Post-91: Korean Monsoon Average Onset Dates, 2005-2013

The Question of When Korea’s Monsoon Begins continues, for some reason, to haunt me, following post-89 and post-90. I remain puzzled about whether the first Monsoon (Jang-Ma [장마] in Korean) on June 17th of this year was “early” or not.

To satiate my curiosity, I tried to find, via online news reports, the reported first Jang-Ma days in Korea over the past ten years. Caveat: I am not so sure about 2011’s truly-very-early date, which may only have influenced Jeju Island.

Dates of the Start of Monsoon Seasons, Jang-Ma, in Mainland Korea
2005: June 26th
2006: June 21st
2007: ?? — I could not find it in the English press
2008: June 17th
2009: June 25th
2010: June 19th
2011: June 10th — “the monsoon…arrive[d] earlier than at any time since the KMA began compiling statistics in 1973”
2012: June 30th
2013: June 17th

The average “Monsoon season onset day” in the past decade, if excluding 2011, is June 22nd; if including 2011’s, it’s June 21st. Both are earlier than the historical average of June 25th-29th. That date range is according to this, which analyzes the entire period of 1778-2004 for Seoul.


Average Rainfall in Seoul, 1778-2004. “Pentad 36” is the five-day period, is June 25th-29th.
From “Variability and Singularity of Seoul, South Korea, Rainy Season (1778–2004)” [Link]

This is a pretty comprehensive academic study, and finds a June 25th-29th average onset day. The government says that the Jang-Ma season for central Korea started, on average, June 24th/25th in the 1981-2010 period.

1778-2004 Average: June 25th-29th
1981-2010 Average: June 24th-25th
2005-2013 Average: June 21st-22nd

My final conclusion: The June 17th 2013 Monsoon was “early”, but not very early. More interestingly, the recent average start day of the Jang-Ma season has been getting earlier, if my attempt to analyze the data is correct. This does not point to a general “warming trend”, because, also ancedotally, I can say that winter was very, very long this year (see post-34 and post-63). Koreans themselves have been saying spring starts later and later each year recently.

In the course of my reading, I saw that Korea’s rainfall is higher today than it was in the 1800s:

Rainfall Statistics in Korea, in millimeters.
Chosun: 1778-1907 / Modern: 1908-2004.
Table 1 from “Variability and Singularity of Seoul, South Korea, Rainy Season (1778–2004)” [Link]

That’s 10% more rain per year in the past century than earlier. Why? I have no idea.
This 85-mm increase in summer rainfall could be interpreted as more days of “regular” (non-torrential) rain, or as more days of true Monsoon, of torrential rain. The following may suggest the latter:

Meteorologists believe Korea is witnessing a rainy season throughout the summer as the country’s climate turns subtropical, with global warming raising the amount of water vapor in the atmosphere. The “August rains” often cause a substantial amount of damage, since torrential downpours are focused on a small area over a short period of time. Between 1954 and 1963, only an average of 1.6 days a year saw 80 mm of rainfall a day. But that increased to 2.3 days between 1994 and 2003. The torrential downpour phenomenon is becoming a feature of our weather. Global warming increases the amount of energy in the atmosphere, leading to more frequent extreme weather conditions, such as typhoons.

I’d like to know more on the topic, but my mind is exhausted from looking to this “First Jang-Ma Day Question” I’ve been on, so I’m throwing in the towel. If someone else who may reads this knows or can guess, leave a comment.