Dynamics of the Indian Monsoon Climate
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Climate Science. Please check back later for the full article.
A lifeline for about one sixth of the world’s population in the subcontinent, the Indian summer monsoon (ISM) is an integral part of the annual cycle of the winds (reversal of winds with seasons) coupled with a strong annual cycle of precipitation (wet summer and dry winter). High socio-economic impacts of ISM rainfall in the region have, for over a century, driven scientists to attempt to predict the year-to-year variations of the ISM rainfall. A remarkably stable phenomenon, making its appearance every year without fail, the year-to-year variation of the ISM climate is rather small (SD of the seasonal mean being 10% of the long-term mean), but it has proven to be an extremely challenging system to predict, with the skill of the most sophisticated models being barely useful and significantly below the level of potential limit on predictability. Understanding what drives the mean ISM climate and its variability on different time scales is critical, therefore, for advancing the skill of monsoon prediction. A conceptual model of ISM helps us to understand what maintains not only the mean ISM but also its variability on inter-annual and longer time scales.
The annual cycle of ISM precipitation could be described as a manifestation of the seasonal migration of the inter-tropical convergence zone (ITCZ), or the zonally oriented cloud (rain) band characterized by a sudden onset. The other important feature of ISM is the deep overturning meridional (regional Hadley circulation) associated with it, driven primarily by latent heat release associated with the ISM (ITCZ) precipitation. The dynamics of monsoon climate, therefore, are an extension of the dynamics of the ITCZ. While the classical land-sea surface temperature gradient model of ISM could explain the seasonal reversal of surface winds, it explains neither the onset nor the deep vertical structure of the ISM circulation. While the surface temperature over land cools after the onset, reversing the north-south surface temperature gradient and making it inadequate to sustain the monsoon after onset, it is the tropospheric temperature gradient that becomes positive at the time of onset and remains strongly positive thereafter, maintaining the monsoon. The change in sign of the tropospheric temperature (TT) gradient is dynamically responsible for a symmetric instability leading to the onset and subsequent northward progression of the ITCZ. The unified model of ISM in terms of the TT gradient provides a platform to understand drivers of ISM variability by identifying processes that affect TT in the north and south and influence the gradient.
While ISM climate on long time scales (e.g., multi-millennium) largely follow the solar forcing, on shorter time scales the ISM variability is governed by internal dynamics arising from ocean-atmosphere-land interactions and feedbacks, regional as well as remote. The predictability of seasonal mean ISM is limited by interactions of the annual cycle and higher frequency monsoon variability within the season. The seminal role of monsoon intra-seasonal oscillation (MISOs) in influencing the seasonal mean and its inter-annual variability is highlighted.