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국제학술지

공지 (2024. 9. 4.)

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Dipole structure of interannual variations in summertime tropical cyclone activity over East Asia

Abstract: The present study examines variations in summertime (July–September) tropical cyclone (TC) activity over East Asia during the period 1951–2003. To represent TC activity, a total of 853 TC best tracks for the period were converted to TC passage frequencies (TPFs) within 5° × 5° latitude–longitude grids; TPFs are defined as the percentage values obtained by dividing the number of TC appearances in each grid box by the total number of TCs each year. Empirical orthogonal function analysis of the TPF showed three leading modes: two tropical modes that represent the long-term trend and the relationship with ENSO and one midlatitude mode that oscillates between south of Korea and southeast of Japan with an interannual time scale. The latter proved to be the most remarkable climatic fluctuation of summertime TC activity in the midlatitudes and is referred to as the East Asian dipole pattern (EADP) in this paper. Anomalous atmospheric flows directly connected to the EADP are an enhanced anticyclonic (cyclonic) circulation centering around Japan when the TPF is high south of Korea (southeast of Japan), thereby showing an equivalent barotropic structure in the entire troposphere. This regional circulation anomaly varies in conjunction with the zonally oriented quasi-stationary Rossby wave train in the upper troposphere. This wave train is meridionally trapped in the vicinity of the summer-mean jet stream; therefore, the mean jet stream alters its internal meandering structure according to the phase of the wave train.
- 작성자Kim et al.
- 작성일2024.06.17
- 조회수13
- 2005
Enhancement of the consistency of MODIS thin cirrus with cloud phase by adding 1.6 ㎛ reflectance

Abstract: Optically thin cirrus derived from the Moderate-resolution Imaging Spectroradiometer (MODIS) instrument is examined in connection with its cloud phase (ice crystal or liquid droplet) for quasi‐single layer thin cirrus (either single-layer thin cirrus or thin cirrus overlying low-level thin clouds) over water surface conditions. Analysing 142 MODIS data samples during the period 1–16 March 2000, it is found that only about 20% of MODIS thin cirrus having a total‐column cloud optical depth less than 2.0 was detected as ice phase. In the present study, MODIS cloud phase datasets are newly reproduced by adding the 1.6 μm reflectance to an existing infrared trispectral algorithm of MODIS which used the 8.7, 11 and 12 mm bands. The percentage of ice phase in the thin cirrus is estimated at about 80% when new cloud phase is incorporated. The increase is found for all regions, in particular in mid-latitudes.
- 작성자Choi et al.
- 작성일2024.06.17
- 조회수6
- 2005
Influence of the Madden-Julian Oscillation on wintertime surface air temperature and cold surges in East Asia

Abstract: The variations of wintertime surface air temperature (SAT) and the occurrences of cold surges over east Asia in association with the Madden and Julian Oscillation (MJO) are examined. The MJO is defined as a combination of the two leading empirical orthogonal function modes of 850 and 200 hPa zonal winds and outgoing longwave radiation in the tropics. The spatial pattern and magnitude of SAT anomalies over east Asia significantly change with respect to MJO phases. Composite analysis reveals that the MJO-SAT relationship is mainly affected by strong temperature advection in the lower troposphere. In addition, occurrences of cold surges are identified using objectively determined synoptic criteria, and their relation to the MJO is analyzed. Most extreme cold surges occur when the MJO convection center is located over the Indian Ocean, and it is suggested that MJO-induced circulation anomalies may reinforce the amplification of normal cold surges to extreme cold surges.
- 작성자Jeong et al.
- 작성일2024.06.17
- 조회수6
- 2005
Interdecadal changes in heavy rainfall in China during the northern summer

Abstract: Using ground observations of daily rainfall over China during summer (June - August) for the period 1961 - 95, we examined interdecadal changes in the distribution of rainy events as a function of the rain rate and assessed their contributions to the total summer rainfall. We found that central China was wetter, and northern and southern China were drier in the decades following 1979 as compared to the earlier decades. The interdecadal changes in the Chinese summer rainfall are closely linked to those in large-scale circulations: the southward displacement of a westerly wind at the levels of 200 and 850 hPa over East Asia and an enhanced 850 hPa northerly wind over central-eastern China. The corresponding 850 hPa relative vorticity increased (decreased) over central (northern and southern) China where the summer rainfall has increased (decreased) during the later decades. Interestingly, the change in the total number of rainy events was found to be negligible during the entire period of data collection. Across the overall region (mostly eastern China), a large amount of interdecadal changes is accounted for by less than 20% of the total number of rainy events in the heavy-to-extremely-heavy categories (≥ ~25 mm day-1). The possible causes-associated with large-scale circulations-and the implications of the present results are discussed.
- 작성자Ho et al.
- 작성일2024.06.17
- 조회수8
- 2005
Possible influence of the Antarctic Oscillation on tropical cyclone activity in the western North Pacific

