northern Hemisphere is associated with strengthening of the Northern Hemisphere monsoons and a weakening of the Southern Hemisphere monsoons. The sea-breeze picture cannot explain this pattern.
What drives the monsoons if they are not giant sea-breezes?
To understand the essential building blocks of the monsoons it helps to ask: what is the minimum complexity that is needed to drive a monsoon? This can be answered by running hierarchies of model simulations where Earth’s features (such as land, mountains, and ocean heat transport) are stripped back to the bare essentials. Unexpectedly, even in “aquaplanet” simulations with a completely uniform surface, monsoon-like seasonal changes in the wind direction and precipitation happen, provided the heat capacity of the surface is small.
In these simulations, when the tropical rainband is near the equator in spring or fall, it sits between two Hadley cells that are similar in strength and extent as an Intertropical Convergence Zone. When one hemisphere warms sufficiently compared to the other, the two-cell pattern is replaced by a single, cross-equatorial Hadley cell, whose ascending branch and associated rainband move into the summer hemisphere subtropics. Consistently, the lower-level winds reverse direction.
What is the concept of a “global monsoon” and how does it better explain the characteristics of monsoons?
The term “global monsoon” has been given to the dominant mode of the seasonal cycle of low-latitude rainfall and circulation, which is a circumglobal annual north-south migration of the tropical rainband. Rather than viewing the regional monsoons as independent circulations driven by land-sea contrast, this perspective views them as seasonal changes in the Hadley circulation and the latitude of the tropical rainfall, with larger amplitude migrations over land. This picture, where inter-hemispheric contrast is the primary driver, matches with the behavior seen in the paleoclimate record. The regional monsoons may vary separately year-to-year and appear independent, but they share similar dynamics and variability on millennial timescales.
What are some of the unresolved questions where additional research, data, or modeling is needed?
Aquaplanet simulations are allowing fundamental controls on the location, intensity, and width of the tropical rainband to be investigated, and multiple theories have been developed for these features based on momentum and energy conservation. More research is needed to connect these theories, to test the timescales on which they apply, and to explore how they might need adapting to account for Earth’s zonal asymmetries. Land-sea contrast may not be the key ingredient for the monsoons, but it remains important in generating the rich regional behavior we experience.
Perhaps the greatest challenge for theory and modeling is to determine how the monsoon systems will change in future climates. State-of-the-art model simulations that are part of Phase 6 of the Coupled Model Intercomparison Project are targeting this question. These data provide a new test for theory and should incite further exciting advances.
—Ruth Geen ([email protected]; 0000-0001-7480-2768), University of Exeter, UK; Simona Bordoni ( 0000-0003-4771-3350), University of Trento, Italy & California Institute of Technology, USA; and David Battisti ( 0000-0003-4871-1293) University of Washington, USA