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국제학술지

공지 (2024. 9. 4.)

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Arctic Oscillation signals in the East Asian summer monsoon

Abstract: The present study examines the relationship between the Arctic Oscillation (AO) and the East Asian summer monsoon. Two rainfall data sets are used. One is obtained from 10 stations along the Yangtze River to the southern Japan and the other from gridded global land rainfall data for the period 1900–1998. All data are high-pass filtered before analyzing to highlight the interannual variability. Results show that the AO significantly influences on year-to-year variations in the East Asian summer monsoon rainfall. When AO leads by one month, the correlation between May–July AO and summer total rainfall is −0.44. When AO leads by two months, correlation becomes −0.32. Of all monthly, May AO shows the strongest connection to the summer monsoon rainfall. Correlation coefficient between them is −0.45. The large-scale atmospheric circulation patterns in East Asia in association with the AO are also evident. A positive phase of the AO in late spring is found to lead to a northward shift in the summertime upper tropospheric jet stream over East Asia. This northward shift of the jet stream is closely related to anomalous sinking motion in 20°–40°N and rising motion in surrounding regions. These changes give rise to a drier condition over the region extending from the Yangtze River valley to the southern Japan and a wetter condition in the southern China. Possible mechanisms connecting the late spring AO and summer monsoon rainfall are suggested.
- 작성자Gong and Ho
- 작성일2024.06.14
- 조회수12
- 2003
A sudden change in summer rainfall characteristics in Korea during the late 1970s

Abstract: We have examined long-term climate change in Korea by studying daily rainfall data over a period of 48 years (1954–2001). The results show that there is a more frequent heavy rainfall anomaly larger than 100 mm per 3 months in recent years. For further investigation, we divide the whole period into two 24 year intervals, 1954–77 and 1978–2001. Two well-defined rainfall peaks occur during summertime in both intervals. During the earlier interval, primary and secondary rainfall peaks are found in early July and early September, respectively. In the later interval, on the other hand, the secondary peak is found in mid–late August, mainly attributed to enhanced heavy rainfall (≥30 mm day−1) events. Although a similar shift occurs in the primary peak, it is much smaller. Thus, the relatively dry spell between the two peaks becomes shorter in the later interval compared with the earlier one. The domain-mean geopotential height at 700 hPa (Φ700) over mid-latitude Asia (30–50° N, 60–120° E) for the summer also experienced a sudden increase in the mid 1970s. A comparison of the spatial distribution of Φ700 between the two intervals shows large positive differences over the central-eastern Asian continent in the later interval. In contrast to the positive anomaly of Φ700 in the later interval, there is a decreasing trend in surface temperature. The increased Φ700 introduces a stronger northerly wind over East Asia and possibly produces a moisture convergence, enhanced convective activity, and heavy rainfall over the region, in particular over Korea and central China.
- 작성자Ho et al.
- 작성일2024.06.14
- 조회수6
- 2003
Detection of large-scale climate signals in spring vegetation index (NDVI) over the Northern Hemisphere

