Basic mechanism for abrupt monsoon transitions

Abstract

Monsoon systems influence the livelihood of hundreds of millions of people. During the Holocene and last glacial period, rainfall in India and China has undergone strong and abrupt changes. Though details of monsoon circulations are complicated, observations reveal a defining moisture-advection feedback that dominates the seasonal heat balance and might act as an internal amplifier, leading to abrupt changes in response to relatively weak external perturbations. Here we present a minimal conceptual model capturing this positive feedback. The basic equations, motivated by observed relations, yield a threshold behavior, robust with respect to addition of other physical processes. Below this threshold in net radiative influx, R c , no conventional monsoon can develop; above R c , two stable regimes exist. We identify a nondimensional parameter l that defines the threshold and makes monsoon systems comparable with respect to the character of their abrupt transition. This dynamic similitude may be helpful in understanding past and future variations in monsoon circulation. Within the restrictions of the model, we compute R c for current monsoon systems in India, China, the Bay of Bengal, West Africa, North America, and Australia, where moisture advection is the main driver of the circulation.Earth system | tipping element | abrupt climate change | atmospheric circulation | nonlinear dynamics M onsoon rainfall shapes regional culture and the livelihoods of hundreds of millions of people (e.g., 1, 2). The future evolution of monsoon rainfall under increasing levels of atmospheric CO 2 and aerosol pollution is highly uncertain (3). Although greenhouse gas abundance tends to increase monsoon rainfall strength (4-6), the situation is more complex with changing aerosol distribution (7,8). Given this large uncertainty in the future forcing of monsoons, it is crucial to understand internal monsoon dynamics, especially with respect to self-amplifying feedbacks, which might result in potentially strong responses to small perturbations. Zickfeld et al. (2005) found two stable states in a simple model of the Indian summer monsoon, which in principle allows for rapid transition between radically different monsoon circulations (9, 10) and thereby identified the Indian monsoon as a potential tipping element of the climate system (11). Evidence for such behavior is found in paleodata that show rapid and strong variations in Indian and East Asian monsoon rainfall (12, 13). These abrupt changes have been linked to climatic events in the North Atlantic for the last glacial period (14, 15) as well as for the Holocene (16,17). Though a physical mechanism for this teleconnection has been suggested (18), relevant climatic signals of the North Atlantic events in Asia (such as temperature and moisture anomalies) are very small (19) indicating that internal feedbacks in monsoon dynamics may have amplified the weak external forcing.Both spatial patterns and temporal evolution of monsoon rainfall are influenced by a number of physical processes ...

Consistent increase in Indian monsoon rainfall and its variability across CMIP-5 models

“…Similarly, the Hadley Centre models (HadGEM2-CC and HadGEM2-ES) and the Institute Pierre Simon Laplace models (IPSL-CM5A-LR and IPSL-CM5A-MR) capture very little rainfall over the all-India region with comparatively higher rainfall over the Himalayan mountains and the Bay of Bengal. As discussed by Levermann et al (2009), the monsoon region can enter a climatic regime in which latent heat transport towards land is insufficient to sustain a monsoon circulation, which may lead to abrupt monsoon transition (Zickfeld et al, 2005) as observed in the past Cook et al, 2010;Sinha et al, 2011). While observations clearly show that the ISM is currently within the active monsoon regime, it is possible that the CMIP-5 models that exhibit a very weak ISM are outside this regime.…”

Warmer and wetter or warmer and dryer? Observed versus simulated covariability of Holocene temperature and rainfall in Asia

“…They may dominate the seasonal heat balance on long timescales, and explain abrupt changes in monsoon rainfall under small changes in external forcing [21]. In the conceptual model of Levermann et al [21], higher summer temperatures increase the seasonal landsea thermal contrast due to the different heat capacity of land and ocean, strengthening the monsoon onset circulation and allowing for more precipitation. Weaknesses in the simulation of these seasonal processes may explain the lack of positive feedbacks between temperature and rainfall changes on long timescales.…”

The Role of Water Vapour in Earth’s Energy Flows

“…5 is manifest most strongly as contrasting precipitation responses in the wet and dry regions of the tropics (e.g., Chou et al 2007;Allan et al 2010). An alternative perspective is to again consider the energy balance: latent heating through condensation of water droplets and eventual precipitation must be balanced by cooling through radiative processes and dry static energy transport, assuming other terms such as sensible heating are small (Muller and O'Gorman 2011); this may also have bearing upon the viability and stability of monsoon systems (Levermann et al 2009). …”