Abrupt transition in organized convection during the monsoon onset in central India and Climate change effect

The Indian summer monsoon (ISM) has profound impacts on the economy and society since it directly affects more than a billion people in the Indian subcontinent. The development of land-ocean thermal contrast during May and June creates meridional temperature and pressure gradients and sets up the ISM circulation. The ISM circulation comprises of two branches, the Arabian Sea (AS) branch that is associated with southwesterly winds blowing from the AS towards the Indian landmass; and the Bay of Bengal (BoB) branch characterized by cyclonic circulation extending from the BoB into central and north India. These two branches dictate the advance of the ISM.

Forecast of the ISM rainfall is challenging even though it has been a research question for many decades. The Indian Meteorological Department forecasts the onset of monsoon over the Kerala, in the south [1]. The recently developed tipping element approach [2] allows forecasting the onset and withdrawal of the monsoon over Central India. However, every state in India desperately needs both forecasts: the onset and withdrawal of the monsoon. Uncertainty and delays (eg. year 2019) in the advance and withdrawal of monsoon results in farmers losing their crop investment. Further, there is no clear consensus on the effect of global warming on the monsoon timing.

Here we explore climate change effects on the advance of the ISM onset towards central India analysing observational data of Outgoing Longwave Radiation (OLR), near-surface air temperature and wind. OLR is a proxy for organized deep tropical convection, wherein low values of OLR correspond to deep clouds with low cloud-top temperatures and high values of OLR correspond to scarcity of clouds.

We use the tipping element approach [2] to reveal tipping in spatially organized rainfall. We find two tipping elements appearing in the AS and the BoB prior to the onset of monsoon in central India (MOC). Maximum fluctuations in the OLR at the tipping elements near MOC indicate deep convection within the two branches of monsoon. The abrupt transition in the OLR at the tipping elements corresponds to the transition from pre-monsoon to monsoon in Central India. We observe an interplay between the temporal dynamics of OLR at these two regions, which indicate the MOC. In these two regions, during the pre-monsoon season the average OLR closely follow each other. Subsequently, the time series of OLR in these two regions diverge from each other, which indicates MOC.

Under climate change, the temporal dynamics of OLR at these two locations show that the transition from pre-monsoon to monsoon has changed from an abrupt transition to a gradual transition in the Bay of Bengal. Furthermore, we identify different spatial patterns of near-air surface temperature, OLR and wind for early, normal and late MOC. We use these patterns as indicators for forecasting advance of ISM.

NBG and ES acknowledge the support of the EPICC project (18_II_149_Global_A_Risikovorhersage) funded by BMU

 

[1] https://mausam.imd.gov.in/

[2] Stolbova, V., E. Surovyatkina, B. Bookhagen, and J. Kurths (2016). GRL 43, 1–9 [doi:10.1002/2016GL068392]