Short Course:
Monsoons - Past, Present and Future

Abstract:

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.

Abstract:

Few doubt that the Tibetan Plateau, bounded on the south by the Himalaya, affects regional climates of Asia. It follows that the growth of that Plateau should have affected regional climates of its surroundings in eastern Asia. Beginning near 10-15 million years ago (Ma), several phenomena occurring on and near Tibet and the Himalaya suggest a rapid change in Asian topography, including both an increase in Tibet’s height and an expansion of the areal extent of high terrain throughout eastern Asia. Concurrently, climates in different parts of Asia changed. Alas, the role of Tibet on regional climate bears resemblance to blind men describing an elephant. The growing Plateau seems to have played different roles insofar as it affected different regional climates that include dust (loess) deposition in North China, rainfall in South China, an aridification of northwest India and Pakistan, and the reputed strengthening of the South Asian (or Indian) monsoon at ~10 Ma. Whether a higher Plateau led to a stronger South Asian monsoon or not, once the holy grail of climate-geodynamic studies may instead prove to be a (doomed) search for the fountain of youth. Although the growth of Tibet surely affected Asian climate, it did so in ways that do not collapse into a simple one-size-fits-all theory.

Abstract:

Our understanding of how the monsoon system responds to different climatic background states comes from the paleoclimate record. This talk will review Late Quaternary paleoclimate data from the Afro-Asian monsoon domain, what these data tell us about the forcing mechanisms driving orbital and millennial-scale behavior of the monsoon, and outstanding issues. I’ll also review evidence for non-linear responses of the monsoon system to orbital forcing and explore possible explanations. Although monsoonal response to orbital changes in Earth’s climate may at first seem straightforward, there is substantial evidence for more complex behavior, highlighting the need to incorporate a more nuanced understanding of climate dynamics into paleoclimate interpretations. Comparisons with climate model simulations are particularly useful, and model-proxy studies have always been a cornerstone of paleo-monsoon research. There are, however, important features of the paleoclimate record that models cannot reproduce, potentially highlighting the importance of including land-surface feedbacks (vegetation and dust emissions) into simulations.

Abstract:

While various scientific assessment bodies have thoughtfully pondered the best estimate of changes in precipitation statistics and associated dynamical changes under global warming, the uncertainty estimates remain much higher than is desirable. A selected review of precipitation and circulation changes in the tropics and subtropics will aim to navigate parts of the piecemeal set of concepts and terminology currently in use to understand this important topic. Statements based on moisture availability — such as the rich-get-richer effect (following the terminology of the late Chia Chou) in which wet areas receive more precipitation and dry areas receive less under warming, or similar formulations relating to sea surface temperature — can yield approximations at sufficiently large scales. At regional scales, dynamical feedbacks associated with changes in circulation often dominate and tend to produce substantial scatter in projections. Related issues tend to arise for changes in statistics of precipitation events. Reducing these uncertainties in future climate projections, while at the same time improving precipitation simulation in current climate, stands as a leading challenge to the community working on moist feedback processes. Characterizing typical error patterns and understanding model sensitivity to parameterizations, including nonlinearity of the sensitivity, each can potentially contribute to identifying and narrowing uncertainties in model ensembles.