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The Asian Monsoon System with Explicit Cloud Simulation

Charlotte DeMott

The Asian monsoon is a seasonal reversal of winds from northeasterlies during northern hemisphere winter to southwesterlies during northern hemisphere summer. This seasonal reversal of winds affects a large portion of the tropics, bringing copious amounts of rain to the densely populated regions of India and southeast Asia. Monsoonal rain does not fall continuously throughout the monsoon season. Rather, it is characterized by intermittent periods of heavy rainfall (onset) interspersed with comparatively dry periods (breaks). The onset-break cycle of monsoon rains repeats every 25~70 days, with 4-6 cycles occurring during a typical monsoon season.

The Asian monsoon is governed by complex interactions of seasonally varying heating over land, ocean-atmosphere feedbacks, and tropical weather disturbances on a variety of time and space scales. Onset-break cycles are closely linked to the slowly eastward-moving 20-90 day intraseasonal oscillation (ISO; Madden and Julian, 1971). Proper simulation of the ISO remains an outstanding challenge for atmospheric general circulation models (GCMs). Reasons for the poor simulation of the ISO are rooted in our limited understanding of what governs the disturbance's initiation, maintenance, and timing, but are generally thought to be linked to GCMs' difficulties in producing a realistic basic state environment, generating sufficient variability of tropical weather disturbances, appropriately moistening the low-level atmosphere, and capturing realistic interactions between the atmosphere and ocean. Through various combinations of these shortcomings, GCMs tend to simulate an Asian monsoon that is too weak, too irregular, and too limited in its spatial extent compared to observations (Waliser et al. 2003; Lin et al. 2008; Bollasina and Nigam, 2009).

A recent simulation with a coupled ocean-atmosphere GCM with explicit convection (Stan et al. 2010) produced an Asian monsoon that compares quite favorably with observations. Comparisons of this simulation to simulations that excluded either ocean-atmosphere coupling or explicit convection provided insight into factors necessary for a successful monsoon simulation. Our findings indicate that explicit simulation of convection is critical to generating tropical variability, while explicit convection and ocean-atmosphere coupling is needed to produce a realistic basic state environment. Proper interaction of tropical weather with the basic state environment is key to the proper simulation of the Asian monsoon. Ocean-atmosphere interactions are thought to be important for two reasons: first, they help determine the basic state environment and, second, they may directly impact the development of tropical weather systems that compose the Asian monsoon.

A surprising result of this study was the role of short-lived tropical weather disturbances on the Asian monsoon. These short-lived disturbances, which are ubiquitous in observations, only emerged in the ocean-atmosphere coupled simulation with explicit convection. Because their lifecycle is so short compared to ocean surface temperature fluctuations, their presence is attributed to interactions of convection and the basic state environment. In both observations and the coupled, explicit-convection simulation, the short-lived disturbances reinforced the onset-break cycle of the monsoon.

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