A Review of Energetic and Dynamic Theories of Monsoons
Assistant Professor - Department of Geology and Geophysics,
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In this talk I will review recent progress in understanding controls on monsoon circulations. Since monsoons are thermally direct circulations ultimately driven by insolation contrasts, the first part of this talk will focus on prognostic theories that constrain monsoon circulations via the atmospheric energy budget, as well as diagnostic theories that relate circulations to the energy of near-surface air through a convective quasi-equilibrium assumption. Energy budget theories have seen recent success in explaining variations in monsoon strength that are observationally associated with high-latitude thermal forcings, many in paleo records, yet we show that large internal feedbacks greatly reduce their quantitative prognostic utility. Convective quasi-equilibrium theories have been used to explain interannual variations in the strength of multiple monsoons, and to improve understanding of the role played by topography in Asian climate. Recent studies show that traditional treatments of convective quasi-equilibrium may need to be altered to consider the energy content of air throughout the entire lower troposphere; this may be particularly important for monsoons because of the dry desert air that flows into nearly all monsoon regions above the convective subcloud layer. In the second part of the talk I review dynamical theories that constrain monsoons via the momentum budget. This includes theories for zonal mean Hadley circulations as well as models that consider the zonally asymmetric response to localized heat sources. Observed nonlinearities in the seasonal evolution of monsoons are most often explained using these dynamical theories. However, I close by showing that the seasonal mean strength of the South Asian monsoon does not seem to exhibit any strong nonlinearities in its response to large forcing perturbations in a global climate model. I document flaws in previously proposed simple models for “tipping points” in monsoons, and argue that simpler explanations of spatial shifts in circulations and near-linear responses to large changes in forcings should be considered before one invoking tipping points, bifurcations, or related dynamics.