During boreal summer, the Indian summer monsoon (ISM) represents one of the most energetic components of the earth’s climate system. The ISM circulation and its related abundant rainfalls are crucial for economy and natural ecosystems of South Asia and extreme states of the ISM can lead to droughts or floods, which can have severe impacts on Indian society. But the influence of the ISM related convective activity is not limited to the Indian region. Acting as a heat source, the ISM can affect the mid-latitude circulation via its interaction with quasi-stationary wave patterns in the mid-latitude regions. Circumglobal wave trains are important for the formation and the maintenance of heat waves and flood events in the mid-latitudes. These wave trains can be modulated the ISM rainfall activity and thus influence ISM intraseasonal and interannual variability. The understanding of these two-way interactions between mid-latitude circumglobal wave trains and monsoon system and the influence of these teleconnections on local and remote surface weather conditions and extreme weather events is fundamental for improving seasonal as well as long-term climatological forecasts of both, the ISM and the mid-latitude summer circulation. During many periods with extreme ISM conditions interactions with the mid-latitude circulation have been observed. The 2010 Indo-Pakistan flooding is an insightful example of such interaction: it occurred simultaneously with a devastating heatwave over Russia and the two events were linked via a wavy jet stream pattern. However, the mechanisms that govern the ISM-mid-latitudes interaction are not yet well understood. Here, I analyse the topic of tropical – extra-tropical interactions in the Northern Hemisphere during boreal summer using a combination of machine learning approaches and state of the art climate model simulations. To improve our understanding of these links, I apply causal discovery tools to assess causal pathways between different components of the ISM circulation system and distinct remote regions including such from the mid-latitudes. Using this technique, the importance and magnitude of tropical and extratropical drivers of the ISM circulation and intraseasonal variability are assessed. As a complementary approach, I use a large ensemble of simulations from an atmospheric model to study the influence of several boundary and surface conditions on concurrent extremes in the ISM – western Russia regions. Both approaches show the importance of a mutual connection between ISM rainfall and the circulation in northern mid-latitudes. In the course of this thesis, I first analyse the 2010 concurrent extremes affecting Russian surface temperatures and Pakistan rainfall using a large ensemble of model simulations. My results show that the atmospheric pattern responsible for those events is a recurrent wave pattern, which is captured by the climate model. Second, I analyse the causal relationships among mid-latitude circulation, the ISM and its tropical drivers using a causal discovery tool able to remove spurious non-causal links among a set of variables selected based on theory. The direction and the sign of these causal links are not affected by the El Niño-Southern Oscillation (ENSO), while the strength of the causal links is modulated by the phase of ENSO. Finally, these causal discovery tools are applied to the problem of seasonal forecasting of the ISM total rainfall amount. Here, I show that causal precursors give useful predictive skill up to 4-month lead time. In conclusion, in this thesis I show that the two-way link between the ISM and the mid-latitude circulation is (i) confirmed in both a causal analysis framework and in experiments using dynamical atmosphere model, (ii) is modulated by ENSO and (iii) is important for the understanding of concurrent mid-latitude – tropical extreme events.
|Award date||8 Sept 2021|
|Publication status||Published - 8 Sept 2021|