Abstract: Climate is one of the determinants driving ecosystems on both local and global scales. During the last two decades, there has occurred a dramatic temperature increase in the northern midlatitudes to high latitudes. The rapid warming has resulted in the promotion of bioactivity and an early growing season. However, the temperature and vegetation changes are not uniform in geographical distribution. In the present study, we analyze the spatial features in the normalized difference vegetation index (NDVI)-temperature relationship over Eurasia and North America in spring for the period 1982–2000. The NDVI data are derived from the Earth Observing System Pathfinder advanced very high resolution radiometer data sets. A singular value decomposition analysis is applied to the covariance matrix of the NDVI and temperature. Most of the squared covariance, 91.6%, is captured by the first seven paired modes. The result clearly indicates that the temperature is a focal factor influencing vegetation activity. Furthermore, those seven paired modes show large-scale features and well-defined patterns. The atmospheric circulation systems, such as the Southern Oscillation, North Atlantic Oscillation/Arctic Oscillation, Pacific/North American pattern, Eurasian pattern, western Pacific pattern, western Atlantic pattern, eastern Atlantic pattern, and North Pacific index, are found to be associated with that. The time coefficient corresponding to the first paired modes, centered on western Siberia, is correlated significantly with the Eurasian teleconnection pattern. Their correlation coefficients are 0.72 and 0.78 for vegetation and temperature, respectively, for the data period. Other modes are also correlated with one or more circulation indices. This implies that the large-scale circulation is essential for understanding the regional response of vegetation to global climate change. Taking all nine circulation indices and time lags into account, a large portion (71%) of the satellite-sensed variance in NDVI could be explained. The temperature-NDVI relationships did not change significantly when the NDVI was rescaled from 1 to 5 degrees, indicating that the singular value decomposition analysis is an appropriate technique for detecting the degree of coupling between vegetation and climate and that the vegetation-temperature relationship presented in this study is robust. Full title: Detection of large-scale climate signals in spring vegetation index (normalized difference vegetation index) over the Northern Hemisphere
- 작성자Gong and Ho
- 작성일2024.06.14
- 조회수7
- 2003
Comparison of different earth radiation budget experiment data sets over tropical oceans

Abstract: We compare radiation budgets derived from different Earth Radiation Budget Experiment (ERBE) archives over the tropical oceans (30S and 30N) from 1985 to 1989. Two ERBE data sets are used. One is taken from the Earth Radiation Budget Satellite (ERBS), and the other from the combined ERBS, National Oceanic and Atmospheric Administration (NOAA) 9, and NOAA 10 satellites. The domain-mean all-sky outgoing longwave radiation (OLR) derived from the combined data set shows a notable change in early 1987 when NOAA 10 replaced NOAA 9. This change is also found in longwave (LW) cloud radiative forcing (CRF), all-sky shortwave (SW) radiation, and SWCRF. The ERBS, however, does not show such changes. We also examine the sensitivity of cloud–radiation interaction to the sea surface temperature (SST) of the tropical oceans. In general, each component of radiative feedbacks derived from the two ERBE data have the same sign, although they show a certain degree of discrepancy in the magnitude. The discrepancy is more notable for averaged quantities over the entire tropical oceans, particularly over the subtropics where convective activities are relatively weak. The combined data show a larger sensitivity of LWCRF and SWCRF to SST than those of the ERBS, consistent with the above results. The response of clouds to an increase in SST has a net cooling effect when using the combined data but has a net heating effect when using the ERBS data (−0.80 W m−2 K−1 versus 0.48 W m−2 K−1). Most of the discrepancies of the net CRF between the two ERBE data sets can be accounted for by the difference in the sensitivity of all-sky OLR (4.52 W m−2 K−1 versus 1.73 W m−2 K−1).
- 작성자Ho et al.
- 작성일2024.06.14
- 조회수6
- 2002
Shift in the summer rainfall over the Yangtze River valley in the late 1970s

Abstract: The summer rainfall over the middle-lower valley of the Yangtze River and over the whole eastern China experienced a notable regime shift in about 1979. This change is consistent with a simultaneous jump-like change in the 500 hPa geopotential height (Φ500) over the northern Pacific. The rainfall over the Yangtze River valley is closely related to the Φ500 averaged over the area 20°–25°N, 125°–140°E, with a correlation coefficient of 0.66 for the period 1958–1999. Since 1980, the subtropical northwestern Pacific high (SNPH) has enlarged, intensified, and extended southwestward. The changes in the SNPH are strongly associated with the variations of the sea surface temperatures (SSTs) of the eastern tropical Pacific and tropical Indian Ocean. The anomalies of these SSTs, responsible primarily for the shift of the summer rainfall over the Yangtze River through the changes in SNPH, precede the Φ500 signals with different leading times.
- 작성자Gong and Ho
- 작성일2024.06.14
- 조회수7
- 2002
The Siberian High and climate change over middle to high latitude Asia

