연구보고서
- 저자
- 이우섭 박사
- 작성일
- 2016.01.23
- 조회
- 148
- 요약
- 목차
Prediction of Korean heat wave variability is a scientific challenge of practical importance. This study investigates the variability of heat waves in South Korea in terms of frequency, duration, and intensity over the past 40 years (1973-2012). Daily maximum temperatures were used to define the indices of extreme months based on the number of days that crossed the thresholds. Empirical orthogonal function (EOF) analysis was applied to the monthly indices. Correlation between the EOF principal components and the time series of other fields allows for plotting maps that highlight the anomalies in the large scale circulation and in the SSTs that are associated with the occurrence of Korean heat waves.
A noteworthy feature exhibited in the results is the north-south dipole mode that dominates the variability of the South China Sea, with opposite signs at the East Asian sector. The positive correlation of the vorticity at 150hPa in the East China Sea induces more convection and diabatic heating, which in turn becomes a source of a Rossby wave-train along the southerly wind that generates positive geo-potential height anomalies around Korea. It can be concluded that Rossby waves are generated by the heat source, with positive correlation to vorticity over South China during July-August.
To investigate the mechanisms behind the circulation changes, we calculated the heat budget correlation map for July-August using thermodynamic equations. Over Korea, the adiabatic atmospheric warming due to the anomalous downward motion of air is dominant. On the other hand, we found a positive correlation between the cyclonic circulation and Korean heat wave from the Tropical Western Pacific to the southern part of China.
This circulation change facilitates the convective activity in the region but it weakens the moist transport from the South China Sea to the eastern part of Korea. Anomalous meridional circulation associated with the Korean heat wave links with the anomalous downward motion over the tropical ocean and the anomalous ascent of air at about 20°N. This leads to extreme (hot and dry) conditions over Korea. The observed adiabatic warming around Korea indicates that the strong positive SST correlation around East Asia is not the primary cause of the Korean heat wave.
Korea experienced extraordinary heat waves in the summers of 1983, 1994 and 2012 with high maximum temperatures. This may be linked to the strong positive IOD index just to the south of Korea. To identify the dynamical evolution and features associated with the Korean heat wave, composites of the dynamical fields (wind, vorticity, and GPH) were tested for the three extreme events (1983, 1994, and 2012). We have calculated the SSTA for July-August (JA) and its standard deviation using the Indian Ocean Dipole Mode Index (IOD). The IOD indices for the abnormal summer years of 1983, 1994, and 2012 show variances of about 1.2σ, 2.8σ, and 1.43σ, respectively. We also show that the Korean hot summer patterns were influenced by the IOD. A cyclonic anomaly was found to the east of the vorticity source region. A Rossby wave train was excited by the vorticity source, propagating northward from southern China.
The IOD-induced cyclonic circulation over the southern part of China and the Western Pacific weakened the seasonal northward wind from the Bay of Bengal, the South China Sea, and the Tropical Western Pacific, prohibiting Korea from receiving the normal moisture from these tropical regions. The anomalously hot summer climate over Korea is explained as a result of the anomalous dynamic heating around Korea and diabatic heating over the eastern part of China. Therefore, the sea surface temperature (SST) anomaly in the Indian Ocean played an important role in forming the Korean heat wave intensity.
As the Korean heat wave event started around early June, the associated downdraft induced adiabatic atmospheric warming and drying through entrainment of dry air from the upper atmosphere. This warmer and drier air reduced precipitation and total cloud amount and decreased soil moisture. The reduced total cloud amount increased downward surface solar radiation with a mean anomaly magnitude of approximately 10 W m-2 over South Korea. Therefore, the maximum temperature at the surface rose.
The decreased soil moisture contributed to a decrease in latent heat flux from the land to the atmosphere. The drier land, increased surface shortwave radiation, and decreased latent heat flux all contributed to the rapid warming of the surface air and land. The large increases in the sensible heat flux (about 6.2 Wm-2 ) and longwave radiation (about 1.2Wm-2 ) reflected the increasing surface temperatures. Also shown is evidence that the Korean heat wave was amplified by positive feedback through changes in surface energy fluxes between the atmospheric high pressure pattern and soil moisture.