Abstract: The present study investigates how large-scale atmospheric circulation in the Southern Hemisphere (SH) modulates tropical cyclone (TC) activity in the western North Pacific (WNP) during a typhoon season (July, August, and September; boreal summer). The variation of the SH circulation of interest is the Antarctic Oscillation (AAO). In the positive phase of AAO relative to its negative phase, two anomalous highs develop over the western Pacific in both hemispheres: a huge anticyclone in southeastern Australia and a relatively weak anticyclone in the East China Sea. These teleconnection patterns are examined and compared with previous analyses. Related to the AAO variations, a statistically significant alteration of TC activities is found over the WNP. The difference in the mean TC passage numbers over the East China Sea (120°–140°E, 20°–40°N) between the eight highest-AAO years and the eight lowest-AAO years is as large as 2, equivalent to a 50–100% increase from the climatology. This change is primarily a result of more TCs forming over the eastern Philippine Sea. On the other hand, TC passage numbers slightly decrease over the South China Sea. These changes in TC activity are predominant in August and are consistent with changes in low-level vorticity over the subtropical WNP. The influence of SH circulation variability on large-scale environments and tropical convection in the subtropical NH suggest a possible usage of AAO variation for long-range forecasting of TC activity over the WNP.
- 작성자Ho et al.
- 작성일2024.06.17
- 조회수8
- 2005
Typhoon track prediction by support vector machine using data reduction methods

Abstract: Typhoon track prediction has mostly been achieved using numerical models which include a high degree of nonlinearity in the computer program. These numerical methods are not perfect and sometimes the forecasted tracks are far from those observed. Many statistical approaches have been utilized to compensate for these shortcomings in numerical modeling. In the present study, a support vector machine, which is well known to be a powerful artificial intelligent algorithm highly available for modeling nonlinear systems, is applied to predict typhoon tracks. In addition, a couple of input dimension reduction methods are also used to enhance the accuracy of the prediction system by eliminating irrelevant features from the input and to improve computational performance.
- 작성자Song et al.
- 작성일2024.06.14
- 조회수14
- 2005
Influences of tropical western and extratropical Pacific SST on East and SE Asian climate in the summers of 1993–94

Abstract: This study emphasizes the importance of sea surface temperature (SST) over the tropical western Pacific and the ocean–atmosphere coupling in the extratropical Pacific for the climate in East and Southeast Asia. Specifically, it demonstrates that the anomalies of tropical SST explain many features of the climate variability in those regions during the summers of 1993 and 1994. Very different atmospheric circulation patterns appeared in East and Southeast Asia between 1993 and 1994. Many regions including northern China, Korea, and Japan suffered from extremely high temperatures and severe droughts in the summer of 1994 but experienced reverse climate anomalies in the summer of 1993. To the south of these regions, the opposite climate patterns occurred. These climate features do not really resemble those associated with the El Niño–Southern Oscillation, which usually exerts a moderate impact on the East Asian climate. However, different SST anomalies have been observed in the tropical western and extratropical Pacific in the spring and summer between these two years. The authors carried out a series of simulations using an atmospheric circulation model and a slab oceanic model to understand the influences of these SST anomalies on the climate features. Both the uncoupled atmospheric and coupled oceanic–atmospheric experiments indicate that the tropical western Pacific SST affects the East and Southeast Asia climate significantly. Warming in the tropical western Pacific produces hot, dry conditions in northern China, Korea, and Japan, and opposite climate signals to their south. These climate anomalies produced by the local SST resemble the observed climate difference between the summers of 1994 and 1993 when positive and negative SST anomalies, respectively, existed in the tropical western Pacific. The coupled experiment also shows that the changes in extratropical atmospheric circulation caused by tropical SST anomalies generate changes in the extratropical Pacific SST, which, in turn, reinforces the climate signals produced by the tropical SST. On the other hand, the uncoupled experiments forced by the extratropical Pacific SST anomalies show that the extratropical SST exerts an insignificant impact on the East and Southeast Asian climate. The change in this SST between 1994 and 1993 generates unrealistic climate patterns in some East Asian regions, accompanying an unnatural shift of the atmospheric circulation. Full title: Influences of tropical western and extratropical Pacific SST on East and Southeast Asian climate in the summers of 1993–94
- 작성자Yoo et al.
- 작성일2024.06.14
- 조회수7
- 2004
Interdecadal changes in summertime typhoon tracks

