By Mike Wong
The Tibetan Plateau is the highest and most extensive plateau in the world, with an average elevation exceeding 4000 metres and stretching over 2.5 million square kilometres. While it is often called the ‘rooftop of the world’, it also serves as the ‘water tower of the world’. Many major rivers in Asia, including the Yangtze, Mekong and the Ganges, originate from the Tibetan Plateau and support the livelihoods of more than 40% of the world’s population living in rapidly developing economies such as China and India.
The Tibetan Plateau is also a crucial component of the Asian climate. During late spring to early summer, its vast and elevated surface heats up rapidly and acts as a highly effective heat source for the atmosphere above. The heating from the plateau’s surface is long believed to be vital in creating ascent and supporting the meridional overturning circulation of the Indian Summer Monsoon.
However, recent studies (e.g. Boos and Kuang 2010, 2013, Wu et al. 2012) questioned the impact of the Tibetan Plateau on the Indian summer monsoon. Using idealised orography in which the Tibetan Plateau is removed, keeping only the Himalayas, these studies demonstrated that the Indian summer monsoon can be maintained in global climate model simulations. Therefore, the current consensus in the literature is that the orographic sheltering provided by the Himalayas is equally important, or perhaps more important, than the elevated surface heating from the Tibetan Plateau in maintaining the Indian summer monsoon.
Funded by the Climate Science for Service Partnership China (CSSP-China), the MESETA project aims to investigate further the role of the Tibetan Plateau in maintaining the Asian summer monsoons. Following previous studies, the Met Office’s global climate model HadGEM3 is used to perform various simulation experiments with idealised orography. Three different modifications to the orography are used to demonstrate the impact of the Tibetan Plateau, Himalayas and the Iranian Plateau on the Asian monsoons (Figure 1a-d). The simulations are performed at N96 resolution (200 km grid spacing at the equator) covering the period between 1981 and 2001, providing 20 years’ worth of data to derive the climatology of the summer monsoons. First, a control experiment is performed using the default orography and Figure 2a shows the summer (June-August) average precipitation and 850 hPa wind. Results from each sensitivity experiment is then compared to the control.
Figure 1. Surface orography used in each experiment: a) Control; b) No Tibetan Plateau; c) Himalayas and Iranian Plateau only and d) Himalayas only.
The first experiment, No Tibetan Plateau (Figure 2b-c), focuses on the impact of the Tibetan Plateau and Himalayas on the monsoons by removing both terrains from the model. Compared to the control experiment, the Indian summer monsoon clearly weakened as indicated by the easterly anomalies over the Arabian Sea. Although there is more rainfall over India compared to the control, it is mostly likely related to HadGEM3’s bias in simulating summer rainfall in the region. There is also a reduction in rainfall over most of China, suggesting a weakened East Asian summer monsoon. Therefore, this experiment shows that without the Tibetan Plateau and the Himalayas, both the Indian summer monsoon and the East Asian summer monsoon will be a lot weaker in intensity.
Figure 2. Summer precipitation and 850 hPa wind (left column) and difference relative to control : a) Control; b-c) No Tibetan Plateau; d-e) Himalayas and Iranian Plateau only; f-g) Himalayas only.
However, things improved greatly in the second experiment when the Himalayas were put back into the model (Himalayas and Iranian Plateau only, Figure 2 d-e). Low level circulation over the Arabian Sea and India is more consistent to the control as the easterly anomalies reduced, while summer rainfall over India is also more similar to the control. Therefore, it seems that the Indian summer monsoon can indeed be maintained even if the elevation of the plateau is drastically reduced as long as the Himalayas are intact. In contrast, the East Asian summer monsoon is more sensitive to the presence of the Plateau as most of China is still receiving less summer rainfall than the control.
While the first two experiments demonstrated the crucial role of the Himalayas, the third experiment, Himalayas only (Figure 2 f-g), focuses on the importance of the Iranian Plateau. In this experiment, the Iranian Plateau is removed leaving only the Himalayan ridges in the model such that their contribution can be isolated. Without the Iranian Plateau, the summer westerlies over the Arabian Sea are weakened and the region is affected by north-easterly wind anomalies, bringing dry continental airmass into the region. Although the weakening of the westerly monsoon is not as significant as in the No Tibetan Plateau experiment, the results here show that the Iranian Plateau also exerts considerable influence on the Indian summer monsoon.
To summarise, the idealised experiments here show that the Indian summer monsoon is not sensitive to the elevation of the Tibetan Plateau as long as the Himalayas and the Iranian Plateau are present. In contrast, the East Asian summer monsoon is more sensitive to the presence of the Tibetan Plateau. Also, it is necessary to consider the impact of model bias as the monsoons in the control experiment are not perfect reconstructions, especially in terms of rainfall. To further examine how much of the results are model dependent, some of the experiments will be repeated in other climate models through the forthcoming Global Monsoon Model Inter-comparison Project (GMMIP). So, if one day the Tibetan Plateau somehow disappeared, don’t worry, the Indian summer monsoon will probably be fine (too bad for East Asian summer monsoon though …).