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2001 Vol. 18, No. 4

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Meridional Wind Stress Anomalies over Tropical Pacific and the Onset of El Nino. Part Ⅰ: Data Analysis
Zhang Renhe, Zhao Gang, Tan Yanke
2001, 18(4): 467-480. doi: 10.1007/s00376-001-0038-9
By using correlation analysis and singular value decomposition (SVD) methods, diagnostic studies are made to investigate the relationship between the meridional wind stress anomalies over the tropical Pacific and El Nino events. The correlation of the sea surface temperature anomalies (SSTA) in the NINO3 region (150°-90°W, 5°S-5°N) and the leading meridional wind stress anomalies shows that the NINO3 SSTA is significantly correlated with the anomalous meridional wind stress convergence in the equatorial eastern Pacific at the leading time of more than six months. With the reduction of the leading time, the correlation becomes stronger and the convergence area with statistical significance enlarges and extends to the west. The coupling patterns between SSTA in tropic Pacificand the leading meridional wind stress anomalies revealed by SVD show the similar feature to that revealed by the correlation analysis. The converging meridional wind stress about the equator in the eastern equatorial Pacific precedes the increasing of SSTA in the central and eastern equatorial Pacific as early as half a year or more. Compared to the leading zonal wind stress anomalies in the tropical western Pacific, it seems that the increasing of the NINO3 SSTA is more related to the convergence of the leading meridional wind stress anomalies in the eastern equatorial Pacific. It is suggested that preceding meridional wind stress anomalies may play an important role in the occurrence of El Nino events.
The Regions with the Most Significant Temperature Trends During the Last Century
Zeng Zhaomei, Yan Zhongwei, Ye Duzheng
2001, 18(4): 481-496. doi: 10.1007/s00376-001-0039-8
Having analyzed a global grid temperature anomaly data set and some sea level pressure data during the last century, we found the following facts. Firstly, the annual temperature change with a warming trend of about 0.6℃/100 years in the tropical area over Indian to the western Pacific Oceans was most closely correlated to the global mean change. Therefore, the temperature change in this area might serve as an indicator of global mean change at annual and longer time scales. Secondly, a cooling of about -0.3℃ / 100 years occurred over the northern Atlantic. Thirdly, a two-wave pattern of temperature change, warming over northern Asia and northwestern America and cooling over the northern Atlantic and the northern Pacific, occurred during the last half century linked to strengthening westerlies over the northern Atlantic and the weakening Siberian High. Fourthly, a remarkable seasonal difference occurred over the Eurasian continent, with cooling (warming) in winter (summer) during 1896-1945, and warming (cooling) in winter (summer) during 1946-1995. The corresponding variations of the North Atlantic Oscillation and the Southern Oscillation were also discussed.
Vertical Circulation Structure, lnterannual Variation Features and Variation Mechanism of Western Pacific Subtropical High
He Jinhai, Zhou Bing, Wen Min, Li Feng
2001, 18(4): 497-510. doi: 10.1007/s00376-001-0040-2
The paper investigates the vertical circulation structure of the western Pacific subtropical high (STH) and its interannual variation features in relation to East Asian subtropical summer monsoon and external thermal forcing by using the high-resolution and good-quality observations from the 1998 South China Sea Summer Monsoon Experiment (SCSMEX), the NCEP 40-year reanalysis data and relevant SST and the STH parameters. It is found that the vertical circulation structures differ greatly in features between quasi-stationary and transient components of the western Pacific STH. When rainstorms happen in the rainkband of East Asian subtropical monsoon on the STH north side, the downdrafts are distinct around the ridge at a related meridian. The sinking at high (low) levels comes from the north (south) side of the STH,thereby revealing that the high is a tie between tropical and extratropical systems. The analyses of this paper suggest that the latent heat release associated with subtropical monsoon precipitation, the offshore SST and East Asian land-sea thermal contrast have a significant effect on the STH interannual anomaly. Our numerical experiment shows that the offshore SSTA-caused sensible heating may excite an anomalous anticyclonic circulation on the west side, which affects the intensity (area) and meridional position of the western Pacific STH.
