By: Amulya Chevuturi
The arrival of the summer monsoon rains over southern China is called Meiyu, literally translated as “plum rains”. These are also called Baiu in Japan and Changma in Korea. As the monsoon progresses, these rain belts first occur over Taiwan, southern China and Okinawa region from early-May to mid-June, then continue towards the Yangtze River valley and the main islands of Japan from mid-June to mid-July, and finally reach the Korean Peninsula and north-eastern China during mid-July to mid-August. This northward march of the rain belts occurs in tandem with the seasonal progression of the East Asian summer monsoon. These rain belts, which last on an average for 8 days, are named after the respective regional names of the season.
Figure 1: Example of the Meiyu—Baiu—Changma rain belt cloud cover (demarcated with a red dotted line) over parts of southern China, Taiwan and Okinawa Islands on 14th June 2017. Source: WorldView NASA.
These rain belts are formed due to monsoon frontal systems, which are strongly influenced by the moisture flow from the monsoon winds in the lower atmosphere. These east-to-west extended frontal systems may last for 3 to 22 days and lead to local-scale variability within the large-scale monsoon season over East Asia. As these frontal systems move north, their characteristics transition from those of tropical disturbances to mid-latitude ones. The rainfall from these systems contributes up to 45% of the total monsoon rainfall over some regions of East Asia. A snapshot of cloud cover during one such rain belt over East Asia, on 14th June 2017, is shown in Figure 1. This weather system led to devastating amounts of rainfall, which lasted almost up to 10 days. It left many displaced people, had damaged infrastructure and ruined crops, due to subsequent floods and landslides.
Figure 2: (a) Mean position of the monsoon front band during May (red), June (purple) and July (blue) and (b) mean June rainfall associated with the mean June frontal band (purple) in ERA5 dataset from 1979-2014.
Researchers in our department, inspired by previous research, have defined an objective method to identify these frontal systems by detecting regions with strong temperature and moisture gradients by using the north-to-south gradient of the equivalent potential temperature. Through this method we can identify the narrow band of the monsoon frontal systems and detect any rainfall within approximately 600 kilometres of the band, as the Meiyu—Baiu—Changma rain belt. Using this method, we have identified the monthly average position of the monsoon front and accompanying rainfall for May, June and July from 1979-2014, using the newest reanalysis dataset from European Centre for Medium-Range Weather Forecasts (ECMWF), named ERA5, shown in Figure 2. We clearly observe the northward progression of the frontal bands from May—June—July, and more rainfall south of the frontal band position. We also found larger contribution to total monsoon rainfall during May than in July, from the monsoon fronts, due to a greater number of frontal systems early in the season.
When we analysed the representation of these monsoon fronts in the UK Met Office model, we found that although the model can generally represent the average position of these frontal systems, it overestimates the associated rainfall. This rainfall error in the model stems from the overall overestimation of the summer monsoon rainfall over East Asia. Further, the monsoon fronts are influenced by the El Niño Southern Oscillation (ENSO) conditions (based on the equatorial Pacific sea surface temperature) in the proceeding winter. Warmer temperatures (El Niño phase), increase the occurrence of these frontal systems, whereas an opposite response is seen due to colder sea surface temperatures (La Niña phase). Although the model correctly depicts this remote teleconnection between ENSO phases and the monsoon fronts, the strength of the remote relationship is much stronger in the model.
The monsoon rainfall from such systems affect the lives of over a billion people. Our continued work for better understanding of these monsoon frontal systems and their representation in the models, will ultimately improve the prediction of rainfall during the monsoon season. This will allow for improved disaster management, agricultural planning and water resource allocation, and thus benefitting the regional economy.