Simona Bordoni – Simulating monsoons in a water-covered earth.
Monsoons are among the most prominent features of the summertime circulation of the Earth’s atmosphere, affecting the climate of nearly one quarter of the globe. They occur in the tropics and subtropics, in regions that are far enough from the equator for a strong seasonality in precipitation and atmospheric circulation to exist, but that are still influenced by tropical balances and dynamics. Monsoons occur over Australia, West Africa, North and South America, but are most extensive and dramatic over Asia. Despite the importance of monsoons in the global circulation of the atmosphere and oceans, and their impact on society, agriculture and economics around the world, the fundamental mechanisms responsible for these circulations are still not completely understood: most notably, what renders the onset of the monsoon precipitation, and the associated changes in circulation patterns, rapid remains to-date unexplained (Fig. 1, top).
The classic view of monsoons, which dates back to Halley (1686), has been founded on the existence of a strong contrast in heating between ocean and land masses, arising from their distribution and their different thermal response to the annual cycle of radiative heating, with monsoons manifesting themselves as giant sea breeze circulations. These circulations form overturning cells, with air flowing across the equator toward the warm land surfaces in the summer hemisphere – such as for instance the Indian subcontinent during the northern hemisphere summer – rising there, flowing back towards the equator at upper levels and sinking in the winter hemisphere.
Using numerical experiments with a General Circulation Model (GCM) in aquaplanet (water-covered earth) configuration, Simona Bordoni has been able to simulate transitions in the tropical overturning circulation that in all essential aspects resemble the onset and end of observed monsoons, even in the absence of land-sea contrasts. In the simulations, the reorganization of the tropical circulation leading to the rapid onset of monsoons is mainly driven by the interaction between the tropical circulation and the large-scale turbulent waves generated in mid-latitudes. These waves, which can have a length-scale of ~ 1000 km and form the familiar mid-latitude weather systems, propagate towards smaller latitudes, i.e. towards the tropical regions, and eventually break, modifying the tropical circulation there. Feedbacks between the tropical circulation, the upper level winds and the propagation characteristics of the waves themselves allow the tropical circulation to change rapidly, resulting in a rapid reversal of the lower-level winds and a rapid onset of heavy rain in subtropical latitudes, as seen in the Earth’s monsoons. (Fig.1 and 2). These feedbacks provide one possible explanation for the rapidity of monsoon onset, which remains unaccounted for in the traditional view of monsoons, because land-sea contrasts can evolve only gradually with season upon heating of the surfaces by sunlight. Ongoing analysis of observational data confirms that similar feedbacks are also acting in Earth’s large-scale monsoons, such as the Asian monsoon.
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Fig. 1: Seasonal cycle of zonal- and pentad-mean precipitation from observations in the Asian monsoon sector (top) and from aquaplanet simulations with an idealized moist GCM (bottom). The contour interval is 1mm day-1 and 2 mm day-1, respectively. Observed precipitation is from the Global Precipitation Climatological Project data for the years 1999 - 2005. Precipitation rates in the simulations can exceed the observed precipitation rates because the lower boundary in the simulations is entirely water-covered. |
| Fig. 2: Seasonal cycle of zonal and pentad-mean lower-level winds at 15N from observations in the Asian monsoon sector (green) and from the aquaplanet simulations (yellow). Observed winds are from the ERA40 reanalysis for the years 1981 – 2000. Lower-level winds are evaluated at 850 hPa. |  |
ASP Spotlight January 2009
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