lnterannual and lnterdecadal Variability of East Asian Acas and Their Impact on Temperature of China in Winter Season for the Last Century
Qian Weihong, Zhang Henian, Zhu Yafen, Dong-Kyou Lee
2001, 18(4): 511-523. doi: 10.1007/s00376-001-0041-1
The interannual and interdecadal variability of the Siberian High (SH) and the Aleutian Low (AL) from aspects of strength and location for the past one hundred years as well as their possible relations with temperature changes over mainland China are investigated. The data sets used are the historical sea level pressure for 1871-1995 and surface air temperature (SAT) over China in the last 100 years. The results show that the SAT in different regions over China, central strength of the SH and the AL, the south-reaching latitude of the 1030 hPa contour of the SH and the pressure gradient between the SH and the AL experienced two obvious changes during this period. One occurred in the 1920s, with a more prominent one in the 1980s. These variations are closely linked with the change of winter temperature over China in the interdecadal timescale. In the last 50 years, there is a remarkable interannual correlation between the strength of Active Centers of Atmosphere (Acas) and the winter temperature of northern and eastern regions in China. The abrupt change of Acas in the 1980s is consistent with the rising of the SAT in China. Since the late 1980s, the atmospheric circulation is experiencing a remarkable modulation, which may cause the interdecadal transition of warming trend.
Topographic Effect on Geostrophic Adjustment and Frontogenesis
Fang Juan, Wu Rongsheng
2001, 18(4): 524-538. doi: 10.1007/s00376-001-0042-0
Three conservative principles: potential vorticity, absolute momentum and potential temperature are used to study the influence of topography on the local frontogenesis and geostrophic adjustment, which are induced by the inhomogeneous thermal fields. It is found that the horizontal distribution of the initial potential temperature and its position relative to the mountain play important roles during the geostrophic adjustment and local frontogenesis. The frontogenesis is weakened by the mountain when the initial thermal perturbation is located at the base of the upwind slope. The frontal discontinuity cannot occur unless the horizontal contrast of the initial potential temperature is great enough. Whereas, the situation is opposite when the initial thermal disturbance is mainly situated near the peak of the mountain. Complementary to the aforementioned cases, the effect of topography on the frontogenesis depends on the stratification of the flow when the initial thermal disturbance lies at the foot of lee slope. For weak stratification, topography is favorable to the formation of frontal discontinuity, vice versa. This discrepancy is attributed to the difference of subsidence warming, caused by the mountain, when the stratification is either strong or weak. Furthermore, the energy conversion ratio between the kinetic and potential energy during the geostrophic adjustment process is also affected by the topography. In contrast to the flat bottom case, the ratio is reduced (increased) when the initial thermal perturbation lies in the up-wind slope (lee slope). The reason is that the gravity force does negative work in the former case while does positive work in the latter case.
Effects of Aerosol Vertical Inhomogeneity on the Upwelling Radiance and Satellite Remote Sensing of Surface Reflectance
Qiu Jinhuan, Nobuo Takeuchi
2001, 18(4): 539-553. doi: 10.1007/s00376-001-0043-z
There are two widely used radiative models without consideration of aerosol inhomogeneity for satellite remote sensing application, the Homogeneous Model and the Two-layer Model with aerosol in the lower layer. In this paper, effects of the aerosol vertical inhomogeneity on upwelling radiance and satellite remote sensing of surface reflectance are analyzed through numerical simulations by using two models. As shown in the simulations by using 24 representative aerosol models, there is often a considerably large error in upwelling radiance calculated by two models (Homogeneous and Two-layer) for the short wavelength channel with strong molecular scattering, owing to the difference between molecular and aerosol scattering properties. For the long wavelength channel, the error is small if aerosol optical parameters are less variable with height, but it could also be significant if there are aerosol layers with different scattering phase functions and single scattering albedo. The radiance errors by the Homogeneous Model and the Two-layer Model can be up to 31.4% and 31.5% for the clean atmosphere, and in case of turbid atmosphere 67.8% and 59.2%,respectively. The radiance error could result in a large uncertainty of surface reflectance retrievals, especially for the short wavelength channel and the strongly absorbing aerosol. For the turbid atmosphere with strongly absorbing aerosol, the Homogeneous Model and the Two-layer Model are not suitable for atmospheric correction application.