Summary: The Siberian High is the most important atmospheric centre of action in Eurasia during the winter months. Here its variability and relationship with temperature and precipitation is investigated for the period 1922 to 2000. The pronounced weakening of the Siberian High during the last ∼ 20 years is its most remarkable feature. Mean temperature, averaged over middle to high latitude Asia (30° E–140° E, 30° N–70° N), is correlated with the Siberian High central intensity (SHCI) with correlation coefficient of − 0.58 (1922–1999), and for precipitation, the correlation coefficient is − 0.44 (1922–1998). Taking the Arctic Oscillation (AO), the SHCI, the Eurasian teleconnection pattern (EU), and the Southern Oscillation (SO) index into account, 72 percent of the variance in temperature can be explained for the period 1949–1997 (for precipitation the variance is 26 percent), with the AO alone explaining 30 percent of the variance, and the Siberian High contributing 24 percent. The precipitation variance explained by the Siberian High is only 9.8 percent of the total.
- 작성자Gong and Ho
- 작성일2024.06.14
- 조회수18
- 2002
Comparison of tropical rainfall between the observed GPCP data and the assimilation products of ECMWF, NCEP/NCAR, ...

Abstract: Combined tropical precipitation of Global Precipitation Climatology Project (GPCP) version 1 is compared with the assimilated precipitation obtained from the European Center for Medium-Range Weather Forecast (ECMWF), the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), and the NASA Goddard Earth Observing System (GEOS-1) reanalysis projects. The analysis focuses on the domain within 30°S-30°N, and the period is from July 1987 to December 1993. Annual-mean precipitation shows that the ECMWF overestimates the precipitation over tropical oceans as compared to the GPCP. On the other hand, the NCEP/NCAR and NASA-GEOS-1 underestimate precipitation of the Inter-Tropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ). Horizontal distributions of the difference in intra-annual and inter-annual precipitation variations between the GPCP and the three reanalysis datasets are similar to those of the difference in annual-mean precipitation. Overall, the ECMWF exhibits the highest standard deviation of precipitation over tropical oceans, followed by the GPCP, NASA-GEOS-1, and NCEP/NCAR. The correlation features of the area-averaged precipitation with the sea surface temperatures over the eastern equatorial Pacific Ocean are also compared among the four products, and their differences are discussed. Full title: Comparison of tropical rainfall between the observed GPCP data and the assimilation products of ECMWF, NCEP/NCAR, and NASA-GEOS-1
- 작성자Lim and Ho
- 작성일2024.06.14
- 조회수9
- 2000
Principal modes of climatological seasonal and intraseasonal variations of the Asian summer monsoon

Abstract: Principal modes of climatological variation of the Asian summer monsoon are investigated. Data used in this study include the high cloud fraction produced by the International Satellite Cloud Climatology Project and sea level pressure, and 850- and 200-mb geopotential heights from ECMWF analysis for the five summers of 1985–89. It is shown that the seasonal evolution of the Asian summer monsoon is adequately described by a few leading EOFs. These EOFs capture the variations of regional rainbands over the East Asian and Indian regions. The first mode is characterized by an increase in large-scale cloud over India and the subtropical western Pacific until mid-August. The second mode depicts large-scale cloud variations associated with the East Asian rainband referred to as Mei Yu and Baiu. This mode is associated with the development of summer monsoon circulation: a low pressure system over the Asian continent and a subtropical high over the Pacific. The third eigenmode is characterized by zonal cloud bands from northern India, crossing the Korean peninsula to Japan, and dryness over the oceans in the south of cloud bands. This mode is related to the mature phase of Changma rainy season in Korea associated with the northward movement of cloud bands and circulation systems from the subtropical western Pacific. This mode appears as a first principal mode of climatological intraseasonal oscillation (CISO) over the entire Asian monsoon region. The CISO mode has a timescale of about 2 months. The northward moving CISO also appears in the 850- and 200-mb geopotential height fields as a first mode of each dataset. Based on the height variations of the CISO mode, it is suggested that the extratropical CISO during summer is related to a regional index cycle associated with the variation of north–south temperature gradient in East Asia.
- 작성자Kang et al.
- 작성일2024.06.14
- 조회수8
- 1999
Anomalous atmospheric hydrologic processes associated with ENSO: Mechanisms of hydrologic cycle-radiation interaction

