Westerlies and the East Asian Summer Monsoon
A graduate seminar that I co-led with Inez Fung on East Asian speleothem records sparked my interest on the dynamics of the East Asian summer monsoon (EASM). It is a curious regional climate system: it is thought of as a monsoon climate and thus many papers on the subject focus on land-ocean contrasts, lower tropospheric moisture transport and feedback from diabatic heating. However, the EASM is also sufficiently far north to be under the influence of the westerlies in early summer. Two features caught my attention: first being its complex seasonal evolution comprised of several intraseasonal stages and abrupt transitions between them; and second being the downstream influence of the westerlies impinging on the Tibetan Plateau. It made me wonder whether the northward migration of the westerlies from South of the Plateu in Spring stage to North of the Plateau by the post-Meiyu stage set and paced the intraseasonal stages (as speculated by this paper). In Chiang et al (2015), we proposed the ‘Jet Transition Hypothesis’ where we hypothesized that East Asian summer monsoon changes were characterized by the timing and duration of its intraseasonal stages, as determined by the timing of the jet migration across the Plateau.
The hypothesis has worked out quite well in a number of scenarios ranging from paleoclimate on orbital and millennial timescales, to interannual variabilty, and to future changes. (Side note: the relative ease with which the hypothesis explained things gave me the confidence that we were on the right track. Most hypotheses tend to not go far!). First, paleoclimate. In Kong et al. 2017 we showed that the early Holocene EASM is marked not by an intensification (as commonly interpreted), but through earlier transitions of its intraseasonal stages; we also showed that this interpretation readily explains the asynchronous Holocene peak seen in EASM paleorecords. Zhang et al. 2018 shows that during stadials, central eastern China is wetter despite the fact that cave d18O records saying otherwise. We explained this as a later termination of the Meiyu stage, which explains the increased rainfall over central eastern China but dry to its north. Using modern-day interannual variability of rainfall d18O, Chiang et al. 2020 showed that enriched oxygen isotope of precipitation indicates reduced summer seasonality of the EASM and the westerlies.
Moving onto interannual variability and future climate: In Chiang et al. 2017 we showed that the well-known ‘tripole’ in EASM rainfall interannual variability comes from variation in the timing of Meiyu termination, modulated by the jet position over the Plateau. Kong and Chiang 2020 argued that El Nino causes a strengthening of the Meiyu rains in the following summer through the former’s influence on the jet position over the Plateau. A similar mechanism appears to operate for the future EASM: in Chiang et al. 2019 we found a remarkable intensification of the pre-Meiyu rainband in the late 21st century under the RCP8.5 scenario, caused by a delayed northward migration of the jet across the Plateau in early summer.
We substantiated the link between EASM intraseasonal stage and the meridional jet position across the Plateau (as first hypothesized explicitly by this paper) in two papers. In Kong and Chiang 2019 we argued that the Meiyu stage terminates when the maximum jet position shifts beyond the northern edge of the Plateau, due to the loss of orographic forcing. We generalized this in Chiang et al. 2020 where we argued that the unique intraseasonal stages of the EASM comes from the presence of the Plateau, and that its seasonality comes from the interaction between two seasonally evolving circulations: namely, monsoonal southerlies that strengthen, and midlatitude northerlies that weaken and eventually disappear, as summer progresses.
Chiang, J.C.H., Fung, I.Y., Wu, C.H., Cai, Y., Edman, J.P., Liu, Y., Day, J.A., Bhattacharya, T., Mondal, Y. and Labrousse, C.A., 2015. Role of seasonal transitions and westerly jets in East Asian paleoclimate. Quaternary Science Reviews, 108, pp.111-129.
Chiang, J.C.H., Swenson, L.M. and Kong, W., 2017. Role of seasonal transitions and the westerlies in the interannual variability of the East Asian summer monsoon precipitation. Geophysical Research Letters, 44(8), pp.3788-3795.
Chiang, J.C.H., Fischer, J., Kong, W. and Herman, M.J., 2019. Intensification of the pre‐Meiyu rainband in the late 21st century. Geophysical Research Letters, 46(13), pp.7536-7545.
Chiang, J.C.H., Herman, M.J., Yoshimura, K. and Fung, I.Y., 2020. Enriched East Asian oxygen isotope of precipitation indicates reduced summer seasonality in regional climate and westerlies. Proceedings of the National Academy of Sciences, 117(26), pp.14745-14750.
Chiang, J.C.H., Kong, W., Wu, C.H. and Battisti, D.S., 2020. Origins of East Asian summer monsoon seasonality. Journal of Climate, 33(18), pp.7945-7965.
Kong, W., Swenson, L.M. and Chiang, J.C.H., 2017. Seasonal transitions and the westerly jet in the Holocene East Asian summer monsoon. Journal of Climate, 30(9), pp.3343-3365.
Kong, W. and Chiang, J.C.H., 2020. Interaction of the westerlies with the Tibetan Plateau in determining the mei-yu termination. Journal of Climate, 33(1), pp.339-363.
Kong, W. and Chiang, J.C.H., 2020. Southward shift of westerlies intensifies the East Asian early summer rainband following El Niño. Geophysical Research Letters, 47(17), p.e2020GL088631.
Zhang, H., Griffiths, M.L., Chiang, J.C.H., Kong, W., Wu, S., Atwood, A., Huang, J., Cheng, H., Ning, Y. and Xie, S., 2018. East Asian hydroclimate modulated by the position of the westerlies during Termination I. Science, 362(6414), pp.580-583.