Origin of the TBO-Interaction between Anomalous East-Asian Winter Monsoon and ENSO Cycle
Li Chongyin, Sun Shuqing, Mu Mingquan
2001, 18(4): 554-566. doi: 10.1007/s00376-001-0044-y
Based on the data analyses by using NCEP/NCAR reanalysis data and other data (OLR, precipitation and temperature), it is shown that the tropospheric circulation and climate in East Asia and the northwestern Pacific region have the evident quasi-biennial oscillation (TBO) feature. It is also shown that anomalous East Asian winter monsoon can impact the atmospheric circulation and climate variations in the following summer, particularly in East Asian region; there is clear interaction between anomalous East Asian winter monsoon and ENSO cycle. The continuous strong (weak) East Asian winter monsoon can excite El Nino (La Nina) through the air-sea interaction, the El Nino (La Nina) event can lead the East Asian winter monsoon to be weak (strong) through the teleconnections or remote responses. The strong or weak winter monsoon and ENSO cycle are linked each other. It can be suggested that interaction between anomalous East Asian winter monsoon and ENSO cycle is a fundamental origin of the TBO.
Interannual to Interdecadal Variation of East Asian Summer Monsoon and its Association with the Global Atmospheric Circulation and Sea Surface Temperature
Xue Feng
2001, 18(4): 567-575. doi: 10.1007/s00376-001-0045-x
The East Asian summer monsoon (EASM) underwent an interdecadal variation with interannual variations during the period from 1958 to 1997, its index tended to decline from a higher stage in the mid-1960's until it reached a lower stage after 1980's. Correlation analysis reveals that EASM is closely related with the global atmospheric circulation and sea surface temperature (SST). The differences between the weak and strong stage of EASM shows that, the summer monsoon circulation over East Asia and North Africa is sharply weakened, in the meantime, the westerlies in high latitudes and the trade--wind over the tropical ocean are also changed significantly. Over the most regions south of the northern subtropics, both air temperature in the lower troposphere and SST tended to rise compared with the strong stage of EASM. It is also revealed that the ocean-atmosphere interaction over the western Pacific and Indian Ocean plays a key role in interannual to interdecadal variation of EASM, most probably, the subtropical Indian Ocean is more important. On the other hand, the ENSO event is less related to EASM at least during the concerned period.
Recent Researches on Aerosol in China
Wang Mingxing, Zhang Renjian, Pu Yifen
2001, 18(4): 576-586. doi: 10.1007/s00376-001-0046-9
In recent years, the Chinese scientists have undertaken numerous researches on aerosol, and made many achievements. This paper reviews the above researches on aerosols which mainly focus on the following three aspects: the characteristics of aerosols, the effects on climate, and dust aerosol research. Present international research plans in relation to aerosols are briefly introduced. The important fields and trends of atmospheric aerosol researches in the near future are also pointed out and discussed.
Predictions of ENSO with a Coupled Atmosphere-Ocean General Circulation Model
Zhou Guangqing, Zeng Qingcun
2001, 18(4): 587-603. doi: 10.1007/s00376-001-0047-8
Predictions of ENSO are described by using a coupled atmosphere-ocean general circulation model. The initial conditions are created by forcing the coupled system using SST anomalies in the tropical Pacific at the background of the coupled model climatology. A series of 24-month hindcasts for the period from November 1981 to December 1997 are carried out to validate the performance of the coupled system. Correlations of SST anomalies in the Nino3 region exceed 0.54 up to 15 months in advance and the rms errors are less than 0.9℃. The system is more skillful in predicting SST anomalies in the 1980s and less in the 1990s. The model skills are also seasonal-dependent, which are lower for the predictions starting from late autumn to winter and higher for those from spring to autumn in a year-time forecast length. The prediction, beginning from March, persists 8 months long with the correlation skill exceeding 0.6, which is important in predictions of summer rainfall in China. The predictions are succesful in many aspects for the 1997-2000 ENSO events.