Abstract: The present work examines interdecadal variations of typhoon tracks in the western North Pacific (WNP) during the boreal summer (June–September) for the period 1951–2001. Typhoon tracks are expressed as percentage values of the total number of typhoon passages into a 5° × 5° latitude–longitude grid box with respect to the total number of typhoons formed in the WNP. The analysis period is divided into two interdecadal periods: ID1 (1951–79) and ID2 (1980–2001). From ID1 to ID2, typhoon passage frequency decreased significantly in the East China Sea and Philippine Sea, but increased slightly in the South China Sea. The time series of typhoon passage frequency over the East China Sea and South China Sea further reveal a regime shift in the late 1970s, while those over the Philippine Sea indicate a continuous downward trend of −9% decade−1. The interdecadal changes in typhoon tracks are associated with the westward expansion of the subtropical northwestern Pacific high (SNPH) in the late 1970s. The expansion of the SNPH to the southeast coast of Asia may result in a larger elliptic pathway of typhoon migration. This is consistent with the westward shift of the typhoon tracks from ID1 to ID2, resulting in an increase of typhoon passage frequency in the South China Sea and a decrease in the East China Sea. The change of typhoon tracks is partly due to the westward shift of major typhoon formation regions associated with a warmer sea surface temperature in the South China Sea. The decreasing typhoon passage frequency over the Philippine Sea is due to less typhoon formation in recent decades. This is consistent with the decreasing cyclonic relative vorticity in the lower troposphere.
- 작성자Ho et al.
- 작성일2024.06.14
- 조회수14
- 2004
Intra-seasonal variability of wintertime temperature over East Asia

Abstract: There has been a profound warming over East Asia during the winter months (November through to March) over the past few decades. The goal of this study is to address the question of whether the daily temperature has become more variable in conjunction with this warming by using observed temperature data obtained from 155 Chinese and Korean stations. Prior to the analysis, the annual cycle is removed to obtain daily temperature anomalies for each winter for each station. Results show that the intra-seasonal variance generally decreases, implying that the daily temperatures are becoming less variable. Considering all stations as a whole, the rate of change is −0.49°C2 per decade (equivalent to −3.59% per decade). The changes are more robust in the northeastern portion of China. In contrast, there are no dominant trends for the skewness coefficients, except for clear negatively skewed trends in northeastern China. These results are consistent with an increase in the number of extremely cold events. Over the region, the frequency of low-temperature extremes (as low as below minus two standard deviations) increases at a rate of change of 0.26 days per decade, significant at the 95% confidence level. Both the Siberian high and Arctic oscillation (AO) exert a notable influence on the temperature variance. Intra-seasonal variance of the Siberian high and AO are significantly correlated with the temperature variance, whereas the seasonal mean state of the AO affects the temperature variance by modulating the high-frequency components of the Siberian high. The intra-seasonal variance of the Siberian high tends to decline at a rate of change of −10.7% per decade, significant at the 99% level; meanwhile, the mean wintertime AOs have strengthened in the last few decades. These two climate features together make a considerable contribution to the changes in intra-seasonal temperature variance in East Asia.
- 작성자Gong and Ho
- 작성일2024.06.14
- 조회수8
- 2004
Upstream subtropical signals preceding the Asian summer monsoon circulation

Abstract: In this study, the authors address several issues with respect to the antecedent signals of the large-scale Asian summer monsoon that were earlier identified by Webster and Yang. In particular, they revisit the changes in the subtropical upper-tropospheric westerlies preceding the monsoon, depict the detailed structure of the monsoon's antecedent signals, and investigate the physical processes from the signals to the monsoon. They also explore the teleconnection of these signals to various large-scale climate phenomena and emphasize the importance of the upstream location of the signals relative to the Tibetan Plateau and the monsoon. Before a strong (weak) Asian summer monsoon, the 200-mb westerlies over subtropical Asia are weak (strong) during the previous winter and spring. A significant feature of these signals is represented by the variability of the Middle East jet stream whose changes are linked to the Arctic Oscillation, North Atlantic Oscillation, El Niño–Southern Oscillation, and other climate phenomena. When this jet stream intensifies and shifts southeastward, cold air intrudes frequently from eastern Europe into the Middle East and southwestern Asia. As a result, in subtropical Asia, snow and precipitation increase, the ground wetness increases, and surface temperature decreases. A strengthening Middle East jet stream is also accompanied by increases in both stationary wave activity flux and higher-frequency eddy activities. The Tibetan Plateau acts to block these westerly activities propagating eastward and increase the persistence of the low-temperature anomalies, which in turn prolongs the atmospheric signals from winter to spring. A strong link is found between the persistent low-temperature anomalies and the decrease in geopotential height over southern Asia, including the Tibetan Plateau, in spring. The latter indicates a late establishment of the South Asian high, and implies a delay in the atmospheric transition from winter to summer conditions and in the development of the summer monsoon. The preceding scenario for a strong Middle East jet stream and a weaker Asian monsoon can be applied accordingly for the discussion of the physical processes from a weak jet stream to a strong monsoon. The current results of the relationship between the extratropical process and Asian monsoon resemble several features of the tropical–extratropical interaction mechanism for the tropospheric biennial oscillation (TBO). While the role of tropical heating is emphasized in the TBO mechanism, compared to the variability of the sea surface temperature related to El Niño–Southern Oscillation, the extratropical process examined in this study is more strongly linked to the Asian summer monsoon.
- 작성자Yang et al.
- 작성일2024.06.14
- 조회수7
- 2004