Abstract: Using reanalysis data from the Goddard Earth Observing System (GEOS) Data Assimilation System, the authors have documented the basic three-dimensional features of anomalous atmospheric hydrologic processes observed during the El Niño–Southern Oscillation (ENSO). The most dominant anomaly pattern features a pair of subtropical temperature maxima straddling the equator in the upper troposphere coupled to a corresponding pair of temperature minima in the lower stratosphere in the form of a dipole. Over the Tropics and subtropics, the water vapor content is increased in regions of large-scale ascent with maximum response in the middle troposphere, whereas substantial drying is found in the descending branches of the Walker and Hadley circulations. While the temperature and moisture patterns in the lower troposphere are thermodynamically linked to the sea surface temperature anomaly pattern, the distribution of temperature and water vapor in the upper troposphere is largely controlled by dynamics and much less by thermodynamics. The troposphere–stratosphere temperature dipole is fundamentally due to the rising of the tropopause associated with hydrostatic expansion and vertical ascent in regions of enhanced deep convection. The rising motion pushes colder upper-tropospheric air into the lower stratosphere where the climatological temperature gradient reverses. No such dipole anomaly exists in the moisture field. Numerical experiments with the GEOS GCM show that while atmospheric dynamics are principally responsible for the generation of the basic structures of the temperature and moisture anomalies observed during ENSO, the interaction between the hydrologic cycle and radiation plays an important role in enhancing and modifying the response. The role of hydrologic cycle–radiation interaction is most important in rendering the atmosphere more unstable both columnwise and locally, through enhanced longwave heating in the middle and lower troposphere and cooling above. The enhanced instability leads to intensified Hadley and Walker circulations, which are accompanied by stronger latent heating and a more vigorous hydrologic cycle. The intensified hydrologic cycle promotes further warming and moistening of the middle and lower troposphere, and cooling and drying in the stratosphere. The radiation–dynamics feedback leads to a new equilibrium climate state in which the increased heat transport by convection into the upper troposphere and stratosphere is balanced by increased radiative cooling, which removes the local excessive heat buildup.
- 작성자Lau et al.
- 작성일2024.06.14
- 조회수11
- 1998
Comparison of model-calculated and ERBE-retrieved clear-sky outgoing longwave radiation

Abstract: In order to validate radiative transfer models and identify sources of errors in the satellite retrieval of radiation budgets, model calculations of clear-sky outgoing longwave radiation (OLR) over oceans are compared with data from the Earth Radiation Budget Experiment (ERBE). The NASA Goddard Earth Observing System (GEOS-1) and the National Centers for Environmental Prediction and National Center for Atmospheric Research reanalyses of temperature and humidity and the Stratospheric Aerosol and Gas Experiment retrieval of the stratospheric humidity are used in the model calculations. Averaged over time (1985–1989) and space (60°S-60°N), the model-calculated clear-sky OLR has a positive bias of 1.9–2.3 W m−2 when compared with that of ERBE. Nearly all of the bias can be accounted for by the inclusion of the absorption due to CO2 in the 4.3-μm band and to the weak CO2 and O3 molecular lines distant from band centers. The use of the two different reanalyses has only a small effect on the flux calculations (∼0.4 W m−2). Consistent with suggestions by previous studies, ERBE is found to overestimate (underestimate) the clear-sky OLR over high humid (dry) regions due to incorrect identification of clear scenes in the ERBE retrievals. The importance of the upper tropospheric humidity in affecting the Earth radiation budget is also investigated. Although only ∼15% of the atmospheric humidity is contained in the region above the 600-hPa level, the upper troposphere is as important as the lower troposphere in contributing to the clear-sky OLR.
- 작성자Ho et al.
- 작성일2024.06.14
- 조회수10
- 1998