A Triangle Mechanism That Brings Hot Summer of East Asia Has Been Revealed
A record-breaking hot and dry summer of 1994 was reported in East Asian countries. Higher than normal air temperatures are observed over the northeastern and eastern China, Korea, and Japan in 1994 summer (June-July-August). Some higher temperatures are also found above the Kuroshio Extension in the northwestern Pacific. Water vapor strongly diverges from the northeastern and eastern China, Korea, and Japan, leading to a severe drought condition in this region. All these are related to the intensified anticyclonic circulation in both the lower and upper tropospheres around East Asia.
The intensification of this anticyclonic circulation, i.e., the cause of this unusual summer of 1994 in East Asia has been found related to an ocean-atmosphere coupled signal in the tropical Indian Ocean, the Indian Ocean Dipole (IOD) (Figure 1 below). In 1994, a strong IOD event was observed without El Niño. This 1994 IOD event continued for more than 8 months from around March through October. It characterizes with the higher temperature in the western equatorial Indian Ocean and the lower temperature in the southeastern Indian Ocean. A triangle mechanism that brings the extremely hot and dry summer of East Asia has been revealed.
Figure 1: Illustration of the triangle teleconnections of the Indian Ocean Dipole
1. IOD-India-Europe connection:
The IOD (1) induces surplus Indian summer monsoon rainfall (2), and intensifies the upward motion over around India. The associated divergent flow in the upper troposphere is found over India and the Tibetan Plateau. This divergent flow progresses westward and converges over the Mediterranean/Sahara region where the decent of air is intensified, constructing a zonal-vertical circulation cell from the northern India towards the Mediterranean/Sahara region. As a result of the atmospheric response to this divergent flow, the summertime anticyclonic circulation in the upper troposphere is excited, which covers west Asia and the northern part of Africa (3).
2. Europe-East Asia connection:
The IOD-induced circulation changes over the Mediterranean/Sahara region and its vicinity must steadily perturb the mid-latitude westerly. The lateral disturbances may also perturb the westerly. The wave energy propagates along the Asian jet stream eastward to East Asia (4), periodically intensifying the aforementioned anticyclonic circulations around East Asia (5) (Reference 1) , and consequently causing extremely hot summer in this region.
3. IOD-China-Japan connection
Water vapor also diverges from the eastern pole of the IOD (a) into the southern part of China, leading to surplus rainfall in this region (b). A cyclonic circulation is induced in the lower troposphere, weakening the moist monsoonal southerly wind that blows normally northward from the Bay of Bengal and the South China Sea to the eastern part of China, Korea and Japan during summertime (Reference 2). The divergent flow in the upper troposphere over the southern part of China emanates towards the Sea of Japan where the decent of air is induced, causing the extremely hot and dry summer conditions in East Asia (c).
Therefore, the 1994 IOD event brings hot and dry summer this year in East Asia through the IOD-China-Japan connection, the IOD-India-Europe connection, and the Europe-East Asia connection.