To clarify, the causal connection between a specific surge in the East Asian Monsoon and an extreme PacNW heat wave a week later is not attribution — it is just a good hurricane analyst taking ten minutes to capture and interpret ECMWF hindcast animations before moving on to cover Elsa and secondary condo collapse.
The degree to which this heat wave can be attributed to the effects of climate change on the EAM is complex and has not yet been determined. We’d like to know if these heat waves are going to be a quasi annual June and July occurrence and if climate change is causing them.
People here know the difference between climate and weather but this event was something different from natural variation (which will continue into the future though differently). Heat waves per se are not unusual. However one was so extreme in so many ways in that it fell too many sigma (std dev) outside the probability distribution applicable to PacNW weather history.
Written records for the East Asian Monsoon go back several thousand years, mostly addressing onset date, seasonal winds, rainfall intensity and rainband geographic extent. EAM is related to but distinct from SEAM, the more familiar southeast Asian monsoon of India.
People are still arguing whether ‘monsoon’ is appropriate for the southwestern US rainy season — which started yesterday but was a non-soon last summer. As noted earlier, the EAM is called meiyu-baiu-changma in China, Japan and Korea respectively (how to pronounce?).
Attribution analysis of one-off events is a fairly new technical topic within climate change. The need comes up all the time: did hurricane such-and-such veer or stall because of it, did it cause Atlantic inflows past Svalbard to slow last year, is the 20-year AZ drought within natural variation, would a GAC2012 repeat this August be surprising, was the late June jet stream wobble over BC driven, is solar cycle 25 affecting ENSO, and so on. It is different from and perhaps more difficult than characterizing future climate directions.
Dissecting out the climate change causal component for a one-time extreme event from ordinary weather weirdness seems to fly in the face of a need for a probabilistic approach. However it’s something climate scientists would like to have in responding to questions from reporters during breaking events.
Whether it can be done persuasively in the case of this heat wave remains to be seen. It may very well have been in combination with ‘perfect storm’ coincidences and locally aggravating conditions.
Impact of Anthropogenic Climate Change on the East Asian Summer Monsoon
C Burke and P Stott 15 Jul 2017
https://doi.org/10.1175/JCLI-D-16-0892.1 open source 26 cites
The East Asian summer monsoon (EASM) is important for bringing rainfall to large areas of China. Historically, variations in the EASM have had major impacts including flooding and drought. The authors present an analysis of the impact of anthropogenic climate change on EASM rainfall in eastern China using a newly updated attribution system. The results suggest that anthropogenic climate change has led to an overall decrease in total monsoon rainfall over the past 65 years and an increased number of dry days.
However, the model also predicts that anthropogenic forcings have caused the most extreme heavy rainfall events to become shorter in duration and more intense. With the potential for future changes in aerosol and greenhouse gas emissions, historical trends in monsoon rainfall may not be indicative of future changes, although extreme rainfall is projected to increase over East Asia with continued warming in the region.
Attribution of extreme precipitation in the lower reaches of the Yangtze River during May 2016
Chunxiang Liet al
https://iopscience.iop.org/article/10.1088/1748-9326/aa9691/meta 2018 open source
May 2016 was the third wettest May on record since 1961 over central eastern China based on station observations, with total monthly rainfall 40% more than the climatological mean for 1961–2013. Accompanying disasters such as waterlogging, landslides and debris flow struck part of the lower reaches of the Yangtze River. Causal influence of anthropogenic forcings on this event is investigated using the newly updated Met Office Hadley Centre system for attribution of extreme weather and climate events. Results indicate that there is a significant increase in May 2016 rainfall in model simulations relative to the climatological period, but this increase is largely attributable to natural variability. El Niño years have been found to be correlated with extreme rainfall in the Yangtze River region in previous studies—the strong El Niño of 2015–2016 may account for the extreme precipitation event in 2016. However, on smaller spatial scales we find that anthropogenic forcing has likely played a role in increasing the risk of extreme rainfall to the north of the Yangtze
A review of climate change attribution studies
P Zhai, B Zhou, Y Chen
https://link.springer.com/article/10.1007/s13351-018-8041-6 2018 Cited by 34
This paper reviews recent progress in climate change attribution studies. The focus is on the
attribution of observed long-term changes in surface temperature, precipitation, circulation,
and extremes, as well as that of specific extreme weather and climate events. .. In terms of extreme weather and climate events, it is clear that attribution studies have provided important new insights into the changes in the intensity or frequency of some of these events caused by anthropogenic climate change. The framing of the research question, the methods selected, and the model and statistical methods used all have influences on the results and conclusions drawn in an event attribution study.
Climate change expected to shift location of East Asian Monsoons
Warming climate could lead to profound changes in the subtropical climate
Lawrence Berkeley National Laboratory November 13, 2019
https://www.sciencedaily.com/releases/2019/11/191113075116.htm Nature Climate Change
The Asian monsoon is closely linked to a planetary-scale tropical air flow which, according to a new study, will most likely shift geographically as the climate continues to warm, resulting in less rainfall in certain regions. Researchers used global climate models to study the Hadley cell. Results suggest that the East Asian Monsoon will shift geographically as the climate continues to warm, and that enhanced warming at the equator will drive this shift.
The Hadley cell consists of two components -- moist air that rises at the equator, or the deep tropics, causing heavy precipitation during monsoons, and dry air that descends in the subtropics on either side of the equator, resulting in dry conditions in the subtropics. Under anthropogenic warming, the dry subtropical part will expand towards the north and south poles, while the moist deep tropical part will get smaller, according to global climate models. The researchers used a worst-case climate change scenario defined by the IPCC to model the climate in the last 30 years of the 21st century.
Large-Scale Dynamics of the Meiyu-Baiu Rainband: Environmental Forcing by the Westerly Jet
Takeaki Sampe and Shang-Ping Xie Jan 2010 cited by 356!
https://journals.ametsoc.org/view/journals/clim/23/1/2009jcli3128.1.xmlMeiyu-baiu is the major rainy season from central China to Japan brought by a zonally elongated rainband from June to mid-July. Large-scale characteristics and environmental forcing of this important phenomenon are investigated based on a reanalysis dataset. The meiyu-baiu rainband is accompanied by a trough of sea level pressure, horizontal shears, and sharp moisture gradients near the surface, a westerly jet tilted northward with height, and large northeastward moisture transport from the south.
The analysis here reveals the westerly jet as an important culprit for meiyu-baiu. Along the rainband, mean ascending motion corresponds well with a band of warm horizontal temperature advection in the midtroposphere throughout summer. This adiabatic induction of upward motion originates from the advection of warm air by the westerlies from the eastern flank of the Tibetan Plateau.
The ascending motion both induces convection and is enhanced by the resultant condensational heating. The westerly jet anchors the meiyu-baiu rainband also by steering transient eddies, creating periods conducive to convection through convective instability and adiabatic updrafts. Indeed, in meiyu-baiu, the probability distribution of convective instability shows large spreads and is strongly skewed, with a sharp cutoff on the unstable side resulting from the effective removal of instability by convection.
Thus, active weather disturbances in the westerly waveguide explain a paradox that convection is active in the meiyu-baiu rainband while mean convective instability is significantly higher to the south over the subtropical North Pacific warm pool.
