1. Why Weather Sometimes Stays the Same for Weeks
Weather forecasts usually show high-pressure and low-pressure systems moving gradually. However, sometimes the same weather conditions continue for several weeks. One cause of this is "blocking." According to the Japan Meteorological Agency (JMA), when the jet stream (westerly winds) blowing at high and mid-latitudes meanders greatly to the north and south, large-scale high-pressure systems can stagnate on the ground. When this happens, similar weather conditions continue for a long time, often leading to extreme weather events [1]. In this article, "blocking" refers to the state where large meanders in the upper atmosphere and a stationary high-pressure system occur together. So, why does a high-pressure system that should move stop moving?
2. Why a Moving High-Pressure System Can Stop
According to a laboratory at the University of Tokyo, high-pressure and low-pressure systems try to move westward. However, the westerly winds in the upper atmosphere try to hold them back. As a result, these systems may remain in the same location [2]. Imagine a person swimming against a river current. If they swim at the same speed as the current, they appear to stay in the same place relative to the bank. However, a high-pressure system does not swim on its own. This is just an image borrowed to explain how two forces balancing each other make the system appear stationary. When a system stops, forecasting becomes difficult. The same laboratory explains that blocking highs often bring diverse extreme weather events. Predicting them is important for long-term forecasts but is considered very difficult [2]. So, how long can these systems actually stay?
3. How a Blocking High Stayed Over Russia for Over a Month in 2010
Research presented at a workshop at the Disaster Prevention Research Institute of Kyoto University states that from late June to mid-August 2010, blocking occurred primarily in Eastern Europe and Western Russia, bringing abnormal high temperatures [5]. This blocking lasted for more than one month. It is reported to have had the longest duration and the greatest intensity among blocking events in Western Eurasia during summer [5]. More than one month is roughly the length of the summer vacation in Japan. In this example, the regions under the stationary high-pressure system suffered from long-term high temperatures. This is a typical pattern where heat continues under a blocking high. However, this is just one example. The summer in Japan did not follow this same pattern.
4. What Caused Japan's Record Heat in June 2022
In late June 2022, Japan experienced record heat. According to JMA data, the average temperature in late June was 4.0°C higher than normal in Eastern Japan and 3.2°C higher in Western Japan. Both were the highest since statistics began in 1946 [3]. In Tokyo, there were 9 consecutive days with a maximum temperature of 35°C or higher (a "scorching day"), which was also the highest since statistics began [3]. The JMA analysis committee identified the main cause as the subtropical jet stream meandering greatly northward near Japan. As a result, the upper-level high pressure and the surface Pacific High both strengthened to record levels [3]. This was a strengthening of high pressure different from blocking. "Scorching heat does not always mean blocking." So, where was the blocking high in that summer?
5. What the Blocking High North of Japan Did in July 2022
According to JMA analysis committee materials, a blocking high formed north of Japan in early July 2022 and continued until mid-July. To its south, over Japan, a trough accompanied by cold air (a cold vortex) persisted in the upper atmosphere [3]. When cold air is in the upper atmosphere, the atmosphere becomes unstable. This contributed to short-duration record heavy rain in various places [3]. The same materials state that the persistence of the northern blocking high and the subtropical jet stream meandering southward near Japan, making it susceptible to upper-level cold air, are "considered" factors in the heavy rain through mid-July. Furthermore, the formation and persistence of the blocking high are said to be related to persistent, large meanders of the polar front jet stream [3]. Weather changes depending on location. In Russia, heat stayed under the high. In Japan in 2022, the blocking high was to the north, and the upper atmosphere over Japan had a trough with cold air to its south. One view is that it is hot under a stationary high, and rain is likely next to it. However, this is not a rule that applies every time.
6. How Long-Lasting Jet Stream Meanders Caused Long-Lasting Weather in 2018
The JMA announced that large meanders of the polar front jet stream and subtropical jet stream in the upper atmosphere persisted, influencing the heavy rain and heat in July 2018 [4]. The heat was attributed to the Pacific High and the upper-level Tibetan High both extending over Japan for a long time. The average temperature in July in Eastern Japan was the highest since statistics began in 1946 [4]. The term "blocking" does not appear in this announcement. Therefore, this article refers to 2018 only as "persistence of meanders." The reason why blocking occurs has not been settled. A model study on the 2010 Russia case suggests that the intensity of the high pressure and heat wave in Western Russia was driven by internal atmospheric dynamics, rather than forced by external conditions like sea surface temperatures, Arctic sea ice, or greenhouse gases [5]. It is also suggested that drought associated with the heat wave may have strengthened the heat wave [5]. Statistical studies show that the frequency increases during La Niña years, so the cause is not determined to be one thing [5]. From these materials, it cannot be decided if it can be summed up as "because of global warming."
7. How to Spot Stationary High-Pressure Systems on Weather Maps
Keeping records of weather maps can sometimes reveal stationary highs. Look at weather maps from newspapers, TV forecasts, or the JMA website at the same time every day, and write the location of the high-pressure center in a notebook. If you continue for about a week, you can see if the high is moving or staying in the same place. This record only shows if the surface high moved. Whether it is blocking requires experts to judge by looking at upper-air flow as well. You may not encounter a stationary high, but simply grasping the movement of moving highs shows how fast normal highs move, helping you see the difference from stationary ones. To read the original source, the JMA press release dated August 22, 2022 (Extreme Weather Analysis Committee) is available in Japanese. Paying attention to where the phrase "considered" is used shows how meteorological experts distinguish between "what is known" and "estimates."