The Influence of Westerlies on Japan's Four Seasons (Part 1)

Introduction

When people hear the phrase "Japan's four seasons," everyone envisions a specific landscape. Cherry blossoms in full bloom in spring, cicadas buzzing amidst towering clouds in summer, mountains painted in autumn colors, and snow covering the earth in winter—these scenes have been depicted countless times in literature, painting, and music, shaping the very sensibility of the Japanese people.

However, the beauty of these four seasons is not something nature provides automatically. Global atmospheric currents, particularly the existence of the "westerlies," create the foundation that vividly colors the seasons of the Japanese archipelago. The westerlies are constant winds that blow from west to east in the mid-latitudes, and Japan lies directly in their path. Therefore, Japan's climate is significantly influenced not only by the temperature difference between north and south or ocean currents, but also by this atmospheric rhythm.

In this article, we will look specifically at how the westerlies create Japan's four seasons, following the flow of spring, summer, autumn, and winter. Furthermore, while touching upon their relationship with agriculture and culture, as well as their connection to the increasing frequency of extreme weather in recent years, I would like to delve into the power of these invisible winds that support "Japan, the land of the four seasons."

Chapter 1: The True Nature of the Westerlies — The Giant Current Circulating the Atmosphere

The Westerlies within the General Atmospheric Circulation

The Earth experiences large temperature differences due to receiving heat from the sun. The equatorial region is heated, while the polar regions are cooled. To resolve this temperature difference, the atmosphere repeats a cycle. Air rises from the equator, flows north, descends, and returns to the equator in the "Hadley cell," followed by the "Ferrel cell" to the north, and the "polar cell"—within these three circulations, Japan is under the influence of the Ferrel cell.

The wind that blows at the surface of the Ferrel cell is the "westerlies," and the current that blows strongly at an altitude of around 10,000 meters is the "jet stream." This jet stream can reach speeds of several tens of meters per second and has enough power to affect the flight of aircraft.

North-South Movement of the Jet Stream

The belt of westerlies is not constant throughout the year. It shifts its position north in summer and south in winter, and sometimes meanders significantly. This north-south oscillation is the major mechanism that brings the four seasons to Japan.

Chapter 2: Spring — The Change of Seasons Announced by the Westerlies

The Mechanism of 'Sankan-Shion' (Three Cold Days and Four Warm Days)

During the transition from winter to spring, cold and warm air masses clash over Japan. As the westerlies meander and carry low-pressure systems one after another from the west, the phenomenon of "sankan-shion" occurs, where cold days and warm days alternate. This repetition makes one feel the arrival of spring on their skin.

Yellow Sand and Spring Storms

The westerlies also carry yellow sand (kosa) from the Chinese continent. The sand, which travels thousands of kilometers, hazes the spring sky and has become a symbolic phenomenon of spring in Japan. Furthermore, when the westerlies combine with low-pressure systems, strong winds known as "haru-ichiban" (the first storm of spring) blow, whipping up spring storms.

Culture and Daily Life

It is no coincidence that the word "spring breeze" appears frequently in waka poetry. The winds created by the meandering of the westerlies must have left a strong impression on the sensibilities of ancient people. In agriculture, too, the low-pressure systems and rain of spring have moistened the fields and announced the beginning of a new farming season.

Chapter 3: Rainy Season (Tsuyu) — The Unique East Asian Rainy Season Created by the Westerlies

Formation of the Baiu Front

From May to June, the westerlies are gradually pushed northward. As a result, warm air and cold air collide over the Japanese archipelago, forming a stationary front. This is the "Baiu front," which creates Japan's unique season of long-lasting rain.

Rain and Rice Cultivation

For ancient people, the rain of the rainy season was a blessing in itself. The rainy season was essential for storing enough water in rice paddies, and without this season, Japanese-style rice cultivation would not have been possible. Words like 'samidare' (early summer rain) and 'minazuki' (the sixth month) were born because daily life was heavily dependent on this rain.

Heavy Rain Disasters and Modern Challenges

However, in recent years, the meandering of the westerlies and rising sea temperatures have caused the rainy season front to stagnate for long periods, leading to an increase in the formation of linear precipitation zones. As a result, heavy rain disasters have begun to occur frequently in various regions. The rainy season, which was once a 'blessing of rain,' has now sometimes become a 'threat of rain' in modern times.