Construction of Explicit Quasi-complete Square Conservative Difference Schemes of Forced Dissipative Nonlinear Evolution Equations
Lin Wantao, Ji Zhongzhen, Wang Bin
2001, 18(4): 604-612. doi: 10.1007/s00376-001-0048-7
Based on the forced dissipative nonlinear evolution equations for describing the motion of atmosphere and ocean, the computational stability of the explicit difference schemes of the forced dissipative nonlinear atmospheric and oceanic equations is analyzed and the computationally stable explicit complete square conservative difference schemes are constructed. The theoretical analysis and numerical experiment prove that the explicit complete square conservative difference schemes are computationally stable and deserve to be disseminated.
On the Annual Cycle Characteristics of the Sea Surface Height in South China Sea
Liu Qinyu, Jia Yinglai, Wang Xiaohua, Yang Haijun
2001, 18(4): 613-622. doi: 10.1007/s00376-001-0049-6
The annual cycle characteristics of the SSH in the South China Sea (SCS) are analyzed based on the Sea Surface Height (SSH) anomaly data from the TOPEX / POSEIDON-ERS altimeter data and the Parallel Ocean Climate Model (POCM) prediction. The results show that the distributions of the SSH anomalies of the SCS in January, March and May, are opposite to those in July, September and November respectively; In January (July) there is the SSH negative (positive) anomaly in the deep water basin and at the Luzon Strait, while there is positive (negative) anomaly on the most of continental shelves in the west and south of South China Sea; In March (September) the SSH anomalies are similar to those in January (July), although their magnitudes have decreased and a small positive (negative) anomaly appears in the center of the South China Sea; The amplitude of the SSH annual cycle reaches its maximum in the Northwest of the Luzon Island; The seasonal variability of the wind stress is dominant in the formation of the SSH seasonal variability.
Analysis of the Characteristics of 30-60 Day Low-Frequency Oscillation over Asia during 1998 SCSMEX
Chen Longxun, Zhu Congwen, Wang Wen, Zhang Peiqun
2001, 18(4): 623-638. doi: 10.1007/s00376-001-0050-0
The wavelet analysis is performed of the mid- and low-latitude circulation index at 850 hPa over East Asia, the East Asian monsoon index and the precipitation over the middle and lower reaches of the Yangtze River during 1998 South China Sea Monsoon Experiment (SCSMEX) from May to August. Analysis shows that distinct 30-60 day low-frequency oscillation (LFO) exists in all of the above elements during the exper iment period. Analysis of low-frequency wind field at 850 hPa from May to August with 5 days interval is performed in this paper. Analysis results reveal that: (1)A low-frequency monsoon circulation system over East Asia, characterized by distinct 30-60 day low-frequency oscillation, exists over 100°-lS0°E of East Asian area from the middle and eastern parts of China continent and the South China Sea to the western Pacific in both the Northern and Southern Hemisphere. The activity of East Asian monsoon is mainly affected by the low-frequency systems in it; (2) All of the tropical monsoon onset over the South China Sea in the fifth pentad of May, the beginning of the Meiyu period and heavy rainfall over the middle and lower reaches of the Yangtze River in mid-June and the heavy rainfall after mid-July are related to the activity of low-frequency cyclone belt over the region, whereas the torrential rainfall over the upper reaches of the Yangtze River in August is associated with the westward propagation of low-frequency anticyclone into the mainland; (3) There are two sources of low-frequency oscillation system over East Asia during SCSMEX. i.e. the equatorial South China Sea (SCS) and mid-high latitudes of the middle Pacific in the Northern Hemisphere. The low-frequency system over SCS propagates northward while that in mid-high latitudes mainly propagates from northeast to southwest. Both of the heavy rainfall over the middle and lower reaches of the Yangtze River in June and July are associated with the northward propagation of the above-mentioned SCS low-frequency systems from the tropical region and the southwestward propagation from mid-high latitudes respectively and their convergence in the middle and lower reaches of the Yangtze River; (4) There are two activities of iow-frequency cyclone and anticyclone belt each in the East Asian monsoon system during May to August. However the activity of these low-frequency circulation systems is not clearly relevant to the low-frequency circulation system in the Indian monsoon system. This means that the low-frequency circulation systems in Indian monsoon and East Asian monsoon are independent of each other. The concept previously put forward by Chinese scholars that the East Asian monsoon circulation system (EAMCS) is relatively independent monsoon circulation system is testified once more in the summer 1998.