Monsoons and their tipping points

The future evolution of monsoon rainfall under increasing levels of atmospheric CO2 and aerosol pollution is highly uncertain, and although it is generally accepted that greenhouse gases tend to increase monsoon strength, distribution of aerosols may have other effects, leaving the overall effect uncertain (Leverman et al. 2009). Summer monsoons are known to vary at orbital and longer timescales as a result of external forcing, but of more interest are the abrupt changes that have occurred without any known forcing over shorter millennial and suborbital scales (Gupta et al. 2003). Millennial-scale abrupt monsoon events within the current interglacial have been linked to North Atlantic climatic changes, according to the hypothesis that increased winter snowfall weakens the monsoon the following summer (Meehl, 1994).

Figure 1 - results of study of G. bulloides and haematite to
show periodically weakened monsoon.

Tipping points have been crossed in monsoon systems before, switching between two stable states. During the Holocene and last glacial period, rainfall in India and China has undergone strong and abrupt changes associated with these states (Leverman et al. 2010). Zickfeld et al. (2005) developed a simple model which identified two stable states in the Indian summer monsoon, potentially allowing for rapid transition between radically different monsoon circulations. We can examine these state shifts by looking at magnetic susceptibility in sediments, oxygen isotopes, dust flux in marine cores, pollen and many others (Feng et al, 2006). Gupta et al (2006) observed seven intervals of weak summer monsoon during the Holocene  (highlighted in the grey bars in Fig. 1) that can be correlated within age uncertainties to millennial-scale Dansgaard-Oescher events in the North Atlantic. These were identified using haematitie and planktonic foraminifera Globigerina bulloides, a unique proxy for monsoon circulation in the tropics that shows linear correlation with the surface cooling due to upwelling (Gupta et al. 2006). These weak phases correlate with North Atlantic cool peroids, and monsoon maxima with warm periods in the North Atlantic. Gupta et al (2006) suggest that the monsoon could be sensitive to relatively small changes in forcing of 0.25% change in solar output, or a 2°C change in sea surface temperature.

Leverman et al (2010) suggest that North Atlantic climatic events had a very small effect, but internal feedback of the monsoon amplified changes. The main feedback is the release of latent heat from precipitation over land, which increases to the temperature difference between land and ocean. This pushes stronger winds from ocean to land, thus increasing advection of moisture over land, enhancing precipitation and associated release of latent heat. If monsoon winds get even slightly weaker, this feedback is reduced significantly. The abrupt transition emerges through an additional stabilizing effect of the direct heat advection which is cooling the atmospheric column and is also reduced for reduced monsoon winds. This continues, along with other feedbacks, until a threshold is reached at which condensation/precipitation cannot provide the latent heat needed to maintain a circulation (Leverman et al, 2010).

Monsoons are depended on by millions of people in some of the world’s most densely populated regions for water for agriculture and fisheries, recharge of aquifers and maintenance of ecosystems amongst others. It is therefore in our best interest to mitigate or at least understand the impact that natural variability and global climate change may have on reaching a critical threshold in monsoon systems and turning of the cycle for good.

Gupta, A.K., D. M. Anderson, J.T. Overpeck (2003) “Abrupt changes in the Asian southwest monsoon during the Holocene and their links to the North Atlantic Ocean”. Nature 421, 354–356.

Levermann, A., J. Schewe, V. Petoukhov, and H. Held (2009) “Basic mechanism for abrupt monsoon transitions” PNAS, vol. 106, 49, 20572–20577

Meehl, G. A. (1994) “Influence of the land surface on the Asian summer monsoon, external conditions versus internal feedbacks”. Journal of Climate. 7, 1033–1049

Zickfeld K, B. Knopf, V. Petoukhov and H. J. Schellnhuber (2005) “Is the Indian summer monsoon stable against global change?” Geophysical Research Letters 32:L15707.